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| author | nfenwick <nfenwick@pglaf.org> | 2025-03-04 01:22:03 -0800 |
|---|---|---|
| committer | nfenwick <nfenwick@pglaf.org> | 2025-03-04 01:22:03 -0800 |
| commit | 770326b6a6caee6abe670461f390f8b603afb212 (patch) | |
| tree | 95bc216888b1eb2b381c728cc4d9fa7a7fa2d4ca /44000-h | |
| parent | 54ec9f8070f1520d5856adacc3c982f0ef8d916d (diff) | |
Diffstat (limited to '44000-h')
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diff --git a/44000-h/44000-h.htm b/44000-h/44000-h.htm new file mode 100644 index 0000000..a99c6aa --- /dev/null +++ b/44000-h/44000-h.htm @@ -0,0 +1,22309 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <title> + Origin of Vertebrates. + </title> + + <style type="text/css"> + /*---------------------------------------- Default settings for tags -------------------------------------*/ + a:hover { color:red } + a:link { color:blue;text-decoration:none; } + a:visited { color:blue;text-decoration:none; } + body { margin-left:10%; margin-right:10%; text-align:justify; } + img { border:0; } + hr { margin-bottom:3ex; margin-left:auto; margin-right:auto; display:block; } + p { margin-bottom:3ex; margin-top:0ex; } + sub { font-style:normal; font-size:80%; } + sup { font-style:normal; font-size:80%; } + table { border-collapse:collapse; } + td { padding:0 0.5em; text-align:left; vertical-align:top; border:0;} + th { padding:1em 0.5em; text-align:center; vertical-align:middle; border:0; font-weight:normal; } + /*------------------------------------------- Paragraph spacings -----------------------------------------*/ + p.sp0 { margin-bottom:0ex; } + p.stanza { margin-top:2ex; } + .poem>p { margin-bottom:0ex; padding-left:3em; text-indent:-3em; text-align:left; } + .smaller>p { margin-bottom:1.5ex; } + .sp2, div.sp2, table.sp2 { margin-bottom:3ex; } + .sp3, div.sp3, table.sp3 { margin-bottom:4.5ex; } + .sp4 { margin-bottom:6ex; } + .sp5, div.sp5, table.sp5 { margin-bottom:7.5ex; } + /*---------------------------------------------- Font sizing ---------------------------------------------*/ + .sc { font-variant:small-caps; } + .xx-smaller { font-size:58%; } + .smaller { font-size:83%; } + .larger { font-size:120%; } + .x-larger { font-size:144%; } + .xxx-larger { font-size:207%; } + /*-------------------------------------------- Text Decoration -------------------------------------------*/ + .gsp { letter-spacing:0.2em } + .gap { display:inline-block; width:2em; } + .hid { visibility:hidden; } + .wnw { white-space:nowrap; } + /*-------------------------------------- Non-Table Text Positioning --------------------------------------*/ + .bq1 { margin-left:3%; margin-right:3%; } + .fcenter { margin:auto; } + .fnote { position:absolute; top:auto; right:88%; vertical-align:super; font-size:66%; text-align:right; } + .foot { margin-left:3%; } + .pagenum { position:absolute; right:5%; font-size:83%; text-align:right; font-style:normal; } + .poem { margin-left:8em; margin-right:8em; margin-bottom:3ex; text-align:left; } + /*---------------------------------------- Table Text Positioning ----------------------------------------*/ + table.tlf { table-layout: fixed; } + .al, .ac>td { text-align:left; } + .ac, .ac>td, .ac>p { text-align:center; } + .ar { text-align:right; } + .aj { text-align:justify; } + .it1 { padding-left:1.0em; text-indent:-1.0em; } + .it1p05 { padding-left:1.5em; text-indent:-1.0em; } + .mc { margin:0 auto 0 auto; } + .vbm { vertical-align:bottom; } + .vmi, .vmi>td { vertical-align:middle; } + /*----------------------------------------------- Borders ------------------------------------------------*/ + table.ba, th.ba, td.ba, .ba>td, .ba>th{ border:1px solid black; } + table.bb, td.bb, th.bb, .bb>th{ border-bottom:1px solid black; } + table.bt, td.bt, th.bt, .bt>td, .bt>th{ border-top:1px solid black; } + td.bl { border-left:1px solid black; } + td.br, th.br, .br>td { border-right:1px solid black; } + td.br0 { border-right:0; } + /*------------------------------------------- Padding and width ------------------------------------------*/ + .transnote { padding:10px; background-color:#ccccff; } + .pl0 { padding-left: 0em; } + .plhs { padding-left: 0.125em; } + .pl2 { padding-left: 2em; } + .pr0 { padding-right: 0em; } + .prhs { padding-right: 0.125em; } + .pr1 { padding-right: 1em; } + .pr2 { padding-right: 2em; } + .pt05 { padding-top:0.5em; } + .pt1 { padding-top:1.0em; } + .pb05 { padding-bottom:0.5em; } + .pb1 { padding-bottom:1.0em; } + .w15 { width:15%; } + .w18 { width:18%; } + .w20 { width:20%; } + .w25 { width:25%; } + .w30 { width:30%; } + .w35 { width:35%; } + .w40 { width:40%; } + .w45 { width:45%; } + .w50 { width:50%; } + .w55 { width:55%; } + .w60 { width:60%; } + .w65 { width:65%; } + .w75 { width:75%; } + .w80 { width:80%; } + .w85 { width:85%; } + .w100 { width:100%; } + /*-------------------------------------- Media-selective formatting --------------------------------------*/ + @media screen,print { + table.handonly { display:none; } + table.nothand { display:table; } + .correction { border-bottom:thin dotted red; } + .poem { margin-left:8em; margin-right:8em; margin-bottom:3ex; text-align:left; } + } + @media handheld { + table.handonly { display:table; } + table.nothand { display:none; } + .correction { border-bottom:0; } + .poem { margin-left:1em; margin-right:1em; margin-bottom:3ex; text-align:left; } + div.w15 { width:30%; } + div.w20 { width:40%; } + div.w25 { width:50%; } + div.w30 { width:60%; } + div.w35, table.w35 { width:70%; } + div.w40 { width:80%; } + div.w45, table.w45 { width:90%; } + div.w50, table.w50 { width:100%; } + div.w55, table.w55 { width:100%; } + table.w60 { width:100%; } + table.w65 { width:100%; } + table.w75 { width:100%; } + table.w85 { width:100%; } + } + </style> +</head> +<body> +<div>*** START OF THE PROJECT GUTENBERG EBOOK 44000 ***</div> + + <table class="sp5 transnote nothand" title="Transcriber's note" summary="Transcriber's note"> + <tr> + <td class="w25">Transcriber's note:</td> + <td>A few typographical errors have been corrected. They appear in the text <span + class="correction" title="explanation will pop up">like this</span>, and the explanation will + appear when the mouse pointer is moved over the marked passage.</td> + </tr> + </table> + + <p class="ac" style="margin-bottom:1.8ex;"><span class="larger">THE</span></p> + + <p class="ac" style="margin-bottom:6.1ex;"><span class="xxx-larger">ORIGIN OF + VERTEBRATES</span></p> + + <p class="ac" style="margin-bottom:1.7ex;"><span class="smaller">BY</span></p> + + <p class="ac" style="margin-bottom:1.3ex;"><span class="larger">WALTER HOLBROOK GASKELL</span></p> + + <p class="ac" style="margin-bottom:5.8ex;"><span class="xx-smaller">M.A., M.D. (CANTAB.), LL.D. + (EDIN. AND McGILL UNIV.); F.R.S.; FELLOW OF TRINITY<br/> + HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE; HONORARY FELLOW<br/> + OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER<br/> + OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC.</span></p> + + <p class="ac" style="margin-bottom:0.6ex;"><span class="larger"><span class="gsp">LONGMANS, GREEN, + AND CO.</span></span></p> + + <p class="ac" style="margin-bottom:0.6ex;"><span class="smaller">39 PATERNOSTER ROW, + LONDON</span></p> + + <p class="ac" style="margin-bottom:0.9ex;"><span class="smaller">NEW YORK, BOMBAY, AND + CALCUTTA</span></p> + + <p class="ac" style="margin-bottom:0.7ex;"><span class="smaller">1908</span></p> + + <p class="sp5 ac" style="margin-bottom:9.6ex;"><span class="smaller"><i>All rights + reserved</i></span></p> + + <p class="sp3 ac" style="margin-bottom:4.3ex;"><span class="x-larger">CONTENTS</span></p> + + <table class="sp5 mc w65" title="Contents" summary="Contents"> + <tr> + <td colspan="2" class="ar smaller">PAGE</td> + </tr> + <tr> + <td class="pl0"><span class="sc">Introduction</span></td> + <td class="ar"><a href="#page1">1</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER I</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Central Nervous + System</span></td> + </tr> + <tr> + <td class="it1">Theories of the origin of vertebrates—Importance of the central nervous + system—Evolution of tissues—Evidence of Palæontology—Reasons for choosing + Ammocœtes rather than Amphioxus for the investigation of this problem—Importance + of larval forms—Comparison of the vertebrate and arthropod central nervous + systems—Antagonism between cephalization and alimentation—Life-history of lamprey, + not a degenerate animal—Brain of Ammocœtes compared with brain of + arthropod—Summary</td> + <td class="ar vbm"><a href="#page8">8</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER II</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Organs of Vision</span></td> + </tr> + <tr> + <td class="it1">Different kinds of eye—Simple and compound retinas—Upright and + inverted retinas—Median eyes—Median or pineal eyes of Ammocœtes and their + optic ganglia—Comparison with other median eyes—Lateral eyes of vertebrates + compared with lateral eyes of crustaceans—Peculiarities of the lateral eye of the + lamprey—Meaning of the optic diverticula—Evolution of vertebrate + eyes—Summary</td> + <td class="ar vbm"><a href="#page68">68</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER III</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Skeleton</span></td> + </tr> + <tr> + <td class="it1">The bony and cartilaginous skeleton considered, not the notochord—Nature + of the earliest cartilaginous skeleton—The mesosomatic skeleton of Ammocœtes; its + topographical arrangement, its structure, its origin in muco-cartilage—The prosomatic + skeleton of Ammocœtes; the trabeculæ and parachordals, their structure, their origin in + white fibrous tissue—The mesosomatic skeleton of Limulus compared with that of + Ammocœtes; similarity of position, of structure, of origin in muco-cartilage—The + prosomatic skeleton of Limulus; the entosternite, or plastron, compared with the trabeculæ of + Ammocœtes; similarity of position, of structure, of origin in fibrous + tissue—Summary</td> + <td class="ar vbm"><a href="#page119">119</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1"> + <p class="sp0">CHAPTER IV</p> + </td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Respiratory + Apparatus</span></td> + </tr> + <tr> + <td class="it1">Branchiæ considered as internal branchial appendages—Innervation of + branchial segments—Cranial region older than spinal—Three-root system of cranial + nerves: dorsal, lateral, ventral—Explanation of van Wijhe's segments—Lateral mixed + root is appendage-nerve of invertebrate—The branchial chamber of + Ammocœtes—The branchial unit, not a pouch but an appendage—The origin of + the branchial musculature—The branchial circulation—The branchial heart of the + vertebrate—Not homologous with the systemic heart of the arthropod—Its formation + from two longitudinal venous sinuses—Summary</td> + <td class="ar vbm"><a href="#page148">148</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER V</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Thyroid Gland</span></td> + </tr> + <tr> + <td class="it1">The value of the appendage-unit in non-branchial segments—The double + nature of the hyoid segment—Its branchial part—Its thyroid part—The double + nature of the opercular appendage—Its branchial part—Its genital part—Unique + character of the thyroid gland of Ammocœtes—Its structure—Its + openings—The nature of the thyroid segment—The uterus of the scorpion—Its + glands—Comparison with the thyroid gland of Ammocœtes—Cephalic generative + glands of Limulus—Interpretation of glandular tissue filling up the brain-case of + Ammocœtes—Function of thyroid gland—Relation of thyroid gland to sexual + functions—Summary</td> + <td class="ar vbm"><a href="#page185">185</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER VI</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Olfactory + Apparatus</span></td> + </tr> + <tr> + <td class="it1">Fishes divided into Amphirhinæ and Monorhinæ—Nasal tube of the + lamprey—Its termination at the infundibulum—The olfactory organs of the scorpion + group—The camerostome—Its formation as a tube—Its derivation from a pair of + antennæ—Its termination at the true mouth—Comparison with the olfactory tube of + Ammocœtes—Origin of the nasal tube of Ammocœtes from the tube of the + hypophysis—Direct comparison of the hypophysial tube with the olfactory tube of the + scorpion group—Summary</td> + <td class="ar vbm"><a href="#page218">218</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER VII</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Prosomatic Segments of Limulus and its + Allies</span></td> + </tr> + <tr> + <td class="it1">Comparison of the trigeminal with the prosomatic region—The prosomatic + appendages of the Gigantostraca—Their number and nature—Endognaths and + ectognath—The metastoma—The coxal glands—Prosomatic region of Eurypterus + compared with that of Ammocœtes—Prosomatic segmentation shown by marks on + carapace—Evidence of cœlomic cavities in Limulus—Summary</td> + <td class="ar vbm"><a href="#page233">233</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1"> + <p class="sp0">CHAPTER VIII</p> + </td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Segments belonging to the Trigeminal + Nerve-Group</span></td> + </tr> + <tr> + <td class="it1">The prosomatic segments of the vertebrate—Number of segments belonging + to the trigeminal nerve-group—History of cranial segments—Eye-muscles and their + nerves—Comparison with the dorso-ventral somatic muscles of the + scorpion—Explanation of the oculomotor nerve and its group of muscles—Explanation + of the trochlear nerve and its dorsal crossing—Explanation of the abducens + nerve—Number of segments supplied by the trigeminal nerves—Evidence of their motor + nuclei—Evidence of their sensory ganglia—Summary</td> + <td class="ar vbm"><a href="#page257">257</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER IX</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Prosomatic Segments of + Ammocœtes</span></td> + </tr> + <tr> + <td class="it1">The prosomatic region in Ammocœtes—The suctorial apparatus of the + adult Petromyzon—Its origin in Ammocœtes—Its derivation from + appendages—The segment of the lower lip or the metastomal segment—The tentacular + segments—The tubular muscles—Their segmental arrangement—Their peculiar + innervation—Their correspondence with the system of veno-pericardial muscles in + Limulus—The old mouth or palæostoma—The pituitary gland—Its comparison with + the coxal gland of Limulus—Summary</td> + <td class="ar vbm"><a href="#page286">286</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER X</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Relationship of Ammocœtes to the most + Ancient Fishes—the Ostracodermata</span></td> + </tr> + <tr> + <td class="it1">Cephalaspis—Ammocœtes only living representative of these ancient + fishes—Formation of cranium—Closure of old mouth—Rohon's primordial + cranium—Primordial cranium of Phrynus and Galeodes—Summary</td> + <td class="ar vbm"><a href="#page326">326</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER XI</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Evidence of the Auditory Apparatus and the + Organs of the Lateral Line</span></td> + </tr> + <tr> + <td class="it1">Lateral line organs—Function of this group of organs—Poriferous + sense-organs on the appendages in Limulus—Branchial sense-organs—Prosomatic + sense-organs—Flabellum—Its structure and position—Sense-organs of + mandibles—Auditory organs of insects and arachnids—Poriferous chordotonal + organs—Balancers of Diptera—Resemblance to organs of + flabellum—Racquet-organs of Galeodes—Pectens of scorpions—Large size of + nerve to all these special sense-organs—Origin of parachordals and auditory + capsule—Reason why VIIth nerve passes in and out of capsule—Evidence of + Ammocœtes—Intrusion of glandular mass round brain into auditory + capsule—Intrusion of generative and hepatic mass round brain into base of + flabellum—Summary</td> + <td class="ar vbm"><a href="#page355">355</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1"> + <p class="sp0">CHAPTER XII</p> + </td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Region of the Spinal Cord</span></td> + </tr> + <tr> + <td class="it1">Difference between cranial and spinal regions—Absence of lateral + root—Meristic variation—Segmentation of cœlom—Segmental excretory + organs—Development of nephric organs; pronephric, mesonephric, + metanephric—Excretory organs of Amphioxus—Solenocytes—Excretory organs of + Branchipus and Peripatus, appendicular and somatic—Comparison of cœlom of + Peripatus and of vertebrate—Pronephric organs compared to coxal glands—Origin of + vertebrate body-cavity (metacœle)—Segmental duct—Summary of formation of + excretory organs—Origin of somatic trunk-musculature—Atrial cavity of + Amphioxus—Pleural folds—Ventral growth of pleural folds and somatic + musculature—Pleural folds of Cephalaspidæ and of Trilobita—Meaning of the ductless + glands—Alteration in structure of excretory organs which have lost their duct in + vertebrates and in invertebrates—Formation of lymphatic glands—Segmental coxal + glands of arthropods and of vertebrates—Origin of adrenals, pituitary body, thymus, + tonsils, thyroid, and other ductless glands—Summary</td> + <td class="ar vbm"><a href="#page385">385</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER XIII</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Notochord and Alimentary Canal</span></td> + </tr> + <tr> + <td class="it1">Relationship between notochord and gut—Position of unsegmented tube of + notochord—Origin of notochord from a median groove—Its function as an accessory + digestive tube—Formation of notochordal tissue in invertebrates from closed portions of + the digestive tube—Digestive power of the skin of Ammocœtes—Formation of + new gut in Ammocœtes at transformation—Innervation of the vertebrate + gut—The three outflows of efferent nerves belonging to the organic system—The + original close contiguity of the respiratory chamber to the cloaca—The elongation of the + gut—Conclusion</td> + <td class="ar vbm"><a href="#page433">433</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1">CHAPTER XIV</td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">The Principles of Embryology</span></td> + </tr> + <tr> + <td class="it1">The law of recapitulation—Vindication of this law by the theory advanced + in this book—The germ-layer theory—Its present position—A physiological not + a morphological conception—New fundamental law required—Composition of adult + body—Neuro-epithelial syncytium and free-living cells—Meaning of the + blastula—Derivation of the Metazoa from the Protozoa—Importance of the central + nervous system for Ontogeny as well as for Phylogeny—Derivation of free-living cells + from germ-cells—Meaning of cœlom—Formation of neural canal—Gastrula + of Amphioxus and of Lucifer—Summary</td> + <td class="ar vbm"><a href="#page455">455</a></td> + </tr> + <tr> + <td colspan="2" class="ac pt1"> + <p class="sp0">CHAPTER XV</p> + </td> + </tr> + <tr> + <td colspan="2" class="ac"><span class="sc">Final Remarks</span></td> + </tr> + <tr> + <td class="it1"> + <p>Problems requiring investigation—</p> + <p>Giant nerve-cells and giant nerve-fibres; their comparison in fishes and arthropods; + blood- and lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; + origin of the sympathetic nervous system; biological test of relationship.</p> + <p class="sp0">Criticisms of Balanoglossus theory—Theory of parallel + development—Importance of the theory advocated in this book for all problems of + Evolution</p> + </td> + <td class="ar vbm"><a href="#page488">488</a></td> + </tr> + <tr> + <td class="pl0 pt1"><span class="sc">Bibliography and Index of Authors</span></td> + <td class="ar pt1"><a href="#page501">501</a></td> + </tr> + <tr> + <td class="pl0 pt1"><span class="sc">General Index</span></td> + <td class="ar pt1"><a href="#page517">517</a></td> + </tr> + </table> + + <div class="poem sp5"> + <p>"<i><span class="sc">Go on and prosper; there is nothing so</span></i></p> + <p><i><span class="sc">useful in science as one of those earthquake</span></i></p> + <p><i><span class="sc">hypotheses, which oblige one to face</span></i></p> + <p><i><span class="sc">the possibility that the solidest-looking</span></i></p> + <p><i><span class="sc">structures may collapse.</span></i>"</p> + <p style="margin-left:7.00em" class="stanza"><span class="sc">Letter from Prof. Huxley + to</span></p> + <p style="margin-left:7.00em"><span class="sc">the Author. June 2, 1889.</span></p> + </div> + + <div><span class="pagenum" id="page1">{1}</span></div> + + <p class="ac">THE</p> + + <p class="sp5 ac" style="margin-bottom:5.3ex;"><span class="x-larger">ORIGIN OF + VERTEBRATES</span></p> + + <p class="sp3 ac"><i>INTRODUCTION</i></p> + + <p>In former days it was possible for a man like Johannes Müller to be a leader both in physiology + and in comparative anatomy. Nowadays all scientific knowledge has increased so largely that + specialization is inevitable, and every investigator is confined more and more not only to one + department of science, but as a rule to one small portion of that department. In the case of such + cognate sciences as physiology and comparative anatomy this limiting of the scope of view is + especially deleterious, for zoology without physiology is dead, and physiology in many of its + departments without comparative anatomy can advance but little. Then, again, the too exclusive + study of one subject always tends to force the mind into a special groove—into a line of + thought so deeply tinged with the prevalent teaching of the subject, that any suggestions which + arise contrary to such teaching are apt to be dismissed at once as heretical and not worthy of + further thought; whereas the same suggestion arising in the mind of one outside this particular + line of thought may give rise to new and valuable scientific discoveries.</p> + + <p>Nothing but good can, in my opinion, result from the incursion of the non-specialist into the + realm of the specialist, provided that the former is in earnest. Over and over again the chemist + has given valuable help to the physicist, and the physicist to the chemist, so closely allied are + the two subjects; so also is it with physiology and anatomy, the two subjects are so + interdependent that a worker in the one may give valuable aid towards the solution of some large + problem which is the special territory of the other.</p> + + <p>It has been a matter of surprise to many how it came about that <span class="pagenum" + id="page2">{2}</span>I, a worker in the physiological laboratory at Cambridge ever since Foster + introduced experimental physiology into English-speaking nations, should have devoted so much time + to the promulgation of a theory of the origin of vertebrates—a subject remote from + physiology, and one of the larger questions appertaining to comparative anatomy. By what process + of thought was I led to take up the consideration of a subject apparently so remote from all my + previous work, and so foreign to the atmosphere of a physiological laboratory?</p> + + <p>It may perhaps be instructive to my readers to see how one investigation leads to another, + until at last, <i>nolens volens</i>, the worker finds himself in front of a possible solution to a + problem far removed from his original investigation, which by the very magnitude and importance of + it forces him to devote his whole energy and time to seeing whether his theory is good.</p> + + <p>In the years 1880-1884 I was engaged in the investigation of the action of the heart, and the + nature of the nerves which regulate that action. In the course of that investigation I was struck + by the ease with which it was possible to distinguish between the fibres of the vagus and + accelerator nerves on their way to the heart, owing to the medullation of the former and the + non-medullation of the latter. This led me to an investigation of the accelerator fibres, to find + out how far they are non-medullated, and so to the discovery that the <i>rami communicantes</i> + connecting together the central nervous system and the sympathetic are in reality single, not + double, as had hitherto been thought; for the grey <i>ramus communicans</i> is in reality a + peripheral nerve which supplies the blood-vessels of the spinal cord and its membranes, and is of + the same nature as the grey accelerators to the heart.</p> + + <p>This led to the conclusion that there is no give and take between two independent nervous + systems, the cerebro-spinal and the sympathetic, as had been taught formerly, but only one nervous + system, the cerebro-spinal, which sends special medullated nerve-fibres, characterized by their + smallness, to the cells of the sympathetic system, from which fibres pass to the periphery, + usually non-medullated. These fine medullated nerves form the system of white <i>rami + communicantes</i>, and have since been called by Langley the preganglionic nerves. Further + investigation showed that such white rami are not universally distributed, but are confined to the + thoracico-lumbar region, where their distribution is easily seen in <span class="pagenum" + id="page3">{3}</span>the ventral roots, for the cells of the sympathetic system are entirely + efferent in nature, not afferent; therefore, the fibres entering into them from the central + nervous system leave the spinal cord by ventral, not dorsal roots.</p> + + <p>Following out this clue, I then found that in addition to this thoracico-lumbar outflow of + efferent ganglionated visceral nerves, there are similar outflows in the cranial and sacral + regions, belonging in the former case especially to the vagus system of nerves, and in the latter + to the system of nerves which pass from the sacral region of the cord to the ganglion-cells of the + hypogastric plexus, and from them supply the bladder, rectum, etc. To this system of nerves, + formerly called the <i>nervi erigentes</i>, I gave the name pelvic splanchnics, in order to show + their uniformity with the abdominal splanchnics. These investigations led to the conclusion that + the organic system of nerves, characterized by the possession of efferent nerve-cells situated + peripherally, arises from the central nervous system by three distinct outflows—cranial, + thoracico-lumbar, and sacral, respectively. To this system Langley has lately given the name + 'autonomic.' These three outflows are separated by two gaps just where the plexuses for the + anterior and posterior extremities come in.</p> + + <p>This peculiar arrangement of the white <i>rami communicantes</i> set me thinking, for the gaps + corresponded to an increase of somatic musculature to form the muscles of the fore and hind limbs, + so that if, as seemed probable, the white <i>rami communicantes</i> arise segmentally from the + spinal cord, then a marked distinction must exist in structure between the spinal cord in the + thoracic region, where the visceral efferent nerves are large in amount and the body musculature + scanty, and in the cervical or lumbar swellings, where the somatic musculature abounds, and the + white <i>rami communicantes</i> scarcely exist.</p> + + <p>I therefore directed my attention in the next place to the structure of the central nervous + system in the endeavour to associate the topographical arrangement of cell-groups in this system + with the outflow of the different kinds of nerve-fibres to the peripheral organs.</p> + + <p>This investigation forcibly impressed upon my mind the uniformity in the arrangement of the + central nervous system as far as the centres of origin of all the segmental nerves are concerned, + <span class="pagenum" id="page4">{4}</span>both cranial and spinal, and also the original + segmental character of this part of the nervous system.</p> + + <p>I could not, therefore, help being struck by the force of the comparison between the central + nervous systems of Vertebrata and Appendiculata as put forward again and again by the past + generation of comparative anatomists, and wondered why it had been discredited. There in the + infundibulum was the old œsophagus, there in the cranial segmental nerves the + infraœsophageal ganglia, there in the cerebral hemispheres and optic and olfactory nerves + the supraœsophageal ganglia, there in the spinal cord the ventral chain of ganglia. But if + the infundibulum was the old œsophagus, what then? The old œsophagus was continuous + with and led into the cephalic stomach. What about the infundibulum? It was continuous with and + led into the ventricles of the brain, and the whole thing became clear. The ventricles of the + brain were the old cephalic stomach, and the canal of the spinal cord the long straight intestine + which led originally to the anus, and still in the vertebrate embryo opens out into the anus. Not + having been educated in a morphological laboratory and taught that the one organ which is + homologous throughout the animal kingdom is the gut, and that therefore the gut of the + invertebrate ancestor must continue on as the gut of the vertebrate, the conception that the + central nervous system has grown round and enclosed the original ancestral gut, and that the + vertebrate has formed a new gut did not seem to me so impossible as to prevent my taking it as a + working hypothesis, and seeing to what it would lead.</p> + + <p>This theory that the so-called central nervous system of the vertebrate is in reality composed + of two separate parts, of which the one, the segmented part, corresponds to the central nervous + system of the highest invertebrates, while the other, the unsegmented tube, was originally the + alimentary canal of that same invertebrate, came into my mind in the year 1887. The following + year, on June 23, 1888, I read a paper on the subject before the Anatomical Society at Cambridge, + which was published in the <i>Journal of Anatomy and Physiology</i>, vol. 23, and more fully in + the <i>Journal of Physiology</i>, vol. 10. Since that time I have been engaged in testing the + theory in every possible way, and have published the results of my investigations in a series of + papers in different journals, a list of which I append at the end of this introductory + chapter.</p> + + <div><span class="pagenum" id="page5">{5}</span></div> + + <p>It is now twenty years since the theory first came into my mind, and the work of those twenty + years has convinced me more and more of its truth, and yet during the whole time it has been + ignored by the morphological world as a whole rather than criticized. Whatever may have been the + causes for such absence of criticism, it is clear that the serial character of its publication is + a hindrance to criticism of the theory as a whole, and I hope, therefore, that the publication of + the whole of the twenty years' work in book-form will induce those who differ from my conclusions + to come forward and show me where I am wrong, and why my theory is untenable. Any one who has been + thinking over any one problem for so long a time becomes obsessed with the infallibility of his + own views, and is not capable of criticizing his own work as thoroughly as others would do. I have + been told that it is impossible for one man to consider so vast a subject with that thoroughness + which is necessary, before any theory can be accepted as the true solution of the problem. I + acknowledge the vastness of the task, and feel keenly enough my own shortcomings. For all that, I + do feel that it can only be of advantage to scientific progress and a help to the solution of this + great problem, to bring together in one book all the facts which I have been able to collect, + which appeal to me as having an important bearing on this solution.</p> + + <p>In this work I have been helped throughout by Miss R. Alcock. It is not too much to say that + without the assistance she has given me, many an important link in the chain of evidence would + have been missing. With extraordinary patience she has followed, section by section, the smallest + nerves to their destination, and has largely helped to free the transformation process in the + lamprey from the mystery which has hitherto enveloped it. She has drawn for me very many of the + illustrations scattered through the pages in this book, and I feel that her aid has been so + valuable and so continuous, lasting as it does over the whole period of the work, that her name + ought fittingly to be associated with mine, if perchance the theory of the Origin of Vertebrates, + advocated in the pages of this book, gains acceptance.</p> + + <p class="sp5">I am also indebted to Mr. J. Stanley Gardiner and to Dr. A. Sheridan Lea for + valuable assistance in preparing this book for the press. I desire to express my grateful thanks + to the former for valuable criticism of the scientific evidence which I have brought <span + class="pagenum" id="page6">{6}</span>forward in this book, and to the latter for his great + kindness in undertaking the laborious task of collecting the proofs.</p> + + <p class="ac">LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CONCERNING THE ORIGIN OF + VERTEBRATES.</p> + + <table class="sp3 mc w50" title="Previous Publications by the Author" + summary="Previous Publications by the Author"> + <tr> + <td>1888.</td> + <td class="it1p05">"Spinal and Cranial Nerves." <i>Proceedings of the Anatomical Society</i>, + June, 1888. <i>Journal of Anatomy and Physiology</i>, vol. xxiii.</td> + </tr> + <tr> + <td class="pt1">1889.</td> + <td class="it1p05 pt1">"On the Relation between the Structure, Function, Distribution, and + Origin of the Cranial Nerves; together with a Theory of the Origin of the Nervous System of + Vertebrata." <i>Journal of Physiology</i>, vol. x., p. 153.</td> + </tr> + <tr> + <td class="pt1">1889.</td> + <td class="it1p05 pt1">"On the Origin of the Central Nervous System of Vertebrates." + <i>Brain</i>, vol. xii., p. 1.</td> + </tr> + <tr> + <td class="pt1">1890.</td> + <td class="it1p05 pt1">"On the Origin of Vertebrates from a Crustacean-like Ancestor." + <i>Quarterly Journal of Microscopical Science</i>, vol. xxxi., p. 379.</td> + </tr> + <tr> + <td class="pt1">1895.</td> + <td class="it1p05 pt1">"The Origin of Vertebrates." <i>Proceedings of the Cambridge + Philosophical Society</i>, vol. ix., p. 19.</td> + </tr> + <tr> + <td class="pt1">1896.</td> + <td class="it1p05 pt1">Presidential Address to Section I. at the meeting of the British + Association for the Advancement of Science in Liverpool. <i>Report of the British + Association</i>, 1896, p. 942.</td> + </tr> + <tr> + <td class="pt1">1899.</td> + <td class="it1p05 pt1">"On the Meaning of the Cranial Nerves." Presidential Address to the + Neurological Society for the year 1899. <i>Brain</i>, vol. xxii., p. 329.</td> + </tr> + </table> + + <p class="sp3">A series of papers on "The Origin of Vertebrates, deduced from the study of + Ammocœtes," in the <i>Journal of Anatomy and Physiology</i>, as follows<span + class="wnw">:—</span></p> + + <table class="sp5 mc w50" title="Previous Publications by the Author, + continued" summary="Previous Publications by the Author, + continued"> + <tr> + <td class="">1898.</td> + <td class="ac prhs">Part</td> + <td class="plhs ar">I.</td> + <td class="it1p05">"The Origin of the Brain," vol. xxxii., p. 513.</td> + </tr> + <tr> + <td></td> + <td class="ac prhs">"</td> + <td class="plhs ar">II.</td> + <td class="it1p05">"The Origin of the Vertebrate Cranio-facial Skeleton," vol. xxxii., p. + 553.</td> + </tr> + <tr> + <td></td> + <td class="ac prhs">"</td> + <td class="plhs ar">III.</td> + <td class="it1p05">"The Origin of the Branchial Segmentation," vol. xxxiii., p. 154.</td> + </tr> + <tr> + <td class="">1899.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">IV.</td> + <td class="it1p05">"The Thyroid, or Opercular Segment: the Meaning of the Facial Nerve," vol. + xxxiii., p. 638.</td> + </tr> + <tr> + <td class="">1900.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">V.</td> + <td class="it1p05">"The Origin of the Pro-otic Segmentation: the Meaning of the Trigeminal and + Eye-muscle Nerves," vol. xxxiv., p. 465.</td> + </tr> + <tr> + <td class="">1900.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">VI.</td> + <td class="it1p05">"The Old Mouth and the Olfactory Organ: the Meaning of the First Nerve," + vol. xxxiv., p. 514.</td> + </tr> + <tr> + <td class="">1900.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">VII.</td> + <td class="it1p05">"The Evidence of Prosomatic Appendages in Ammocœtes, as given by the + Course and Distribution of the Trigeminal Nerve," vol. xxxiv., p. 537.</td> + </tr> + <tr> + <td class="">1900.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">VIII.</td> + <td class="it1p05">"The Palæontological Evidence: Ammocœtes a Cephalaspid," vol. + xxxiv., p. 562.</td> + </tr> + <tr> + <td class="">1901.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">IX.</td> + <td class="it1p05">"The Origin of the Optic Apparatus: the Meaning of the Optic Nerves," vol. + xxxv., p. 224.</td> + </tr> + <tr> + <td class=""><span class="pagenum" id="page7">{7}</span> + <p class="sp0">1902.</p> + </td> + <td class="ac prhs">"</td> + <td class="plhs ar">X.</td> + <td class="it1p05">"The Origin of the Auditory Organ: the Meaning of the VIIIth Cranial + Nerve," vol. xxxvi., p. 164.</td> + </tr> + <tr> + <td class="">1903.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">XI.</td> + <td class="it1p05">"The Origin of the Vertebrate Body-cavity and Excretory Organs: the Meaning + of the Somites of the Trunk and of the Ductless Glands," vol. xxxvii., p. 168.</td> + </tr> + <tr> + <td class="">1905.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">XII.</td> + <td class="it1p05">"The Principles of Embryology," vol. xxxix., p. 371.</td> + </tr> + <tr> + <td class="">1906.</td> + <td class="ac prhs">"</td> + <td class="plhs ar">XIII.</td> + <td class="it1p05">"The Origin of the Notochord and Alimentary Canal," vol. xl., p. 305.</td> + </tr> + </table> + + <div><span class="pagenum" id="page8">{8}</span></div> + + <p class="ac">CHAPTER I</p> + + <p class="ac"><i>THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Theories of the origin of vertebrates.—Importance of the central nervous + system.—Evolution of tissues.—Evidence of Palæontology.—Reasons for choosing + Ammocœtes rather than Amphioxus.—Importance of larval forms.—Comparison of + the vertebrate and arthropod central nervous systems.—Antagonism between cephalization and + alimentation.—Life-history of lamprey: not a degenerate animal.—Brain of + Ammocœtes compared with brain of arthropod.—Summary.</p> + </div> + + <p>At the present time it is no longer a debatable question whether or no Evolution has taken + place. Since the time of Darwin the accumulation of facts in its support has been so overwhelming + that all zoologists look upon this question as settled, and desire now to find out the manner in + which such evolution has taken place. Here two problems offer themselves for investigation, which + can be and are treated separately—the one dealing with the question of those laws of + heredity and variation which have brought about in the past and are still causing in the present + the evolution of living beings, <i>i.e.</i> the causes of evolution; the other concerned with the + relationship of animals, or groups of animals, rather than with the causes which have brought + about such relationship, <i>i.e.</i> the sequence of evolution.</p> + + <p>It is the latter problem with which this book deals, and, indeed, not with the whole question + at all, but only with that part of it which concerns the origin of vertebrates.</p> + + <p>This problem of the sequence of evolution is of a twofold character: first, the finding out of + the steps by which the higher forms in any one group of animals have been evolved from the lower; + and secondly, the evolution of the group itself from a lower group.</p> + + <p>In any classification of the animal kingdom, it is clear that large groups of animals exist + which have so many common characteristics as to necessitate their being placed in one larger group + or kingdom; <span class="pagenum" id="page9">{9}</span>thus zoologists are able to speak + definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Cœlenterata, + Mollusca, etc. In each of these groups affinities can be traced between the members, so that it is + possible to speak of the progress from lower to higher members of the group, and it is + conceivable, given time to work out the details, that the natural relationships between the + members of the whole group will ultimately be discovered.</p> + + <p>Thus no one can doubt that a sequence of the kind has taken place in the Vertebrata as we trace + the progress from the lowest fishes to man, and already the discoveries of palæontology and + anatomy give us a distinct clue to the sequence from fish to amphibian, from amphibian to reptile, + from reptile to mammal on the one hand, and to bird on the other. That the different members of + the vertebrate group are related to each other in orderly sequence is no longer a matter of doubt; + the connected problems are matters of detail, the solution of which is certain sooner or later. + The same may be said of the members of any of the other great natural groups, such as the + Arthropoda, the Annelida, the Echinodermata, etc.</p> + + <p>It is different, however, when an attempt is made to connect two of the main divisions + themselves. It is true enough that there is every reason to believe that the arthropod group has + been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be + looked on as forming one big division, the Appendiculata, all the members of which will some day + be arranged in orderly sequence, but the same feeling of certainty does not exist in other + cases.</p> + + <p class="sp3">In the very case of the origin of the Appendiculata we are confronted with one of + the large problems of evolution—the origin of segmented from non-segmented animals—the + solution of which is not yet known.</p> + + <p class="ac"><span class="sc">Theories of the Origin of Vertebrates.</span></p> + + <p>The other large problem, perhaps the most important of all, is the question of the relationship + of the great kingdom of the Vertebrata: from what invertebrate group did the vertebrate arise?</p> + + <p>The great difficulty which presents itself in attempting a solution of this question is not so + much, as used to be thought, the difficulty of deriving a group of animals possessing an internal + bony and <span class="pagenum" id="page10">{10}</span>cartilaginous skeleton from a group + possessing an external skeleton of a calcareous or chitinous nature, but rather the difficulty + caused by the fundamental difference of arrangement of the important internal organs, especially + the relative positions of the central nervous system and the digestive tube.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig001.png" id="fig1"><img style="width:100%" src="images/fig001.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 1.</span>—<span class="sc">Arrangement of Organs in the + Vertebrate (A) and Arthropod (B).</span></p> + <p class="sp0"><i>Al</i>, gut; <i>H</i>, heart; <i>C.N.S.</i>, central nervous system; V, + ventral side; D, dorsal side.</p> + </div> + </div> + + <p>Now, if we take a broad and comprehensive view of the invertebrate kingdom, without arguing out + each separate case, we find that it bears strongly the stamp of a general plan of evolution + derived from a cœlenterate animal, whose central nervous system formed a ring surrounding + the mouth. Then when the radial symmetry was given up, and an elongated, bilateral, segmented form + evolved, the central nervous system also became elongated and segmented, but, owing to its + derivation from an oral ring, it still surrounded the mouth-tube, or œsophagus, and thus in + its highest forms is divided into supra-œsophageal and infra-œsophageal nervous + masses. These latter <span class="pagenum" id="page11">{11}</span>nervous masses are of necessity + ventral to the digestive tube, because the mouth of the cœlenterate is on the ventral side. + The striking characteristic, then, of the invertebrate kingdom is the situation of a large portion + of the central nervous system ventrally to the alimentary canal and the piercing of the nervous + system by a tube—the œsophagus—leading from the mouth to the alimentary canal. + The equally striking characteristic of the vertebrate is the dorsal position of the central + nervous system and the ventral position of the alimentary canal combined with the absence of any + piercing of the central nervous system by the œsophagus.</p> + + <p>So fundamentally different is the arrangement of the important organs in the two groups that it + might well give rise to a feeling of despair of ever hoping to solve the problem of the Origin of + Vertebrates; and, to my mind, this is the prevalent feeling among morphologists at the present + time. Two attempts at solution have been made. The one is associated with the name of Geoffrey St. + Hilaire, and is based on the supposition that the vertebrate has arisen from the invertebrate by + turning over on its back, swimming in this position, and so gradually converting an originally + dorsal surface into a ventral one, and <i>vice versâ</i>; at the same time, a new mouth is + supposed to have been formed on the new ventral side, which opened directly into the alimentary + canal, while the old mouth, which had now become dorsal, was obliterated.</p> + + <p>The other attempt at solution is of much more recent date, and is especially associated with + the name of Bateson. It supposes that bilaterally symmetrical, elongated, segmented animals were + formed from the very first in two distinct ways. In the one case the digestive tube pierced the + central nervous system, and was situated dorsally to its main mass. In the other case the + segmented central nervous system was situated from the first dorsally to the alimentary canal, and + was not pierced by it. In the first case the highest result of evolution led to the Arthropoda; in + the second case to the Vertebrata.</p> + + <p>Neither of these views is based on evidence so strong as to cause universal acceptance. The + great difficulty in the way of accepting the second alternative is the complete absence of any + evidence, either among animals living on the earth at the present day or among those known to have + existed in the past, of any such chain of intermediate animal forms as must, on this hypothesis, + have existed in order to link together the lower forms of life with the vertebrates.</p> + + <div><span class="pagenum" id="page12">{12}</span></div> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig002.png" id="fig2"><img style="width:100%" src="images/fig002.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 2.</span>—<span class="sc">Larval + Balanoglossus</span> (from the Royal Natural History).</p> + </div> + </div> + + <p>It has been supposed that the Tunicata and the Enteropneusta (<i>Balanoglossus</i>) (Fig. <a + href="#fig2">2</a>) are members of this missing chain, and that in Amphioxus the vertebrate + approaches in organization to these low invertebrate forms. The tunicates, indeed, are looked upon + as degenerate members of an early vertebrate stock, which may give help in picturing the nature of + the vertebrate ancestor but are not themselves in the direct line of descent. Balanoglossus is + supposed to have arisen from the Echinodermata, or at all events to have affinities with them, so + that to fill up the enormous gap between the Echinodermata and the Vertebrata on this theory there + is absolutely nothing living on the earth except Balanoglossus, Rhabdopleura, and Cephalodiscus. + The characteristics of the vertebrate upon which this second theory is based are the notochord, + the respiratory character of the anterior part of the alimentary canal, and the tubular nature of + the central nervous system; it is claimed that in Balanoglossus the beginnings of a notochord and + a tubular central nervous system are to be found, while the respiratory portion of the gut is + closely comparable to that of Amphioxus.</p> + + <p>The strength of the first theory is essentially based on the comparison of the vertebrate + central nervous system with that of the segmented invertebrate, annelid or arthropod. In the + latter the central nervous system is composed of—</p> + + <p>1. The supra-œsophageal ganglia, which give origin to the nerves of the eyes and + antennules, <i>i.e.</i> to the optic and olfactory nerves, for the first pair of antennæ are + olfactory in function. These are connected with the infra-œsophageal ganglia by the + œsophageal commissures which encircle the œsophagus.</p> + + <p>2. The infra-œsophageal ganglia and the two chains of ventral ganglia, which are + segmentally-arranged sets of ganglia. Of these, <span class="pagenum" id="page13">{13}</span>each + pair gives rise to the nerves of its own segment, and these nerves are not nerves of special sense + as are the supra-œsophageal nerves, but motor and sensory to the segment; nerves by the + agency of which food is taken in and masticated, respiration is effected, and the animal moves + from place to place.</p> + + <p>In the vertebrate the central nervous system consists of—</p> + + <p>1. The brain proper, from which arise only the olfactory and optic nerves.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig003.png" id="fig3"><img style="width:100%" src="images/fig003.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 3.</span>—<span class="sc">Vertebrate Central + Nervous System compared with the Central Nervous System and Alimentary Canal of the + Arthropod.</span></p> + <p>A. Vertebrate central nervous system. <i>S. Inf. Br.</i>, supra-infundibular brain; <i>I. + Inf. Br.</i>, infra-infundibular brain and cranial segmental nerves; <i>C.Q.</i>, corpora + quadrigemina; <i>Cb.</i>, cerebellum; <i>C.C.</i>, crura cerebri; <i>C.S.</i>, corpus + striatum; <i>Pn.</i>, pineal gland.</p> + <p class="sp0">B. Invertebrate central nervous system. <i>S. Œs. G.</i>, + supra-œsophageal ganglia; <i>I. Œs. G.</i>, infra-œsophageal ganglia; + <i>Œs. Com.</i>, œsophageal commissures.</p> + </div> + </div> + + <p>2. The region of the mid-brain, medulla oblongata, and spinal cord; from these arises a series + of nerves segmentally arranged, which, as in the invertebrate, gives origin to the nerves + governing mastication, respiration, and locomotion.</p> + + <p>Further, the vertebrate central nervous system possesses the peculiarity, found nowhere else, + of being tubular, and the tube is of a striking character. In the spinal region it is a small, + simple canal of uniform calibre, which at the front end dilates to form the ventricles of the + region of the brain. From that part of this dilated <span class="pagenum" + id="page14">{14}</span>portion, known as the third ventricle, a narrow tube passes to the ventral + surface of the brain. This tube is called the <i>infundibulum</i>, and, extraordinary to relate, + lies just anteriorly to the exits of the third cranial or oculomotor nerves; in other words, it + marks the termination of the series of spinal and cranial segmental nerves. Further, on each side + of this infundibular tube are lying the two thick masses of the <i>crura cerebri</i>, the strands + of fibres which connect the higher brain-region proper with the lower region of the medulla + oblongata and spinal cord. Not only, then, are the nerve-masses in the two systems exactly + comparable, but in the very place where the œsophageal tube is found in the invertebrate, + the infundibular tube exists in the vertebrate, so that if the words infundibular and + œsophageal are taken to be interchangable, then in every respect the two central nervous + systems are comparable. The brain proper of the vertebrate, with its olfactory and optic nerves, + becomes the direct descendant of the supra-œsophageal ganglia; the crura cerebri become the + œsophageal commissures, and the cranial and spinal segmental nerves are respectively the + nerves belonging to the infra-œsophageal and ventral chain of ganglia.</p> + + <p>This overwhelmingly strong evidence has always pointed directly to the origin of the vertebrate + from some form among the segmented group of invertebrates, annelid or arthropod, in which the + original œsophagus had become converted into the infundibulum, and a new mouth formed. So + far, the position of this school of anatomists was extremely sound, for it is impossible to + dispute the facts on which it is based. Still, however, the fact remained that the gut of the + vertebrate lies ventrally to the nervous system, while that of the invertebrate lies dorsally; + consequently, since the infundibulum was in the position of the invertebrate œsophagus, it + must originally have entered into the gut, and since the vertebrate gut was lying ventrally to it, + it could only have opened into that gut in the invertebrate stage by the shifting of dorsal and + ventral surfaces. From this argument it followed that the remains of the original mouth into which + the infundibulum, <i>i.e.</i> œsophagus, opened were to be sought for on the dorsal side of + the vertebrate brain. Here in all vertebrates there are two spots where the roof of the brain is + very thin, the one in the region of the pineal body, and the other constituting the roof of the + fourth ventricle. Both of these places have had their advocates as the position of the old mouth, + the former being upheld by Owen, the latter by Dohrn.</p> + + <div><span class="pagenum" id="page15">{15}</span></div> + + <p>The discovery that the pineal body was originally an eye, or, rather, a pair of eyes, has + perhaps more than anything else proved the impossibility of accepting this reversal of surfaces as + an explanation of the genesis of the vertebrate from the annelid group. For whereas a pair of eyes + close to the mid-dorsal line is not only likely enough, but is actually found to exist among large + numbers of arthropods, both living and extinct, a pair of eyes situated close to the mid-ventral + line near the mouth is not only unheard of in nature, but so improbable as to render impossible + the theory which necessitates such a position.</p> + + <p>Yet this very discovery gives the strongest possible additional support to the close identity + in the plan of the central nervous system of vertebrate and appendiculate.</p> + + <p>A truly paradoxical situation! The very discovery which may almost be said to prove the truth + of the hypothesis, is the very one which has done most to discredit it, because in the minds of + its authors the only possible solution of the transition from the one group to the other was by + means of the reversal of surfaces.</p> + + <p>Still, as already said, even if the theory advanced to explain the facts be discredited, the + facts remain the same; and still to this day an explanation is required as to why such + extraordinary resemblances should exist between the two nervous systems, unless there is a genetic + connection between the two groups of animals. An explanation may still be found, and must be + diligently sought for, which shall take into account the strong evidence of this relationship + between the two groups, and yet not necessitate any reversal of surfaces. It is the object of this + book to consider the possibility of such an explanation.</p> + + <p>What are the lines of investigation most likely to meet with success? Is it possible to lay + down any laws of evolution? It is instructive to consider the nature of the investigations which + have led to the two theories just mentioned, for the fundamental starting-point is remarkably + different in the two cases. The one theory is based upon the study of the vertebrate itself, and + especially of its central nervous system, and its supporters and upholders have been and are + essentially anatomists, whose chief study is that of vertebrate and human anatomy. The other + theory is based upon the study of the invertebrate, and consists especially of an attempt to find + in the invertebrate some structure resembling a notochord, such <span class="pagenum" + id="page16">{16}</span>organ being considered by them as the great characteristic of the + vertebrate; indeed, so much is this the case, that a large number of zoologists speak now of + Chordata rather than of Vertebrata, and in order to emphasize their position follow Bateson, and + speak of the Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the Enteropneusta as + Hemi-chordata, and even of Actinotrocha (to use Masterman's term), as Diplo-chordata.</p> + + <p>The upholders of this theory lay no stress on the nature of the central nervous system in + vertebrates, they are essentially zoologists who have made a special study of the invertebrate + rather than of the vertebrate.</p> + + <p>Of these two methods of investigating the problem, it must be conceded that the former is more + likely to give reliable results. By putting the vertebrate to the question in every possible way, + by studying its anatomy and physiology, both gross and minute, by inquiring into its past history, + we can reasonably hope to get a clue to its origin, but by no amount of investigation can we tell + with any certainty what will be its future fate; we can only guess and prophesy in an uncertain + and hesitating manner. So it must be with any theory of the origin of vertebrates, based on the + study of one or other invertebrate group. Such theory must partake rather of the nature of + prophecy than of deduction, and can only be placed on a firm basis when it so happens that the + investigation of the vertebrate points irresistibly to its origin from the same group; in fact, + "never prophesy unless you know."</p> + + <p class="sp3">The first principle, then, I would lay down is this: In order to find out the + origin of vertebrates, inquire, in the first place, of the vertebrate itself.</p> + + <p class="ac"><span class="sc">Importance of the Central Nervous System.</span></p> + + <p>Does the history of evolution pick out any particular organ or group of organs as more + necessary than another for upward progress? If so, it is upon that organ or group of organs that + special stress must be laid.</p> + + <p>Since Darwin wrote the "Origin of Species," and laid down that the law of the 'survival of the + fittest' is the factor upon which evolution depends, it has gradually dawned upon the scientific + mind that 'the fittest' may be produced in two diametrically opposite ways: <span class="pagenum" + id="page17">{17}</span>either by progress upwards to a superior form, or by degeneration to a + lower type of animal. The principle of degeneration as a factor in the formation of groups of + animals, which are thereby enabled to survive, is nowadays universally admitted. The most striking + example is to be found in the widely distributed group of Tunicata, which live, in numbers of + instances, a sedentary life upon the rocks, have the appearance of very low forms of animal life, + propagate by budding, have lost all the characteristics of higher forms, and yet are considered to + be derived from an original vertebrate stock. Such degenerate forms remain degenerate, and are + never known to regenerate and again to reach the higher stage of evolution from which they arose. + Such forms are of considerable interest, but cannot help, except negatively, to decide what factor + is especially important for upward progress.</p> + + <p>At the head of the animal race at the present day stands man, and in mankind itself some races + are recognized as higher than others. Such recognition is given essentially on account of their + greater brain-power, and without doubt the great characteristic which puts man at the head is the + development of his central nervous system, especially of the region of the brain. Not only is this + point most manifest in distinguishing man from the lower animals, but it applies to the latter as + well. By the amount of convolution of the brain, the amount of grey matter in the cerebral + hemispheres, the enlargement and increasing complexity of the higher parts of the central nervous + system, the anthropoid apes are differentiated from the lower forms, and the higher mammals from + the lower. In the recent work of Elliot Smith, and of Edinger, most conclusive proof is given that + the upward progress in the vertebrate phylum is correlated with the increase of brain-power, and + the latter writer shows how steady and remarkable is the increase in substance and in complexity + of the brain-region as we pass from the fishes, through the amphibians and reptiles, to the birds + and mammals.</p> + + <p>The study of the forms which lived on the earth in past ages confirms and emphasizes this + conclusion, for it is most striking to see how small is the cranium among the gigantic Dinosaurs; + how in the great reptilian age the denizens of the earth were far inferior in brain-power to the + lords of creation in after-times.</p> + + <p>What applies to the vertebrate phylum applies also to the invertebrate groups. Here also an + upward progress is recognized as we <span class="pagenum" id="page18">{18}</span>pass from the + sponges to the arthropods—a progress which is manifested, first by the concentration of + nervous material to form a central nervous system, and then by the increase in substance and + complexity of that nervous system to form a higher and a higher type, until the culmination is + reached in the nervous system of the scorpions and spiders. No upward progress is possible with + degeneration of the central nervous system, and in all those cases where a group owes its + existence to degeneration, the central nervous system takes part in the degeneration.</p> + + <p>This law of the paramount importance of the growth of the central nervous system for all upward + progress in the evolution of animals receives confirmation from the study of the development of + individuals, especially in those cases where a large portion of the life of the animal is spent in + a larval condition, and then, by a process of transformation, the larva changes into the adult + form. Such cases are well known among Arthropoda, the familiar instance being the change from the + larval caterpillar to the adult imago. Among Vertebrata, the change from the tadpole to the frog, + from the larval form of the lamprey (<i>Ammocœtes</i>) to the adult form + (<i>Petromyzon</i>), are well-known instances. In all such cases the larva shows signs of having + attained a certain stage in evolution, and then a remarkable transformation takes place, with the + result that an adult animal emerges, whose organization reaches a higher stage of evolution than + that of the larva.</p> + + <p>This transformation process is characterized by a very great destruction of the larval tissues + and a subsequent formation of new adult tissues. Most extensive is the destruction in the + caterpillar and in the larval lamprey. But one organ never shares in this process of histolysis, + and that is the central nervous system; amidst the ruins of the larva it remains, leading and + directing the process of re-formation. In the Arthropoda, the larval alimentary canal may be + entirely destroyed and eaten up by phagocytes, but the central nervous system not only remains + intact but increases in size, and by the concentration and cephalization of its + infra-œsophageal ganglia forms in the adult a central nervous system of a higher type than + that of the larva.</p> + + <p>So, too, in the transformation of the lamprey, there is not the slightest trace of any + destruction in the central nervous system, but simply a development and increase in nervous + material, which <span class="pagenum" id="page19">{19}</span>results in the formation of a brain + region more like that of the higher vertebrates than exists in Ammocœtes.</p> + + <p>In these cases the development is upward—the adult form is of a higher type than that of + the larva. It is, however, possible for the reverse to occur, so that the individual development + leads to degeneration, not to a higher type. Instances are seen in the Tunicata, and in various + parasitic arthropod forms, such as Lernæa, etc. In these cases, the transformation from the larval + to the adult form leads to degradation, and in this degradation the central nervous system is + always involved.</p> + + <p>It is perhaps a truism to state that upward progress is necessarily accompanied by increased + development of the central nervous system; but it is necessary to lay special stress upon the + importance of the central nervous system in all problems of evolution, because there is, in my + opinion, a tendency at the present time to ignore this factor to too great an extent.</p> + + <p>The law of progress is this—The race is not to the swift, nor to the strong, but to the + wise.</p> + + <p>This law carries with it the necessary corollary that the immediate ancestor of the vertebrate + must have had a central nervous system nearly approaching that of the lowest undegenerated + vertebrate. Among all the animals living on the earth at the present time, the highest + invertebrate group, the Arthropoda, possesses a central nervous system most closely resembling + that of the vertebrate.</p> + + <p class="sp3">The law, then, of the paramount importance of a steady development of the central + nervous system for the upward progress of the animal kingdom, points directly to the arthropod as + the most probable ancestor of the vertebrate.</p> + + <p class="ac"><span class="sc">Evolution of Tissues.</span></p> + + <p>In the whole scheme of evolution we can recognize, not only an upward progress in the + organization of the animal as a whole, but also a distinct advance in the structure of the tissues + composing an individual, which accompanies that upward progress. Thus it is possible to speak of + an evolution of the supporting tissues from the simplest form of connective tissue up to cartilage + and thence to bone; of the contractile tissues, from the simplest contractile protoplasm <span + class="pagenum" id="page20">{20}</span>to unstriped muscle, and thence to the highest forms of + striated muscle; of the nervous connecting strands, from undifferentiated to fine strands, then to + thicker, more separated ones, resembling non-medullated fibres, and finally to well-differentiated + separate fibres, each enclosed in a medullated sheath.</p> + + <p>In the connective tissue group, bone is confined to the vertebrates, cartilage is found among + invertebrates, and the closest resemblance to vertebrate embryonic or parenchymatous cartilage is + found in the cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus, more than any + other invertebrate, possesses a fibrous connective tissue resembling that of vertebrates.</p> + + <p>In the muscular group, Biedermann, who has made a special study of the physiology of striated + muscle, says that among invertebrates the striated muscle of the arthropod group resembles most + closely that of the vertebrate.</p> + + <p class="sp3">In the nervous group the resemblance between the nerve-fibres of Limulus and + Ammocœtes, both of which are devoid of any marked medullary sheath, is very apparent, and + Retzius points out that the only evidence of medullation, so characteristic of the vertebrates, is + found in a species of prawn (<i>Palæmon</i>). In all these cases the nearest resemblance to the + vertebrate tissues is to be found in the arthropod.</p> + + <p class="ac"><span class="sc">The Evidence of Palæontology.</span></p> + + <p>Perhaps the most important of all the clues likely to help in the solution of the origin of + vertebrates is that afforded by Geology, for although the geological record is admittedly so + imperfect that we can never hope by its means alone to link together the animals at present in + existence, yet it does undoubtedly point to a sequence in the evolution of animal forms, and gives + valuable information as to the nature of such sequence. In different groups of animals there are + times when the group can be spoken of as having attained its most flourishing period. During these + geological epochs the distribution of the group was universal, the numbers were very great, the + number of species was at the maximum, and some of them had attained a maximal size. Such races + were at that time dominant, and the struggle for existence was essentially among members of the + same group. At the present time the dominant race is man, and the <span class="pagenum" + id="page21">{21}</span>struggle for existence is essentially between the members of that race, and + not between them and any inferior race.</p> + + <p>The effect of such conditions is, as Darwin has pointed out, to cause great variation in that + group; in consequence of that variation and that dominance the evolution of the next higher group + is brought about from some member of the dominant group. Thus the present age is the outcome of + the Tertiary period, a time when giant mammals roamed the earth and left as their successors the + mammals of the present day; a time of dominance of quadruped mammals; a time of which the period + of maximum development is long past, and we now see how the dominance of the biped mammal, man, is + accompanied by the rapid diminution and approaching extermination of the larger mammals. No + question can possibly arise as to the immediate ancestor of the biped mammal; he undoubtedly arose + from one of the dominant quadrupedal mammals.</p> + + <p>Passing along to the next evidence of the rocks, we find an age of reptiles in the Mesozoic + period. Here, again, the number and variety is most striking; here, again, the size is enormous in + comparison with that of the present-day members of the group. This was the dominant race at the + time when the birds and mammals first appeared on the earth, and anatomists recognize in these + extinct reptilian forms two types; the one bird-like, the other more mammalian in character. From + some members of the former group birds are supposed to have been evolved, and mammals from members + of the other group. There is no question of their origin directly from lower fish-like forms; the + time of their appearance on the earth, their structure, all point irresistibly to the same + conclusion as we have arrived at from the consideration of the origin of the biped from the + quadruped mammal, viz. that birds and mammals arose, in consequence of the struggle for existence, + from some members of the reptilian race which at that time was the dominant one on earth.</p> + + <p>Passing down the geological record, we find that when the reptiles first appear in the + Carboniferous age there is abundant evidence of the existence of numbers of amphibian forms. At + this time the giant Labyrinthodonts flourished. Here among the swamps and marshes of the + coal-period the prevalent vertebrate was amphibian in structure. Their variety and number were + very great, and at that period they attained their greatest size. Here, again, from the geological + record we draw the same conclusion as before, that the reptiles arose from the race which was then + predominant on the earth—the Amphibia.</p> + + <div><span class="pagenum" id="page22">{22}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig004.png" id="fig4"><img style="width:100%" src="images/fig004.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 4.—Plan of Geological Strata.</span> (From <span + class="sc">Lankester</span>.)</p> + </div> + </div> + + <div><span class="pagenum" id="page23">{23}</span></div> + + <p>Again, another point of great interest is seen here, and that is that these Labyrinthodonts, as + Huxley has pointed out, possess characters which bring them more closely than the amphibians of + the present day into connection with the fishes; and further, the fish-like characters they + possessed are those of the Ganoids, the Marsipobranchs, the Dipnoans, and the Elasmobranchs, + rather than of the Teleosteans.</p> + + <p>Now, it is a striking fact that the ancient fishes at the time when the amphibians appeared had + not reached the teleostean stage. The ganoids and elasmobranchs swarmed in the waters of the + Devonian and Carboniferous times. Dipnoans and marsipobranchs were there, too, in all probability, + but teleosteans do not appear until the Mesozoic period. The very kinds of fish, then, which + swarmed in the seas at that time, and were the predominant race before the Carboniferous epoch, + are those to which the amphibians at their first appearance show the closest affinity. Here, + again, the same law appears; from the predominant race at the time, the next higher race arose, + and arose by a most striking modification, which was the consequence of altering the medium in + which it lived. By coming out of the water and living on the land, or, rather, being able to live + partly on land and partly in the water, by the acquisition of air-breathing respiratory organs or + lungs in addition to, and instead of, water-breathing organs or gills, the amphibian not only + arose from the fish, but made an entirely new departure in the sequence of progressive forms.</p> + + <p>This was a most momentous step in the history of evolution—one fraught with mighty + consequences and full of most important suggestions.</p> + + <p>From this time onwards the struggle for existence by which upward progress ensued took place on + the land, not in the sea, and, as has been pointed out, led to the evolution of reptiles from + amphibians, birds and quadrupedal mammals from reptiles, and man from quadrupeds. In the sea the + fishes were left to multiply and struggle among themselves, their only opponents being the giant + cephalopods, which themselves had been evolved from a continual succession of the Mollusca. For + this reason the struggle for existence between the fishes and the higher race evolved from them + did not <span class="pagenum" id="page24">{24}</span>take place until some members of that higher + race took again to the water, and so competed with the fish-tribe in their own element.</p> + + <p>Another most important conclusion to be derived from the uprising of the Amphibia is that at + that time there was no race of animals living on the land which had a chance against them. No race + of land-living animals had been evolved whose organization enabled them to compete with and + overcome these intruders from the sea in the struggle for existence. For this reason that the + whole land was their own, and no serious competition could arise from their congeners, the fish, + they took possession of it, and increased mightily in size; losing more and more the habit of + going into the water, becoming more and more truly terrestrial animals. Henceforth, then, in + trying to find out the sequence of evolution, we must leave the land and examine the nature of the + animals living in the sea; the air-breathing animals which lived on the land in the Upper Silurian + and Devonian times cannot have reached a stage of organization comparable with that of the fishes, + seeing how easily the amphibians became dominant.</p> + + <p>We arrive, then, at the conclusion that the ancestors of the fishes must have lived in the sea, + and applying still the same principles that have held good up to this time, the ancestors of the + fishes must have arisen from some member of the race predominant at the time when they first + appeared, and also the earliest fishes must have much more closely resembled the ancestral form + than those found in later times or at the present day.</p> + + <p>What, then, is the record of the rocks at the time of the first appearance of fish-like forms? + What kind of fishes were they, and what was the predominant race at the time?</p> + + <p>We have now reached the Upper Silurian and Lower Devonian times, and most instructive and + suggestive is the revelation of the rocks. Here, when the first vertebrates appeared, the sea was + peopled with corals, brachiopods, early forms of cephalopods, and other invertebrates; but, above + all, with the great tribe of trilobites (Fig. <a href="#fig6">6</a>) and their successors. From + the trilobites arose, as evidenced by their larval form, the king-crab group, called the Xiphosura + (Fig. <a href="#fig5">5</a>). Closely connected with them, and forming intermediate stages between + trilobites and king-crabs, numerous forms have been discovered, known as Belinurus, Prestwichia, + Hemiaspis, Bunodes, etc. (Fig. <a href="#fig5">5</a> and Fig. <a href="#fig12">12</a>). From them + also arose the most striking group <span class="pagenum" id="page25">{25}</span>of animals which + existed at this period—the giant sea-scorpions, or Gigantostraca. This group was closely + associated with the king-crabs, and the two groups together are classified under the title + Merostomata.</p> + + <div class="ac w45 fcenter sp3"> + <a href="images/fig005.png" id="fig5"><img style="width:100%" src="images/fig005.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 5</span> (from <span class="sc">H. + Woodward</span>).—1. <i>Limulus polyphemus</i> (dorsal aspect). 2. <i>Limulus,</i> + young, in trilobite stage. 3. <i>Prestwichia rotundata.</i> 4. <i>Prestwichia Birtwelli.</i> + 5. <i>Hemiaspis limuloides.</i> 6. <i>Pseudoniscus aculeatus.</i></p> + </div> + </div> + + <p>The appearance of these sea-scorpions is given in Figs. 7 and 8, representing Stylonurus, + Slimonia, Pterygotus, Eurypterus. They must have been in those days the tyrants of the deep, for + specimens of Pterygotus have been found over six feet in length.</p> + + <p>At this time, then, by every criterion hitherto used, by the multitude of species, by the size + of individual species, which at this period reached the maximum, by their subsequent decay and + final extinction, we must conclude that these forms were in their zenith, that the predominant + race at this time was to be found in this group of arthropods. Just previously, the sea swarmed + with trilobites, and right into the period when the Gigantostraca flourished, the trilobites <span + class="pagenum" id="page26">{26}</span>are still found of countless forms, of great difference in + size. The whole period may be spoken of as the great trilobite age, just as the Tertiary times + form the mammalian age, the Mesozoic times the reptilian age, etc. From the trilobites the + Gigantostraca and Xiphosura arose, as evidenced by the embryology of Limulus, and, therefore, in + the term trilobite age would be included the whole of those peculiar forms which are classified by + the names Trilobita, Gigantostraca, Xiphosura, etc. Of all these the only member alive at the + present time is Limulus, or the King-Crab.</p> + + <table class="mc tlf sp2 w45" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:44%;"><a href="images/fig006.png" id="fig6"><img + style="width:100%" src="images/fig006.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:44%;"><a href="images/fig007.png" id="fig7"><img + style="width:100%" src="images/fig007.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 6.—A Trilobite</span> (<i><span + class="correction" title="Original reads 'Dalmatites' (which is an + ammonite)">Dalmanites</span></i>) (after <span class="sc">Pictet</span>). Dorsal view.</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 7.</span>—<i>Eurypterus remipes</i> (after + <span class="sc">Nieskowski</span>). Dorsal view.</p> + </div> + </td> + </tr> + </table> + + <p>As, however, the term 'trilobite' does not include the members of the king-crab or sea-scorpion + groups, it is advisable to use some other term to represent the whole group. They cannot be called + crustaceans or arachnids, for in all probability they gave origin to both; the nearest approach to + the Trilobite stage of development at the present time is to be found perhaps in Branchipus (Fig. + <a href="#fig10">10</a>) and Apus (Fig. <a href="#fig9">9</a>), just as the nearest approach to + the Eurypterid <span class="pagenum" id="page27">{27}</span>form is Limulus. Crustaceans such as + crabs and lobsters are of much later origin, and do not occur in any quantity until the late + Mesozoic period. The earliest found, a kind of prawn, occurs in the Carboniferous age.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig008.png" id="fig8"><img style="width:100%" src="images/fig008.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 8.</span>—A, <i>Pterygotus Osiliensis</i> (from + <span class="sc">Schmidt</span>). B, <i>Stylonurus Logani</i> (from <span + class="sc">Woodward</span>). C, <i>Slimonia acuminata</i> (from <span + class="sc">Woodward</span>).</p> + </div> + </div> + + <p>Korschelt and Heider have accordingly suggested the name <i>Palæostraca</i> for this whole + group, and <i>Protostraca</i> for the still earlier <span class="pagenum" + id="page28">{28}</span>arthropod-like animals which gave origin to the trilobites themselves. This + name I shall adopt, and speak, therefore, of the <i>Palæostraca</i> as the dominant race at the + time when vertebrates first appeared.</p> + + <p>If, then, there is no break in the law of evolution here, the race which was predominant at the + time when the vertebrate first appeared must have been that from which the first fishes arose, and + these fishes must have resembled, not the crustacean proper, or the arachnid proper, but a member + of the palæostracan group. Moreover, just as the Labyrinthodonts show special affinities to the + fishes which were then living, so we should expect that the forms of the earliest fish would + resemble the arthropodan type dominant at the time more closely than the fish of a later era.</p> + + <p>At first sight it seems too great an absurdity even to imagine the possibility of any genetic + connection between a fish and an arthropod, for to the mind's eye there arises immediately the + picture of a salmon or a shark and a lobster or a spider. So different in appearance are the two + groups of animals, so different their methods of locomotion, that it is apparently only an inmate + of a lunatic asylum who could possibly suggest such a connection. Much more likely is it that a + fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is + therefore to the annelids that the upholders of the theory of the reversal of surfaces look for + the ancestor of the vertebrate.</p> + + <table class="mc tlf sp2 w85" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:47%;"><a href="images/fig009.png" id="fig9"><img + style="width:47%" src="images/fig009.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:43%;"><a href="images/fig010.png" id="fig10"><img + style="width:100%" src="images/fig010.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 9.</span>—<i>Apus</i> (from the Royal Natural + History). Dorsal view.</p> + </div> + </td> + <td> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 10.</span>—<i>Branchipus stagnalis.</i> (From + <span class="sc">Claus</span>.)</p> + </div> + </td> + </tr> + </table> + + <div><span class="pagenum" id="page29">{29}</span></div> + + <p>We must endeavour to dismiss from our imagination such forms as the salmon and shark as + representatives of the fish-tribe, and the lobster and spider of the arthropods, and try to + picture the kind of animals living in the seas in the early Devonian and Upper Silurian times, and + then we find, to our surprise, that instead of the contrast between fishes and arthropods being so + striking as to make any comparison between the two seem an absurdity, the difficulty in the last + century, and even now, is to decide in many cases whether a fossil is an arthropod or a fish.</p> + + <p>I have shown what kind of animal the palæostracan was like. What information is there of the + nature of the earliest vertebrate?</p> + + <p>The most ancient fishes hitherto discovered have been classified by Lankester and Smith + Woodward into the three orders, Heterostraci, Osteostraci, and Antiarcha. Of these the + Heterostraci contain the genera Pteraspis and Cyathaspis, and are the very earliest vertebrates + yet discovered, being found in the Lower Silurian. The Osteostraci are divided into the + Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper Silurian and Devonian beds. The + Antiarcha, comprising Pterichthys and Bothriolepis, belong to the Devonian and are not found in + Silurian deposits. This, then, is the order of their appearance—Pteraspis, Cephalaspis, and + Pterichthys.</p> + + <p>In none of these families is there any resemblance to an ordinary fish. In no case is there any + sign of vertebræ or of jaws. They, like the lampreys, were all agnathostomatous. Strange indeed is + their appearance, and it is no wonder that there should have been a difficulty in deciding whether + they were fish or arthropod. Their great characteristic is their buckler-plated cephalic shield, + especially conspicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give the dorsal + shields of Pteraspis, Auchenaspis, Pterichthys, and Bothriolepis.</p> + + <p>In 1904, Drevermann discovered a mass of <i>Pteraspis Dunensis</i> embedded in a single stone, + showing the same kind of head-shield as <i>P. rostrata</i>, but the rostrum was longer and the + spine at the extremity of the head-shield much longer and more conspicuous. The whole shape of the + animal as seen in this photograph recalls the shape of a Hemiaspid rather than of a fish. It is, + then, natural enough for the earlier observers to have looked upon such a fossil as related to an + arthropod rather than a fish.</p> + + <div><span class="pagenum" id="page30">{30}</span></div> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig011.jpg" id="fig11"><img style="width:100%" src="images/fig011.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 11.</span>—<i>Pteraspis dunensis</i> (from <span + class="sc">Drevermann</span>). Dorsal view of body and spine on the right side. Head-end, + showing long rostrum on the left side.</p> + </div> + </div> + + <table class="mc tlf sp2 w35" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:42%;"><a href="images/fig012.jpg" id="fig12"><img + style="width:100%" src="images/fig012.jpg" alt="" title=""/></a></td> + <td class="vbm ac" style="width:42%;"><a href="images/fig013.jpg" id="fig13"><img + style="width:100%" src="images/fig013.jpg" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 12.</span>—<i>Bunodes lunula.</i> (From <span + class="sc">Schmidt</span>.)</p> + </div> + </td> + <td> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 13.</span>—<i>Auchenaspis (Thyestes) + verrucosus</i>, natural size. (From <span class="sc">Woodward</span>.)</p> + </div> + </td> + </tr> + </table> + + <div><span class="pagenum" id="page31">{31}</span></div> + + <p>In Figs. 12 and 13 I have placed side by side two Silurian fossils which are found in the same + geological horizon. They are both life size and possess a general similarity of appearance, yet + the one is a Cephalaspidian fish known by the name of <i>Auchenaspis</i> or <i>Thyestes + verrucosa</i>, the other a Palæostracan called <i>Bunodes lunula</i>.</p> + + <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations"> + <tr class="vmi"> + <td class="vbm ac" style="width:42%;"><a href="images/fig014.jpg" id="fig14"><img + style="width:100%" src="images/fig014.jpg" alt="" title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 14.—Dorsal Head-shield of</span> <i>Thyestes (Auchenaspis) + verrucosus</i>. (From <span class="sc">Rohon</span>.)</p> + <p class="sp0"><i>Fro.</i>, narial opening; <i>l.e.</i>, lateral eyes; <i>gl.</i>, + glabellum or plate over brain; <i>Occ.</i>, occipital region.</p> + </div> + </td> + <td class="vbm ac" style="width:46%;"><a href="images/fig015.png" id="fig15"><img + style="width:100%" src="images/fig015.png" alt="" title=""/></a> + <div class="smaller aj it1"> + <p class="sp0 ac"><span class="sc">Fig. 15.</span>—<i><span class="correction" + title="Original reads 'Ptericthys'.">Pterichthys</span>.</i></p> + </div> + </td> + </tr> + </table> + + <p>In a later chapter I propose to discuss the peculiarities and the nature of the head-shields of + these earliest fishes, in connection with the question of the affinities of the animals which bore + them. At this point of my argument I want simply to draw attention to the undoubted fact of the + striking similarity in appearance between the <span class="pagenum" + id="page32">{32}</span>earliest fishes and members of the Palæostraca, the dominant race of + arthropods which swarmed in the sea at the time: a similarity which could never have been + suspected by any amount of investigation among living forms, but is immediately revealed when the + ages themselves are questioned.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig016.png" id="fig16"><img style="width:100%" src="images/fig016.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 16.</span>—<i>Bothriolepis.</i> (After <span + class="sc">Patten</span>.)</p> + <p class="sp0"><i>An.</i>, position of anus.</p> + </div> + </div> + + <p>I have not reproduced any of the attempted restorations of these old forms, as usually given in + the text-books, because all such restorations possess a large element of fancy, due to the + personal bias of the observer. I have put in Rohon's idea of the general shape of Tremataspis + (Fig. <a href="#fig17">17</a>) in order to draw attention to the lamprey-like appearance of the + fish according to his researches (<i>cf.</i> Fig. <a href="#fig18">18</a>).</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig017.jpg" id="fig17"><img style="width:100%" src="images/fig017.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 17.</span>—<span class="sc">Restoration of</span> + <i>Tremataspis</i>. (After <span class="sc">Rohon</span>, slightly modified.)</p> + </div> + </div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig018.png" id="fig18"><img style="width:100%" src="images/fig018.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 18.</span>—<i>Ammocœtes.</i></p> + </div> + </div> + + <p class="sp3">The argument, then, from geology, like that from comparative anatomy and from the + consideration of the importance of the central nervous system in the upward development of the + animal race, not only points directly to the arthropod group as the ancestor of the <span + class="pagenum" id="page33">{33}</span>vertebrate, but also to a distinct ancient type of + arthropod, the Palæostracan, the only living example of which is the King-Crab or Limulus; while + the nearest approach to the trilobite group among living arthropods are Branchipus and Apus. It + follows, therefore, that for the following up of this clue, Limulus especially must be taken into + consideration, while Branchipus and Apus are always to be kept in mind.</p> + + <p class="ac"><span class="sc">Ammocœtes rather than Amphioxus is the Best Subject for + Investigation.</span></p> + + <p>It is not, however, Limulus that must be investigated in the first instance, but the vertebrate + itself; for it can never be insisted on too often that in the vertebrate itself its past history + will be found, but that Limulus cannot reveal the future of its race. What vertebrate must be + chosen for investigation? Reasons have been given why our attention should be fixed upon the + king-crab rather than on the lobster on the invertebrate side; what is the most likely animal on + the vertebrate side?</p> + + <p>From the evidence already given it is manifest that the earliest mammal belonged to the lowest + group of mammals; that the birds on their first appearance presented reptilian characteristics, + that the earliest reptiles belonged to a low type of reptile, that the amphibians at their first + appearance were nearer in type to the fishes than were the later forms. As each of these groups + advances in number and power, specialization takes place in it, and the latest developed members + become further and further removed in type from the earliest. So also it must have been with the + origin of fishes: here too, in the quest for information as to the structure and nature of the + first-formed fishes, we must look to the lowest rather than to the highest living members of the + group.</p> + + <p>The lowest fish-like animal at present living is Amphioxus, and on this ground it is argued + that the original vertebrate must have approached in organization to that of Amphioxus; it is upon + the comparison between the structure of Amphioxus and that of Balanoglossus, that the theory of + the origin of vertebrates from forms like the latter animal is based. For my own part, I think + that in the first instance, at all events, Amphioxus should be put on one side, although of course + its structure must always be kept in mind, for the following reasons<span + class="wnw">:—</span></p> + + <div><span class="pagenum" id="page34">{34}</span></div> + + <p>Amphioxus, like the tunicates, does not possess the characteristics of other vertebrates. In + all vertebrates above these forms the great characteristic is a well-defined brain-region from + which arise nerves to organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for + the pigmented spot at the anterior extremity of the brain-region is no eye but only a mass of + pigment, and the so-called olfactory pit is a very rudimentary and inferior organ of smell. In + connection with the nearly complete absence of these two most important sense-organs, the most + important part of the central nervous system, the region corresponding to the cerebral + hemispheres, is also nearly completely absent.</p> + + <p>Now, the history of the evolution of the central nervous system in the animal race points + directly to its formation as a concentrated mass of nervous material at the anterior extremity of + the body, in consequence of the formation of special olfactory and visual organs at that + extremity. As already stated, the concentration of nervous material around the mouth as an oral + ring was its beginning. In connection with this there arose special sense-organs for the guidance + of the animal to its food which took the form of olfactory and optic organs. With the shifting + from the radial to the elongated form these sense-organs remained at the anterior or mouth-end of + the animal, and owing to their immense importance in the struggle for existence, that part of the + central nervous system with which they were connected developed more than any other part, became + the leader to which the rest of the nervous system was subservient, and from that time onwards the + development of the brain-region was inevitably associated with the upward progress of animal + life.</p> + + <p>To those who believe in Evolution and the Darwinian theory of the survival of the fittest, it + is simply inconceivable that a soft-bodied animal living in the mud, blind, with a rudimentary + brain and rudimentary olfactory organs, such as is postulated when we think of Balanoglossus and + Amphioxus, should hold its own and come victorious out of the struggle for existence at a time + when the sea was peopled with powerful predaceous scorpion- and crab-like armour-plated animals + possessing a well-developed brain, good eyes and olfactory organs, and powerful means of + locomotion. Wherever in the scale of animal development Amphioxus may ultimately be placed, it + cannot be looked upon as the type of the earliest formed fishes such as appeared in Silurian + times.</p> + + <div><span class="pagenum" id="page35">{35}</span></div> + + <p>The next lowest group of living fishes is the Marsipobranchii which include the lampreys and + hag-fishes. To these naturally we must turn for a clue as to the organization of the earliest + fish, for here we find all the characteristics of the vertebrates represented: a well-formed + brain-region, well-developed eyes and nose, cranial nerves directly comparable with those of other + vertebrates, and even the commencement of vertebræ.</p> + + <p>Among these forms the lamprey is by far the best for investigation, not only because it is + easily obtainable in large quantities, but especially because it passes a large portion of its + existence in a larval condition, from which it emerges into the adult state by a wonderful process + of transformation, comparable in extent with the transformation of the larval caterpillar into the + adult imago. So long does the lamprey live in this free larval condition, and so different is it + in the adult stage, that the older anatomists considered that the two states were really different + species, and gave the name of <i>Ammocœtes branchialis</i> to the larval stage, while the + adult form was called <i>Petromyzon planeri</i>, or <i>Petromyzon fluviatilis</i>.</p> + + <p>This long-continued free-living existence in the larval or Ammocœtes stage makes the + lamprey, more than any other type of lowly organized fish, invaluable for the present + investigation, for throughout the animal kingdom it is recognized that the larval form approaches + nearer to the ancestral type than the adult form, whether the latter is progressive or degenerate. + Not only are the tissues formed during the stages which are passed through in a free-living larval + form, serviceable tissues comparable to those of adult life, but also these stages proceed at so + much slower a rate than do those in the embryo <i>in utero</i> or in the egg, as to make the + larval form much more suitable than the embryo for the investigation of ancestral problems. It is + true enough that the free life of the larva may bring about special adaptations which are not of + an ancestral character, as may also occur during the life of the adult; but the evidence is very + strong that although some of the peculiarities of the larva may be due to such cœnogenetic + factors, yet on the whole many of them are due to ancestral characters, which disappear when + transformation takes place, and are not found in the adult.</p> + + <p>Thus if it be supposed that the amphibian arose from the fish, the tadpole presents more + resemblance to the fish than the frog. If <span class="pagenum" id="page36">{36}</span>it be + supposed that the arthropod arose from the segmented worm, the caterpillar bears out the + suggestion better than the adult imago. If it be supposed that the tunicate arose from a stock + allied to the vertebrate, it is because of the peculiarities of the larva that such a supposition + is entertained. So, too, if it be supposed that the fish arose from a member of the arthropod + group, the larval form of the fish is most likely to give decisive information on the point.</p> + + <p class="sp3">For all these reasons the lowest form of fish to be investigated, in the hopes of + finding out the nature of the earliest formed fish, is not Amphioxus, but Ammocœtes, the + larval form of the lamprey—a form which, as I hope to satisfy my reader after perusal of + subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living + vertebrate.</p> + + <p class="ac"><span class="sc">Comparison of Central Nervous Systems of Vertebrate and Arthropod + without Reversal of Surfaces.</span></p> + + <p>So far different lines of investigation all point to the origin of the vertebrate from + arthropods, the group of arthropods in question being now extinct, the nearest living + representative being Limulus; also to the fact that of the two theories of the origin of + vertebrates, that one which is based on the resemblance between the central nervous systems of the + Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in accordance with this + evidence than the other, which is based mainly on the supposed possession of a notochord among + certain animals.</p> + + <p>How is it, then, that this theory has been discredited and lost ground? Simply, I imagine, + because it was thought to necessitate the turning over of the animal. Let us, then, again look at + the nervous system of the vertebrate, and see whether there is any such necessity.</p> + + <p>As previously mentioned, the comparison of the two central nervous systems showed such close + resemblances as to force those anatomists who supported this theory to the conclusion that the + infundibular tube was in the position of the original œsophagus; they therefore looked for + the remains of a mouth opening in the dorsal roof of the brain, but did not attempt to explain the + extraordinary fact that the infundibular tube is only a ventral offshoot from the tube of the + central nervous system. Yet this latter tube <span class="pagenum" id="page37">{37}</span>is one, + if not the most striking, of the peculiarities which distinguish the vertebrate; a tubular central + nervous system such as that of the vertebrate is totally unlike any other nervous system, and the + very fact that the two nervous systems of the vertebrate and arthropod are so similar in their + nervous arrangements, makes it still more extraordinary that the nervous system should be grouped + round a tube in the one case and not in the other.</p> + + <p>Now, in the arthropod the œsophagus leads directly into the stomach, which is situated + in the head-region, and from this a straight intestine passes directly along the length of the + body to the anus, where it terminates. The relations of mouth, œsophagus, alimentary canal, + and nervous system in these animals are represented in the diagram (Fig. <a + href="#fig3">3</a>).</p> + + <p>Any tube, therefore, such as that of the infundibulum, which would represent the + œsophagus of such an animal, must have opened into the mouth on the ventral side, and into + the stomach on the dorsal side, and the lining epithelium of such an œsophagus must have + been continuous with that of the stomach, and so of the whole intestinal tract.</p> + + <p>Supposing, then, the animal is not turned over, but that the dorsal side still remains dorsal + and ventral ventral, then the original mouth-opening of the œsophagus must be looked for on + the ventral surface of the vertebrate brain in the region of the pituitary body or hypophysis, and + on the dorsal side the tube representing the œsophagus must be continuous with a large + cephalically dilated tube, which ought to pass into a small canal, to run along the length of the + body and terminate in the anus.</p> + + <p>This is exactly what is found in the vertebrate, for the infundibular tube passes into the + third ventricle of the brain, which forms, with the other ventricles of the brain, the large + dilated cephalic portion of the so-called nerve tube, and at the junction of the medulla oblongata + and spinal cord, this dilated anterior part passes into the small, straight, central canal of the + spinal cord, which in the embryo terminates in the anus by way of the neurenteric canal. If the + animal is regarded as not having been turned over, then the conclusion that the infundibulum was + the original œsophagus leads immediately to the further conclusion that the ventricles of + the vertebrate brain represent the original cephalic stomach, and the central canal of the spinal + cord the straight intestine of the arthropod ancestor.</p> + + <div><span class="pagenum" id="page38">{38}</span></div> + + <p>For the first time a logical, straightforward explanation is thus given of the peculiarities of + the tube of the central nervous system, with its extraordinary termination in the anus in the + embryo, its smallness in the spinal cord, its largeness in the brain region, and its offshoot to + the ventral side of the brain as the infundibular channel. It is so clear that, if the + infundibular tube be looked on as the old œsophagus, then its lining epithelium is the + lining of that œsophagus; and the fact that this lining epithelium is continuous with that + of the third ventricle, and so with the lining of the whole nerve-tube, must be taken into account + and not entirely ignored as has hitherto been the case. If, then, we look at the central nervous + system of the vertebrate in the light of the central nervous system of the arthropod without + turning the animal over, we are led immediately to the conclusion that what has hitherto been + called the vertebrate nervous system is in reality composed of two parts, viz. a nervous part + comparable in all respects with that of the arthropod ancestor, which has grown over and included + into itself, to a greater or less extent, a tubular part comparable in all respects with the + alimentary canal of the aforesaid ancestor. If this conclusion is correct, it is entirely wrong to + speak of the vertebrate central nervous system as being tubular, for the tube does not belong to + the nervous system, but was originally a simple epithelial tube, such as characterizes the + œsophagus, cephalic stomach, and straight intestine of the arthropod.</p> + + <p>Here, then, is the crux of the position—either the so-called nervous tube of the + vertebrate is composed of two separate factors, consisting of a true non-tubular nervous system + and a non-nervous epithelial tube, these two elements having become closely connected together; or + it is composed of one factor, an epithelial tube which constitutes the nervous system, its + elements being all nervous elements.</p> + + <p>If this latter hypothesis be accepted, then it is necessary to explain why parts of that tube, + such as the roof of the fourth ventricle, the choroid plexuses of the various ventricles, which + are parts of the original roof inserted into the ventricles, are not composed of nervous material, + but form simple single-layered epithelial sheets, which by no possibility can be included among + functional nervous structures. The upholders of this hypothesis can only explain the nature of + these thin epithelial parts of the nervous tube in one of two ways; either the tube was originally + formed of nervous <span class="pagenum" id="page39">{39}</span>material throughout, and for some + reason parts of it have lost their nervous function and thinned down; or else these thin + epithelial parts are on their way to become nervous material, are still in an embryonic condition, + and are of the nature of epiblast-epithelium, from which the central nervous system originally + arose.</p> + + <p>The first explanation is said to be supported by embryology, for at first the nerve-tube is + formed in a uniform manner, and then later, parts of the roof appear to thin out and so form the + thin epithelial parts. If this were the right explanation, then it ought to be found that in the + lowest vertebrates there is greater evidence of a uniformly nervous tube than in the higher + members of the group: while conversely, if, on the contrary, as we descend the vertebrate phylum, + it is found that more and more of the tube presents the appearance of a single layer of + epithelium, and the nervous material is limited more and more to certain parts of that tube, then + the evidence is strong that the tubular character of the central nervous system is not due to an + original nervous tube, but to a non-nervous epithelial tube with which the original nervous system + has become closely connected.</p> + + <p>The comparison of the brain region of the different groups of vertebrates (Fig. <a + href="#fig19">19</a>) is most instructive, for it demonstrates in the most conclusive manner how + the roof of the nervous tube in that region loses more and more its nervous character, and takes + on the appearance of a simple epithelial tube, as we descend lower and lower; until at last, in + the brain of Ammocœtes, as represented in the figures, the whole of the brain-roof, from + the region of the pineal eye to the commencement of the spinal cord, is composed of fold upon fold + of a thin epithelial membrane forming an epithelial bag, which is constricted in only one place, + where the fourth cranial nerve crosses over it.</p> + + <p>Further, the brain of Ammocœtes (Fig. <a href="#fig20">20</a>) shows clearly not only + that it is composed of two parts, an epithelial tube and a nervous system, but also that the + nerve-masses are arranged in the same relative position with respect to this tube as are the + nerve-masses in the invertebrate with respect to the cephalic stomach and œsophagus. This + evidence is so striking, so conclusive, that it is impossible to resist the conclusion that the + tube did not originate as part of the central nervous system, but was originally independent of + the central nervous system, and has been invaded by it.</p> + + <div><span class="pagenum" id="page40">{40}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig019.png" id="fig19"><img style="width:100%" src="images/fig019.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 19.—Comparison of Vertebrate Brains.</span></p> + <p class="sp0"><i>CB.</i>, cerebellum; <i>PT.</i>, pituitary body; <i>PN.</i>, pineal body; + <i>C. STR.</i>, corpus striatum; <i>G.H.R.</i>, right ganglion habenulæ. <i>I.</i>, olfactory; + <i>II.</i>, optic nerves.</p> + </div> + </div> + + <div><span class="pagenum" id="page41">{41}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig020.png" id="fig20"><img style="width:100%" src="images/fig020.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 20.—Brain of Ammocœtes.</span></p> + <p class="sp0">A, dorsal view; B, lateral view; C, ventral view. <i>C.E.R.</i>, cerebral + hemispheres; <i>G.H.R.</i>, right ganglion habenulæ; <i>PN.</i>, right pineal eye; + <i>CH<sub>2</sub></i>, <i>CH<sub>3</sub></i>, choroid plexuses; <i>I.-XII.</i> cranial nerves; + <i>C.P.</i>, <i>Conus post-commissuralis</i>.</p> + </div> + </div> + + <div><span class="pagenum" id="page42">{42}</span></div> + + <p>The second explanation is hardly worth serious consideration, for it supposes that the nervous + system, for no possible reason, was laid down in its most important parts—the + brain-region—as an epithelial tube with latent potential nervous functions; that even up to + the highest vertebrate yet evolved these nervous functions are still in abeyance over the whole of + the choroid plexuses and the roof of the fourth ventricle. Further, it supposes that this + prophetic epithelial tube originally developed into true nervous material only in certain parts, + and that these parts, curiously enough, formed a nervous system absolutely comparable to that of + the arthropod, while the dormant prophetic epithelial part was formed so as just to mimic, in + relation to the nervous part, the alimentary canal of that same arthropod.</p> + + <p>The mere facts of the case are sufficient to show the glaring absurdity of such an explanation. + This is not the way Nature works; it is not consistent with natural selection to suppose that in a + low form nervous material can be laid down as non-nervous epithelial material in order to provide + in some future ages for the great increase in the nervous system.</p> + + <p>Every method of investigation points to the same conclusion, whether the method is + embryological, anatomical, or pathological.</p> + + <p>First, take the embryological evidence. On the ground that the individual development + reproduces to a certain extent the phylogenetic development, the peculiarities of the formation of + the central nervous system in the vertebrate embryo ought to receive an appropriate explanation in + any theory of phylogenetic development. Hitherto such explanation has been totally lacking; any + suggestion of the manner in which a tubular nervous system may have been formed takes no account + whatever of the differences between different parts of the tube; its dilated cephalic end with its + infundibular projection ventrally, its small straight spinal part, and its termination in the + anus. My theory, on the other hand, is in perfect harmony with the embryological history, and + explains it point by point.</p> + + <p>From the very first origin of the central nervous system there is evidence of two + structures—the one nervous, and the other an epithelial surface-layer which ultimately forms + a tube; this was first described by Scott in Petromyzon, and later by Assheton in the frog. In the + latter case the external epithelial layer is pigmented, while the underlying nervous layer + contains no pigment; a marked <span class="pagenum" id="page43">{43}</span>and conspicuous + demarcation exists, therefore, between the two layers from the very beginning, and it is easy to + trace the subsequent fate of the two layers owing to this difference of pigmentation. The + pigmented cells form the lining cells of the central canal, and becoming elongated, stretch out + between the cells of the nervous layer; while the latter, on their side, invade and press between + the pigmented cells. In this case, owing to the pigmentation of the epithelial layer, embryology + points out in the clearest possible manner how the central nervous system of the vertebrate is + composed of two structures—an epithelial non-nervous tube, on the outside of which the + central nervous system was originally grouped; how, as development proceeds, the elements of these + two structures invade each other, until at last they become so involved together as to give rise + to the conception that we are dealing with one single nerve tube. It is impossible for embryology + to give a clearer clue to the past history than it does in this case, for it actually shows, step + by step, how the amalgamation between the central nervous system and the old alimentary canal took + place.</p> + + <p>Further, consider the shape of the tube when it is first formed, how extraordinary and + significant that is. It consists of a simple dilated anterior end leading into a straight tube, + the lumen of which is much larger than that of the ultimate spinal canal, and terminates by way of + the neurenteric canal in the anus.</p> + + <p>Why should the tube take this peculiar shape at its first formation? No explanation is given or + suggested in any text-book of embryology, and yet it is so natural, so simple: it is simply the + shape of the invertebrate alimentary canal with its cephalic stomach and straight intestine ending + in the anus. Again embryology indicates most unmistakably the past history of the race. How are + the nervous elements grouped round this tube when it is first formed? Here embryology shows that a + striking difference exists between the part of the tube which forms the spinal cord and the + dilated cephalic part. Fig. <a href="#fig21">21</a>, A (2), represents the relation between the + nervous masses and the epithelial tube in the first instance. At this stage the nervous material + in the spinal cord lies laterally and ventrally to this tube, and at a very early stage the white + anterior commissure is formed, joining together these two lateral masses; as yet there is no sign + of any posterior fissure, the tube with its open lumen extends right to the dorsal surface.</p> + + <div><span class="pagenum" id="page44">{44}</span></div> + + <p>The interpretation of this stage is that in the invertebrate ancestor the nerve-masses were + situated laterally and ventrally to the epithelial tube, and were connected together by + commissures on the ventral side of the tube (Fig. <a href="#fig21">21</a>, A (1)); in other words, + the chain of ventral ganglia and their transverse commissures lying just ventrally to the + intestine, which are so characteristic of the arthropod nervous system, is represented at this + stage.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig021.png" id="fig21"><img style="width:100%" src="images/fig021.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 21.—A, Method of Formation of the Vertebrate Spinal + Cord from the Ventral Chain of Ganglia and the Intestine of an Arthropod, represented in 1; B, + Method of Formation of the Vertebrate Medulla Oblongata from the Infra-œsophageal + Ganglia and the Cephalic Stomach of an Arthropod.</span></p> + </div> + </div> + + <p>Subsequently, by the growth dorsalwards of nervous material to form the posterior columns, the + original epithelial tube is compressed dorsally and laterally to such an extent that those parts + lose all signs of lumen, the one becoming the posterior fissure and the others the <i>substantia + gelatinosa Rolandi</i> on each side. The original tube is thus reduced to a small canal formed by + its ventral portion only (Fig. <a href="#fig21">21</a>, A (3)). In this way the spinal cord is + formed, and the walls of the original epithelial tube are finally visible only as the lining of + the central canal (Fig. <a href="#fig21">21</a>, A (4)).</p> + + <p>When we pass to the brain-region, to the anterior dilated portion of the tube, embryology tells + a different story. Here, as in the spinal cord, the nervous masses are grouped at first laterally + and ventrally to the epithelial tube, as is seen in Fig. <a href="#fig21">21</a>, B (2), but owing + to the large size of its lumen here, the nervous material is not able to enclose it completely, as + in the case of the spinal cord; <span class="pagenum" id="page45">{45}</span>consequently there is + no posterior fissure formed; but, on the contrary, the dorsal roof, not enclosed by the + nerve-masses, remains epithelial, and so forms the membranous roof of the fourth ventricle and of + the other ventricles of the brain (Fig. <a href="#fig21">21</a>, B (3)). In the higher animals, + owing to the development of the cerebrum and cerebellum, this membranous roof becomes pushed into + the larger brain cavity, and thus forms the choroid plexuses of the third and lateral ventricles. + In the lower vertebrates, as in Ammocœtes and the Dipnoi, it still remains as a dorsal + epithelial roof and forms a most striking characteristic of such brains.</p> + + <p>In this part of the nervous system, then, the nervous material is all grouped in its original + position on the ventral side of the tube; and yet it is the same nervous material as that of the + spinal cord, all the elements are there, giving origin here to the segmental cranial nerves just + as lower down they give rise to the segmental spinal nerves, connecting together the separate + segments each with the other and all with the higher brain-centres—the supra-infundibular + centres—just as they do in the spinal region.</p> + + <p>Why should there be this striking difference between the formation of the infra-infundibular + region of the brain and that of the spinal cord? Do the advocates of the origin of vertebrates + from Balanoglossus give the slightest reason for it? They claim that their view also provides a + tubular nervous system for the vertebrate, but give not the slightest sign or indication as to why + the nervous material should be grouped entirely on the ventral side of an epithelial tube in the + infra-infundibular region and yet surround it in the spinal cord region. And the explanation is so + natural, so simple: embryology does its very best to tell us the past history of the race, if only + we look at it the right way.</p> + + <p>The infra-infundibular nervous mass is naturally confined to the ventral side of the epithelial + tube, because it represents the infra-œsophageal ganglia, situated as they are on the + ventral side of the cephalic stomach, and, owing to the size of the stomach, they could not + enclose it by dorsal growth, as they do in the case of the formation of the spinal cord (Fig. <a + href="#fig21">21</a>, B (1)). Still these nervous masses have grown dorsalwards, have commenced to + involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in + Ammocœtes, in which animal a ventral portion of the epithelial bag has been evidently + compressed and its lumen finally obliterated <span class="pagenum" id="page46">{46}</span>by the + growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this + obliterated portion still leaves its mark as the <i>raphé</i> or seam, which is so characteristic + of the infra-infundibular portion of the brain.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig022.png" id="fig22"><img style="width:100%" src="images/fig022.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 22.—Horizontal Section through the Brain of + Ammocœtes.</span></p> + <p class="sp0"><i>Cr.</i>, membranous cranium; <i>I</i>, olfactory nerves; <i>l.v.</i>, + lateral ventricles; <i>gl.</i>, glandular tissue which fills up the cranial cavity.</p> + </div> + </div> + + <p>Here, again, it is seen how simple is the explanation of a peculiarity which has always puzzled + anatomists—why should there be this seam in the infra-infundibular portion of the brain and + not in the supra-infundibular or in the spinal cord? The corresponding compression in the upper + brain-region forms the lateral ventricles, as is seen in the accompanying figure of the brain of + Ammocœtes (Fig. <a href="#fig22">22</a>).</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig023.png" id="fig23"><img style="width:100%" src="images/fig023.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 23.—Section through Rhomboidal Sinus of + Bird.</span></p> + </div> + </div> + + <p>In yet another instance it is seen how markedly the nervous masses are arranged in the same + position with respect to the central tube as are the nerve ganglia with respect to the intestinal + tube in the case of the invertebrate. Thus in birds a portion of the spinal cord in the + lumbo-sacral region presents a very different appearance from the rest of the cord; it is known as + the rhomboidal sinus, and a section of the cord of an adult pigeon across this region is given in + Fig. <a href="#fig23">23</a>. As is seen, the nervous portions are entirely confined to two masses + connected together by the white anterior commissures which are situated laterally and ventrally to + a median gelatinous mass; the small central canal is visible and <span class="pagenum" + id="page47">{47}</span>the whole dorsal area of the cord is taken up by a peculiar non-nervous + wedge-shaped mass of tissue. At its first formation this portion of the cord is formed exactly in + the same manner as the rest of the cord; instead, however, of the nervous material invading the + dorsal part of the tube to form the posterior fissure, it has been from some cause unable to do + so, the walls of the original non-nervous tube have become thickened dorsally, been transformed + into this peculiar tissue, and so caused the peculiar appearance of the cord here. The nervous + parts have not suffered in their development; the mechanism for walking in the bird is as well + developed as in any other animal; their position only is different, for they still retain the + original ventro-lateral position, but the non-nervous tube, the remains of the old intestine, has + undergone a peculiar gelatinous degeneration just where it has remained free from invasion by the + nervous tissue.</p> + + <p>Throughout the whole of that part of the nervous system which gives origin to the cranial and + spinal segmental nerves, the evidence is absolutely uniform that the nervous material was + originally arranged bilaterally and ventrally on each side of the central tube, exactly in the + same way as the nerve-masses of the infra-œsophageal and ventral chain of ganglia are + arranged with respect to the cephalic stomach and straight intestine of the arthropod. But, in + addition, we find in the vertebrate nervous masses, the cerebral hemispheres, the corpora + quadrigemina and the cerebellum situated on the dorsal side of the central tube in the + brain-region; this nervous material is, however, of a different character to that which gives + origin to the spinal and cranial segmental nerves. How is the presence of these dorsal masses to + be explained on the supposition that the dilated anterior part of the nerve-tube was originally + the cephalic stomach of the arthropod ancestor? The cerebral hemispheres are simple enough, for + they represent the supra-œsophageal ganglia, which of necessity, as they increased in size, + would grow round the anterior end of the cephalic stomach and become more and more dorsal in + position.</p> + + <p>The difficulty lies rather in the position of the cerebellum and corpora quadrigemina, and the + solution is as simple as it is conclusive.</p> + + <p>Let us again turn to embryology and see what help it gives. In all vertebrates the dilated + anterior portion of the nerve-tube does not, <span class="pagenum" id="page48">{48}</span>as it + grows, increase in size uniformly, but a constriction appears on its dorsal surface at one + particular place, so as to divide it into an anterior and posterior vesicle; then the latter + becomes divided into two portions by a second constriction. In this way three cerebral vesicles + are formed; these three primary cerebral vesicles indicate the region of the fore-brain, + mid-brain, and hind-brain respectively. Subsequently the first cerebral vesicle becomes divided + into two to form the prosencephalon and thalamencephalon, while the third cerebral vesicle is also + divided into two to form the region of the cerebellum and medulla oblongata.</p> + + <p>These constrictions are in the position of commissural bands of nervous matter; of these the + limiting nervous strands between the thalamencephalon and mesencephalon and between the + mesencephalon and the hind-brain are of primary importance. The first of these commissural bands + is in the position of the posterior commissure connecting the two optic thalami. In close + connection with this are found, on the mid-dorsal region, the two pineal eyes with their optic + ganglia, the so-called <i>ganglia habenulæ</i>. From these ganglia a peculiar tract of fibre, + known as Meynert's bundle, passes on each side to the ventral infra-infundibular portion of the + brain. In other words, the first constriction of the dilated tube is due to the presence and + growth of nervous material in connection with the median pineal eyes. Here in precisely the same + spot, as will be fully explained in the next chapter, there existed in the arthropod ancestor a + pair of median eyes situated dorsally to the cephalic stomach, the pre-existence of which explains + the reason for the first constriction.</p> + + <p>The second primary constriction separating the mid-brain from the hind-brain is still more + interesting, for it is coincident with the position of the trochlear or fourth cranial nerve. In + all vertebrates without exception this nerve takes an extraordinary course; all other nerves, + whether cranial or spinal, pass ventralwards to reach their destination. This nerve passes + dorsalwards, crosses its fellow mid-dorsally in the valve of Vieussens, where the roof of the + brain is thin, and then passes out to supply the superior oblique muscle of the eye of the + opposite side. The two nerves form an arch constricting the dilated tube at this place. In the + lowest vertebrate (Ammocœtes) the constriction formed by this nerve-pair is evident not + only in the embryonic condition as in other vertebrates, but during the whole larval stage. As + Fig. <a href="#fig20">20</a>, A and B, shows, the whole of the dorsal <span class="pagenum" + id="page49">{49}</span>region of the brain up to the region of the pineal eye and <i>ganglion + habenulæ</i> is one large membranous bag, except for the single constriction where the fourth + nerve on each side crosses over. The explanation of this peculiarity is given in Chapter VII., and + follows simply from the facts of the arrangement of that musculature in the scorpion-group which + gave rise to the eye-muscles of the vertebrate.</p> + + <p>In Ammocœtes both cerebellum and posterior corpora quadrigemina can hardly be said to + exist, but upon transformation a growth of nervous material takes place in this region, and it is + seen that this commencing cerebellum and the corpora quadrigemina arise from tissue that is + present in Ammocœtes along the course of the fourth nerve.</p> + + <p>Here, then, again Embryology does its best to tell us how the vertebrate arose. The formation + of the two primary constrictions in the dilated anterior vesicle whereby the brain is divided into + fore-brain, mid-brain, and hind-brain is simply the representation ontogenetically of the two + nerve-tracts which crossed over the cephalic stomach in the prevertebrate stage, in consequence of + the mid-dorsal position of the pineal eyes and of the insertion of the original superior oblique + muscles.</p> + + <p>The subsequent constriction by which the prosencephalon is separated from the thalamencephalon + is in the position of the anterior commissure, that commissure which connects the two + supra-infundibular nerve-masses, and is one of the first-formed commissures in every vertebrate. + This naturally is simply the commissure between the two supra-œsophageal ganglia; anterior + to it, in the middle line, equally naturally, the anterior end of the old stomach wall still + exists as the <i>lamina terminalis</i>.</p> + + <p>The other division in the hind-brain region, which separates the region of the cerebellum from + the medulla oblongata, is due to the growth of the cerebellum, and indicates its posterior limit. + In such an animal as the lamprey, where the cerebellum is only commencing, this constriction does + not occur in the embryo.</p> + + <p>From such simple beginnings as are seen in Ammocœtes, the higher forms of brain have + been evolved, to culminate in that of man, in which the massive cerebrum and cerebellum conceals + all sign of the dorsal membranous roof, those parts of the simple epithelial tube which still + remain being tucked away into the cavities to form the various choroid plexuses.</p> + + <div><span class="pagenum" id="page50">{50}</span></div> + + <p>In the whole evolution from the brain of Ammocœtes to that of man, the same process is + plainly visible, viz. growth and extension of nervous material over the epithelial tube; extension + dorsally and posteriorly of the supra-infundibular nervous masses (as seen in Fig. <a + href="#fig19">19</a>), combined with a dorsal growth of parts of the infra-infundibular nervous + masses to form the cerebellum and posterior corpora quadrigemina.</p> + + <p>Especially instructive is the formation of the cerebellum. It consists at first of a small mass + of nervous tissue accompanying the fourth nerve, then by the growth of that mass surrounding and + constricting a fold of the membranous roof, the <i>worm</i> of the cerebellum is formed, as in the + dog-fish. This very constriction causes the membrane to be thrown into a lateral fold on each + side, as seen in Fig. <a href="#fig24">24</a>, and in the dog-fish the nervous material on each + side, known as the fimbriæ, is already commencing to grow from the ventral mass of the medulla + oblongata to surround these lateral membranous folds. These <i>fimbriæ</i> develop more and more + in higher forms, and thus form the cerebellar hemispheres.</p> + + <p>Not only does comparative anatomy confirm the teachings of embryology, but also pathology gives + its quota in the same direction.</p> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig024.png" id="fig24"><img style="width:43%" src="images/fig024.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 24.—Cerebellum of Dog-fish.</span></p> + <p class="sp0"><i>v</i>, worm of cerebellum; <i>IV.</i>, membranous roof of fourth ventricle + continuous with the membranous folds on each side. Through these the fimbriæ (<i>fb.</i>) can + be dimly seen.</p> + </div> + </div> + + <p>One of the striking facts about malformations and disease of the central nervous system is the + frequency of cystic formations; <i>spina bifida</i> is a well-known instance. These cysts are + merely epithelial non-nervous cysts formed from the epithelium of the central canal, difficult to + understand if the whole nerve tube is one and entirely nervous, either actually or potentially, + but natural and easy if we are really dealing with a simple epithelial tube on the outside of + which the nervous material was originally grouped. The cystic formation belongs naturally enough + to this tube, not to the nervous system.</p> + + <p>Again, where animals such as lizards have grown a new tail, owing to the breaking off of the + original one, it is found that the central canal extends into this new tail for some distance, but + not <span class="pagenum" id="page51">{51}</span>the nervous material surrounding it; all the + nerves supplying the new tail arise from the uninjured spinal cord above, the central canal with + its lining layer of epithelial cells alone grows into the new-formed appendage.</p> + + <p>To all intents and purposes the same thing is seen in the termination of the spinal cord in a + bird-embryo; more and more, as the end of the tail is approached, does the nervous matter of the + spinal cord grow less and less, until at last a naked central canal with its lining epithelium is + alone left to represent the so-called nerve-tube.</p> + + <p>All these different methods of investigation lead irresistibly to the one conclusion that the + tubular nature of the central nervous system has been caused by the central nervous system + enclosing to a greater or less extent a pre-existing, non-nervous, epithelial tube.</p> + + <p class="sp3">This must always be borne strictly in mind. The problem, therefore, which presents + itself is the comparison of these two factors separately, in order to find out the relationship of + the vertebrate to the invertebrate. The nervous system without the tube must be compared to other + nervous systems, and the tube must be considered apart from the nervous system.</p> + + <p class="ac"><span class="sc">The Principle of Concentration and Cephalization.</span></p> + + <p>The central nervous system of the vertebrate resembles that of all the Appendiculata in the + fact that it is composed of segments joined together which give origin to segmental nerves. There + is, however, a great difference between the two systems: the division into separate segments is + not obvious to the eye in the vertebrate nervous system, while in the invertebrate we can see that + it is composed of a series of separate pairs of ganglia joined together longitudinally by nervous + strands known as connectives and transversely by the nerve-commissures. Such a simple segmented + system is found in the segmented worms, and in the lower arthropods, such as Branchipus, no great + advance has been made on that of the annelid. In the higher forms, however, a greater and greater + tendency to fusion of separate ganglia exists, especially in the head-region, so that the + infra-œsophageal ganglia, which, in the lower forms are as separate as those of the ventral + chain, in the higher forms are fused together to form a single nervous mass.</p> + + <div><span class="pagenum" id="page52">{52}</span></div> + + <p>This is the great characteristic of the advancement of the central nervous system among the + Invertebrata, its concentration in the region of the head. It may be called the principle of + cephalization, and is characteristic not only of higher organization in a group, but also of the + adult as distinguished from the larval form. Thus in the imago greater concentration is found than + in the caterpillar.</p> + + <p>The segmented annelid type of nervous system consists of a supra-œsophageal ganglion, + composed of the fused ganglia belonging to the pre-oral segments, and an infra-œsophageal + chain of separate ganglia. With the concentration and modification around the mouth of the most + anterior locomotor appendages to form organs for prehension and mastication of food, a + corresponding concentration and fusion of the ganglia belonging to these segments takes place, so + that finally, in the higher annelids, and in most of the great arthropod group, a fusion of a + number of the most anterior ganglia has taken place to form the infra-œsophageal + ganglion-mass.</p> + + <p>The infra-œsophageal ganglia which are the first to fuse are those which supply the most + anterior portion of the animal with nerves, and include always those anterior appendages which are + modified for mastication purposes. To this part the name <i>prosoma</i> has been given; in many + cases it forms a well-defined, distinct portion of the animal.</p> + + <p>Succeeding this prosoma or masticatory region, there occurs in all gill-bearing arthropods a + respiratory region, in many cases more or less distinctly defined, which has received the name of + <i>mesosoma.</i> The rest of the body is called the <i>metasoma</i>.</p> + + <p>In accordance with this nomenclature the central nervous system of many of the Arthropoda may + be divided as follows<span class="wnw">:—</span></p> + + <p>1. Pre-oral, or supra-œsophageal ganglia.</p> + + <p>2. Infra-oral, or infra-œsophageal ganglia and ventral chain, which consist of three + groups: prosomatic, mesosomatic, and metasomatic ganglia.</p> + + <p>The infra-œsophageal ganglion-mass, then, in most of the Arthropoda may be spoken of as + formed by the fusion of the prosomatic or mouth-ganglia, the mesosomatic and metasomatic remaining + separate and distinct. The number of ganglia which have fused may be observed by examination of + the embryo, in which it is easy to see indications of the individual ganglia or <i>neuromeres</i>, + although all such indication has disappeared in the adult; thus the <span class="pagenum" + id="page53">{53}</span>infra-œsophageal ganglia of the cray-fish have been shown to be + constituted of six prosomatic ganglia.</p> + + <p>In Fig. <a href="#fig25">25</a> I give figures of the central nervous system (with the + exception of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus, Scorpio, + Androctonus, Thelyphonus, and Ammocœtes. In all the figures the supra-œsophageal + ganglia are lined horizontally, and their nerves shown, viz. optic (lateral eyes (II) and median + eyes (II′)), olfactory (I) (first antennæ, camerostome, nose); then come the prosomatic + ganglia (dotted), with their nerves (A) supplying the mouth parts, and the second antennæ or + cheliceræ; then the mesosomatic (lined horizontally), with their nerves (B) supplying respiratory + appendages. These figures show that the concentrated brain mass around the œsophagus of an + arthropod which has arrived at the stage of Astacus, is represented by the supra-œsophageal + ganglia and the fused prosomatic ganglia.</p> + + <p>The next stage in the evolution of the brain is seen in the gradual inclusion of the + mesosomatic ganglia, one after the other, into the infra-œsophageal mass of the already + fused prosomatic ganglia. With this fusion is associated the loss of locomotion in these + mesosomatic appendages, and their entire subservience to the function of respiration. Dana urges + that cephalization is a consequence of functional alteration in the appendages, from organs of + locomotion to those of mastication and respiration. Whether this be true or not, it is certainly a + fact that in Limulus, the ganglion supplying the first mesosomatic appendage has fused with the + prosomatic, infra-œsophageal mass. It is also a fact that the prosomatic appendages are the + organs of mastication, their basal parts being arranged round the mouth so as to act as foot-jaws, + while the mesosomatic appendages, though still free to move, have been reduced to such an extent + as to consist mainly of their basal parts, which are all respiratory in function, except in the + case of the first pair, where they carry the terminal ducts of the genital organs. In the next + stage, that, of the scorpion, in which the mesosomatic appendages have lost all power of free + locomotion, and have become internal branchiæ, another mesosomatic ganglion has fused with the + brain mass, while in Androctonus two of the branchial mesosomatic ganglia have fused; and finally, + in Thelyphonus and Phrynus, all the mesosomatic ganglia have coalesced with the fused prosomatic + ganglia, while the metasomatic ganglia have themselves fused together in the caudal region to form + what is known as the caudal brain.</p> + + <div><span class="pagenum" id="page54">{54}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig025.png" id="fig25"><img style="width:100%" src="images/fig025.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 25.—Comparison of Invertebrate Brains from + Branchipus to Ammocœtes.</span></p> + </div> + </div> + + <div><span class="pagenum" id="page55">{55}</span></div> + + <p>The brain in these animals may be spoken of as composed of three parts—(1) the fused + supra-œsophageal ganglia, (2) the fused prosomatic ganglia, and (3) the fused mesosomatic + ganglia. Such a brain is strictly homologous with the vertebrate brain, which also is built up of + three parts—(1) the part in front of the notochord, the prechordal or supra-infundibular + brain, which consists of the cerebral hemispheres, together with the basal and optic ganglia and + corresponds, therefore, to the supra-œsophageal mass, with its olfactory and optic + divisions lying in front of the œsophagus; (2 and 3) the epichordal brain, composed of (2) + a trigeminal and (3) a vagus division, of which the first corresponds strictly to the fused + prosomatic ganglia, and the second to the fused mesosomatic ganglia. Further, just as in the + embryo of an arthropod it is possible, with more or less accuracy, to see the number of neuromeres + or original ganglia which have fused to form the supra- and infra-œsophageal portions of + its brain, so also in the embryo of a vertebrate we are able at an early stage to gain an + indication, more or less accurate, of the number of neuromeres which have built up the vertebrate + brain. The further consideration of these neuromeres, and the evidence they afford as to the + number of the prosomatic and mesosomatic ganglia which have formed the epichordal part of the + vertebrate brain, must be left to the chapter on the segmentation of the cranial nerves.</p> + + <p class="sp3">The further continuation of this process of concentration of separate segments, + together with the fusion of the nervous system with the tube of the alimentary canal, leads in the + simplest manner to the formation of the spinal cord of the vertebrate from the metasomatic ganglia + of the ventral chain of the arthropod.</p> + + <p class="ac"><span class="sc">The Antagonism between Cephalization and Alimentation.</span></p> + + <p>This concentration of the nervous system in the head-region, together with an actual increase + in the bulk of the cephalic nervous masses, constitutes the great principle upon which the law of + upward progress or evolution in the animal kingdom is based, and it illustrates in a striking + manner the blind way in which natural selection works; for, as already explained, the central + nervous system arose as a ring round the mouth, in consequence of which, with the progressive + <span class="pagenum" id="page56">{56}</span>evolution of the animal kingdom, the œsophagus + necessarily pierced the central nervous system at the cephalic end. At the same time, the very + fact that the evolution was progressive necessitated the concentration and increase of the nervous + masses in this very same œsophageal region.</p> + + <p>Progress on these lines must result in a crisis, owing to the inevitable squeezing out of the + food-channel by the increasing nerve-mass; and, indeed, the fact that such a crisis had in all + probability arisen at the time when vertebrates first appeared is apparent when we examine the + conditions at the present time.</p> + + <p>Those invertebrates whose central nervous system is most concentrated at the cephalic end + belong to the arachnid group, among which are included the various living scorpion-like animals, + such as Thelyphonus, Androctonus, etc.</p> + + <p>As already mentioned, the giants of the Palæostracan age were Pterygotus, Slimonia, etc., all + animals of the scorpion-type—in fact, sea-scorpions. Now, all these animals, spiders and + scorpions, without exception, are blood-suckers, and in all of them the concentrated cephalic mass + of nervous material surrounds an œsophagus the calibre of which is so small that nothing + but a fluid pabulum can be taken into the alimentary canal; and even for that purpose a special + suctorial apparatus has in some species been formed on the gastric side of the œsophagus + for the purpose of drawing blood through this exceedingly narrow tube.</p> + + <p>In Fig. <a href="#fig25">25</a> this increasing antagonism between brain-power and + alimentation, as we pass from such a form as Branchipus to the scorpion, is illustrated, and in + Fig. <a href="#fig26">26</a> the relative sizes of the œsophagus and the brain-mass + surrounding it is shown. The section shows that the food channel is surrounded by the white and + grey matter of the brain as completely as the central canal of the spinal cord of the vertebrate + is surrounded by the white and grey nervous material.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig026.png" id="fig26"><img style="width:100%" src="images/fig026.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 26.—Transverse Section through the Brain of a Young + Thelyphonus.</span></p> + <p class="sp0"><i>A</i>, supra-œsophageal ganglia; <i>B</i>, infra-œsophageal + ganglia; <i>Al</i>, œsophagus.</p> + </div> + </div> + + <div><span class="pagenum" id="page57">{57}</span></div> + + <p>Truly, at the time when vertebrates first appeared, the direction and progress of variation in + the Arthropoda was leading, owing to the manner in which the brain was pierced by the + œsophagus, to a terrible dilemma—either the capacity for taking in food without + sufficient intelligence to capture it, or intelligence sufficient to capture food and no power to + consume it.</p> + + <p>Something had to be done—some way had to be found out of this difficulty. The atrophy of + the brain meant degeneration and the reduction to a lower stage of organization, as is seen in the + Tunicata. The further development of the brain necessitated the establishment of a new method of + alimentation and the closure of the old œsophagus, its vestiges still remaining as the + infundibular canal of the vertebrate, meant the enormous upward stride of the formation of the + vertebrate.</p> + + <p>At first sight it might appear too great an assumption even to imagine the possibility of the + formation of a new gut in an animal so highly organized as an arthropod, but a little + consideration will, I think, show that such is not the case.</p> + + <p>In the higher animals we are accustomed to speak of certain organs as vital and necessary for + the further existence of the animal; these are essentially the central nervous system, the + respiratory system, the circulatory system, and the digestive system. Of these four vital systems + the first cannot be touched without the chance of degeneration; but that is not the case with the + second. The passage from the fish to the amphibian, from the water-breathing to the air-breathing + animal, has actually taken place, and was effected by the modification of the swim-bladder to form + new respiratory organs—the lung; the old respiratory organs—the gills—becoming + functionless, but still persisting in the embryo as vestiges. The necessity arose in consequence + of the passage of the animal from water to land, and with this necessity nature found a means of + overcoming the difficulty; air-breathing vertebrates arose, and from the very fact of their being + able to extend over the land-surfaces, increased in numbers and developed in complexity in the + manner already sketched out.</p> + + <p>For a respiratory system all that is required is an arrangement <span class="pagenum" + id="page58">{58}</span>by means of which blood should be brought to the surface, so as to + interchange its gases with those of the external medium; and it is significant to find that of all + vertebrates the Amphibia alone are capable of an effective respiration by means of the skin.</p> + + <p>As to the circulatory system, it is exceedingly easily modified. An animal such as Amphioxus + has no heart; in some the heart is systemic, in others branchial; in some there are more than one + heart; in others there are contractile veins in addition to a heart. There is no difficulty here + in altering and modifying the system according to the needs of the individual.</p> + + <p>For a digestive system all that is required is an arrangement for the digestion and absorption + of food, a mechanism which can arise easily if some of the cells of the skin possess digestive + power. Now Miss Alcock has shown that some of the surface-cells of crustaceans secrete a fluid + which possesses digestive powers, and she has also shown that certain of the cells in the skin of + Ammocœtes possess digestive power.</p> + + <p>The difficulty, then, of forming a new digestive system in the passage from the arthropod to + the vertebrate is very much the same as the difficulty in forming a new respiratory system in the + passage from the water-breathing fish to the air-breathing amphibian—a change which does not + strike us as inconceivable, because we know it has taken place.</p> + + <p>The whole argument so far leads to the conclusion that vertebrates arose from ancient forms of + arthropods by the formation of a new alimentary canal, and the enclosure of the old canal by the + growing central nervous system. If this conclusion is true, then it follows that we possess a + well-defined starting-point from which to compare the separate organs of the arthropod with those + of the vertebrate, and if, in consequence of such working hypothesis, each organ of the arthropod + is found in the vertebrate in a corresponding position and of similar structure, then the truth of + the starting-point is proved as fully as can possibly be expected by deductive methods. It is, in + fact, this method of comparative anatomy which has proved the descent of man from the ape, the + frog from the fish, etc.</p> + + <p class="sp3">Let us, then, compare all the organs of such a low vertebrate as Ammocœtes + with those of an arthropod of the ancient type.</p> + + <div><span class="pagenum" id="page59">{59}</span></div> + + <p class="ac"><span class="sc">Life History of the Lamprey—not a Degenerate + Animal.</span></p> + + <p>The striking peculiarity of the lamprey is its life-history. It lives in fresh water, spending + a large portion of its life in the mud during the period of its larval existence: then comes a + somewhat sudden transformation-stage, characterized, as in the lepidopterous larva, by a process + of histolysis, by which many of the larval tissues are destroyed and new ones formed, with the + result that the larval lamprey, or Ammocœtes, is transformed into the adult lamprey, or + Petromyzon. This transformation takes place in August, at all events in the neighbourhood of + Cambridge, and later in the year the transformed lamprey migrates to the sea, grows in size and + maturity, and returns to the river the following spring up to its spawning beds, where it spawns + and forthwith dies. How long it lives in the Ammocœtes stage is unknown; I myself have kept + some without transformation for four years, and probably they live in the rivers longer than that + before they change from their larval state. It is absolutely certain that very much the longest + part of the animal's life is spent in the larval stage, and that with the maturity of the sexual + organs and the production of the fertilized ova the life of the individual ends.</p> + + <p>Now, the striking point of this transformation is that it produces an animal more nearly + comparable with higher vertebrates than is the larval form; in other words, the transformation + from larva to adult is in the direction of upward progress, not of degeneration. It is, therefore, + inaccurate to speak of the adult lamprey as degenerate from a higher race of fishes represented by + its larval form—Ammocœtes. Its transformation does not resemble that of the + tunicates, but rather that of the frog, so that, just as in the case of the tadpole, the + peculiarities of its larval form may be expected to afford valuable indications of its immediate + ancestry. The very peculiarities to which attention must especially be paid are those discarded at + transformation, and, as will be seen, these are essentially characteristic of the invertebrate and + are not found in the higher vertebrates. In fact, the transformation of the lamprey from the + Ammocœtes to the Petromyzon stage may be described as the casting off of many of its + ancestral invertebrate characters and the putting on of the characteristics of the vertebrate + type. It is this double individuality of the lamprey, together with its long-continued existence + in the larval form, which makes Ammocœtes more <span class="pagenum" + id="page60">{60}</span>valuable than any other living vertebrate for the study of the stock from + which vertebrates sprang.</p> + + <p>Many authorities hold the view that the lamprey, like Amphioxus, must be looked upon as + degenerate, and therefore as no more suitable for the investigation of the problem of vertebrate + ancestry than is Amphioxus itself. This charge of degeneracy is based on the statement that the + lamprey is a parasite, and that the eyes in Ammocœtes are under the skin. The whole + supposition of the degeneracy of the Cyclostomata arose because of the prevailing belief of the + time that the earliest fishes were elasmobranchs, and therefore gnathostomatous. From such + gnathostomatous fishes the cyclostomes were supposed to have descended, having lost their jaws and + become suctorial in habit in consequence of their parasitism.</p> + + <p>The charge of parasitism is brought against the lamprey because it is said to suck on to fishes + and so obtain nutriment. It is, however, undoubtedly a free-swimming fish; and when we see it + coming up the rivers in thousands to reach the spawning-beds, and sucking on to the stones on the + way in order to anchor itself against the current, or holding on tightly during the actual process + of spawning, it does not seem justifiable to base a charge of degeneration upon a parasitic habit, + when such so-called habit simply consists in holding on to its prey until its desires are + satisfied. If, of course, its suctorial mouth had arisen from an ancestral gnathostomatous mouth, + then the argument would have more force.</p> + + <p>Dohrn, however, gives absolutely no evidence of a former <span class="correction" + title="Original reads 'gnathostomotous'">gnathostomatous</span> condition either in Petromyzon or, + in its larval state, Ammocœtes. He simply assumes that the Cyclostomata are degenerated + fishes and then proceeds to point out the rudiments of skeleton, etc., which they still possess. + Every point that Dohrn makes can be turned round; and, with more probability, it can be argued + that the various structures are the commencement of the skeletal and other structures in the + higher fishes, and not their degenerated remnants. Compare the life-history of the lamprey and of + the tunicate. In the latter case we look upon the animal as a degenerate vertebrate, because the + larval stage alone shows vertebrate characteristics; when transformation has taken place, and the + adult form is reached, the vertebrate characteristics have vanished, and the animal, instead of + reaching a higher grade, has sunk lower in the scale, the central nervous system especially having + lost all <span class="pagenum" id="page61">{61}</span>resemblance to that of the vertebrate. In + the former case a transformation also takes place, a marvellous transformation, characterized by + two most striking facts. On the one hand, the resulting animal is more like a higher vertebrate, + for, by the formation of new cartilages, its cranial skeleton is now comparable with that of the + higher forms, and the beginnings of the spinal vertebræ appear; by the increased formation of + nervous material, its brain increases in size and complexity, so as to compare more closely with + higher vertebrate brains; its eyes become functional, and its branchiæ are so modified, + simultaneously with the formation of the new alimentary canal in the cranial region, that they now + surround branchial pouches which are directly comparable to those of higher vertebrates. On the + other hand, the transformation process is equally characterized by the throwing off of tissues and + organs, one and all of which are comparable in structure and function with corresponding + structures in the Arthropoda—the thyroid of the Ammocœtes, the tentacles, the + muco-cartilage, the tubular muscles, all these structures, so striking in the Ammocœtes + stage, are got rid of at transformation. Here is the true clue. Here, in the throwing off of + invertebrate characters, and the taking on of a higher vertebrate form, especially a higher brain, + not a lower one, Petromyzon proclaims as clearly as is possible that it is not a degenerate + elasmobranch, but that it has arisen from Ammocœtes-like ancestors, even though Myxine, + Amphioxus, and the tunicates be all stages on the downward grade from those same + Ammocœtes-like ancestors.</p> + + <p class="sp3">As to the eyes, they are functional in the adult form and as serviceable as in any + fish. There is no sign of degeneracy; it is only possible to speak of a retarded development which + lasts through the larval stage.</p> + + <p class="ac"><span class="sc">Comparison of Brain of Ammocœtes with that of an + Arthropod.</span></p> + + <p>Seeing that the steady progress of the development of the central nervous system is the most + important factor in the evolution of animals, it follows that of all organs of the body, the + central nervous system must be most easily comparable with that of the supposed ancestor. I will, + therefore, start by comparing the brain of Ammocœtes with that of arthropods, especially of + Limulus and of the scorpion-group.</p> + + <div><span class="pagenum" id="page62">{62}</span></div> + + <p>The supra-infundibular portion of the brain in vertebrates corresponds clearly to the + supra-œsophageal portion of the invertebrate brain in so far that in both cases here is the + seat of the will. Voluntary action is as impossible to the arthropod deprived of its + supra-œsophageal ganglia as to the vertebrate deprived of its cerebrum. It corresponds, + also, in that from it arise the nerves of sight and smell and no other nerves; this is also the + case with the supra-œsophageal ganglia, for from a portion of these ganglia arise the + nerves to the eyes and the nerves to the first antennæ, of which the latter are olfactory in + function. Thus, in the accompanying figure, taken from Bellonci, it is seen that the + supra-œsophageal ganglia consist of a superior segment corresponding to the cerebrum, a + middle segment from which arise the nerves to the lateral eyes and to the olfactory antennæ, + corresponding to the basal ganglia of the brain and the optic lobes, and, according to Bellonci, + of an inferior segment from which arise the nerves to the second pair of antennæ. This last + segment is not supra-œsophageal in position, but is situated on the œsophageal + commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in + reality the first ganglion of the infra-œsophageal series, and not to belong to the + supra-œsophageal group.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig027.png" id="fig27"><img style="width:100%" src="images/fig027.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 27.</span>—<span class="sc">The Brain of</span> + <i>Sphæroma serratum</i>. (After <span class="sc">Bellonci</span>.)</p> + <p class="sp0"><i>Ant. I.</i> and <i>Ant. II.</i>, nerves to 1st and 2nd antennæ. + <i>f.br.r.</i>, terminal fibre layer of retina; <i>Op. g. I.</i>, first optic ganglion; <i>Op. + g. II.</i>, second optic ganglion; <i>O.n.</i>, optic nerve-fibres forming an optic + chiasma.</p> + </div> + </div> + + <p>Further, in Limulus, in the scorpion-group, and in all the extinct <span class="pagenum" + id="page63">{63}</span>Eurypteridæ—in fact, in the Palæostraca generally—there are two + median eyes in addition to the lateral eyes, which were innervated from these ganglia.</p> + + <p>In Ammocœtes, then, if the supra-infundibular portion of the brain really corresponds to + the supra-œsophageal of the palæostracan group, we ought to find, as indeed is the case, an + optic apparatus consisting of two lateral eyes and two median eyes, innervated from the + supra-infundibular brain-mass, and an olfactory apparatus built up on the same lines as in the + scorpion-group, also innervated from this region. If, in addition, it be found that those two + median eyes are degenerate eyes of the same type as the median eyes of Limulus and the + scorpion-group, then the evidence is so strong as to amount to a proof of the correctness of the + theory. This evidence is precisely what has been obtained in recent years, for the vertebrate did + possess two median eyes in addition to the two lateral ones, and these two median eyes are + degenerate eyes of the type found in the median eyes of arthropods and are not of the vertebrate + type. Moreover, as ought also to be the case, they are most evident, and one of the pair is most + nearly functional in the lowest perfect vertebrate, Ammocœtes.</p> + + <p>Of all the discoveries made in recent years, the discovery that the pineal gland of the + vertebrate brain was originally a pair of median eyes is by far the most important clue to the + ancestry of the vertebrate, for not only do they correspond exactly in position with the median + eyes of the invertebrates, but, being already degenerate and functionless in the lowest + vertebrate, they must have been functional in a pre-vertebrate stage, thus giving the most direct + clue possible to the nature of the pre-vertebrate stage. It is especially significant that in + Limulus they are already partially degenerated. What, then, ought to be the structure and relation + to the brain of the median and lateral eyes of the vertebrate if they originated from the + corresponding organs of some one or other member of the palæostracan group?</p> + + <p class="sp3">This question will form the subject of the next chapter.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The object of this book is to attempt to find out from what group of invertebrates the + vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which + evolution takes place.</p> + <div><span class="pagenum" id="page64">{64}</span></div> + <p>A review of the animal kingdom as a whole leads to the conclusion that the upward development + of animals from an original cœlenterate stock, in which the central nervous system + consists of a ring of nervous material surrounding the mouth, has led, in consequence of the + elaboration of the central nervous system, to a general plan among the higher groups of + invertebrates in the topographical arrangement of the important organs. The mouth is situated + ventrally, and leads by means of the œsophagus into an alimentary canal which is situated + dorsally to the central nervous system. Thus the œsophagus pierces the central nervous + system and divides it into two parts, the supra-œsophageal ganglia and the + infra-œsophageal ganglia. This is an almost universal plan among invertebrates, but + apparently does not hold for vertebrates, for in them the central nervous system is always + situated dorsally and the alimentary canal ventrally, and there is no piercing of the central + nervous system by an œsophagus.</p> + <p>Yet a remarkable resemblance exists between the central nervous system of the vertebrate and + that of the higher invertebrates, of so striking a character as to compel one school of + anatomists to attempt the derivation of vertebrates from annelids. Now, the central nervous + system of vertebrates forms a hollow tube, and a diverticulum of this hollow tube, known as the + infundibulum, passes to the ventral surface of the brain in the very position where the + œsophagus would have been if that brain had belonged to an annelid or an arthropod. This + school of anatomists therefore concluded that this infundibular tube represented the original + invertebrate œsophagus which had become closed and no longer opened into the alimentary + canal owing to the formation of a new mouth in the vertebrate. As, however, the alimentary canal + of the vertebrate is ventral to the central nervous system, and not dorsal, as in the + invertebrate, it follows that the remains of the original invertebrate mouth into which the + œsophagus (in the vertebrate the infundibular tube) must have opened must be searched for + on the dorsal side of the vertebrate; and so the theory was put forward that the vertebrate had + arisen from the annelid by the reversal of surfaces, the back of the one animal becoming the + front of the other.</p> + <p>The difficulties in the way of accepting such reversal of surfaces have proved insuperable, + and another school has arisen which suggests that evolution has throughout proceeded on two + lines, the one forming groups of animals in which the central nervous system is pierced by the + food-channel and the gut therefore lies dorsally to it, the other in which the central nervous + system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the + highest products of the evolution have become markedly segmented animals, in the former, + annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is + based is the existence of low forms of animals, known as the <i>Enteropneusta</i>, the best + known example of which is called <i>Balanoglossus</i>; they are looked upon as aberrant annelid + forms by many observers.</p> + <p>This theory does not attempt to explain the peculiarities of the tube of the vertebrate + central nervous system, or to account for the extraordinary resemblance between the structure + and arrangement of the central nervous systems of vertebrates and of the highest invertebrate + group.</p> + <p>Neither of these theories is satisfactory or has secured universal acceptance. The problem + must be considered entirely anew. What are the guiding principles in this investigation?</p> + <div><span class="pagenum" id="page65">{65}</span></div> + <p>The evolution of animal life on this earth can clearly, on the whole, be described as a + process of upward progress culminating in the highest form—man; but it must always be + remembered that whole groups of animals such as the Tunicata have been able to survive owing to + a reverse process of degeneration.</p> + <p>If there is one organ more than another which increases in complexity as evolution proceeds, + which is the most essential organ for upward progress, surely it is the central nervous system, + especially that portion of it called the brain. This consideration points directly to the origin + of vertebrates from the most highly organized invertebrate group—the Arthropoda—for + among all the groups of animals living on the earth in the present day they alone possess a + central nervous system closely comparable with that of vertebrates. Not only has an upward + progress taken place in animals as a whole, but also in the tissues themselves a similar + evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely + those of the arthropod than of any other invertebrate group.</p> + <p>The evidence of geology points to the same conclusion, for the evidence of the rocks shows + that before the highest mammal—man—appeared, the dominant race was the mammalian + quadruped, from whom the highest mammal of all—man—sprung; then comes, in Mesozoic + times, the age of reptiles which were dominant when the mammal arose from them. Preceding this + era we find in Carboniferous times that the amphibian was dominant, and from them the next + higher group—the reptiles—arose. Below the Carboniferous come the Devonian strata + with their evidence of the dominance of the fish, from whom the amphibian was directly evolved. + The evidence is so clear that each succeeding higher form of vertebrate arose from the highest + stage reached at the time, as to compel one to the conclusion that the fishes arose from the + race which was dominant at the time when the fishes first appeared. This brings us to the + Silurian age, in which the evidence of the rocks points unmistakably to the sea-scorpions, + king-crabs, and trilobites as being the dominant race. It was preceded by the great trilobite + age, and the whole period, from the first appearance of the trilobite to the time of dwindling + away of the sea-scorpions, may be designated the Palæostracan age, using the term Palæostraca to + include both trilobites and the higher scorpion and king-crab forms evolved from them. The + evidence of geology then points directly and strongly to the origin of vertebrates from the + Palæostraca—arthropod forms which were not crustacean and not arachnid, but gave origin + both to the modern-day crustaceans and arachnids. The history of the rocks further shows that + these ancient fishes, when they first appeared, resembled in a remarkable manner members of the + palæostracan group, so that again and again palæontologists have found great difficulty in + determining whether a fossil is a fish or an arthropod. Fortunately, there is still alive on the + earth one member of this remarkable group—the Limulus, or King-Crab. On the vertebrate + side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large + portion of its existence in a larval stage, known as the Ammocœtes stage of the lamprey, + because it was formerly considered to be a separate species and received the name of + Ammocœtes. The larval stages of any animal are most valuable for the study of ancestral + problems, so that it is most fortunate for the solution of the ancestry of vertebrates that + Limulus on the one side and Ammocœtes on the other are <span class="pagenum" + id="page66">{66}</span>available for thorough investigation and comparison. There are no + trilobites still alive, but in Branchipus and Apus we possess the nearest approach to the + trilobite organization among living crustaceans.</p> + <p>So strongly do all these different lines of argument point to the origin of vertebrates from + arthropods as to make it imperative to reconsider the position of that school of anatomists who + derived vertebrates from annelids by reversing the back and front of the animal. Let us not turn + the animal over, but re-consider the position, the infundibular tube of the vertebrate still + representing the œsophagus of the invertebrate, the cerebral hemispheres and basal + ganglia the supra-œsophageal ganglia, the <i>crura cerebri</i> the œsophageal + commissures, and the infra-infundibular part of the brain the infra-œsophageal ganglia. + It is immediately apparent that just as the invertebrate œsophagus leads into the large + cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the + third ventricle, which, together with the other ventricles, forms in the embryo a large anterior + dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight + narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of + the spinal cord. In the arthropod the intestine terminates in the anus; in the vertebrate embryo + the canal of the spinal cord terminates in the anus by way of the neurenteric canal. Keep the + animal unreversed, and immediately the whole mystery of the tubular nature of the central + nervous system is revealed, for it is seen that the nervous matter, which corresponds bit by bit + with that of the arthropod, has surrounded to a greater or less extent and amalgamated with the + tube of the arthropod alimentary canal, and thus formed the so-called central nervous system of + the vertebrate.</p> + <p>The manner in which the nervous material has invaded the walls of the tube is clearly shown + both by the study of the comparative anatomy of the central nervous system in the vertebrate and + also by its development in the embryo.</p> + <p class="sp0">This theory implies that the vertebrate alimentary canal is a new formation + necessitated by the urgency of the case, and, indeed, there was cause for urgency, for the + general plan of the evolution of the invertebrate from the cœlenterate involved the + piercing of the anterior portion of the central nervous system by the œsophagus, while, + at the same time, upward progress meant brain-development; brain-development meant concentration + of nervous matter at the anterior end of the animal, with the result that in the highest + scorpion and spider-like animals the brain-mass has so grown round and compressed the food-tube + that nothing but fluid pabulum can pass through into the stomach; the whole group have become + blood-suckers. These kinds of animals—the sea-scorpions—were the dominant race when + the vertebrates first appeared: here in the natural competition among members of the dominant + race the difficulty must have become acute. Further upward evolution demanded a larger and + larger brain with the ensuing consequence of a greater and greater difficulty of food-supply. + Nature's mistake was rectified and further evolution secured, not by degeneration in the + brain-region, for that means degradation not upward progress, but by the formation of a new + food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus + the great and mighty kingdom of the Vertebrata was evolved with its culminating + organism—man—whose massive brain with all its possibilities could never have been + evolved if he had still been <span class="pagenum" id="page67">{67}</span>compelled to pass the + whole of his food through the narrow œsophageal tube, still existent in him as the + infundibular tube. This, then, is the working hypothesis upon which this book is written. If + this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of + surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it + follows that we have various fixed points of comparison in the central nervous systems of the + two groups of animals from which to search for further clues. It further follows that from such + starting-point every organ of importance in the body of the arthropod ought to be visible in the + corresponding position in the vertebrate, either as a functional or rudimentary organ. The + subsequent chapters will deal with this detailed comparison of organs in the arthropod and + vertebrate respectively.</p> + </div> + + <div><span class="pagenum" id="page68">{68}</span></div> + + <p class="ac">CHAPTER II</p> + + <p class="ac"><i>THE EVIDENCE OF THE ORGANS OF VISION</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Different kinds of eye.—Simple and compound retinas.—Upright and + inverted retinas.—Median eyes.—Median or pineal eyes of Ammocœtes and their + optic ganglia.—Comparison with other median eyes.—Lateral eyes of vertebrates + compared with lateral eyes of crustaceans.—Peculiarities of the lateral eye of the + lamprey.—Meaning of the optic diverticula.—Evolution of vertebrate + eyes.—Summary.</p> + </div> + + <p class="ac"><span class="sc">The Different Kinds of Eye.</span></p> + + <p>In all animals the eyes are composed of two parts. 1. A set of special sensory cells called the + retina. 2. A dioptric apparatus for the purpose of forming an image on the sensory cells. The + simplest eye is formed from a modified patch of the surface-epithelium; certain of the hypodermal + cells, as they are called, elongate, and their cuticular surface becomes bulged to form a simple + lens. These elongated cells form the retinal cells, and are connected with the central nervous + system by nerve-fibres which constitute an optic nerve; the cells themselves may contain + pigment.</p> + + <p>The more complicated eyes are modifications of this type for the purpose of making both the + retina and the dioptric apparatus more perfect. According to a very prevalent view, these + modifications have been brought about by invaginations of the surface-epithelium. Thus if ABCD + (Fig. <a href="#fig28">28</a>) represents a portion of the surface-epithelium, the chitinous + cuticle being represented by the dark line, with the hypodermal cells beneath, and if the part C + is modified to form an optic sense-plate, then an invagination occurring between A and B will + throw the retinal sense-cells with the optic nerve further from the surface, and the layers B and + A between the retina and the source of light will be available for the formation of the dioptric + apparatus. The lens is now formed from the cuticular surface of A, and the <span class="pagenum" + id="page69">{69}</span>hypodermal cells of A elongate to form the layer known by the name of + corneagen, or vitreogen, the cells of B remaining small and forming the pre-retinal layer of + cells. The large optic nerve end-cells of the retinal layer, C, take up the position shown in the + figure, and their cuticular surface becomes modified to form rods of varying shape called + rhabdites, which are attached to the retinal cells. Frequently the rhabdites of neighbouring cells + form definite groups, each group being called a rhabdome. Whatever shape they take it is + invariably found that these little rods (bacilli), or rhabdites, are modifications of the + cuticular surface of the cells which form the retinal layer. Also, as must necessarily be the case + from the method of formation, the optic nerve arises from the nuclear end of the retinal cells, + never from the bacillary end. As in the case first mentioned, so in this case, the light strikes + direct upon the bacillary end of the retinal cells; such eyes, therefore, are eyes with an + <i>upright retina</i>.</p> + + <div class="ac w45 fcenter sp3"> + <a href="images/fig028.png" id="fig28"><img style="width:100%" src="images/fig028.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 28.—Diagram of Formation of an Upright Simple + Retina.</span></p> + </div> + </div> + + <p>It may happen that the part invaginated is the optic sense-plate itself, as would be the case + if in the former figure, instead of C, the part B was modified to form a sense-plate. This will + give rise to an eye of a character different from the former (Fig. <a href="#fig29">29</a>). The + optic nerve-fibres now lie between the source of light and the retinal end-cells, the layer A as + before forms the cuticular lens, and its hypodermal cells elongate to form the corneagen; there is + no pre-retinal layer, but, on the contrary, a post-retinal layer, C, called the tapetum, and, as + is seen, the light passes through the retinal layer to the <span class="pagenum" + id="page70">{70}</span>tapetum. The cuticular surface of the retinal cells forming the rods or + bacilli is directed towards the tapetal layer away from the source of light, and the nuclei of the + retinal cells are pre-bacillary in position, in contradistinction to the upright eye, where they + are post-bacillary. The retinal end-cells are devoid of pigment, the pigment being in the tapetal + layer.</p> + + <p>Such an eye, in contradistinction to the former type, is an eye with an <i>inverted retina</i>; + but still the same law holds as in the former case—the optic nerve-fibres enter at the + nuclear ends of the cells, and the rods are formed from the cuticular surface.</p> + + <p>In these eyes the pigmented tapetal layer is believed to act as a looking-glass; the dioptric + apparatus throws the image on to its shiny surface, from whence it is reflected directly on to the + rods, which are in close contact with the tapetum. A similar process has been suggested in the + case of the mammalian lateral eye, with its inverted retina. Johnson describes the post-retinal + pigmented layer as being frequently coloured and shiny, and imagines that it reflects the image + directly back on to the rods.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig029.png" id="fig29"><img style="width:100%" src="images/fig029.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 29.—Diagram of Formation of an Inverted Simple + Retina.</span></p> + <p class="sp0">The arrow shows the direction of the source of light in this as in the + preceding figure. In both figures the cuticular rhabdites are represented by thick black + lines.</p> + </div> + </div> + + <p>Thus we see that eyes can be placed in different categories, <i>e.g.</i> those with an upright + retina and those with an inverted retina; also, according to the presence or absence of a tapetum, + eyes have been grouped as tapetal or non-tapetal. All the eyes considered so far are called simple + eyes, or ocelli; and a number of ocelli may be <span class="pagenum" + id="page71">{71}</span>contiguous though separate, as in the lateral eyes of the scorpion. They + may, however, come into close contact and form one single, large, compound eye. Such ocelli, in a + very large number of cases, retain each its own dioptric apparatus, and therefore the external + appearance of the compound eye represents not a single lens, but a large number of facets, as is + seen in the eyes of insects. Owing to these differences, eyes have been divided into simple and + compound, and into facetted and non-facetted.</p> + + <p>Yet another complication occurs in the formation of eyes, which is, perhaps, the most important + of all: the retinal portion of the eye, instead of consisting of simple retinal cells, with their + accompanying rhabdites, may include within itself a portion of the central nervous system.</p> + + <p>The rationale of such a formation is as follows: The external covering of the body is formed by + a layer of external epithelial cells—the ectodermal cell-layer—and an underlying + neural layer, of which the latter gives origin to the central nervous system. As development + proceeds, this central nervous system sinks inwards, leaving as its connection with the ectoderm + the sensory nerves of the skin. That part of the neural layer which underlies the optic plate + forms the optic ganglion, and when the central nervous system leaves the surface to take up its + deeper position, the strand of nerve-fibres known as the optic nerve, is left connecting it with + the retinal cells as seen in Figs. 28, 29. It may, however, happen that part of the optic ganglion + remains at the surface, in close connection with the retinal end-cells, when the rest of the + central nervous system sinks inwards. The retina of such an eye is composed of the combined optic + ganglion and retinal end-cells; the strand of nerve-fibres which is left as the connection between + it and the rest of the brain, which is also called the optic nerve, is not a true peripheral + nerve, as in the first case, but rather a tract of fibres connecting two parts of the brain, of + which one has remained at the periphery. Such a retina, in contradistinction to the first kind, + may be called a <i>compound retina</i>.</p> + + <p>The optic ganglion, as seen in eyes with a simple retina, consists of a cortical layer of + small, round nerve-cells, and an internal medulla of fine nerve-fibres, which form a thick network + known as 'Punctsubstanz,' or in modern terminology, 'Neuropil.' Fibres which pass into this + 'neuropil' from other parts of the brain connect them with the optic ganglion.</p> + + <div><span class="pagenum" id="page72">{72}</span></div> + + <p>At the present time, owing to the researches of Golgi, Ramón y Cajal, and others, the nervous + system is considered to be composed of a number of separate nerve-units, called neurones, each + neurone consisting of a nerve-cell with its various processes; one of these—the + neuraxon—constitutes the nerve-fibre belonging to that nerve-cell, the other + processes—the dendrites—establish communication with other neurones. The place where + these processes come together is called a synapse, and the tangle of fine fibres formed at a + number of synapses forms the 'neuropil.'</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig030.png" id="fig30"><img style="width:100%" src="images/fig030.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 30.—Diagram of Formation of an Upright Compound + Retina.</span></p> + <p class="sp0"><i>ABCD</i>, as in Fig. <a href="#fig28">28</a>. <i>Op. g. I.</i> and <i>Op. g. + II.</i>, two optic ganglia which combine to form the retinal ganglion, <i>Rt. g.</i></p> + </div> + </div> + + <p>When, therefore, a compound retina is formed by the amalgamation of the ectodermal + part—the retinal cells proper—with the neurodermic part—to which the name + 'retinal ganglion' may be given,—such a retina consists of neuropil substance and + nerve-cells, as well as the retinal end-cells. In all such compound retinas, the retinal ganglion + is not single, but two optic ganglia at least are included in it, so that there are two sets of + nerve-cells and two synapses are always formed; one between the retinal end-cells and the neurones + of the first optic ganglion, which may be called the ganglion of the retina, the other between the + first and second ganglia, which, seeing that the neuraxons of its cells form the optic nerve, may + be called the ganglion of the optic nerve. The 'neuropil' formed by these synapses forms the + molecular layers of the compound retina, and the cells themselves form the nuclear layers. Thus an + upright compound retina, formed in the same way as the upright simple retina, would be illustrated + by Fig. <a href="#fig30">30</a>.</p> + + <div><span class="pagenum" id="page73">{73}</span></div> + + <p>Further, in precisely the same way as in the case of the simple retina, such a compound retina + may be upright or inverted. Thus, in the lateral eyes of crustaceans and insects, a compound + retina of this kind is formed, which is upright; while in the vertebrates the compound retina of + the lateral eyes is inverted.</p> + + <p>The compound retina of vertebrates is usually described as composed of a series of layers, + which may be analyzed into their several components as follows<span + class="wnw">:—</span></p> + + <table class="sp2 mc nothand" title="Layers of a compound retina" + summary="Layers of a compound retina"> + <tr> + <td class="pr0" rowspan="3">Layer of rods and cones<br/> + External nuclear layer<br/> + External molecular layer<br/> + Internal nuclear layer<br/> + Internal molecular layer<br/> + Optic nerve-cell layer<br/> + Layer of optic nerve fibres</td> + <td class="vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">retina proper</td> + <td class="vmi pl0 pr0"><img src="images/rbrace1.png" style="height:3.3ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0" colspan="3">Ectodermic part</td> + </tr> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">ganglion of retina</td> + <td class="vmi pl0 pr0" rowspan="2"><img src="images/rbrace3.png" style="height:7.0ex; + width:1em;" alt="brace" /></td> + <td class="vmi pl0 pr0" rowspan="2">retinal<br/> + ganglion</td> + <td class="vmi pl0 pr0" rowspan="2"><img src="images/rbrace2.png" style="height:4.5ex; + width:1em;" alt="brace" /></td> + <td class="vmi pl0 pr0" rowspan="2">neurodermic<br/> + part</td> + </tr> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">ganglion of optic nerve</td> + </tr> + </table> + +<!-- Trimmed down version for handheld --> + + <table class="sp3 mc handonly" title="Layers of a compound retina" + summary="Layers of a compound retina"> + <tr class="ba"> + <td>Layer of rods and cones</td> + <td rowspan="3" class="vmi">retina proper</td> + <td rowspan="3" colspan="2" class="vmi">Ectodermic part</td> + </tr> + <tr class="ba"> + <td>External nuclear layer</td> + </tr> + <tr class="ba"> + <td rowspan="2">External molecular layer</td> + </tr> + <tr class="ba"> + <td rowspan="3" class="vmi">ganglion of retina</td> + <td rowspan="6" class="vmi">retinal<br/> + ganglion</td> + <td rowspan="6" class="vmi">neurodermic<br/> + part</td> + </tr> + <tr class="ba"> + <td>Internal nuclear layer</td> + </tr> + <tr class="ba"> + <td rowspan="2">Internal molecular layer</td> + </tr> + <tr class="ba"> + <td rowspan="3" class="vmi">ganglion of optic nerve</td> + </tr> + <tr class="ba"> + <td>Optic nerve-cell layer</td> + </tr> + <tr class="ba"> + <td>Layer of optic nerve fibres</td> + </tr> + </table> + +<!-- End of trimmed down version for handheld --> + + <p>The difference between the development of these two types of eye—those with a simple + retina and those with a compound retina—has led, in the most natural manner, to the + conception that the retina is developed, in the higher animals, sometimes from the cells of the + peripheral epidermis, sometimes from the tissue of the brain—two modes of development termed + by Balfour 'peripheral' and 'cerebral.' An historical survey of the question shows most + conclusively that all investigators are agreed in ascribing the origin of the simple retina to the + peripheral method of development, the retina being formed from the hypodermal cells by a process + of invagination, while the cerebral type of development has been described only in the development + of the compound retina. The natural conclusion from this fact is that, in watching the development + of the compound retina, it is more difficult to differentiate the layers formed from the epidermal + retinal cells and those formed from the epidermal optic ganglion-cells, than in the case of the + simple retina, where the latter cells withdraw entirely from the surface. This is the conclusion + to which Patten has come, and, indeed, judging from the text-book of Korschelt and Heider, it is + the generally received opinion of the day that, as far as the Appendiculata are concerned, the + retina, in the true sense—the retinal end-cells, with their cuticular rods,—is formed, + in all cases, from the peripheral cells of the hypodermal layer, the cuticular rods being + modifications of the general cuticular surface of the body. The apparent cerebral development of + the crustacean <span class="pagenum" id="page74">{74}</span>retina, as quoted from Bobretsky by + Balfour, is therefore in reality the development of the retinal ganglion, and not of the retina + proper.</p> + + <p class="sp3">There is, I imagine, a universal belief that the natural mode of origin of a + sense-organ, such as the eye, must always have been from the cells forming the external surface of + the animal, and that direct origin from the central nervous system is <i>a priori</i> most + improbable. It is, therefore, a matter of satisfaction to find that the evidence for the latter + origin has universally broken down, with the single exception of the eyes of vertebrates and their + degenerated allies; a fact which points strongly to the probability that a reconsideration of the + evidence upon which the present teaching of the origin of the vertebrate eye is based will show + that here, too, a confusion has arisen between that part formed from the epidermal surface and + that from the optic ganglion.</p> + + <p class="ac"><span class="sc">The Median or Pineal Eyes.</span></p> + + <p>Undoubtedly, in recent times, the most important clue to the ancestry of vertebrates has been + given by the discovery that the so-called pineal gland in the vertebrate brain is all that remains + of a pair of median or pineal eyes, the existence of which is manifest in the earliest + vertebrates; so that the vertebrate, when it first arose, possessed a pair of median eyes as well + as a pair of lateral eyes. The ancestor of the vertebrate, therefore, must also have possessed a + pair of median eyes as well as a pair of lateral eyes.</p> + + <p>Very instructive, indeed, is the evidence with regard to these median eyes, for one of the + great characteristics of the ancient palæostracan forms is the invariable presence of a pair of + median eyes as well as a pair of lateral eyes. In the living representative of such + forms—Limulus—the pair of median eyes (Fig. <a href="#fig5">5</a>) is well shown, and + it is significant that here, according to Lankester and Bourne, these eyes are already in a + condition of degeneration; so also in many of the Palæostraca (Fig. <a href="#fig7">7</a>) the + lateral eyes are the large, well-developed eyes, while the median eyes resemble those of Limulus + in their insignificance.</p> + + <p>We see, then, that in the dominant arthropod race at the time when the fishes first appeared, + the type of eyes consisted of a pair of well-developed lateral eyes and a pair of insignificant, + partially degenerated, median eyes. Further, according to all palæontologists, <span + class="pagenum" id="page75">{75}</span>in the best-preserved head-shields of the most ancient + fishes, especially well seen in the Osteostraci, in Cephalaspis, Tremataspis, Auchenaspis, + Keraspis, a pair of large, prominent lateral eyes existed, between which, in the mid-line, are + seen a pair of small, insignificant median eyes.</p> + + <p>The evidence of the rocks, therefore, proves that the pair of median eyes which were originally + the principal eyes (Hauptaugen), had already, in the dominant arthropod group been supplanted by a + pair of lateral eyes, and had, in consequence, become small and insignificant, at the time when + vertebrates first appeared. This dwindling process thus initiated in the arthropod itself has + steadily continued ever since through the whole development of the vertebrates, with the result + that, in the highest vertebrates, these median or pineal eyes have become converted into the + pineal gland with its 'brain-sand.'</p> + + <p>In the earliest vertebrate these median eyes may have been functional; they certainly were more + conspicuous than in later forms. Alone among living vertebrates the right median eye of + Ammocœtes is so perfect and the skin covering it so transparent that I have always felt + doubtful whether it may not be of use to the animal, especially when one takes into consideration + the undeveloped state of the lateral eyes in this animal, hidden as they are under the skin. Thus + the one living vertebrate which is comparable with these extinct fishes is the one in which one of + the pineal eyes is most well defined, most nearly functional.</p> + + <p>Before passing to the consideration of the structure of the median eyes of Ammocœtes, it + is advisable to see whether these median eyes in other animals, such as arachnids and crustaceans, + belong to any particular type of eyes, for then assuredly the median eyes of Ammocœtes + ought to belong to the same type if they are derived from them.</p> + + <p>In the specialized crustacean, as in the specialized vertebrate, the median eyes have + disappeared, at all events in the adult, but still exist in the primitive forms, such as + Branchipus, which resemble the trilobites in some respects. On the other hand, the median eyes + have persisted, and are well developed in the arachnids, both scorpions and spiders possessing a + well-developed pair. The characteristics of the median eyes must then be especially sought for in + the arachnid group.</p> + + <p>Both scorpions and spiders possess many eyes, of which two are <span class="pagenum" + id="page76">{76}</span>always separate and median in position, while the others form lateral + groups; all these eyes possess a simple retina and a simple corneal lens. Grenacher was the first + to point out that in the spiders two very distinct types of eye are found. In the one the retina + is upright; in the other the retina is inverted, and the eye possesses a tapetal layer. The + distribution of these two types is most suggestive, for the inverted retina is always found in the + lateral eyes, never in the two median eyes; these always possess a simple upright retina.</p> + + <p class="sp3">In the crustaceans, the lateral eyes differ also from the median eyes, but not in + the same way as in the arachnids; for here both types of eye possess an upright retina, but the + retina of the lateral eyes is compound, while that of the median eyes is simple. In other words, + the median eyes are in all cases eyes with a simple upright retina and a simple cuticular lens, + while the retina of the lateral eyes is compound or may be inverted, according as the animal in + question possesses crustacean or arachnid affinities. The lateral eye of the vertebrate, + possessing, as it does, an inverted compound retina, indicates that the vertebrate arose from a + stock which was neither arachnid nor crustacean, but gave rise to both groups—in fact, was a + member of the great palæostracan group. What, then, is the nature of the median eyes in the + vertebrate?</p> + + <p class="ac"><span class="sc">The Median Eyes of Ammocœtes.</span></p> + + <p>The evidence of Ammocœtes is so conclusive that I, for one, cannot conceive how it is + possible for any zoologist to doubt whether the parietal organ, as they insist on calling it, had + ever been an eye, or rather a pair of eyes.</p> + + <p>Anyone who examines the head of the larval lamprey will see on the dorsal side, in the median + line, first, a somewhat circular orifice—the unpaired nasal opening; and then, tailwards to + this, a well-marked circular spot, where the skin is distinctly more transparent than elsewhere. + This spot coincides in position with the underlying dorsal pineal eye, which shines out + conspicuously owing to the glistening whiteness of its pigment. Upon opening the brain-case the + appearance as in Fig. <a href="#fig20">20</a> is seen, and the mass of the right <i>ganglion + habenulæ</i> (<i>G.H.R.</i>), as it has been called, stands out conspicuously as well as the right + or dorsal pineal eye (<i>Pn.</i>). Both eye and ganglion appear at first sight to be one-sided, + but further examination shows that a left <i>ganglion habenulæ</i> is present, though much smaller + than on <span class="pagenum" id="page77">{77}</span>the right side. In connection with this is + another eye-like organ—the left or ventral pineal eye,—much more aborted, much less + like an eye than the dorsal one; so also there are two bundles of peculiar fibres called Meynert's + bundles, which connect this region with the infra-infundibular region of the brain; of these, the + right Meynert's bundle is much larger than the left.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig031.jpg" id="fig31"><img style="width:100%" src="images/fig031.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 31.—One of a Series of Horizontal Sections through + the Head of Ammocœtes.</span></p> + <p class="sp0"><i>l.m.</i>, upper lip muscles; <i>m.c.</i>, muco-cartilage; <i>n.</i>, nose; + <i>na.c.</i>, nasal cartilage; <i>pn.</i>, right pineal eye and nerve; <i>g.h.r.</i>, right + <i>ganglion habenulæ</i>; <i>s.m.</i>, somatic muscles; <i>cr.</i>, membranous wall of + cranium; <i>ch.</i>, choroid plexus; <i>gl.</i>, glandular substance and pigment filling up + brain-case.</p> + </div> + </div> + + <div><span class="pagenum" id="page78">{78}</span></div> + + <table class="mc tlf sp2 w60" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:45%;"><a href="images/fig032.png" id="fig32"><img + style="width:100%" src="images/fig032.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:45%;"><a href="images/fig033.png" id="fig33"><img + style="width:100%" src="images/fig033.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 32.—Eye of Acilius Larva, with its Optic + Ganglion.</span></p> + <p class="sp0">On the right side the nerve end-cells have been drawn free from + pigment.</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 33.—Pineal Eye of Ammocœtes, with + its</span> <i>Ganglion Habenulæ.</i></p> + <p class="sp0">On the left side the eye is drawn as it appeared in the section. On the + right side I have removed the pigment from the nerve end-cells, and drawn the eye as, in + my opinion, it would appear if it were functional.</p> + </div> + </td> + </tr> + </table> + + <p>This difference between right and left indicates a greater degeneration on the left side, and + points distinctly to a close relationship between the nerve-masses known as <i>ganglia + habenulæ</i> and the median eyes. In my opinion this ganglion is, in part, at all events, the + optic ganglion of the median eye on each side. It is built up on the same type as the optic + ganglia of invertebrate simple eyes, with a cortex of small round cells and a medulla of fine + nerve-fibres. Into this ganglion, on the right side, there passes a very well-defined + nerve—the nerve of the dorsal eye. The eye itself with its nerve, <i>pn.</i>, and its optic + ganglion, <i>g.h.r.</i>, is beautifully shown by means of a horizontal section through the head of + Ammocœtes (Fig. <a href="#fig31">31</a>). Originally, as described by Scott, the eye stood + vertically <span class="pagenum" id="page79">{79}</span>above its optic ganglion, and presented an + appearance remarkably like Fig. <a href="#fig32">32</a>, which represents one of the simple eyes + and optic ganglia of a larva of Acilius as described by Patten; then, with the forward growth of + the upper lip, the right pineal eye was dragged forward and its nerve pulled horizontally over the + <i>ganglion habenulæ</i>. For this reason the eye, nerve, and ganglion are better shown in a + nearly horizontal than in a transverse section.</p> + + <p>The optic nerve belonging to this eye is most evident and clearly shown in Fig. <a + href="#fig31">31</a>, and in the series of consecutive sections which follow upon this section; no + doubt can arise as to the structure in question having been the nerve of the eye, even though, as + is possible, it does not contain any functional nerve-fibres.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig034.png" id="fig34"><img style="width:56%" src="images/fig034.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 34.—Horizontal Section through Brain of Ammocœtes, to + show the Left, or Ventral Pineal Eye.</span></p> + <p class="sp0"><i>pn.<sub>2</sub></i>, left or ventral pineal eye; <i>pn.<sub>1</sub></i>, + last remnant of right, or dorsal pineal eye; <i>g.h.r.</i>, right <i>ganglion habenulæ</i>; + <i>g.h.l.<sub>1</sub></i>, <i>g.h.l.<sub>3</sub></i>, parts of left <i>ganglion habenulæ</i>; + <i>pi.</i>, fold of <i>pia mater</i> which separates the left <i>ganglion habenulæ</i> from + the left pineal eye; <i>f.</i>, strands of nerve-fibres connecting the left eye with its + ganglion, <i>g.h.l.<sub>3</sub></i>; <i>V<sub>3</sub></i>, third ventricle; <i>V.aq.</i>, + ventricle of aquæduct.</p> + </div> + </div> + + <p>The second, ventral or left, eye, belonging to the left ganglion habenulæ is very different in + appearance, being much less evidently an eye. Fig. <a href="#fig34">34</a> is one of the same + series of horizontal sections as Fig. <a href="#fig31">31</a>, <i>pn.<sub>1</sub></i> being the + last remnant of the right, or dorsal, eye, while <i>pn.<sub>2</sub></i> shows the left, or + ventral, eye with its connection with the left <i>ganglion habenulæ</i>.</p> + + <div><span class="pagenum" id="page80">{80}</span></div> + + <p class="sp3">In a series of sections I have followed the nerve of the right pineal eye to its + destination, as described in my paper in the <i>Quarterly Journal of Microscopical Science</i>, + and have found that it enters into the <i>ganglion habenulæ</i> just as the nerve to any simple + eye enters into its optic ganglion. This nerve, as I have shown, is a very distinct, well-defined + nerve, with no admixture of ganglion-cells or of connective tissue, very different indeed to the + connection between the left pineal eye and its optic ganglion. Here there is no defined nerve at + all; but the cells of the <i>ganglion habenulæ</i> stretch right up to the remains of the eye + itself. Seeing, then, that both the eye and ganglion on this side have reached a much further + grade of degeneration than on the right side, it may be fairly concluded that the original + condition of these two median eyes is more nearly represented by the right eye, with its + well-defined nerve and optic ganglion, than by the left eye, or by the eyes in lizards and other + animals which do not show so well-defined a nerve as is possessed by Ammocœtes. Quite + recently Dendy has examined the two median eyes in the New Zealand lamprey <i>Geotria + australis</i>. In this species the second eye is much better defined than in the European lamprey, + and its connection with the <i>ganglion habenulæ</i> is more nerve-like. In neither eye, however, + is the nerve so clean cut and isolated as is the nerve of the dorsal, or right, eye in the + Ammocœtes stage of <i>Petromyzon Planeri</i>; in both, cells resembling those of the cortex + of the <i>ganglion habenulæ</i> and connective tissues are mixed up with the nerve-fibres which + pass from each eye to its respective optic ganglion.</p> + + <p class="ac"><span class="sc">The Right Pineal Eye of Ammocœtes.</span></p> + + <p>The optic fibres of the right median eye of Ammocœtes are connected with a well-defined + retina, the limits of which are defined by the white pigment so characteristic of this eye. This + pigment is apparently calcium phosphate, which still remains as the 'brain-sand' of the human + pineal gland. The cells, which are hidden by this pigment, were described by me in 1890 as the + retinal end-cells with large nuclei. In 1893, Studniçka examined them more closely, and concluded + that the retinal cells are of two kinds: the one, nerve end-cells, the sensory cells proper; the + other, pigmented epithelial cells, which surround the sense-cells. The sense-cells may contain + some of the white pigment, but not so much as the other cells. Similarly, in the <span + class="pagenum" id="page81">{81}</span>median eyes of Limulus, Lankester and Bourne find it + difficult to determine how far the retinal end-cells contain pigment and how far that pigment + really is in the cells surrounding these nerve end-cells.</p> + + <p>The interior of the eye presents the appearance of a cavity in shape like a cornucopia, the + stalk of which terminates at the place where the nerve enters. This cavity is not empty, but the + posterior part of it is filled with the termination of the nerve end-cells of the retina, as + pointed out by me and confirmed by Studniçka. These terminations are free from pigment, and + contain strikingly translucent bodies, which I have described in my paper in the <i>Quarterly + Journal</i>, and called rhabdites, for they present the same appearance and are situated in the + same position as are many of the rhabdites on the terminations of the retinal end-cells of + arthropod eyes. Studniçka has also seen these appearances, and figures them in his second paper on + the nerve end-cells of the pineal eye of Ammocœtes.</p> + + <p>Up to this point the following conclusions may be drawn<span class="wnw">:—</span></p> + + <div class="bq1 sp2"> + <p>1. Ammocœtes possesses a pair of median eyes, just as was the case with the most + ancient fishes, and with the members of the contemporary palæostracan group.</p> + <p>2. The retina of one of these eyes is well-defined and upright, not inverted, and therefore + in this respect agrees with that of all median eyes.</p> + <p>3. The presence of nerve end-cells, with pigment either in them or in cells around them, to + the unpigmented ends of which translucent bodies resembling rhabdites are attached, is another + proof that this retina agrees with that of the median eyes of arthropods.</p> + <p class="sp0">4. The simple nature of the nerve with its termination in an optic ganglion + closely resembling in structure an arthropod optic ganglion, together with Studniçka's statement + that the nerve end-cells pass directly into the nerve, points directly to the conclusion that + this retina is a simple, not a compound, retina, and that it therefore in this respect also + agrees with the retina of all median eyes.</p> + </div> + + <p>With respect to this last conclusion, neither I myself nor Studniçka have been able to see any + definite groups of cells between the nerve end-cells and the optic nerve such as a compound retina + necessitates.</p> + + <div><span class="pagenum" id="page82">{82}</span></div> + + <p>On the other hand, Dendy describes in the New Zealand lamprey, <i>Geotria australis</i>, a + cavity where the nerve enters into the eye, which he calls the atrium. This cavity is distinct + from the main cavity of the eye, and is separated from it by a mass of cells similar in appearance + to those of the cortex of the <i>ganglion habenulæ</i>. In these two eyes then, groups of cells, + resembling in appearance those belonging to an optic ganglion, exist in the eyes themselves. This + atrium is evidently that part of the central cavity which I have called the handle of the + cornucopia in the European lamprey, and the very fact that it is separated from the rest of the + central cavity is evidence that we are dealing here with a later stage in the history of the + pineal eyes than in the case of the Ammocœtes of <i>Petromyzon Planeri</i>. Taking also + into consideration the continuity of the mass of small ganglion-cells which surround this atrium + with the cells of the <i>ganglion habenulæ</i> by means of the similar cells scattered along the + course of the nerve, and also bearing in mind the fact already stated that in the more degenerate + left eye of Ammocœtes the cells of the <i>ganglion habenulæ</i> extend right up to the eye + itself, it seems more likely than not that these cells do not represent the original optic + ganglion of a compound retina, but rather the subsequent invasion, by way of the pineal nerve, of + ganglion-cells belonging to a portion of the brain. In the last chapter it has been suggested that + the presence of the trochlear or fourth cranial nerve has given rise to the formation of the + cerebellum by a similar spreading.</p> + + <p>There is certainly no appearance in the least resembling a compound retina such as is seen in + the vertebrate or crustacean lateral eye. In the median eyes of scorpions and of Limulus, cells + are found within the capsule of the eye among the nerve-fibres and the nerve end-cells. These are + especially numerous in the median eyes of Limulus, as described by Lankester and Bourne, and are + called by them intrusive connective tissue cells. The meaning of these cells is not, to my mind, + yet settled. It is sufficient for my purpose to point out that the presence of cells interneural + in position among the nerve end-cells of the retina of the median eyes of Ammocœtes is more + probable than not, on the assumption that the retina of these eyes is built up on the same plan as + that of the median eyes in Limulus and the scorpions.</p> + + <p class="sp3">It is further to be borne in mind that these specimens of <i>Geotria</i> worked at + by Dendy were in the 'Velasia' stage of the New Zealand <span class="pagenum" + id="page83">{83}</span>lamprey, and correspond, therefore, more nearly to the Petromyzon than to + the Ammocœtes stage of the European lamprey.</p> + + <p class="ac"><span class="sc">The Dioptric Apparatus.</span></p> + + <p>Besides the retina, all eyes possess a dioptric apparatus. What is the evidence as to its + nature in these vertebrate median eyes? Lankester and Bourne have divided the eyes of scorpions + and Limulus into two kinds, monostichous and diplostichous. In the first the retinal cells are + supposed to give rise to not only rhabdites but also the cuticular chitinous lens, so that the eye + is one-layered; in the second the lens is formed by a well-marked hypodermal layer, in front of + the retina, composed of elongated cells, so that these eyes are two-layered or diplostichous. The + lateral eyes, according to them, are all monostichous, but the median eyes are diplostichous.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig035.png" id="fig35"><img style="width:100%" src="images/fig035.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 35.—Eye of Acilius Larvæ.</span> (After <span + class="sc">Patten</span>.)</p> + <p class="sp0"><i>l.</i>, chitinous lens; <i>c.</i>, corneagen; <i>pr.</i>, pre-retinal layer; + <i>rh.</i>, rhabdites; <i>ret.</i>, retinal end-cells.</p> + </div> + </div> + + <p>This distinction is not considered valid by other observers. Thus, <span class="pagenum" + id="page84">{84}</span>as already indicated, Patten looks on all these eyes as three-layered, and + states that in all cases a corneagen or vitreogen layer exists, which gives origin to the lens. + For my own part I agree with Patten, but we are not concerned here with the lateral eyes. It is + sufficient to note that all observers are agreed that the median eyes are characterized by this + well-marked cell-layer, the so-called vitreous or corneagen layer of cells.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig036.png" id="fig36"><img style="width:100%" src="images/fig036.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 36.—Eye of Hydrophilus Larva, with the Pigment over the Retinal + End-cells.</span></p> + <p class="sp0"><i>l.</i>, chitinous lens; <i>c.</i>, corneagen; <i>pr.</i>, pre-retinal layer; + <i>rh.</i>, rhabdites; <i>ret.</i>, retinal end-cells.</p> + </div> + </div> + + <p>This layer (<i>c.</i>, Fig. <a href="#fig35">35</a>) is composed of much-elongated cells of the + hypodermal layer, in each of which the large nucleus is always situated towards the base of the + cell. The space between it and the retina contains, according to Patten the cells of the + pre-retinal layer <i>(pr.)</i>. These may be so few and insignificant as to give the impression + that the vitreous layer is immediately adjacent to the retina (<i>ret.</i>).</p> + + <p>Let us turn now to the right pineal eye of Ammocœtes (Fig. <a href="#fig37">37</a>) and + see what its further structure is. The anterior part of the eye is free from pigment, and is + composed, as is seen in hæmatoxylin or carmine specimens, of an inner layer of nuclei which are + frequently arranged in a wavy line. From this nucleated layer, strands of tissue, free from + nuclei, pass to the anterior edge of the eye.</p> + + <p>In the horizontal longitudinal sections it is seen that these strands are confined to the + middle of the eye. On each side of them the nuclear layer reaches the periphery, so that if we + consider these strands to represent long cylindrical cells, as described by Beard, then the + anterior wall may be described as consisting of long cylindrical cells, which are flanked on + either side by shorter cells of a similar kind. The nuclei at the base of these cylindrical cells + are not all alike. We see, in the first place, large nuclei resembling the large nuclei belonging + to the nerve end-cells; these are the nuclei of <span class="pagenum" id="page85">{85}</span>the + long cylindrical cells. We see also smaller nuclei in among these larger ones, which look like + nuclei of intrusive connective tissue, or may perhaps form a distinct layer of cells, situated + between the cells of the anterior wall and the terminations of the nerve end-cells already + referred to.</p> + + <p>These elongated cells are in exactly the same position and present the same appearance as the + cells of the corneagen layer of any median eye. Like the latter they are free from pigment and + never show with osmic staining any sign of the presence of translucent rhabdite-like bodies, such + as are seen in the termination of the retinal cells, and like the latter their nuclei are at the + base. The resemblance between this layer and the corneagen cells of any median eye is absolute. + Between it and the terminations of the retinal cells there exists some ill-defined material + certainly containing cells which may well correspond to Patten's pre-retinal layer of cells.</p> + + <p>Retina, corneagen, nerve, optic ganglion, all are there, all in their right position, all of + the right structure, what more is needed to complete the picture?</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig037.png" id="fig37"><img style="width:100%" src="images/fig037.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 37.—Pineal Eye of Ammocœtes, with its</span> + <i>Ganglion Habenulæ</i>.</p> + </div> + </div> + + <p>In order to complete the dioptric apparatus a lens is necessary. Where, then, is the lens in + these pineal eyes? In all the arachnid eyes, whether median or lateral, the lens is a single + corneal lens composed of the external cuticle, which is thickened over the corneagen cells. This + thickened cuticle is composed of chitin, and is not cellular, but is dead material formed out of + the living underlying corneagen cells. Such a lens is in marked contrast to the lens of the + lateral vertebrate eye, which is formed by living cells themselves. This <span class="pagenum" + id="page86">{86}</span>thickening of the cuticular layer to form a lens could only exist as long + as that layer is absolutely external, so that the light strikes immediately upon it; for, if from + any cause the eye became situated internally, the place of such a lens must be filled by the + structures situated between it and the surface, and the thickened cuticle would no longer be + formed.</p> + + <p>In all vertebrates these pineal eyes are separated from the external surface by a greater or + less thickness of tissues; in the case of Ammocœtes, as is seen in Fig. <a + href="#fig31">31</a>, the eye lies within the membranous cranial wall, and is attached closely to + it. The position, then, of the cuticular, or corneal lens, as it is often called, on the + supposition that this is a median eye of the arachnid type, is taken by the membranous cranium, + and, as I have described in my paper in the <i>Quarterly Journal</i>, on carefully lifting the eye + in the fresh condition from the cranial wall, it can be seen under a dissecting microscope that + the cranial wall often forms at this spot a lens-like bulging, which fits the shallow concavity of + the surface of the eye, and requires some little force to separate it from the eye.</p> + + <p>As will appear in a subsequent chapter, this cranial wall has been formed by the growth, + laterally and dorsally, of a skeletal structure known by the name of the <i>plastron</i>. The last + part of it to be completed would be that part in the mid-dorsal line, where apparently, in + consequence of the insinking of the degenerating eyes, a dermal and subdermal layer already + intervened between the source of light and the eyes themselves.</p> + + <p>When the membranous cranium was completed in the mid-dorsal region, it was situated here as + elsewhere just internally to the subdermal layer, and therefore enclosed the pineal eyes. This, to + my mind, is the reason why the pineal eyes, which, in all other respects, conform to the type of + the median eyes of an arachnid-like animal, do not possess a cuticular lens. Other observers have + attempted to make a lens out of the elongated cells of the anterior wall of the eye (my corneagen + layer), but without success.</p> + + <p>Studniçka, who calls this layer the <i>pellucida</i>, does not look upon it as the lens, but + considers, strangely enough, that the translucent appearances at the ends of each nerve end-cell + represent a lens for that cell, so that every nerve end-cell has its own lens. Still more strange + is it that, holding this view, he should yet consider these knobs <span class="pagenum" + id="page87">{87}</span>to be joined by filaments to the cells in the anterior wall of the eye, a + conception fatal to the action of such knobs as lenses.</p> + + <p class="sp3">The discovery that the vertebrate possesses, in addition to the lateral eyes, a + pair of median eyes, which are most conspicuous in the lowest living vertebrate, together with the + fact that such eyes are built up on the same plan as the median eyes of living crustaceans or + arachnids, not only with respect to the eye itself but also to its nerve and optic ganglion, + constitutes a fact of the very greatest importance for any theory of the origin of vertebrates; + especially in view of the further fact, that similar eyes in the same position are found not only + in all the members of the Palæostraca, but also in all those ancient forms (classed as fishes) + which lived at that time. At one and the same moment it proves the utter impossibility of + reversing dorsal and ventral surfaces, points in the very strongest manner to the origin of the + vertebrate from some member or other of the palæostracan group, and insists that the advocates of + the origin of vertebrates from the Hemichordata, etc., should give an explanation of the presence + of these two median eyes of a more convincing character than that given here.</p> + + <p class="ac"><span class="sc">The Lateral Eyes.</span></p> + + <p>Turning now to the consideration of the lateral eyes, we see that these eyes in the arachnids + often possess an inverted retina, in the crustaceans always an upright retina. In the arachnids + they possess a simple retina, while in the crustaceans their retina is compound; so that in the + latter case the so-called optic nerve is in reality a tract of fibres connecting together the + brain-region with a variable number of optic ganglia, which have been left at the periphery in + close contact with the retinal cells, when the brain sunk away from the superficial epithelial + covering.</p> + + <p>There is, then, this difference between the lateral eyes of crustaceans and arachnids, that the + retina of the former is compound, but never inverted, while that of the latter may be inverted, + but is always simple.</p> + + <p>The retina of the lateral eyes of the vertebrate resembles both of these, for it is compound, + as in the crustacean, and inverted as in the arachnid.</p> + + <p>It must always be borne in mind that in the palæostracan epoch <span class="pagenum" + id="page88">{88}</span>the dominant race was neither crustacean nor arachnid, but partook of the + characters of both; also, as is characteristic of dominance, there was very great variety of form, + so that it seems more probable than not that some of these forms may have combined the arachnid + and crustacean characteristics to the extent of possessing lateral eyes with an inverted yet + compound retina. A certain amount of evidence points in this direction. As already stated, the + compound retina which characterizes the vertebrate lateral eyes is characteristic of all facetted + eyes, and in the trilobites facetted lateral eyes are commonly found. From this it may be + concluded that many of the trilobites possessed eyes with a compound retina. There have, however, + been found in certain species, e.g. <i>Harpes vittatus</i> and <i>Harpes ungula</i>, lateral eyes + which were not facetted, and are believed by Korschelt and Heider to be of an arachnid nature. + They say, "Palæontologists have appropriately described them as ocelli, although, from a + zoological point of view, they do not deserve this name, having most probably arisen in a way + similar to that conjectured in connection with the lateral eyes of scorpions." If this conjecture + is right, then in these forms the retina may have been inverted, but because they belonged to the + trilobite group, the retina was most probably compound, so that here we may have had the + combination of the arachnid and crustacean characteristics. On the other hand, in some forms of + Branchipus, and many of the Gammaridæ, a single corneal lens is found in conjunction with an eye + of the crustacean type, so that a non-facetted lateral eye, found in a fossil form, would not + necessarily imply the arachnid type of eye with the possibility of an inverted retina. Whatever + may be the ultimate decision upon these particular forms, the striking fact remains, that both in + the vertebrate and in the arachnid the median eyes possess a simple upright retina, while the + lateral eyes possess an inverted retina, and that both in the vertebrate and the crustacean the + median eyes possess a simple upright retina, while the lateral eyes possess a compound retina.</p> + + <p>The resemblance of the retina of the lateral eyes of vertebrates to that of the lateral eyes of + many arthropods, especially crustaceans, has been pointed out by nearly every one who has worked + at these invertebrate lateral eyes. The foundation of our knowledge of the compound retina is + Berger's well-known paper, the results of which are summed up by him in the following two main + conclusions.</p> + + <div><span class="pagenum" id="page89">{89}</span></div> + + <p>1. The optic ganglion of the Arthropoda consists of two parts, of which the one stands in + direct inseparable connection with the facetted eye, and together with the layer of retinal rods + forms the retina of the facetted eye, while the other part is connected rather with the brain, and + is to be considered as an integral part of the brain in the narrower sense of the word.</p> + + <div class="ac w30 fcenter sp3"> + <a href="images/fig038.png" id="fig38"><img style="width:100%" src="images/fig038.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 38.—The Retina of Musca.</span> (After <span + class="sc">Berger</span>.)</p> + <p class="sp0"><i>Br.</i>, brain; <i>O.n.</i>, optic nerve; <i>n.l.o.g.</i>, nuclear layer of + ganglion of optic nerve; <i>m.l.</i>, molecular layer (Punktsubstanz); <i>n.l.r.g.i.</i> and + <i>n.l.r.g.o.</i>, inner and outer nuclear layers of the ganglion of the retina; + <i>f.br.r.</i>, terminal fibre-layer of retina; <i>r.</i>, layer of retinal end-cells + (indicated only).</p> + </div> + </div> + + <p>2. In all arthropods examined by him, the retina consists of five layers, as follows<span + class="wnw">:—</span></p> + + <div class="poem"> + <p>(1) The layer of rods and their nuclei.</p> + <p>(2) The layer of nerve-bundles.</p> + <p>(3) The nuclear layer.</p> + <p>(4) The molecular layer.</p> + <p>(5) The ganglion cell layer.</p> + </div> + + <p>Berger passes under review the structure and arrangement of the optic ganglion in a large + number of different groups of arthropods, and concludes that in all cases one part of the optic + ganglion is always closely attached to the visual end-cells, and this combination he calls the + retina. On the other hand, the nerve-fibres which connect the peripheral part of the optic + ganglion with the brain, the so-called optic nerve, are by no means homologous in the different + groups; for in some cases, as in many of the stalk-eyed crustaceans, the whole optic ganglion is + at the periphery, while in others, as in the Diptera, only the retinal ganglion is at the + periphery, and the nerve-stalk connects this with the rest of the optic ganglion, the latter being + fused with the main brain-mass. In the Diptera, in fact, according to Berger, the optic nerve + <span class="pagenum" id="page90">{90}</span>and retina are most nearly comparable to those of the + vertebrate. For this reason I give Berger's picture of the retina of Musca (Fig. <a + href="#fig38">38</a>), in order to show the arrangement there of the retinal layers.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig039.png" id="fig39"><img style="width:100%" src="images/fig039.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 39.</span>—<span class="sc">The Brain of</span> + <i>Sphæroma serratum</i>. (After <span class="sc">Bellonci</span>.)</p> + <p class="sp0"><i>Ant. I.</i> and <i>Ant. II.</i>, nerves to 1st and 2nd antennæ. + <i>f.br.r.</i>, terminal fibre-layer of retina; <i>Op. g. I.</i>, first optic ganglion; <i>Op. + g. II.</i>, second optic ganglion; <i>O.n.</i>, optic nerve-fibres forming an optic + chiasma.</p> + </div> + </div> + + <p>In Branchipus and other primitive Crustacea, Berger also finds the same retinal layers, but is + unable to distinguish in the brain the rest of the optic ganglion. Judging from Berger's + description of Branchipus, and Bellonci's of Sphæroma, it would almost appear as though the + cerebral part of the retina in the higher forms originated from two ganglionic enlargements, an + external and internal enlargement, as Bellonci calls them. The external ganglion (<i>Op. g. + I.</i>, Fig. <a href="#fig39">39</a>) may be called the ganglion of the retina, the cells of which + form the nuclear layer of the higher forms, and the internal ganglion (<i>Op. g. II.</i>, Fig. <a + href="#fig39">39</a>), from which the optic nerve-fibres to the brain arise, may therefore be + called the ganglion of the optic nerve. Bellonci describes how in this latter ganglion cells are + found very different to the small ones of the external ganglion or ganglion of the retina. So also + in Branchipus, judging from the pictures of Berger, Claus, and from my own observations + (<i>cf.</i> Fig. <a href="#fig46">46</a>, in which the double nature of the retinal ganglion is + indicated), the peripheral part of the optic ganglion—<i>i.e.</i> the retinal + ganglion—may be spoken <span class="pagenum" id="page91">{91}</span>of as composed of two + ganglia. The external of these is clearly the ganglion of the retina; its cells form the nuclear + layer, the striking character of which, and close resemblance to the corresponding layer in + vertebrates, is shown by Claus' picture, which I reproduce (Fig. <a href="#fig40">40</a>). The + internal ganglion with which the optic nerve is in connection contains large ganglion cells, + which, together with smaller ones, form the ganglionic layer of Berger.</p> + + <p>The most recent observations of the structure of the compound retina of the crustacean eye are + those of Parker, who, by the use of the methylene blue method, and Golgi's method of staining, has + been able to follow out the structure of the compound retina in the arthropod on the same lines as + had already been done for the vertebrate. These two methods have led to the conclusion that the + arthropod central nervous system and the vertebrate central nervous system are built up in the + same manner—viz. by means of a series of ganglia connected together, each ganglion being + composed of nerve-cells, nerve-fibres, and a fine reticulated substance called by Leydig in + arthropods 'Punktsubstanz,' and known in vertebrates and in invertebrates at the present time as + 'neuropil.' A further analysis resolves the whole system into a combination of groups of neurones, + the cells and fibres of which form the cells and fibres of the ganglia, while their dendritic + connections with the terminations of other neurones, together with the neuroglia-cells form the + 'neuropil.' As is natural to expect, that part of the central nervous system which helps to form + the compound retina is built up in the same manner as the rest of the central nervous system.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig040.png" id="fig40"><img style="width:37%" src="images/fig040.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 40.—Bipolar Cells of Nuclear Layer in Retina of + Branchipus.</span> (After <span class="sc">Claus</span>.)</p> + <p class="sp0"><i>f.br.r.</i>, terminal fibre-layer of retina; <i>n.l.r.g.</i>, bipolar cells + of the ganglion of the retina = inner nuclear layer; <i>m.l.</i>, Punktsubstanz = inner + molecular layer; <i>b.m.</i>, basement membrane formed by neurilemma round central nervous + system.</p> + </div> + </div> + + <p>Thus, according to Parker, the mass of nervous tissue which occupies the central part of the + optic stalk in Astacus is composed <span class="pagenum" id="page92">{92}</span>of four distinct + ganglia; the retina is connected with the first of these by means of the retinal fibres, and the + optic nerve extends proximally from the fourth ganglion to the brain. Each ganglion consists of + ganglion-cells, nerve-fibres, and 'neuropil,' and, in addition, supporting cells of a neuroglial + type. By means of the methylene blue method and the Golgi method, it is seen that the retinal + end-cells, with their visual rods, are connected with the fibres of the optic nerve by means of a + system of neurones, the synapses of which take place in and help to form the 'neuropil' of the + various ganglia. Thus, an impulse in passing from the retina to the brain would ordinarily travel + over five neurones, beginning with one of the first order and ending with one of the fifth. He + makes five neurones although there are only four ganglia, because he reckons the retinal cell with + its elongated fibre as a neurone of the first order, such fibre terminating in dendritic processes + which form synapses in the 'neuropil' of the first ganglion with the neurones of the second + order.</p> + + <p>Similarly the neurones of the second order terminate in the 'neuropil' of the second ganglion, + and so on, until we reach the neurones of the fifth order, which terminate on the one hand in the + 'neuropil' of the fourth ganglion, and on the other pass to the optic lobes of the brain by their + long neuraxons—the fibres of the optic nerve.</p> + + <p>He compares this arrangement with that of Branchipus, Apus, Estheria, Daphnia, etc., and shows + that in the more primitive crustaceans the peripheral optic apparatus was composed, not of four + but of two optic ganglia, not, therefore, of five but of three neurones, viz.—</p> + + <p>1. The neurone of the first order—<i>i.e.</i> the retinal cell with its fibre + terminating in the 'neuropil' of the first optic ganglion (ganglion of the retina).</p> + + <p>2. The neurone of the second order, which terminates in the 'neuropil' of the second ganglion + (ganglion of the optic nerve).</p> + + <p>3. The neurone of the third order, which terminates in the optic lobes of the brain by means + of its neuraxons (the optic nerve).</p> + + <p>We see, then, that the most recent researches agree with the older ones of Berger, Claus, and + Bellonci, in picturing the retina of the primitive crustacean forms as formed of two ganglia only, + and not of four, as in the specialized crustacean group the Malacostraca.</p> + + <div><span class="pagenum" id="page93">{93}</span></div> + + <p>The comparison of the arthropod compound retina with that of the vertebrate shows, as one would + expect upon the theory of the origin of vertebrates put forward in this book, that the latter + retina is built up of two ganglia, as in the more primitive less specialized crustacean forms. The + modern description of the vertebrate retina, based upon the Golgi method of staining, is exactly + Parker's description of the simpler form of crustacean retina in which the 'neuropil' of the first + ganglion is represented by the external molecular layer, and that of the second ganglion by the + internal molecular layer; the three sets of neurones being, according to Parker's terminology<span + class="wnw">:—</span></p> + + <p>1. The neurones of the first order—viz. the visual cells—the nuclei of which form + the external nuclear layer, and their long attenuated processes form synapses in the external + molecular layer with</p> + + <p>2. The neurones of the second order, the cells of which form the internal nuclear layer, and + their processes form synapses in the internal molecular layer with</p> + + <p>3. The neurones of the third order, the cells of which form the ganglionic layer and their + neuraxons constitute the fibres of the optic nerve which end in the optic lobes of the brain.</p> + + <p class="sp3">Strictly speaking, of course, the visual cells with their elongated processes have + no right to be called neurones: I only use Parker's phraseology in order to show how closely the + two retinas agree even to the formation of synapses between the fine drawn-out processes of the + visual cells and the neurones of the ganglion of the retina.</p> + + <p class="ac"><span class="sc">The Retina of the Lateral Eye of Ammocœtes.</span></p> + + <p>As in the case of all other organs, it follows that if we are dealing here with a true genetic + relationship, then the lower we go in the vertebrate kingdom the more nearly ought the structure + of the retina to approach the arthropod type. It is therefore a matter of intense interest to + determine the nature of the retina in Ammocœtes in order to see whether it differs from + that of the higher vertebrates, and if so, whether such differences are explicable by reference to + the structure of the arthropod eye.</p> + + <p>Before describing the structure of this retina it is necessary to clear away a remarkable + misconception, shared among others by <span class="pagenum" id="page94">{94}</span>Balfour, that + this eye is an aborted eye, and that it cannot be considered as a primitive type. Thus Balfour + says: "Considering the degraded character of the Ammocœte eye, evidence derived from its + structure must be received with caution," and later on, "the most interesting cases of partial + degeneration are those of Myxine and the Ammocœte. The development of such aborted eyes has + as yet been studied only in the Ammocœte, in which it resembles in most important features + that of other Vertebrata."</p> + + <p>Again and again the aborted character of the eye is stated to be evidence of degeneration in + the case of the lamprey. What such a statement means, why the eye is in any way to be considered + as aborted, is to me a matter of absolute wonderment: it is true that in the larval form it lies + under the skin, but it is equally true that at transformation it comes to the surface, and is most + evidently as perfect an eye as could be desired. There is not the slightest sign of any + degeneration or abortion, but simply of normal development, which takes a longer time than usual, + lasting as it does throughout the life-time of the larval form.</p> + + <p>Kohl, who has especially studied degenerated vertebrate eyes, discusses with considerable + fulness the question of the Ammocœtes eye, and concludes that in aborted eyes a retarded + development occurs, and this applies on the whole to Ammocœtes, "but with the important + difference that in this case the period of retarded development is not followed by a stoppage, but + on the contrary by a period of very highly intensified progressive development during the + metamorphosis," with the result that "the adult eye of <i>Petromyzon Planeri</i> does not diverge + from the ordinary type."</p> + + <p>Referring in his summing up to this retarded development, he says: "Such reminiscences of + embryonic conditions are after all present here and there in normally developed organs, and by no + means entitle us to speak of abnormal development."</p> + + <p>The evidence, then, is quite clear that the eye of Petromyzon, or, indeed, of the full-grown + Ammocœtes, is in no sense an abnormal eye, but simply that its development is slow during + the ammocœte stage. The retina of Petromyzon was figured and described by Langerhans in + 1873. He describes it as composed of the following layers<span class="wnw">:—</span></p> + + <div class="poem"> + <p style="margin-left:0.35em">(1) <i>Membrana limitans interna.</i></p> + <p style="margin-left:0.35em">(2) Thick inner molecular layer.</p> + <p style="margin-left:0.35em">(3) Optic fibre layer.</p> + <p style="margin-left:0.35em">(4) Thick inner nuclear layer.</p> + <p style="margin-left:0.35em">(5) Peculiar double-layered ganglionic layer.</p> + <p style="margin-left:0.35em">(6) External molecular layer.</p> + <p style="margin-left:0.35em">(7) External nuclear layer.</p> + <p style="margin-left:0.35em">(8) <i>Membrana limitans externa.</i></p> + <p style="margin-left:0.35em">(9) Layer of rods.</p> + <p>(10) Pigment-epithelium.</p> + </div> + + <div><span class="pagenum" id="page95">{95}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig041.png" id="fig41"><img style="width:100%" src="images/fig041.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 41.—Retina and Optic Nerve of Petromyzon. (After + Müller and Langerhans.)</span></p> + <p class="sp0">On the left side the Müllerian fibres and pigment-epithelium are represented + alone. The retina is divided into an epithelial part, <i>C</i> (the layer of visual + rod-cells), and a neurodermal or cerebral part which is formed of, <i>A</i>, the ganglion of + the optic nerve and, <i>B</i>, the ganglion of the retina. 1, int. limiting membrane; 2, int. + molecular layer with its two layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear + layer; 5, double row of tangential fulcrum cells; 6, layer of terminal retinal fibres; 7, ext. + nuclear layer; 8, ext. limiting membrane; 9, layer of rods; 10, layer of pigment-epithelium. + <i>D</i>, axial cell layer (Axenstrang) in optic nerve. The layer 6 is drawn rather too + thick.</p> + </div> + </div> + + <p>He points out especially the peculiarity of layer (2) (2, Fig. <a href="#fig41">41</a>), the + inner molecular, in which two rows of nuclei are arranged with great regularity, the one row + closely touching the <i>membrana limitans interna</i>, the other at the inner boundary of the + middle third of the <span class="pagenum" id="page96">{96}</span>molecular layer. Of these two + rows of nuclei, he describes the innermost as composed almost entirely of large nuclei belonging + to ganglion cells, while the outermost is composed mainly of distinctly smaller nuclei, which in + staining and appearance appear to belong not to nerve-cells but to the true reticular tissue of + the molecular layer.</p> + + <p>He also draws special attention to the remarkable layer (5) (5, Fig. <a href="#fig41">41</a>), + which is not found in the retina of the higher vertebrates, the cells of which, in his opinion, + are of the nature of ganglion-cells.</p> + + <p>W. Müller, in 1874, gave a most careful description of the eye of Ammocœtes and + Petromyzon, and traced the development of the retina; the subsequent paper of Kohl does not add + anything new, and his drawings are manifestly diagrams, and do not represent the appearances so + accurately as Müller's illustrations. In the accompanying figure (Fig. <a href="#fig41">41</a>) I + reproduce on the right-hand side Müller's picture of the retina of Petromyzon, but have drawn it, + as in Langerhans' picture, at the place of entry of the optic nerve.</p> + + <p>From his comparison of this retina with a large number of other vertebrate retinas, he comes to + the conclusion that the retina of all vertebrates is divisible into</p> + + <div class="bq1 sp2"> + <p><i>A.</i> An ectodermal (epithelial) part consisting of the layer of the visual cells, + and</p> + <p class="sp0"><i>B.</i> A neurodermal (cerebral) part which forms the rest of the retina.</p> + </div> + + <p>Further, Müller points out that the neuroderm gives origin throughout the central nervous + system to two totally different structures, on the one hand to the true nervous elements, on the + other to a system of supporting cells and fibres which cannot be classed as connective tissue, for + they do not arise from mesoblast, and are therefore called by him 'fulcrum-cells.' In the retina + he recognizes two distinct groups of such supporting structures—(1) a system of radial + fibres with well-marked elongated nuclei, which extend between the two limiting layers, and form + at their outer ends a membrane-like expansion which was originally the outer limit of the retina, + but becomes afterwards co-terminous with the <i>membrana limitans externa</i>, owing to the + piercing through it of the external limbs of the rods. This system, which is known by the name of + the radial Müllerian fibres (shown on the left-hand side of Fig. <a href="#fig41">41</a>), has no + connection with (2) the spongioblasts and neurospongium, which form a framework of neuroglia, in + which the terminations of the <span class="pagenum" id="page97">{97}</span>optic ganglion and of + the retinal ganglion ramify to form the molecular layers.</p> + + <p>It is evident from Fig. <a href="#fig41">41</a> that the retina of Ammocœtes and + Petromyzon differs in a striking manner from the typical vertebrate retina. The epithelial part + (C) remains the same—viz. the visual rods, the external limiting membrane, and the external + nuclear layer; but the cerebral part, the retinal ganglion (A and B), is remarkably different. It + is true, it consists in the main of the small-celled mass known as the inner nuclear layer, and of + the reticulated tissue or 'neuropil' known as the inner molecular layer, just as in all other + compound retinal eyes; but neither the ganglion cell-layer nor the optic fibre-layer is clearly + defined as separate from this molecular layer; on the contrary, it is matter of dispute as to what + cells represent the ganglionic layer of higher vertebrates, and the optic fibres do not form a + distinct innermost layer, but pass into the inner molecular layer at its junction with the inner + nuclear layer. A comparison of this innermost part of the retina (A, Fig. 41), with the + corresponding part in Berger's picture of Musca (<i>n.l.o.g.</i>, Fig. <a href="#fig38">38</a>), + shows a most striking similarity between the two. In both cases the fibres of the optic nerve + (<i>O.n.</i>, Fig. <a href="#fig38">38</a>) which cross at their entrance pass into the 'neuropil' + of this part of the retinal ganglion, and are connected probably (though that is not proved in + either case) with the cells of the ganglionic layer. In both cases we find two well-marked + parallel rows of cells in this part of the retina, of which one, the innermost, is composed in + Ammocœtes of large ganglion-cells, and the other mainly of smaller, deeper staining cells + apparently supporting in function. Similarly, also, in Branchipus, as I conclude from my own + observations as well as from those of Berger and Claus, the ganglionic layer is composed partly of + true ganglion-cells and partly of supporting cells arranged in a distinct layer. This part, then, + of the retina of Ammocœtes is remarkably like that of a typical arthropod retina, and forms + that part of the retinal ganglion which may be called the ganglion of the optic nerve.</p> + + <p>Next comes the ganglion of the retina (B, Fig. <a href="#fig41">41</a>) (Parker's first optic + ganglion), the cells of which form the small bipolar granule-cells of the inner nuclear layer; + granule-cells arranged in rows just as they are shown in Claus' picture of the same layer in the + retina of Branchipus (Fig. <a href="#fig40">40</a>), just as they are found in the cortical layers + of the optic ganglion of the pineal eye (<i>ganglion habenulæ</i>), in the <span class="pagenum" + id="page98">{98}</span>optic lobes and other parts of the Ammocœtes brain, or in the + cortical layers of the optic ganglia of all arthropods.</p> + + <p>Between this small-celled nuclear layer (4, Fig. <a href="#fig41">41</a>) and the layer of + nuclei of the visual rod cells (7, Fig. <a href="#fig41">41</a>) (the external nuclear layer), we + find in the eye of Ammocœtes and Petromyzon two well-marked rows of cells of a most + striking character—viz. the two remarkably regular rows of large epithelial-like cells with + large conspicuous nuclei, which give the appearance of two opposing rows of limiting epithelium + (5, Fig. <a href="#fig41">41</a>), already mentioned in connection with the researches of + Langerhans and W. Müller. Here, then, is a striking peculiarity of the retina of the lamprey, and + according to Müller the obliteration of these two layers can be traced as we pass upwards in the + vertebrate kingdom. Among fishes, they are especially well seen in the perch; in the higher + vertebrates the whole layer is only a rudiment represented, he thinks, by the simple layer of + round cells which lies close against the inner surface of the layer of terminal fibres + (Nervenansätze), and is especially evident in birds and reptiles. In man and the higher mammals + they are probably represented by the horizontal cells of the outer part of the inner nuclear + layer.</p> + + <p>Seeing, then, that they are most evident in Ammocœtes, and become less and less marked + in the higher vertebrates, it is clear that their origin cannot be sought among the animals higher + in the scale than Ammocœtes, but must, therefore, be searched for in the opposite + direction.</p> + + <p>Müller describes them as forming a very conspicuous landmark in the embryology of the retina, + dividing it distinctly into two parts, an outer thinner, and an inner somewhat thicker part, the + zone formed by them standing out conspicuously on account of the size and regularity of the cells + and their lighter appearance when stained. Thus in his description of the retina of an + Ammocœtes 95 mm. in length, he says, "The layer of pale tangentially elongated cells formed + a double layer and produced the appearance of a pale, very characteristic zone between the outer + and inner parts of the retina."</p> + + <p>Let us now turn to the retina of the crustacean and see whether there is any evidence there + that the retina is divisible into an outer and inner part, separated by a zone of + characteristically pale staining cells with conspicuous nuclei. The most elaborate description of + the development of the retina of Astacus is given by Reichenbach, <span class="pagenum" + id="page99">{99}</span>according to whom the earliest sign of the formation of the retina is an + ectodermic involution (Augen-einstülpung), which soon closes, so that the retinal area appears as + a thickening. In close contiguity to this thickening, the thickening of the optic ganglion arises, + so that that part of the optic ganglion which will form the retinal ganglion fuses with the + thickened optic plate and forms a single mass of tissue. Later on a fold (Augen-falte) appears in + this mass of tissue, in consequence of which it becomes divided into two parts. The lining walls + of this fold form a double row of cells, the nuclei of which are most conspicuous because they are + larger and lighter in colour than the surrounding nuclei, so that by this fold the retina is + divided into an outer and an inner wall, the line of demarcation being conspicuous by reason of + these two rows of large, lightly-staining nuclei.</p> + + <p>Reichenbach is unable to say that this secondary fold is coincident with the primary + involution, and that therefore the junction between the two rows of large pale nuclei is the line + of junction between the retinal ganglion and the retina proper, because all sign of the primary + involution is lost before the secondary fold appears.</p> + + <p>Parker compares the appearances in the lobster with Reichenbach's description in the crayfish, + and says that he finds only a thickening, no primary involution; at the same time he expressly + states that in the very early stages his material was deficient, and that he had not grounds + sufficient to warrant the statement that no involution occurs. He also finds that in the lobster + the ganglionic tissue which arises by proliferation is divided into an outer and inner part; the + separation is effected by a band of large, lightly-staining nuclei, which, in position and + structure, resemble the band figured by Reichenbach. According to Parker, then, the line of + separation indicated in the development by Reichenbach's outer and inner walls is not the line of + junction between the retina and the retinal ganglion, as Reichenbach was inclined to think, but + rather a separation of two rows of large ganglion-cells belonging to the retinal ganglion.</p> + + <p>The similarity between these conspicuous layers of lightly-staining cells in Ammocœtes + and in crustaceans is remarkably close, and in both cases observers have found the same difficulty + in interpreting their meaning. In each case one group of observers looks upon them as + ganglion-cells, the other as supporting structures. Thus in the lamprey, Müller considers them to + belong to the supporting elements, while Langerhans and Kohl describe them as a double <span + class="pagenum" id="page100">{100}</span>layer of ganglion-cells. In the crustacean, Berger in + Squilla, Grenacher in Mysis, and Parker in Astacus, look upon them as supporting elements, while + Viallanes in Palinurus considers them to be true ganglionic cells.</p> + + <p>Whatever the final interpretation of these cells may prove to be, we may, it seems to me, + represent an ideal compound retina of the crustacean type by combining the investigations of + Berger, Claus, Reichenbach, and Parker in the following figure.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig042.png" id="fig42"><img style="width:46%" src="images/fig042.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 42.—Ideal Diagram of the Layers in a Crustacean + Eye.</span></p> + <p class="sp0">The retina is divided into an epithelial part, <i>C</i> (the layer of retinular + cells and rhabdomes), and a neurodermal or cerebral part, which is formed of, <i>A</i>, the + ganglion of the optic nerve, and, <i>B</i>, the ganglion of the retina. 1, optic nerve fibres + which cross at their entrance into the retina; 2, int. molecular layer with its two rows of + cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5, + layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer + of crystalline cones; 9, cornea.</p> + </div> + </div> + + <p>The comparison of this figure (Fig. <a href="#fig42">42</a>) with that of the Petromyzon retina + (Fig. <a href="#fig41">41</a>) shows how great is the similarity of the latter with the arthropod + type, and how the very points in which it deviates from the recognized vertebrate type are + explainable by comparison with that of the arthropod. The most striking difference between the + retinas in the two figures is that the layer of terminal nerve fibres (5, Fig. <a + href="#fig42">42</a>), which, after all, are only the elongated terminations of the retinal cells + belonging to Parker's neurones of the first order, is very much longer than in Petromyzon or in + any vertebrate, for the external molecular layer (6, Fig. <a href="#fig41">41</a>) (Müller's layer + of Nervenansätze) is very short and inconspicuous (in Fig. <a href="#fig41">41</a> it is drawn too + thick).</p> + + <p>Turning from the retina to the fibres of the optic nerve we again find a remarkable + resemblance, for in Ammocœtes, as pointed out by <span class="pagenum" + id="page101">{101}</span>Langerhans and carefully figured by Kohl, a crossing of the fibres of the + optic nerve occurs as the nerve leaves the retina, just as is so universally the case in all + compound retinas. To this crossing Kohl has given the name <i>chiasma nervi optici</i>, in + distinction to the cerebral chiasma, which he calls <i>chiasma nervorum opticorum</i>. Further, we + find that even this latter chiasma is well represented in the arthropod brain; thus Bellonci in + Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a true optic chiasma, the only + difference in opinion being, whether the crossing of the optic nerves is complete or not. + Especially instructive are Bellonci's figures and description. He describes the brain of Sphæroma + as composed of three segments—a superior segment, the cerebrum proper, a middle segment, and + an inferior segment; the optic fibres, as is seen in Fig. <a href="#fig39">39</a>, after crossing, + pass direct into the middle segment, in the ganglia of which they terminate. From this segment + also arises the nerve to the first antenna of that side—<i>i.e.</i> the olfactory nerve. The + optic part, then, of this middle segment is clearly the brain portion of the optic ganglionic + apparatus, and may be called the optic lobes, in contradistinction to the peripheral part, which + is usually called the optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as + already mentioned. These optic lobes are therefore homologous with the optic lobes of the + vertebrate brain.</p> + + <p>The resemblance throughout is so striking as to force one to the conclusion that the retina of + the vertebrate eye is a compound retina, composed of a retina and retinal ganglion of the type + found in arthropods. From this it follows that the development of the vertebrate retina ought to + show the formation of (1) an optic plate formed from the peripheral epidermis and not from the + brain; (2) a part of the brain closely attached to this optic plate forming the retinal ganglion, + which remains at the surface when the rest of the optic ganglion withdraws; (3) an optic nerve + formed in consequence of this withdrawal, as the connection between the retinal and cerebral parts + of the optic ganglion.</p> + + <p class="sp3">This appears to me exactly what the developmental process does show according to + Götte's investigations. He asserts that the retina arises from an optic plate, being the optical + portion of his 'Sinnes-platte.' At an early stage this is separated by a furrow (Furche) from the + general mass of epidermal cells which ultimately form the brain. This separation then vanishes, + and the retina and brain-mass <span class="pagenum" id="page102">{102}</span>become inextricably + united into a mass of cells, which are still situated at the surface. By the closure of the + cephalic plate and the withdrawal of the brain away from the surface, a retinal mass of cells is + left at the surface connected with the tubular central nervous system by the hollow optic + diverticulum or primary optic vesicle. If we regard only the retinal and nervous elements, and for + the moment pay no attention to the existence of the tube, Götte's observation that the true retina + has been formed from the optic plate (Sinnes-platte) to which the retinal portion of the brain + (retinal ganglion) has become firmly fixed, and that then the optic nerve has been formed by the + withdrawal of the rest of the brain (optic lobes), is word for word applicable to the description + of the development of the compound retina of the arthropod eye, as has been already stated.</p> + + <p class="ac"><span class="sc">The Significance of the Optic Diverticula.</span></p> + + <p>The origin of the retina from an optic epidermal plate in vertebrates, as in all other animals, + brings the cephalic eyes of all animals into the same category, and leaves the vertebrate eye no + longer in an isolated and unnatural position. In one point the retina of the vertebrate eye + differs from that of a compound retina of an invertebrate; in the former, a striking supporting + tissue exists, known as Müller's fibres, which is absent in the latter. This difference of + structure is closely associated with another of the same character as in the central nervous + system, viz. the apparent development of the nervous part from a tube. We see, in fact, that the + retinal and nervous arrangements of the vertebrate eye are comparable with those of the arthropod + eye, in precisely the same way and to the same extent as the nervous matter of the brain of the + vertebrate is comparable with the brain of the arthropod. In both cases the nervous matter is, in + structure, position, and function, absolutely homologous; in both cases there is found in the + vertebrate something extra which is not found in the invertebrate—viz. a hollow tube, the + walls of which, in the case of the brain, are utilized as supporting tissues for the nerve + structures. The explanation of this difference in the case of the brain is the fundamental idea of + my whole theory, namely, that the hollow tube is in reality the cephalic stomach of the + invertebrate, around which the nervous brain-matter was originally grouped in precisely the same + manner as in the invertebrate. What, then, are the optic diverticula?</p> + + <div><span class="pagenum" id="page103">{103}</span></div> + + <p>"The formation of the eye," as taught by Balfour, "commences with the appearance of a pair of + hollow outgrowths from the anterior cerebral vesicle. These outgrowths, known as the optic + vesicles, at first open freely into the cavity of the anterior cerebral vesicle. From this they + soon, however, become partially constricted, and form vesicles united to the base of the brain by + comparatively narrow, hollow stalks, the rudiments of the optic nerves."</p> + + <p>"After the establishment of the optic nerves, there takes place (1) the formation of the lens, + and (2) the formation of the optic cup from the walls of the primary optic vesicle."</p> + + <p>He then goes on to explain how the formation of the lens forms the optic cup with its double + walls from the primary optic vesicle, and says—</p> + + <p>"Of its double walls, the inner, or anterior, is formed from the front portion, the outer, or + posterior, from the hind portion of the wall of the primary optic vesicle. The inner, or anterior, + which very speedily becomes thicker than the other, is converted into the retina; in the outer, or + posterior, which remains thin, pigment is eventually deposited, and it ultimately becomes the + tesselated pigment-layer of the choroid."</p> + + <p>The difficulties in connection with this view of the origin of the eye are exceedingly great, + so great as to have caused Balfour to discuss seriously Lankester's suggestion that the eye must + have been at one time within the brain, and that the ancestor of the vertebrate was therefore a + transparent animal, so that light might get to the eye through the outer covering and the + brain-mass; a suggestion, the unsatisfactory nature of which Balfour himself confessed. Is there + really evidence of any part of either retina or optic nerve being formed from the epithelial + lining of the tube?</p> + + <p>This tube is formed as a direct continuation of the tube of the central nervous system, and we + can therefore apply to it the same arguments as have been used in the discussion of the meaning of + the latter tube. Now, the striking point in the latter case is the fact that the lining membrane + of the central canal is in so many parts absolutely free from nervous matter, and so shows, as in + the so-called choroid plexuses, its simple, non-nervous epithelial structure. This also we find in + the optic diverticulum. Where there is no evidence of any invasion of the tube by nervous + elements, there it retains its simple non-nervous character of a tube composed of a single layer + of <span class="pagenum" id="page104">{104}</span>epithelial cells—viz. in that part of the + tube which, as Balfour says, remains thin, in which pigment is eventually deposited, and which + ultimately becomes the tesselated pigment-layer of the choroid. Nobody has ever suggested that + this pigment-layer is nervous matter, or ever was, or ever will be, nervous matter; it is in + precisely the same category as the membranous roof of the brain in Ammocœtes, which never + was, and never will be, nervous matter. Yet, according to the old embryology both in the case of + the eye and the brain, the pigment-layer and the so-called choroid plexuses are a part of the + tubular nervous system.</p> + + <p>Turning now to the optic nerve, Balfour describes it as derived from the hollow stalk of the + optic vesicle. He says—</p> + + <p>"At first the optic nerve is equally continuous with both walls of the optic cup, as must of + necessity be the case, since the interval which primarily exists between the two walls is + continuous with the cavity of the stalk. When the cavity within the optic nerve vanishes, and the + fibres of the optic nerve appear, all connection is ruptured between the outer wall of the optic + cup and the optic nerve, and the optic nerve simply perforates the outer wall, and becomes + continuous with the inner one."</p> + + <p>In this description Balfour, because he derived the optic nerve fibres from the epithelial wall + of the optic stalk, of necessity supposed that such fibres originally supplied both the outer and + inner walls of the optic cup and, therefore, seeing that when the fibres of the optic nerve appear + they do not supply the outer wall, he supposes that their original connection with the outer wall + is ruptured, because a discontinuity of the epithelial lining takes place coincidently with the + appearance of the optic nerve-fibres, and, according to him, the optic nerve simply perforates the + outer wall and becomes continuous with the inner one. This last statement is very difficult to + understand. I presume he meant that some of the fibres of the optic nerve supplied from the + beginning the inner wall of the optic cup, but that others which originally supplied the outer + wall were first ruptured, then perforated the outer wall, and finally completed the supply to the + inner wall or retina.</p> + + <p>This statement of Balfour's is the necessary consequence of his belief, that the epithelial + cells of the optic stalk gave rise to the fibres of the optic nerve. If, instead of this, we + follow Kölliker and His, who state that the optic nerve-fibres are formed outside the <span + class="pagenum" id="page105">{105}</span>epithelial walls of the optic stalk, and that the cells + of the latter form supporting structures for the nerve-fibres, then the position of the optic + nerve becomes perfectly simple and satisfactory without any rupturing of its connection with the + outer wall and subsequent perforation, for the optic nerve-fibres from their very first appearance + pass directly to supply the retina—<i>i.e.</i> the inner wall of the optic cup and nothing + else.</p> + + <p>They pass, as is well known, without any perforation by way of the choroidal slit to the inner + surface of the inner wall (retina) of the optic cup; then, when the choroidal slit becomes closed + by the expansion of the optic cup, the optic nerve naturally becomes situated in the centre of the + base of the cup and spreads over its inner surface as that surface expands.</p> + + <p>A section across the optic cup at an early stage at the junction of the optic stalk and optic + cup would be represented by the upper diagram in Fig. 43; at a later stage, when the choroidal + slit is closed, by the lower diagram.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig043.png" id="fig43"><img style="width:45%" src="images/fig043.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 43.—Diagram of the Relation of the Optic Nerve to + the Optic Cup.</span></p> + <p class="sp0">The upper diagram represents a stage before the formation of the choroidal + slit, the lower one the stage of closure of the choroidal slit. <i>R.</i>, retina; + <i>O.n.</i>, optic nerve; <i>p.</i>, pigment epithelium.</p> + </div> + </div> + + <p>The evident truth of this manner of looking at the origin of the optic nerve is demonstrated by + the appearance of the optic nerve in Ammocœtes and Petromyzon. In the latter, although the + development is complete, and the eye, and consequently also the optic nerve-fibres, are fully + functional, there is still present in the axial core of the nerve a row of epithelial cells + (Axenstrang) which are altered so as to form supporting structures, in the same way as a row of + epithelial cells in the retina is altered to form the system of supporting cells known by the name + of the Müllerian fibres.</p> + + <p>The origin of this axial core of cells is perfectly clear, as has been pointed out by W. + Müller. He says—</p> + + <p>"The development of the optic nerve shows peculiarities in <span class="pagenum" + id="page106">{106}</span>Petromyzon of such a character as to make this animal one of the most + valuable objects for deciding the various controversial questions connected with the genesis of + its elements. The lumen of the stalk of the primary optic vesicle is obliterated quite early by a + proliferation of its lining epithelium. Also the original continuity of this epithelium with that + of the pigment-layer is at an early period interrupted at the point of attachment of the optic + stalk. This interruption occurs at the time when the fibres of the optic nerve first become + visible."</p> + + <p>Further on he says—</p> + + <p>"The epithelium of the optic stalk develops entirely into supporting cells, which in Petromyzon + fill up the original lumen and so form an axial core (Axenstrang) to the nerve-fibres which are + formed entirely outside them; the projections of these supporting cells are directed towards the + periphery, and so separate the bundles of the optic nerve-fibres. The mesodermal coat of the optic + stalk takes no part in this separation; it simply forms the connective tissue sheath of the optic + nerve. The development of the optic nerve in the higher vertebrates also obeys the same law, as I + am bound to conclude from my own observations."</p> + + <p>The evidence, then, of Ammocœtes is very conclusive. Originally a tube composed of a + single layer of epithelial cells became expanded at the anterior end to form a bulb. On the + outside of this tube or stalk the fibres of the optic nerve make their appearance, arising from + the ganglion-cell layer of the retina, and, passing over the surface of the epithelial tube at the + choroidal fissure, proceed to the brain by way of the optic chiasma. Owing to the large number of + fibres, their crossing at the junction of the stalk with the bulb, and the narrowness at this + neck, the obliteration of the lumen of the tube which takes place in the stalk is carried out to a + still greater extent at this narrow part. The result of this is that all continuity of the + cell-layers of the original tube of the optic stalk with those of both the inner and outer walls + of the bulb is interrupted, and all that remains in this spot of the original continuous line of + cells which connected the tube of the stalk with that of the bulb are possibly some of the groups + of cells which are found scattered among the fibres of the optic nerve at their entrance into the + retina. Such separation of the originally continuous elements of the epithelial wall of the optic + stalk, which is apparent only at this neck of the nerve in Petromyzon, takes place <span + class="pagenum" id="page107">{107}</span>along the whole of the optic nerve in the higher + vertebrates, so that no continuous axial core of cells exist, but only scattered supporting + cells.</p> + + <p>If further proof in support of this view be wanted, it is given by the evidence of physiology, + which shows that the fibres of the optic nerve are not different from other nerve-fibres of the + central nervous system, but that they degenerate when separated from their nerve-cell, and that + the nerve-cell of which the optic nerve-fibre is a process is the large ganglion-cell of the + ganglionic layer of the retina. The origin of the ganglionic layer of the retina cannot therefore + be separated from that of the optic nerve-fibres. If the one is outside the epithelial tube, so is + the other, and what holds true of the ganglionic layer must hold good of the rest of the retinal + ganglion and, from all that has been said, of the retina itself. We therefore come to the + conclusion that the evidence is distinctly in favour of the view, that the retina and optic nerve + in the true sense are structures which originally were outside a non-nervous tube, but, just like + the central nervous system as a whole, have amalgamated so closely with the elements of this tube + as to utilize them for supporting structures. One part of this non-nervous tube, its dorsal wall, + like the corresponding part of the brain-tube, still retains its original character, and by the + deposition of pigment has been pressed into the service of the eye to form the pigmented + epithelial layer.</p> + + <p>We can, however, go further than this, for we know definitely in the case of the retina what + the fate of the epithelial cells lining this tube has been. They have become the system of + supporting structures known as Müllerian fibres.</p> + + <p>The epithelial layer of the primary optic vesicle can be traced into direct continuity with the + lining epithelium of the brain cavity, as a single layer of epithelial cells in the core of the + optic nerve, forming the optic stalk, which, in consequence of close contact, becomes the + well-known axial layer of supporting cells. This epithelial layer of the optic stalk then expands + to form the optic bulb, the outer or dorsal wall of which still remains as a single layer of + epithelium and becomes the layer of pigment epithelium. This layer of epithelium becomes doubled + on itself by the approximation of the inner or ventral wall of the optic cup to the outer or + dorsal wall in consequence of the presence of the lens, and still remaining a single layer, forms + the <i>pars ciliaris retinæ</i>; then suddenly, at the <i>ora <span class="pagenum" + id="page108">{108}</span>serrata</i>, the single epithelial layer vanishes, and the layers of the + retina take its place. It has long been known, however, that even throughout the retina this + single epithelial layer still continues, being known as the fibres of Müller. This is how the fact + is described in the last edition of Foster's "Text-book of Physiology," p. 1308—</p> + + <p>"Stretching radially from the inner to the outer limiting membrane in all regions of the retina + are certain peculiar-shaped bodies known as the radial fibres of Müller. Each fibre is the outcome + of the changes undergone by what was at first a simple columnar epithelial cell. The changes are, + in the main, that the columnar form is elongated into that of a more or less prismatic fibre, the + edges of which become variously branched, and that while the nucleus is retained the cell + substance becomes converted into neuro-keratin. And, indeed, at the <i>ora serrata</i> the fibres + of Müller may be seen suddenly to lose their peculiar features and to pass into the ordinary + columnar cells which form the <i>pars ciliaris retinæ</i>."</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig044.png" id="fig44"><img style="width:60%" src="images/fig044.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 44.—Diagram representing the Single-layered Epithelial Tube of + the Vertebrate Eye after Removal of the Nervous and Retinal Elements.</span></p> + <p class="sp0"><i>O.n.</i>, axial core of cells in optic nerve; <i>p.</i>, pigment epithelium; + <i>p.c.r., pars ciliaris retinæ</i>; <i>m.f.</i>, Müllerian fibres; <i>l.</i>, lens.</p> + </div> + </div> + + <p>It is then absolutely clear that the essential parts of the eye may be considered as composed + of two parts—</p> + + <p>1. A tube or diverticulum from the tube of the central nervous system, composed throughout of a + single layer of epithelium, which forms the supporting axial cells in the optic nerve, the pigment + epithelium and the Müllerian fibres of the retina. Such a tube would be represented by the + accompanying Fig. <a href="#fig44">44</a>, and the left side of Fig. <a href="#fig41">41</a>.</p> + + <p>2. The retina proper with the retinal ganglion and the optic nerve-fibres as already described. + In this part supporting elements are found, just as in any other compound retina, of the nature of + neuroglia, which are independent of the Müllerian fibres.</p> + + <div><span class="pagenum" id="page109">{109}</span></div> + + <p>Of these two parts we have already seen that the second is to all intents and purposes a + compound retina of a crustacean eye, and seeing that the single-layered epithelial tube is + continuous with the single-layered epithelial tube of the central nervous system—<i>i.e.</i> + with the cephalic part of the gut of the arthropod ancestor—it follows with certainty that + the ancestor of the vertebrates must have possessed two anterior diverticula of the gut, with the + wall of which, near the anterior extremity, the compound retina has amalgamated on either side, + just as the infra-œsophageal ganglia have amalgamated with the ventral wall of the main + gut-tube. In this way, and in this way alone, does the interpretation of the structure of the + vertebrate lateral eye harmonize in the most perfect manner with the rest of the conclusions + already arrived at.</p> + + <p>The question therefore arises:—Have we any grounds for believing that the ancient forms + of primitive crustaceans and primitive arachnids, which were so abundant in the time when the + Cephalaspids appeared, possessed two anterior diverticula of the stomach, such as the + consideration of the vertebrate eye strongly indicates must have been the case?</p> + + <p>The beautiful pictures of Blanchard, and his description, show how, on the arachnid side, + paired diverticula of the stomach are nearly universal in the group. Thus, although they are not + present in the scorpions, still, in the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most + marked characteristic of the stomach-region is the presence of four pairs of cœcal + diverticula, which spread laterally over the prosomatic region. In the spiders the number of such + diverticula increases, and the whole prosomatic region becomes filled up with these tubes. + Blanchard considers that they form nutrient tubes for the direct nutrition of the organs in the + prosoma, especially the important brain-region of the central nervous system. He points out that + these animals are blood-suckers, and that, therefore, their food is already in a suitable form for + purposes of nutrition when it is taken in by them, so that, as it were, the anterior part of the + gut is transformed into a series of vessels or diverticula conveying blood directly to the + important organs in the prosoma, by means of which they obtain nourishment in addition to their + own blood-supply.</p> + + <p>The universality of such diverticula among the arachnids makes it highly probable that their + progenitors did possess an alimentary canal with one or more pairs of anterior diverticula. In the + <span class="pagenum" id="page110">{110}</span>vertebrate, however, the paired diverticula are + associated with a compound retina, a combination which does not occur among living arachnids; we + must, therefore, examine the crustacean group for the desired combination, and naturally the most + likely group to examine is the Phyllopoda, especially such primitive forms as Branchipus and + Artemia, for it is universally acknowledged that these forms are the nearest living + representatives of the trilobites. If, therefore, it be found that the retina and optic nerve in + Artemia is in specially close connection with an anterior diverticulum of the gut on each side, + then it is almost certain that such a combination existed also in the trilobites.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig045.png" id="fig45"><img style="width:100%" src="images/fig045.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 45.—Section through one of the two Anterior + Diverticula of the Gut in Artemia and the Retinal Ganglion.</span></p> + <p class="sp0">The section is through the extreme anterior end of the diverticulum, thus + cutting through many of the columnar cells at right angles to their axis. <i>Al.</i>, gut + diverticulum; <i>rt. gl.</i>, retinal ganglion.</p> + </div> + </div> + + <div><span class="pagenum" id="page111">{111}</span></div> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig046.png" id="fig46"><img style="width:100%" src="images/fig046.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 46.—The Brain, Eyes, and Anterior Termination of the Alimentary + Canal of Artemia, viewed from the Dorsal Aspect.</span></p> + <p class="sp0"><i>Br.</i>, brain; <i>l.e.</i>, lateral eyes; <i>c.e.</i>, median eyes; + <i>Al.</i>, alimentary canal.</p> + </div> + </div> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig047.png" id="fig47"><img style="width:100%" src="images/fig047.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 47.—A, The Formation of the Retina of the Eye of + Ammocœtes</span> (after <span class="sc">Scott</span>); <span class="sc">B, The + Formation of the Retina of the Eye of Ammocœtes, on my theory</span>.</p> + <p class="sp0"><i>R.</i>, retina; <i>l.</i>, lens; <i>O.n.</i>, optic nerve fibres; + <i>Al.</i>, cephalic end of invertebrate alimentary canal; <i>V.</i>, cavity of ventricles of + brain; <i>Al.d.</i>, anterior diverticulum of alimentary canal; <i>op.d.</i>, optic + diverticulum.</p> + </div> + </div> + + <p>My friend Mr. W. B. Hardy has especially investigated the nervous system of Artemia. In the + course of his work he cut serial sections through the whole animal, and, as mentioned in my paper + in the <i>Journal of Anatomy and Physiology</i>, he discovered that the retinal ganglion of each + lateral eye is so closely attached to the end of the corresponding diverticulum of the gut that + the lining cells of the ventral part of the diverticulum form a lining to the retinal ganglion + (Fig. <a href="#fig45">45</a>). In this animal there are only two gut-diverticula, which are + situated most anteriorly. I have plotted out this series of sections by means of a camera lucida, + with the result that the retina appears as a bulging attached ventro-laterally to the extremity of + each gut-diverticulum, as is shown in Fig. <a href="#fig46">46</a>. It is instructive to compare + with this figure Scott's picture of the developing eye in Ammocœtes, where he figures the + retina as <span class="pagenum" id="page112">{112}</span>a bulging attached ventrally to the + extremity of the narrow tube of the optic diverticulum. In Fig. <a href="#fig47">47</a>, A, I + reproduce this figure of Scott, and by the side of it, Fig. <a href="#fig47">47</a>, B, I have + represented the origin of the vertebrate eye as I believe it to have occurred.</p> + + <p>We see, then, this very striking fact, that in the most primitive of the Crustacea, not only + are there two anterior diverticula of the gut, but also the retinal ganglion of the lateral eye is + in specially close connection with the end of the diverticulum on each side. In fact, we find in + the nearest living representative of the trilobites a retina and retinal ganglion and optic nerve, + closely resembling that of the vertebrate, in close connection with an epithelial tube which has + nothing to do with the organ of sight, but is one of a pair of anterior gut-diverticula. It is + impossible to obtain more decisive evidence that the trilobites possessed a pair of + gut-diverticula surrounded to a greater or less extent by the retina and optic nerve of each + lateral eye.</p> + + <p>Such anterior diverticula are commonly found in the lower Crustacea; they are usually known by + the name of liver-diverticula, but as they take no part in digestion, and, on the contrary, + represent that part of the gut which is most active in absorption, the term liver is not + appropriate, and it is therefore better to call them simply the pair of anterior diverticula. Our + knowledge of their function in Daphnia is given in a paper by Hardy and M‘Dougall, which + does not appear to be widely known. Hardy succeeded in feeding Daphnia with yolk of egg in which + carmine grains were mixed, and was able in the living animal to watch the whole process of + deglutition, digestion, and absorption. The food, which is made into a bolus, is moved down to the + middle region of the gut, and there digestion takes place. Then by an antiperistaltic movement the + more fluid products of the digestion-process are sent right forward into the two anterior + diverticula, where the single layer of columnar cells lining these diverticula absorbs these + products, the cells becoming thickly studded with fat-drops after a feed of yolk of egg. The + carmine particles, which were driven forward with the proteid- and fat-particles, are not + absorbed, but are at intervals driven back by contractions of the anterior diverticula to the + middle region of the gut.</p> + + <p>These observations prove most clearly that the anterior diverticula have a special nutrient + function, being the main channels by which new nutrient material is brought into the body, and, as + <span class="pagenum" id="page113">{113}</span>pointed out by the authors, it is a remarkable + exception in the animal kingdom that absorption should occur in that portion of the gut which is + anterior to the part in which digestion occurs. In all these animals the two anterior diverticula + extend forwards over the brain, and, as we have seen in Artemia, the anterior extremity of each + one is so intimately related to a part of the brain—viz. the retinal ganglion—as to + form a lining membrane to that mass of nerve-cells. It follows, therefore, that the nutrient fluid + absorbed by the cells of this part of the gut-diverticulum must be primarily for the service of + the retinal ganglion. In fact, the relations of this anterior portion of the gut to the brain as a + whole suggest strongly that the marked absorptive function of this anterior portion of the gut + exists in order to supply nutrient material in the first place to the most vital, most important + organ in the animal—the brain and its sense-organs. This conclusion is borne out by the fact + that in these lower crustaceans the circulation of blood is of a very inefficient character, so + that the tissues are mainly dependent for their nutrition on the fluid immediately surrounding + them. It stands to reason that the establishment of the anterior portion of the gut as a nutrient + tube to the brain would necessitate a closer and closer application of the brain to that tube, so + that the process of amalgamation of the brain with the single layer of columnar epithelial cells + which constitutes the wall of the gut (which we see in its initial stage in the retinal ganglion + of Artemia), would tend rapidly to increase as more and more demands were made upon the brain, + until at last both the supra- and infra-œsophageal ganglia, as well as the retinal ganglia + and optic nerves, were in such close intimate connection with the ventral wall of the anterior + portion of the gut and its diverticula as to form a brain and retina closely resembling that of + Ammocœtes.</p> + + <p>Such an origin for the lateral eyes of the vertebrate explains in a simple and satisfactory + manner why the vertebrate retina is a compound retina, and why both retina and optic nerve have an + apparent tubular development.</p> + + <p>At the same time one discrepancy still exists which requires consideration—viz. in no + arthropod eye possessing a compound retina is the retina inverted. All the known cases of + inversion among arthropods occur in eyes, the retina of which is simple, and are all natural + consequences of the process of invagination by which <span class="pagenum" + id="page114">{114}</span>the retina is formed. On the other hand, eyes with an inverted compound + retina are not entirely unknown among invertebrates, for the eyes of Pecten and of Spondylus + possess a retina which is inverted after the vertebrate fashion and still may be spoken of as + compound rather than simple. It is clear that an invagination, the effect of which is an inversion + of the retinal layer, would lead to the same result, whether the retinal optic nerves were short + or long, whether, in fact, a retinal ganglion existed or not. Undoubtedly the presence of the + retinal ganglion tends greatly to obscure any process of invagination, so that, as already + mentioned, many observers, with Parker, consider the retina of the crustacean lateral eye to be + formed by a thickening only, without any invagination, while Reichenbach says an obscure + invagination does take place at a very early stage. So in the vertebrate eye most observers speak + only of a thickening to form the retina, but Götte's observation points to an invagination of the + optic plate at an early stage. So also in the eye of Pecten, Korschelt and Heider consider that + the thickening, by which the retina is formed according to Patten, in reality hides an + invagination process by means of which, as Bütschli suggests, an optic vesicle is formed in the + usual manner. The retina is formed from the anterior wall of this vesicle, and is therefore + inverted.</p> + + <p>The origin of the inverted retina of the vertebrate eye does not seem to me to present any + great difficulty, especially when one takes into consideration the fact that the retina is + inverted in the arachnid group, only in the lateral eyes. The inversion is usually regarded as + associated with the tubular formation of the vertebrate retina, and it is possible to suppose that + the retina became inverted in consequence of the involvement of the eye with the gut-diverticulum. + I do not myself think any such explanation is at all probable, because I cannot conceive such a + process taking place without a temporary derangement—to say the least of it—of the + power of vision, and as I do not believe that evolution was brought about by sudden, startling + changes, but by gradual, orderly adaptations, and as I also believe in the paramount importance of + the organs of vision for the evolution of all the higher types of the animal kingdom, I must + believe that in the evolution from the Arthropod to the Cephalaspid, the lateral eyes remained + throughout functional. I therefore, for my own part, would say that the inversion of the <span + class="pagenum" id="page115">{115}</span>retina took place before the complete amalgamation with + the gut-diverticulum, that, in fact, among the proto-crustacean, proto-arachnid forms there were + some sufficiently arachnid to have an inverted retina, and at the same time sufficiently + crustacean to possess a compound retina, and therefore a compound inverted retina after the + vertebrate fashion existed in combination with the anterior gut-diverticula. Thus, when the eye + and optic nerve sank into and amalgamated with the gut-diverticulum, neither the dioptric + apparatus nor the nervous arrangements would suffer any alteration, and the animal throughout the + whole process would possess organs of vision as good as before or after the period of + transition.</p> + + <p>Further, not only the retina but also the dioptric apparatus of the vertebrate eye point to its + origin from a type that combined the peculiarities of the arachnids and the crustaceans. In the + former it is difficult to speak of a true lens, the function of a lens being undertaken by the + cuticular surface of the cells of the corneagen (Mark's 'lentigen'), while in the latter, in + addition to the corneal covering, a true lens exists in the shape of the crystalline cones. + Further, these crustacean lenses are true lenses in the vertebrate sense, in that they are formed + by modified hypodermal cells, and not bulgings of the cuticle, as in the arachnid. We see, in + fact, that in the compound crustacean eye an extra layer of hypodermal cells has become inserted + between the cornea and the retina to form a lens. So also in the vertebrate eye the lens is formed + by an extra layer of the epidermal cells between the cornea and the retina. The fact that the + vertebrate eye possesses a single lens, though its retina is composed of a number of ommatidia, + while the crustacean eye possesses a lens to each ommatidium, may well be a consequence of the + inversion of the vertebrate retina. It is most probable, as Korschelt and Heider have pointed out, + that the retina of the arachnid eyes is composed of a number of ommatidia, just as in the + crustacean eyes and in the inverted eyes it is probable that the image is focussed on to the + pigmented tapetal layer, and thence reflected on to the percipient visual rods. In such a method + of vision a single lens is a necessity, and so it must also be if, as I suppose, eyes existed with + an inverted compound retina. Owing to the crustacean affinities of such eyes, a lens would be + formed and the retina would be compound: owing to the arachnid affinities, the retina would be + inverted and the hypodermal cells which formed the lens would be massed <span class="pagenum" + id="page116">{116}</span>together to form a single lens, instead of being collected in groups of + four to form a series of crystalline cones.</p> + + <p>To sum up: The study of the vertebrate eyes, both median and lateral, leads to most important + conclusions as to the origin of the vertebrates, for it shows clearly that whereas, as pointed out + in this and subsequent chapters, their ancestors possessed distinct arachnid characteristics, yet + that they cannot have been specialized arachnids, such as our present-day forms, but rather they + were of a primitive arachnid type, with distinct crustacean characteristics: animals that were + both crustacean and arachnid, but not yet specialized in either direction: animals, in fact, of + precisely the kind which swarmed in the seas at the time when the vertebrates first made their + appearance. In the opinion of the present day, the ancestral forms of the Crustacea, which were + directly derived from the Annelida, may be classed as an hypothetical group the Protostraca, the + nearest approach to which is a primitive Phyllopod.</p> + + <p>"Starting from the Protostraca," say Korschelt and Heider, "according to the present condition + of our knowledge, we may, as has been already remarked, assume three great series of development + of the Arthropodan stock, by the side of which a number of smaller independent branches have been + retained. One of these series leads through the hypothetical primitive Phyllopod to the Crustacea; + the second through the Palæostraca (Trilobita, Gigantostraca, Xiphosura) to the Arachnida; the + third through forms resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and the + Tardigrada must probably be regarded as smaller independent branches of the Arthropodan + stock."</p> + + <p class="sp3">To these "three great series of development of the Arthropodan stock" the evidence + of Ammocœtes shows that a fourth must be added, which, starting also from the Protostraca, + and closely connected with the second, palæostracan branch, leads through the Cephalaspidæ to the + great kingdom of the Vertebrata. Such a direct linking of the earliest vertebrates with the + Annelida through the Protostraca is of the utmost importance, as will be shown later in the + explanation of the origin of the vertebrate cœlom and urinary apparatus.</p> + + <div><span class="pagenum" id="page117">{117}</span></div> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The most important discovery of recent years which gives a direct clue to the ancestry of the + vertebrates is undoubtedly the discovery that the pineal gland is all that remains of a pair of + median eyes which must have been functional in the immediate ancestor of the vertebrate, seeing + how perfect one of them still is in Ammocœtes. The vertebrate ancestor, then, possessed + two pairs of eyes, one pair situated laterally, the other median. In striking confirmation of + the origin of the vertebrate from Palæostracans it is universally admitted that all the + Eurypterids and such-like forms resembled Limulus in the possession of a pair of median eyes, as + well as of a pair of lateral eyes. Moreover, the ancient mailed fishes the Ostracodermata, which + are the earliest fishes known, are all said to show the presence of a pair of median eyes as + well as of a pair of lateral eyes. This evidence directly suggests that the structure of both + the median and lateral vertebrate eyes ought to be very similar to that of the median and + lateral arthropod eyes. Such is, indeed, found to be the case.</p> + <p>The retina of the simplest form of eye is formed from a group of the superficial epidermal + cells, and the rods or rhabdites are formed from the cuticular covering of these cells; the + optic nerve passes from these cells to the deeper-lying brain. This kind of retina may be called + a simple retina, and characterizes the eyes, both median and lateral, of the scorpion group.</p> + <p>In other cases a portion of the optic ganglion remains at the surface, when the brain sinks + inwards, in close contiguity to the epidermal sense-cells which form the retina; a tract of + fibres connects this optic ganglion with the underlying brain, and is known as the optic nerve. + Such a retina may be called a compound retina and characterizes the lateral eyes of both + crustaceans and vertebrates. Also, owing to the method of formation of the retina by + invagination, the cuticular surface of the retinal sense-cells, from which the rods are formed, + may be directed towards the source of light or away from it. In the first case the retina may be + called upright, in the second inverted.</p> + <p>Such inverted retinas are found in the vertebrate lateral eyes and in the lateral eyes of the + arachnids, but not of the crustaceans.</p> + <p>The evidence shows that all the invertebrate median eyes possess a simple upright retina, and + in structure are remarkably like the right median or pineal eye of Ammocœtes; while the + lateral eyes possess, as in the crustaceans, an upright compound retina, or, as in many of the + arachnids, a simple inverted retina. The lateral eyes of the vertebrates alone possess a + compound inverted retina.</p> + <p>This retina, however, is extraordinarily similar in its structure to the compound crustacean + retina, and these similarities are more accentuated in the retina of the lateral eye of + Petromyzon than that of the higher vertebrates.</p> + <p>The evidence afforded by the lateral eye of the vertebrate points unmistakably to the + conclusion that the ancestor of the vertebrate possessed both crustacean and arachnid + characters—belonged, therefore, to a group of animals which gave rise to both the + crustacean and arachnid groups. This is precisely the position of the Palæostracan group, which + is regarded as the ancestor of both the crustaceans and arachnids. <span class="pagenum" + id="page118">{118}</span>In two respects the retina of the lateral eyes of vertebrates differs + from that of all arthropods, for it possesses a special supporting structure, the Müllerian + fibres, which do not exist in the latter, and it is developed in connection with a tube, the + optic diverticulum, which is connected on each side with the main tube of the central nervous + system. These two differences are in reality one and the same, for the Müllerian fibres are the + altered lining cells of the optic diverticulum, and this tube has the same significance as the + rest of the tube of the nervous system; it is something which has nothing to do with the nervous + portion of the retina but has become closely amalgamated with it. The explanation is, word for + word, the same as for the tubular nervous system, and shows that the ancestor of the vertebrate + possessed two anterior diverticula of its alimentary canal which were in close relationship to + the optic ganglion and nerve of the lateral eye on each side. It is again a striking coincidence + to find that Artemia, which with Branchipus represents a group of living crustaceans most nearly + allied to the trilobites, does possess two anterior diverticula of the gut which are in + extraordinarily close relationship with the optic ganglia of the retina of the lateral eyes on + each side.</p> + <p class="sp0">The evidence of the optic apparatus of the vertebrate points most remarkably to + the derivation of the Vertebrata from the Palæostraca.</p> + </div> + + <div><span class="pagenum" id="page119">{119}</span></div> + + <p class="ac">CHAPTER III</p> + + <p class="ac"><i>THE EVIDENCE OF THE SKELETON</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">The bony and cartilaginous skeleton considered, not the notochord.—Nature + of the earliest cartilaginous skeleton.—The mesosomatic skeleton of Ammocœtes; its + topographical arrangement, its structure, its origin in muco-cartilage.—The prosomatic + skeleton of Ammocœtes; the trabeculæ and parachordals, their structure, their origin in + white fibrous tissue.—The mesosomatic skeleton of Limulus compared with that of + Ammocœtes; similarity of position, of structure, of origin in muco-cartilage.—The + prosomatic skeleton of Limulus; the entosternite or plastron compared with the trabeculæ of + Ammocœtes; similarity of position, of structure, of origin in fibrous + tissue.—Summary.</p> + </div> + + <p>The explanation of the two optic diverticula given in the last chapter accounts in the same + harmonious manner for every other part of the tube around which the central nervous system of the + vertebrate has been grouped. The tube conforms in all respects to the simple epithelial tube which + formed the alimentary canal of the ancient type of marine arthropods such as were dominant in the + seas when the vertebrates first appeared. The whole evidence so far is so uniform and points so + strongly in the direction of the origin of vertebrates from these ancient arthropods, as to make + it an imperative duty to proceed further and to compare one by one the other parts of the central + nervous system, together with their outgoing nerves in the two groups of animals.</p> + + <p>Before proceeding to do this, it is advisable first to consider the question of the origin of + the vertebrate skeletal tissues, for this is the second of the great difficulties in the way of + deriving vertebrates from arthropods, the one skeleton being an endo-skeleton composed of + cartilage and bone, and the other an exo-skeleton composed of chitin. Here is a problem of a + totally different kind to that we have just been considering, but of so fundamental a character + that it must, if possible, be solved before passing on to the consideration of the cranial nerves + and the organs they supply.</p> + + <div><span class="pagenum" id="page120">{120}</span></div> + + <p class="sp3">Is there any evidence which makes it possible to conceive the method by which the + vertebrate skeleton may have arisen from the skeletal tissues of an arthropod? By the vertebrate + skeleton I mean the bony and cartilaginous structures which form the backbone, the cranio-facial + skeleton, the pectoral and pelvic girdles, and the bones of the limbs. I do not include the + notochord in these skeletal tissues, because there is not the slightest evidence that the + notochord played any part in the formation of these structures; the notochordal tissue is + something <i>sui generis</i>, and never gives rise to cartilage or bone. The notochord happens to + lie in the middle line of the body and is very conspicuous in the lowest vertebrate; with the + development of the backbone the notochord becomes obliterated more and more, until at last it is + visible in the higher vertebrates only in the embryo; but that obliteration is the result of the + encroachment of the growing bone-masses, not the cause of their growth. Although, then, the + notochord may in a sense be spoken of as the original supporting axial rod of the vertebrate, it + is so different to the rest of the endo-skeleton, has so little to do with it, that the + consideration of its origin is a thing apart, and must be treated by itself without reference to + the origin of the cartilaginous and bony skeleton.</p> + + <p class="ac"><span class="sc">The Commencement of the Bony Skeleton in the Vertebrate.</span></p> + + <p>What is the teaching of the vertebrate? What evidence is there as to the origin of the bony + skeleton in the vertebrate phylum itself?</p> + + <p>The axial bony skeleton of the higher Mammalia consists of two parts, (1) the vertebral column + with its attached bony parts, and (2) the cranio-facial skeleton. Of these two parts, the bony + tissue of the first arises in the embryo from cartilage, of the second partly from cartilage, + partly from membrane.</p> + + <p>In strict accordance with their embryonic origin is their phylogenetic origin: as we pass from + the higher vertebrates to the lower these structures can be traced back to a cartilaginous and + membranous condition, so that, as Parker has shown, the cranio-facial bony skeleton of the higher + vertebrates can be derived directly from a non-bony cartilaginous skeleton, such as is seen in + Petromyzon and the cartilaginous fishes.</p> + + <p>Balfour, in his "Comparative Embryology," states that the <span class="pagenum" + id="page121">{121}</span>primitive cartilaginous cranium is always composed of the following + parts<span class="wnw">:—</span></p> + + <p>1. A pair of cartilaginous plates on each side of the cephalic section of the notochord known + as the parachordals (<i>pa.ch.</i>, Fig. <a href="#fig49">49</a>; <i>iv.</i>, Fig. <a + href="#fig48">48</a>). These plates, together with the notochord (<i>ch.</i>) enclosed between + them, form a floor for the hind and mid-brain.</p> + + <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:43%;"><a href="images/fig048.png" id="fig48"><img + style="width:100%" src="images/fig048.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:43%;"><a href="images/fig049.png" id="fig49"><img + style="width:100%" src="images/fig049.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 48.—Embryo Pig, two-thirds of an inch long</span> (from + <span class="sc">Parker</span>), <span class="sc">Elements of Skull seen from + below</span>.</p> + <p class="sp0"><i>ch.</i>, notochord; <i>iv.</i>, parachordals; <i>au.</i>, auditory + capsule; <i>py.</i>, pituitary body; <i>tr.</i>, trabecula; <i>ctr.</i>, trabecular cornu; + <i>pn.</i>, pre-nasal cartilage; <i>ppg.</i>, palato-pterygoid tract; <i>mn.</i>, + mandibular arch; <i>th.h.</i>, first branchial arch; <i>VII.-XII.</i>, cranial nerves.</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 49.—Head of Embryo Dog-fish</span> (from <span + class="sc">Parker</span>), <span class="sc">Basal View of Cranium from above</span>.</p> + <p class="sp0"><i>ol.</i>, olfactory sacs; <i>au.</i>, auditory capsule; <i>py.</i>, + pituitary body; <i>pa.ch.</i>, parachordal cartilage; <i>tr.</i>, trabecula; <i>inf.</i>, + infundibulum; <i>pt.s.</i>, pituitary space; <i>e.</i>, eye.</p> + </div> + </td> + </tr> + </table> + + <p>2. A pair of bars forming the floor for the fore-brain, known as the trabeculæ (<i>tr</i>). + These bars are continued forward from the parachordals. They meet posteriorly and embrace the + front end of the notochord, and after separating for some distance bend in again in such a way as + to enclose a space—the pituitary space (<i>pt.s.</i>). In <span class="pagenum" + id="page122">{122}</span>front of this space they remain in contact, and generally unite. They + extend forward into the nasal region (<i>pn.</i>).</p> + + <p>3. The cartilaginous capsules of the sense organs. Of these the auditory (<i>au.</i>) and the + olfactory capsules (<i>ol.</i>) unite more or less intimately with the cranial walls; while the + optic capsules, forming the usually cartilaginous sclerotics, remain distinct.</p> + + <p>The parachordals and notochord form together the basilar plate, which forms the floor for that + section of the brain belonging to the primitive postoral part of the head, and its extent + corresponds roughly to that of the basioccipital of the adult skull.</p> + + <p>The trabeculæ, so far as their mere anatomical relations are concerned, play the same part in + forming the floor for the front cerebral vesicle as do the parachordals for the mid- and + hind-brain. They differ, however, from the parachordals in one important feature, viz. that except + at their hinder end they do not embrace the notochord. The notochord always terminates at the + infundibulum, and the trabeculæ always enclose a pituitary space, in which lies the infundibulum + (<i>inf.</i>) and the pituitary body (<i>py.</i>).</p> + + <p>In the majority of the lower forms the trabeculæ arise quite independently of the parachordals, + though the two sets of elements soon unite.</p> + + <p>The trabeculæ are usually somewhat lyre-shaped, meeting in front and behind, and leaving a + large pituitary space between their middle parts. Into this space the whole base of the fore-brain + primitively projects, but the space itself gradually becomes narrowed until it usually contains + only the pituitary body.</p> + + <p>The trabecular floor of the brain does not long remain simple. Its sides grow vertically + upwards, forming a lateral wall for the brain, in which in the higher types, two regions may be + distinguished, viz. an alisphenoidal region behind, growing out from what is known as the + basisphenoidal region of the primitive trabeculæ, and an orbito-sphenoidal region in front, + growing out from the presphenoidal region of the trabeculæ. These plates form at first a + continuous lateral wall of the cranium. The cartilaginous walls which grow up from the trabecular + floor of the cranium generally extend upwards so as to form a roof, though almost always an + imperfect roof, for the cranial cavity.</p> + + <p>The basi-cranial cartilaginous skeleton reduces itself always into trabeculæ and parachordals + with olfactory and auditory cartilaginous capsules.</p> + + <div><span class="pagenum" id="page123">{123}</span></div> + + <p>In addition, a branchial skeleton exists, which consists of a series of bars known as the + branchial bars, so situated as to afford support to the successive branchial pouches. An anterior + arch known as the mandibular arch (Fig. <a href="#fig50">50</a>, <i>Mn.</i>), placed in front of + the hyo-mandibular cleft, and a second arch, known as the hyoid arch (<i>Hy.</i>), placed in front + of the hyo-branchial cleft, are developed in all types; the succeeding arches are known as the + true branchial arches (<i>Br.</i>), and are only fully developed in the Ichthyopsida. In all cases + of jaw-bearing (gnathostomatous) vertebrates the first arch has become a supporting skeleton for + the mouth (Fig. <a href="#fig51">51</a>), and in the higher vertebrates in combination with the + second or hyoid arch takes part in the formation of the ear-bones.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig050.png" id="fig50"><img style="width:62%" src="images/fig050.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 50.—Head of Embryo Dog-fish, eleven lines + long.</span> (From <span class="sc">Parker</span>.)</p> + <p class="sp0"><i>Tr.</i>, trabecula; <i>Mn.</i>, mandibular cartilage; <i>Hy.</i>, hyoid + arch; <i>Br<sub>1</sub>.</i>, first branchial arch; <i>Na.</i>, olfactory sac; <i>E.</i>, eye; + <i>Au.</i>, auditory capsule; <i>Hm.</i>, hemisphere; <i>C<sub>1</sub></i>, + <i>C<sub>2</sub></i>, <i>C<sub>3</sub></i>, cerebral vesicles.</p> + </div> + </div> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig051.png" id="fig51"><img style="width:100%" src="images/fig051.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 51.—Skull of Adult Dog-fish, Side View.</span> (From <span + class="sc">Parker</span>.)</p> + <p class="sp0"><i>cr.</i>, cranium; <i>Br.</i>, branchial arches; <i>Mn.</i> + <i>Hy.</i>, + mandibular and hyoid arches.</p> + </div> + </div> + + <p>The true branchial arches persist, to a certain extent, in the Amphibia, and become still more + degenerated in the Amniota (reptiles, birds, and mammals) in correlation with the total + disappearance of a branchial respiration at all periods of their life. <span class="pagenum" + id="page124">{124}</span>Their remnants become more or less important parts of the hyoid bone, and + are employed solely in support of the tongue.</p> + + <p>In no single animal is there any evidence that the foremost arch, the mandibular, is a true + branchial arch. As low down as the Elasmobranchs it becomes divided into two elements which form + respectively the upper and lower jaws; the hyoid arch, on the other hand, although it has altered + its form and acquired the secondary function of supporting the mandibular arch, still retains its + respiratory function.</p> + + <p>The evidence afforded by the mode of formation of the skeletal tissues of vertebrates down to + the Elasmobranchs indicates that the primitive cranial skeleton arose from two paired basal + cartilages, the parachordals and trabeculæ, to which were attached respectively cartilaginous + cases enclosing the organs of hearing and smell. In addition, the branchial portion of the cranial + region was provided with cartilaginous bars arranged serially for the support of the branchiæ, + with the exception of the foremost, the mandibular bar, which formed supporting tissues for the + mouth—the upper and lower jaws.</p> + + <p>Just as in past times the spinal nerves and the segments they supplied were supposed to + represent the type on which the original vertebrate was built, so also the spinal vertebræ + afforded the type of the segmented skeleton, and the anatomists of those days strove hard to + resolve the cranio-facial skeleton into a series of modified vertebræ. Owing especially to the + labours of Huxley, who showed that the segmentation in the head-region was essentially a + segmentation due to the presence of branchial bars, this conception was finally laid to rest and + nowadays it is admitted to be hopeless to resolve the cranium into vertebral segments. Still, + however, the vertebrate is a segmented animal and its segmented nature is visible in the cranial + region, so far as the skeletal tissues are concerned, in the shape of the series of branchial and + visceral bars.</p> + + <p>To this segmentation the name of 'branchiomeric' has been given, while that due to the presence + of vertebræ is called 'mesomeric.'</p> + + <p>As we have seen, the internal bony skeleton of the vertebrate commences as a cartilaginous and + membranous skeleton. For this reason the preservation of such skeletons is impossible in the + fossil form, unless the cartilage has become impregnated with lime salts, so that there is but + little hope of ever obtaining traces of such <span class="pagenum" + id="page125">{125}</span>structures in the fossils of the Silurian age either among the vertebrate + or invertebrate remains. Fortunately for this investigation there are still living on the earth + two representatives of that age; on the invertebrate side Limulus, and on the vertebrate side + Ammocœtes.</p> + + <p>The Elasmobranchs represent the most primitive of the gnathostomatous vertebrates. Below them + come the Agnatha, known as the cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon) + and the hag-fishes (Myxine).</p> + + <p>The skeleton of Petromyzon (Fig. <a href="#fig52">52</a>) consists of a cranio-facial skeleton + composed of a cartilaginous unsegmented cranium, with the basal trabeculæ and parachordals and a + series of branchial and visceral cartilaginous bars forming the so-called branchial basket-work; + to these must be added auditory and nasal capsules. In contradistinction to this elaborate + cranio-facial skeleton, the spinal vertebral skeleton is represented only by segmentally arranged + small pieces of cartilage formed in the connective tissue dissepiments between segmented sheets of + body-muscles (myotomes).</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig052.png" id="fig52"><img style="width:100%" src="images/fig052.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 52.—Skeleton of Petromyzon.</span> (From <span + class="sc">Parker</span>.)</p> + <p class="sp0"><i>na.</i>, nasal capsule; <i>au.</i>, auditory capsule; <i>nc.</i>, + notochord.</p> + </div> + </div> + + <p>But Petromyzon is derived from Ammocœtes by a remarkable process of transformation, and + a most important part of that transformation is the formation of new cartilaginous structures. + Thus we see that in Ammocœtes there is no sign of a cartilaginous vertebral column; at + transformation the rudimentary vertebræ of Petromyzon are formed. In Ammocœtes the + brain-case is a simple fibrous membranous covering; at transformation this becomes cartilaginous. + In Ammocœtes there are no cartilaginous structures corresponding to the sub-ocular arches; + these are all formed at transformation. It follows, that we can trace back the bony skeleton of + the vertebrate head to the skeleton of Ammocœtes, and we may therefore conclude <span + class="pagenum" id="page126">{126}</span>that the primitive cartilaginous skeleton of the + vertebrate consisted of the following structures (Fig. <a href="#fig53">53</a>, B), viz. the + branchial bars forming a basket-work, the trabeculæ and parachordals, the auditory and nasal + capsules—a clear proof that the cranial skeleton is older than the spinal. Of these + structures the branchial bars are the only evidently segmented parts.</p> + + <div class="ac w50 fcenter sp3"> + <a href="images/fig053.png" id="fig53"><img style="width:50%" src="images/fig053.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 53.—Comparison of Cartilaginous Skeleton of Limulus + and Ammocœtes.</span></p> + <p>A, Diagram of cartilaginous skeleton of Limulus. <i>Soft cartilage</i>, entapophysial + ligaments, deep black; branchial bars simply hatched; <i>hard cartilage</i>, lateral trabeculæ + of entosternite, netted; <i>Ph.</i>, position of pharynx.</p> + <p class="sp0">B, Diagram of cartilaginous skeleton of Ammocœtes. <i>Soft + cartilage</i>, sub-chordal cartilaginous bands, deep black; branchial basket-work (first + formed part), simply hatched; <i>hard cartilage</i>, cranio-facial skeleton, trabeculæ, + parachordals and auditory capsules, netted; <i>Inf.</i>, position of tube of infundibulum (old + œsophagus).</p> + </div> + </div> + + <p class="ac"><span class="sc">The Soft Cartilage of the Branchial Skeleton of + Ammocœtes.</span></p> + + <p>The study of Ammocœtes gives yet another clue to the nature of the earliest skeleton, + for these two marked groups of cartilage—the branchial and basi-cranial—are + characterized by a difference in structure as well as a difference in topographical position. J. + Müller was the first to point out that these two sets of cartilages differ in appearance and + constitution, and he gave to them the name of yellow and grey cartilage. Parker has described them + fully under the terms soft and hard cartilage, terms which Schaffer has also used, and I shall + also make use of them here. The whole of the branchial cartilaginous skeleton is composed of soft + cartilage, while the basi-cranial skeleton, consisting of trabeculæ, parachordals, and auditory + capsule, is composed <span class="pagenum" id="page127">{127}</span>of hard cartilage, the only + soft cartilage in this region being that which forms the nasal capsule, not represented in Fig. <a + href="#fig53">53</a>, B.</p> + + <p>These two groups of cartilage arise independently, so that at first the basi-cranial system is + quite separate from the branchial, and only late in the history of the animal is a junction + effected between the branchial system and the trabeculæ and parachordals, an initial separation + which is especially striking when we consider that in this animal all the cartilaginous structures + of any one system are continuous: there is no sign of anything in the nature of joints.</p> + + <p>Of these two main groups, the branchial cartilages are formed first in the embryo, a fact which + suggests that they are the most primitive of the vertebrate cartilages, and that, therefore, the + first true formation of cartilage in the invertebrate ancestor may be looked for in the shape of + bars supporting the branchial mechanism. The evidence of the origin of the cartilaginous + structures in Ammocœtes is given by Shipley in the following words<span + class="wnw">:—</span></p> + + <p>"The branchial bases are the first part of the skeleton to appear. They arise about the 24th + day as straight bars of cartilage, lying external and slightly posterior to the branchial + vessel.</p> + + <p>"The first traces of the basi-cranial skeleton appear on the 30th day as two rods of + cartilage—the trabeculæ."</p> + + <p>Our attention must, in the first place, be directed to this branchial basket-work of + Ammocœtes.</p> + + <p>Underlying the skin of Ammocœtes in the branchial region is situated the sheet of + longitudinal body-muscles, divided into a series of segments or myotomes, which forms the somatic + muscles so characteristic of all fishes. This muscular sheet is depicted on the left-hand side of + Fig. <a href="#fig54">54</a>. It does not extend over the lower lip or over that part in the + middle line where the thyroid gland is situated. In these parts a sheet of peculiar tissue known + by the name of muco-cartilage lies immediately under the skin, covering over the thyroid gland and + lower lip. The somatic muscular sheet with the superjacent skin can be stripped off very easily + owing to the vascularity and looseness of the tissue immediately underlying it. When this is done + the branchial basket-work comes beautifully into view as is seen on the right-hand side of Fig. <a + href="#fig54">54</a>. It forms a cage within which the branchiæ and their muscles lie entirely + concealed.</p> + + <p>This is the great characteristic of this most primitive form of the branchial cartilaginous + bars and distinguishes it from the branchial <span class="pagenum" id="page128">{128}</span>bars + of other higher fishes, in that it forms a system of cartilages which lie external to the + branchiæ—an extra-branchial system.</p> + + <p>This branchial basket-work is simpler in Ammocœtes than in Petromyzon, and its actual + starting-point consists of a main transverse bar corresponding to each branchial segment; from + this transverse bar the system of longitudinal bars by which the basket-work is formed has sprung. + These transverse bars arise from a cartilaginous longitudinal rod, situated close against the + notochord on each side. These rods may be called the subchordal cartilaginous bands (Fig. <a + href="#fig53">53</a>), and, according to the observations of Schneider and others, each subchordal + band does not form at first a continuous cartilaginous rod, but the cartilage is conspicuous only + at the places where the transverse bars arise. In the youngest Ammocœtes examined by + Schaffer, he could find no absolute discontinuity of the cartilage except between the first two + transverse bars, but he says that the thinning between the transverse bars was so marked as to + make it highly probable that at an earlier stage there was discontinuity. The whole system of + branchial bars and subchordal rods is at first absolutely disconnected from the cranial system of + trabeculæ and parachordals, and only later do the two systems join.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig054.png" id="fig54"><img style="width:47%" src="images/fig054.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 54.—Ventral View of Head Region of Ammocœtes.</span></p> + <p class="sp0"><i>Th.</i>, thyroid gland; <i>M.</i>, lower lip, with its muscles.</p> + </div> + </div> + + <p>These observations on Ammocœtes lead most definitely to the conclusion that the + starting-point of the whole cartilaginous skeleton of the vertebrate consisted of a series of + transverse cartilaginous bars, for the purpose of supporting branchial segments; these were + connected with two axial longitudinal cartilaginous rods, which at first contained cartilage only + near the places of junction of the branchial <span class="pagenum" id="page129">{129}</span>bars. + This system may be called the mesosomatic skeleton, as it is entirely confined to the branchial or + mesosomatic region.</p> + + <p class="sp3">In addition to this primitive cartilaginous framework, which was formed for the + support of the mesosomatic or respiratory segments, but at a slightly later period in the + phylogenetic history, a separate cartilaginous system was formed for the support of the prosomatic + segments, viz. the trabeculæ and parachordals with the auditory capsules: a system which was at + first entirely separated from the mesosomatic, and, as we shall see, is more advanced in structure + than the branchial system. Later still, the story is completed at the time of transformation to + Petromyzon by the formation of the simple cartilaginous skull and the rudimentary vertebræ, the + structure of which is also of a more advanced type.</p> + + <p class="ac"><span class="sc">The Structure of the Soft Branchial Cartilage.</span></p> + + <p>Having considered the topographical position of the primitive branchial cartilaginous skeleton, + we may now inquire, What was its structure and how was it formed?</p> + + <p>In the higher vertebrates various forms of cartilage are described, viz. hyaline, + fibro-cartilage, elastic cartilage, and parenchymatous cartilage. Of these, the parenchymatous + cartilage is looked upon as the most primitive form, because it preserves without modification the + characters of embryonic cartilage.</p> + + <p>Embryology, then, would lead to the belief that the earliest form of cartilage in the + vertebrate kingdom ought to be of this type, viz. large cells, each of which is enclosed in a + simple capsule, so that the capsules of the cells form the whole of the matrix, and thus form a + simple homogeneous honeycomb-structure, in the alveoli of which the cartilage-cells lie singly. + If, then, the branchial cartilages of Ammocœtes are, as has just been argued, the + representatives of the cartilaginous skeleton of the primitive vertebrate, it is reasonable to + suppose that they should resemble in structure this embryonic cartilage. Such is undoubtedly the + case: all observers who have described the branchial basket-work of Ammocœtes or Petromyzon + have been struck with the extremely primitive character of the cartilage, and the last observer + (Schaffer) describes it as composed of thin walls of homogeneous material, in which there are no + lines of separation, which form a simple honeycomb-structure, in the alveoli <span class="pagenum" + id="page130">{130}</span>of which the separate cells lie singly. These branchial cartilages are + each surrounded by a layer of perichondrium, and in Fig. <a href="#fig55">55</a>, A, I give a + picture of a section of a portion of one of the bars.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig055.jpg" id="fig55"><img style="width:100%" src="images/fig055.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 55.—A, Branchial Cartilage of Ammocœtes, + stained with Thionin. B, Branchial Cartilage of Limulus, stained with Thionin.</span></p> + </div> + </div> + + <p class="sp3">Hence we see that structurally as well as topographically the branchial bars of + Ammocœtes justify their claim to be considered as the origin of the vertebrate + cartilaginous framework.</p> + + <p class="ac"><span class="sc">On the Structure of the Muco-cartilage in + Ammocœtes.</span></p> + + <p>We can, however, go further than this, and ask how this cartilage itself is formed in + Ammocœtes? The answer is most definite, most instructive and suggestive, for in all cases + this particular kind of cartilage is formed from, or at all events in, a peculiar fibrous tissue, + which was called by Schneider "Schleim-Knorpel," or muco-cartilage, a tissue which is + distinguishable from other connective tissues, not only by its structural peculiarities, but also + by its strong affinity for all dyes which differentiate mucoid or chondro-mucoid substances.</p> + + <p>This muco-cartilage is thus described by Schneider:—The perichondrium in Ammocœtes + is not confined to the true cartilaginous structures, but extends itself in the form of thin + plates in definite directions. Between these plates of perichondrium a peculiar tissue (Fig. <a + href="#fig56">56</a>)—the muco-cartilage—exists, consisting of fibrillæ, whose + direction is mainly at right angles to the planes of the perichondrial plates, with star-shaped + cells in among them, and with the spaces between the fibrillæ filled up with a semi-fluid + mass.</p> + + <div><span class="pagenum" id="page131">{131}</span></div> + + <p>From this tissue all the primitive cartilages which resemble the branchial bars are formed, + either by the invasion of chondroblasts from the surrounding perichondrium, or by the + proliferation and encapsulation of the cells of the muco-cartilage itself.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig056.png" id="fig56"><img style="width:100%" src="images/fig056.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 56.—Section of Muco-cartilage from Dorsal + Head-plate of Ammocœtes.</span></p> + </div> + </div> + + <p>This very distinctive tissue—the muco-cartilage—is of very great importance in all + questions of the origin of the skeletal tissues. In all descriptions of the skeletal tissues it + has been practically disregarded until recent years when, besides my own observations, its + distribution has been mapped out by Schaffer. Thus Parker, in his well-known description of the + skeleton of the marsipobranch fishes, does not even mention its existence. Its importance is shown + by its absolute disappearance at transformation and its non-occurrence in any of the higher + vertebrates. It is entirely confined to the head-region, and its distribution there is most + suggestive, for, as will be described fully later on, it forms a skeleton which both in structure + and position resembles very closely the head-shields of cephalaspidian fishes. At the present part + of my argument its more immediate interest lies in the method of tracing this tissue. For this + purpose I made use of the micro-chemical reaction of thionin, a dye which, as shown by Hoyer, + stains all mucin-containing substances a bright purple. Schaffer made use of a corresponding + basophil stain, hæmalum. When stained with thionin, the matrix, or ground-substance of the + branchial cartilages as well as the matrix or semi-fluid substance in which the fibrils of the + muco-cartilaginous cells are embedded take on a deep purple colour, while the fibrous material of + the cranial walls and other connective tissue strands, such as the perichondrium, are coloured + light blue. Muco-cartilage, then, may be described as a peculiar form of connective tissue which + differs from other connective tissue not only in its appearance but in <span class="pagenum" + id="page132">{132}</span>its chemical composition, for unlike white fibrous tissue it contains a + large amount of mucin, and this tissue is the forerunner of the earliest cartilaginous vertebrate + skeleton, the branchial bars of Ammocœtes.</p> + + <p>The conclusions to which we are led by the study of the structure, position, and mode of origin + of these primitive cartilages of Ammocœtes may be thus summed up<span + class="wnw">:—</span></p> + + <p>1. The immediate ancestor of the vertebrate must have possessed a peculiar fibrous + tissue—the ground-substance of which stained deep purple with thionin—in which + cartilage arose.</p> + + <p>2. The cartilage so formed was not like hyaline cartilage, but resembled in a striking manner + parenchymatous cartilage.</p> + + <p class="sp3">3. This cartilage was situated partly in two axial longitudinal bands, partly as + transverse bars, which supported the branchial apparatus.</p> + + <p class="ac"><span class="sc">The Prosomatic or Basi-cranial Skeleton of + Ammocœtes.</span></p> + + <p>Before searching for any evidence of a similar tissue in any invertebrate group, it is + advisable to consider the other portion of the cartilaginous skeleton of Ammocœtes, which + consists of the trabeculæ, parachordals and auditory capsules—the basi-cranial + skeleton—and is composed of hard, not soft cartilage.</p> + + <p>This basi-cranial skeleton represented in Fig. <a href="#fig53">53</a>, B, is confined to the + region of the notochord, the cranial walls being composed entirely of a white fibrous membrane. It + is separated at first entirely from the sub-chordal portion of the branchial basket-work, and is + composed of a foremost part, the trabeculæ (<i>Tr.</i>), and of a hindermost part, the + parachordals (<i>Pr.ch.</i>), which are characterized by the attachment on each side of the large + auditory capsule (<i>Au.</i>). In Ammocœtes the trabecular bars are continuous with the + parachordals, the junction being marked by a small lateral projection on each side, which at + transformation is seen to play an important part in the formation of the sub-ocular arch. The + trabecular bar lies close against the notochord on each side up to its termination; it then bends + away from the middle line and curves round until it meets its fellow on the opposite side, thus + forming, as it were, the head of a racquet of which the notochord forms the splice in the handle. + The strings of the racquet are represented by a thin membrane, in the centre of which the position + of the infundibulum (<i>Inf.</i>) of the <span class="pagenum" id="page133">{133}</span>brain can + be clearly seen. In an earlier stage of Ammocœtes the two trabecular horns do not meet, but + are separated by connective tissue, which afterwards becomes cartilaginous.</p> + + <p>As far, then, as the topography of this basi-cranial skeleton is concerned, the striking points + are—the shape of the trabecular portion, diverging as it does around the infundibulum, and + the presence on the parachordal portion of the two large auditory capsules.</p> + + <p>These two points indicate, on the hypothesis that infundibulum and œsophagus are + convertible terms, that two supporting structures of a cartilaginous nature must have existed in + the ancestor of the vertebrate, the first of which surrounded the œsophagus, and the second + was in connection with its auditory apparatus.</p> + + <div class="ac w45 fcenter sp3"> + <a href="images/fig057.jpg" id="fig57"><img style="width:100%" src="images/fig057.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 57.—A, Cartilage of Trabeculæ of Ammocœtes, + stained with Hæmatoxylin and Picric Acid. B, Nests of Cartilage Cells in Entosternite of + Hypoctonus, stained with Hæmatoxylin and Picric Acid.</span></p> + </div> + </div> + + <p class="ac"><span class="sc">Structure of the Hard Cartilages.</span></p> + + <p>The structure of this hard cartilage of the trabeculæ and auditory capsules resembles that of + the soft, in so far that it consists of large cells with a comparatively small amount of + intercellular substance. Schaffer, who has described it lately, considers that it is a nearer + approach to hyaline cartilage than the soft, but yet cannot be called hyaline cartilage in the + usual sense of the term. Its peculiarities and its differences from the soft are especially well + seen by its staining reactions. I have myself been particularly struck with the effect of + picrocarmine or combined hæmatoxylin and picric acid <span class="pagenum" + id="page134">{134}</span>staining (Fig. <a href="#fig57">57</a>). In the case of the soft + cartilage the capsular substance stains respectively a brilliant red or blue, while that of the + hard cartilage is coloured a deep yellow, so that the junction between the parachordals and the + branchial cartilages is beautifully marked out. Then, again, with thionin, which gives so marked a + reaction in the case of the soft cartilage, the hard cartilage of the auditory capsule is not + stained at all, and in the trabeculæ the deep purple colour is confined to the mucoid + cement-substance between the capsules, just as Schaffer has stated. The same kinds of reactions + have been described by Schaffer: thus by double staining with hæmalum-eosin the hard cartilage + stains red, the soft blue; and he points out that even with over-staining by hæmalum the auditory + capsule remains colourless, just as I have noticed with thionin. He infers, precisely as I have + done from the thionin reaction, that chondro-mucoid, which is so marked a constituent of the soft + cartilage and of the muco-cartilage, is absent or present in but slight quantities in the hard + cartilage. Similarly, he points out that double staining with tropœolin-methyl-violet + stains the hard cartilage a bright orange colour, and the soft cartilage a violet.</p> + + <p>The evidence, then, shows clearly that a marked chemical difference exists between these two + cartilages, which may be expressed by saying that the one contains very largely a basophil + substance, which we may speak of as belonging to the class of chondro-mucoid substances, while the + other contains mainly an oxyphil substance, probably a chondro-gelatine substance.</p> + + <p>We may perhaps go further and attribute this difference of composition to a difference of + origin; for whereas the soft cartilage is invariably formed in a special tissue, the + muco-cartilage, which shows by its reaction how largely it is composed of a mucoid substance, the + hard cartilage is certainly, in the case of the cartilage of the cranium where its origin has been + clearly made out, formed in the membranous tissue of the cranium of + Ammocœtes—<i>i.e.</i> in a tissue which stains light blue with thionin, and contains + a gelatinous rather than a mucoid substratum.</p> + + <p>The best opportunity of finding out the mode of origin of the hard cartilage is afforded at the + time of transformation, when so much of this kind of cartilage is formed anew. Unfortunately, it + is very difficult to obtain the early transformation stages, consequently we cannot be said to + possess any really exhaustive and <span class="pagenum" id="page135">{135}</span>definite account + of how the new cartilages are formed. Bujor, Kaensche, and Schaffer all profess to give a more or + less definite account of their formation, and the one striking impression left on the mind of the + reader is how their descriptions vary. In one point only are they agreed, and in that I also agree + with them, viz. the manner in which the new cranial walls are formed. Schaffer describes the + process as the invasion of chondroblasts into the homogeneous fibrous tissue of the cranial walls. + Such chondroblasts not only form the cartilaginous framework, but also assimilate the fibrous + tissue which they invade, so that finally all that remains of the original fibrous matrix in which + the cartilage was formed are these lines of cement-substance between the groups of cartilage + cells, which, containing some basophil material, are marked out, as already mentioned (Fig. <a + href="#fig57">57</a>).</p> + + <p>We may therefore conclude, from the investigation of Ammocœtes, that the front part of + the basi-cranial skeleton arose as two trabecular bars, to which muscles were attached, situated + bilaterally with respect to the central nervous system. These bars were composed of tendinous + material with a gelatinous rather than a mucoid substratum, in which nests of cartilage-cells were + formed, the cartilaginous material formed by these cells being of the hard variety, not staining + with thionin, and staining yellow with picro-carmine, etc. By the increase of such nests and the + assimilation of the intermediate fibrous material, the original fibro-cartilage was converted into + the close-set semi-hyaline cartilage of the trabeculæ and auditory capsules, in which the fibrous + material still marks out by its staining-reaction the limits of the cell-clusters.</p> + + <p>Such I gather to be Schaffer's conclusions, and they are certainly borne out by my own and Miss + Alcock's observations. As far as we have had an opportunity of observing at present, the first + process at transformation appears to consist of the invasion of the fibrous tissue of the cranial + wall by groups of cells which form nests of cells between the fibrous strands. These nests of + cells form round themselves capsular material, and thus form cell-territories of cartilage, which + squeeze out and assimilate the surrounding fibrous tissue, until at last all that remains of the + original fibrous matrix is the lines of cement-substance which mark out the limits of the various + cell-groups.</p> + + <p>At present I am inclined to think that both soft and hard cartilage originate in a very similar + manner, viz. by the formation of capsular <span class="pagenum" id="page136">{136}</span>material + around the invading chondroblasts, and that the difference in the resulting cartilage is mainly + due to the difference in chemical composition of the matrix of the connective tissue which is + invaded. Thus the difference may be formulated as follows<span class="wnw">:—</span></p> + + <p>The hard cartilage is formed by the invasion of chondroblasts into a fibrous tissue, which + contains a gelatinous rather than a mucoid substratum, in contradistinction to the soft cartilage + which is formed, probably also by the invasion of chondroblasts, in a tissue—the + muco-cartilage—which contains a specially mucoid substratum.</p> + + <p class="sp3">Such, then, is the very clearly defined starting-point of the vertebrate + skeleton—two distinct formations of different histological and chemical structure,—the + one forming a segmented branchial skeleton, the other a non-segmented basi-cranial skeleton.</p> + + <p class="ac"><span class="sc">The Cartilaginous Skeleton of Limulus.</span></p> + + <p>Among the whole of the invertebrates at present living on the earth, is there any sign of an + internal cartilaginous skeleton that will give a direct clue to the origin of the primitive + vertebrate skeleton? The answer to this question is most significant: only one animal among all + those at present known possesses a cartilaginous skeleton, which is directly comparable with that + of Ammocœtes, and here the comparison is very close—only one animal among the + thousands of living invertebrate forms, and that animal is the only representative still surviving + of the palæostracan group, which was the dominant race when the vertebrate first made its + appearance. The Limulus, or king-crab, possesses a segmented branchial internal cartilaginous + skeleton (Fig. <a href="#fig53">53</a>, A), made up of the same kind of cartilage as the branchial + skeleton of Ammocœtes, confined to the mesosomatic or branchial region, just as in + Ammocœtes, forming, as in Ammocœtes, cartilaginous bars supporting the branchiæ, and + these bars are situated externally to the branchiæ, as in Ammocœtes. In addition this + animal possesses a basi-cranial internal semi-cartilaginous unsegmented plate known as the + entosternite or plastron situated, with respect to the œsophagus, similarly to the position + of the trabeculæ with respect to the infundibulum in Ammocœtes. Moreover, the cartilaginous + cells in this tissue differ from those in the branchial region, in precisely the same manner as + the hard cartilage differs from the soft in Ammocœtes.</p> + + <div><span class="pagenum" id="page137">{137}</span></div> + + <p class="sp3">This plastron, it is true, is found in other animals, all of which are members of + the scorpion tribe, except in one instance, and this, strikingly enough, is the crustacean + Apus—a strange primitive form, which is acknowledged to be the nearest representative of the + Trilobita still living on the earth. None of these forms, however, possess any sign of an internal + cartilaginous branchial skeleton, such as is possessed by Limulus. Scorpions, Apus, Limulus, are + all surviving types of the stage of organization which had been reached in the animal world when + the vertebrate first appeared.</p> + + <p class="ac"><span class="sc">The Mesosomatic or Respiratory Skeleton of Limulus, composed of + Soft Cartilage.</span></p> + + <p>Searching through the literature of the histology of the cartilaginous tissues in invertebrate + animals, to see whether any cartilage had been described similar to that seen in the branchial + cartilages of Ammocœtes, and whether such cartilage, if found, arose in a fibrous tissue + resembling muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's article on the + tropho-skeletal tissues of Limulus, a picture of the cartilage of Limulus, which would have passed + muster for a drawing of the branchial cartilage of Ammocœtes. This clue I followed out in + the manner described in my former paper in the <i>Journal of Anatomy and Physiology</i>, and + mapped out the topography of this remarkable tissue.</p> + + <p>Limulus, like other water-dwelling arthropods, breathes by means of gills attached to its + appendages. These gill-bearing appendages are confined to the mesosomatic region, as is seen in + Fig. <a href="#fig59">59</a>; and these appendages are very different to the ordinary locomotor + appendages, which are confined to the prosomatic region. Each appendage, as is seen in Fig. <a + href="#fig58">58</a>, consists mainly of a broad, basal part, which carries the gill-book on its + under surface; the distal parts of the appendage have dwindled to mere rudiments and still exist, + not for locomotor purposes, but because they carry on each segment organs of special importance to + the animal (see Chapter XI.). As is seen in Fig. <a href="#fig58">58</a>, the basal parts of each + pair of appendages form a broad, flattened paddle, by means of which the animal is able to swim in + a clumsy fashion. Very striking and suggestive is the difference between these gill-bearing + mesosomatic appendages and the non-gill-bearing locomotor appendages of the prosoma.</p> + + <div><span class="pagenum" id="page138">{138}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig058.png" id="fig58"><img style="width:100%" src="images/fig058.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 58.—Transverse Section through the Mesosoma of Limulus, to show + the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or Branchial + Appendage.</span></p> + <p class="sp0">In each figure the branchial cartilaginous bar, <i>Br.C.</i>, has been exposed + by dissection on one side. <i>Ent.</i>, entapophysis; <i>Ent.l.</i>, entapophysial ligament + cut across; <i>Br.C.</i>, branchial cartilaginous bar, which springs from the entapophysis; + <i>H.</i>, heart; <i>P.</i>, pericardium; <i>Al.</i>, alimentary canal; <i>N.</i>, nerve cord; + <i>L.V.S.</i>, longitudinal venous sinus; <i>Dv.</i>, dorso-ventral somatic muscle; + <i>Vp.</i>, veno-pericardial muscle.</p> + </div> + </div> + + <p>At the base of each of these appendages, where it is attached to the body of the animal, the + external chitinous surface is characterized by a peculiar stumpy, rod-like marking, and upon + removing the chitinous covering, this surface-appearance is seen to correspond to a well-marked + rod of cartilage (<i>Br.C.</i>), which extends from the body <span class="pagenum" + id="page139">{139}</span>of the animal well into each appendage. This bar of cartilage arises on + each side from the corresponding entapophysis (<i>Ent.</i>), which is the name given to a + chitinous spur which projects a short distance (Fig. <a href="#fig58">58</a>, B) into the animal + from the dorsal side, for the purpose of giving attachment to various segmental muscles. These + entapophyses are formed by an invagination of the chitinous surface on the dorsal side and are + confined to the mesosomatic region, so that the mesosomatic carapace indicates, by the number of + entapophyses, the number of segments in that region, in contradistinction to the prosomatic + carapace, which gives no indication on its surface of the number of its components.</p> + + <p>Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each + spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial + ligament (<i>Ent.l.</i>) (Fig. <a href="#fig58">58</a>), and in this tissue cartilage is formed. + Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a + concentration of such groups always takes place at each entapophysis, forming here a solid mass of + cartilage, from which the massive cartilaginous bar of each branchial appendage arises.</p> + + <p class="sp3">Further, not only is this cartilage exactly similar to parenchymatous cartilage, as + it occurs in the branchial cartilages of Ammocœtes, but also its matrix stains a brilliant + purple with thionin in striking contrast to the exceedingly slight light-blue colour of the + surrounding perichondrium. In its chemical composition it shows, as might be expected, that it is + a cartilage containing a very large amount of some mucin-body.</p> + + <p class="ac"><span class="sc">The Muco-cartilage of Limulus.</span></p> + + <p>The resemblance between this structure and that of the branchial bars of Ammocœtes does + not end even here, for, as already mentioned, the cartilage originates in a peculiar connective + tissue band, the entapophysial ligament, and this tissue bears the same relation in its chemical + reactions to the ordinary connective tissue of Limulus, as muco-cartilage does to the white + fibrous tissue of Ammocœtes. The white connective tissue of Limulus, as already stated, + resembles that of the vertebrate more than does the connective tissue of any other invertebrate, + and, similarly to that of Ammocœtes, does not stain, or gives only a light-blue tinge with + thionin. The tissue of <span class="pagenum" id="page140">{140}</span>the entapophysial ligament, + on the contrary, just like muco-cartilage, takes on an intense purple colour when stained with + thionin. It possesses a mucoid substratum, just as does muco-cartilage, and in both cases a + perfectly similar soft cartilage is born from it.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig059.jpg" id="fig59"><img style="width:100%" src="images/fig059.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 59.—Diagram of Limulus, to show the Nerves to the + Appendages (1-13) and the Branchial Cartilages.</span></p> + <p class="sp0">The branchial cartilages and the entapophysial ligaments are coloured blue, the + branchiæ red. <i>gl.</i>, generative and hepatic glands surrounding the central nervous system + and passing into the base of the flabellum (<i>fl.</i>).</p> + </div> + </div> + + <p>One difference, however, exists between the branchial cartilages of these two animals; the + innermost axial layer of the branchial bar of Limulus is very apt to contain a specially hard + substance, apparently chalky in nature, so that it breaks up in sections, and gives the appearance + of a broken-down spongy mass; if, however, the tissue is first placed in a solution of + hydrochloric acid, it then cuts easily, and the whole tissue is seen to be of the same structure + throughout, the main difference being that the capsular spaces in the axial region are much larger + and much more free from cell-protoplasm than are those of the smaller younger cells near the + periphery.</p> + + <div><span class="pagenum" id="page141">{141}</span></div> + + <p>I have attempted in Fig. <a href="#fig53">53</a> to represent this close resemblance between + the segmented branchial skeleton of Limulus and of Ammocœtes, a resemblance so close as to + reach even to minute details, such as the thinning out of the cartilage in the subchordal bands + and entapophysial ligaments respectively between the places where the branchial bars come off.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig060.jpg" id="fig60"><img style="width:100%" src="images/fig060.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 60.—Diagram of Ammocœtes cut open to show the + Lateral System of Cranial Nerves</span> <i>V., VII., IX., X.</i>, <span class="sc">and the + Branchial Cartilages.</span></p> + <p class="sp0">The branchial cartilages and sub-chordal ligaments are coloured blue, the + branchiæ red. <i>gl.</i>, glandular substance surrounding the central nervous system and + passing into the auditory capsule with the auditory nerve (<i>VIII.</i>).</p> + </div> + </div> + + <p>In Fig. <a href="#fig59">59</a> I have shown the prosoma and mesosoma of Limulus, and indicated + the nerves to the appendages together with the mesosomatic cartilaginous skeleton.</p> + + <p class="sp3">In Fig. <a href="#fig60">60</a> I have drawn a corresponding picture of the + prosomatic and mesosomatic region of Ammocœtes with the corresponding nerves <span + class="pagenum" id="page142">{142}</span>and cartilages. In this figure the animal is supposed to + be slit open along the ventral mid-line and the central nervous system exposed.</p> + + <p class="ac"><span class="sc">The Prosomatic Skeleton of Limulus, composed of Hard + Cartilage.</span></p> + + <p>The rest of the primitive vertebrate skeleton arose in the prosomatic region, and formed a + support for the base of the brain. This skeleton was composed of hard cartilage, and arose in + white fibrous tissue containing gelatin rather than mucin.</p> + + <p>Is there, then, any peculiar tissue of a cartilaginous nature in Limulus and its allies, + situated in the prosomatic region, which is entirely separate from the branchial cartilaginous + skeleton, which acts as a supporting internal framework, and contains a gelatinous rather than a + mucoid substratum?</p> + + <p>It is a striking fact, common to the whole of the group of animals to which our inquiries, + deduced from the consideration of the structure of Ammocœtes, have, in every case, led us + in our search for the vertebrate ancestor, that they do possess a remarkable internal + semi-cartilaginous skeleton in the prosomatic region, called the entosternite or plastron, which + gives support to a large number of the muscles of that region; which is entirely independent of + the branchial skeleton, and differs markedly in its chemical reactions from that cartilage, in + that it contains a gelatinous rather than a mucoid substratum.</p> + + <p>In Limulus it is a large, tough, median plate, fibrous in character, in which are situated rows + and nests of cartilage-cells. The same structure is seen in the plastron of Hypoctonus, of + Thelyphonus, and to a certainty in all the members of the scorpion group. Very different is the + behaviour of this tissue to staining from that of the branchial region. No part of the plastron + stains purple with thionin; it hardly stains at all, or gives only a very slight blue colour. In + its chemical composition there is a marked preponderance of gelatin with only a slight amount of a + mucin-body. In some cases, as in Hypoctonus (Fig. <a href="#fig57">57</a>, B) and Mygale, the + capsules of the cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while the + fibres between the cell-nests stain a blue-brown colour, partly from the hæmatoxylin, partly from + the picric acid.</p> + + <p>All the evidence points to the plastron as resembling the basi-cranial skeleton of + Ammocœtes in its composition and in the origin <span class="pagenum" + id="page143">{143}</span>of its cells in a white fibrous tissue. What, then, is its topographical + position? It is in all cases a median structure lying between the cephalic stomach and the + infra-œsophageal portion of the central nervous system, and in all cases it possesses two + anterior horns which pass around the œsophagus and the nerve-masses which immediately + enclose the œsophagus (Fig. <a href="#fig61">61</a>, A). These lateral horns, then, which + lie laterally and slightly ventral to the central nervous system, and are called by Ray Lankester + and Benham the sub-neural portion of the entosternite, are very nearly in exactly the position of + the racquet-shaped head of the trabeculæ in Ammocœtes. It is easy to see that, with a more + extensive growth of the nervous material dorsally, such lateral horns might be caused to take up a + still more ventral position. Now, these two lateral horns of the plastron of Limulus are continued + along its whole length so as to form two thickened lateral ridges, which are conspicuous on the + flat surface of the rest of this median plate. In other cases, as in the Thelyphonidæ, the + plastron consists mainly of these two lateral ridges or trabeculæ, as they might be called, and + Schimkéwitsch, who more than any one else has made a comparative study of the entosternite, + describes it as composed in these animals of two lateral trabeculæ crossed by three transverse + trabeculæ. I myself can confirm his description, and give in Fig. <a href="#fig61">61</a>, B, the + appearance of the entosternite of Thelyphonus or of Hypoctonus. The supra-œsophageal + ganglia and part of the infra-œsophageal ganglia fill up the space <i>Ph.</i>; stretching + over the rest of the infra-œsophageal mass is a transverse trabecula, which is very thin; + then comes a space in which is seen the rest of the infra-œsophageal mass, and then the + posterior part of the plastron, ventrally to which lies the commencement of the ventral + nerve-cord.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig061.png" id="fig61"><img style="width:100%" src="images/fig061.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 61.—A, Entosternite of Limulus; B, Entosternite of + Thelyphonus.</span></p> + <p class="sp0"><i>Ph.</i>, position of pharynx.</p> + </div> + </div> + + <div><span class="pagenum" id="page144">{144}</span></div> + + <p>In these forms, in which the central nervous system is more concentrated towards the cephalic + end than in Limulus, the whole of the concentrated brain-mass is separated from the gut only by + this thin transverse band of tissue. Judging, then, from the entosternite of Thelyphonus, it is + not difficult to suppose that a continuation of the same growth of the brain-region of the central + nervous system would cause the entosternite to be separated into two lateral trabeculæ, which + would then take up the ventro-lateral position of the two trabeculæ of Ammocœtes.</p> + + <p>On the other hand, it might be that two lateral trabeculæ, similar to those of Thelyphonus and + situated on each side of the central nervous system, were the original form from which, by the + addition of transverse fibres running between the gut and nervous system, the entosternite of + Thelyphonus and of the scorpions, etc., was formed. From an extensive consideration of the + entosternite in different animals, Schimkéwitsch has come to the conclusion that this latter + explanation is the true one. He points out that the lateral trabeculæ can be distinguished from + the transverse by their structure, being much more cellular and less fibrous, and the + cell-cavities more rounded, or, as I should express it, the two lateral trabeculæ are more + cartilaginous, while the transverse are more fibrous. Schimkéwitsch, from observations of + structure and from embryological investigations, comes to the conclusion that the entosternite was + originally composed of two parts—</p> + + <p>1. A transverse muscle corresponding to the adductor muscle of the shell of certain + crustaceans, such as Nebalia.</p> + + <p>2. A pair of longitudinal mesodermic tendons, which may have been formed originally out of a + number of segmentally arranged mesodermic tendons, and are crossed by the fibrils of the + transverse muscular bundles.</p> + + <p>These paired tendons of the entosternite he considers to correspond to the intermuscular + tendons, situated lengthways, which are found in the ventral longitudinal muscles of most + arthropods.</p> + + <p>It is clear from these observations of Schimkéwitsch, that the essential part of the + entosternite consists of two lateral trabeculæ, which were originally tendinous in nature and have + become of the nature of cartilaginous tissue by the increase of cellular elements in the matrix of + the tissue: these two trabeculæ function as supports for the attachment of muscles, which are + specially attached at certain places. At these places transverse fibres belonging to some <span + class="pagenum" id="page145">{145}</span>of the muscular attachments cross between the two + longitudinal trabeculæ, and so form the transverse trabeculæ.</p> + + <p>I entirely agree with Schimkéwitsch that the nests of cartilage-cells are much more extensive + in, and indeed nearly entirely confined to, these two lateral trabeculæ in the entosternite of + Hypoctonus. Ray Lankester describes in the entosternite of Mygale peculiar cell-nests strongly + resembling those of Hypoctonus, and he also states that they are confined to the lateral portions + of the entosternite.</p> + + <p>From this evidence it is easy to see that that portion of the basi-cranial skeleton known as + the trabeculæ may have originated from the formation of cartilage in the plastron or entosternite + of a palæostracan animal. Such an hypothesis immediately suggests valuable clues as to the origin + of the cranium and of the rest of the basi-cranial skeleton—the parachordals and the + auditory capsules. The former would naturally be a dorsal extension of the more membranous portion + of the plastron, in which, equally naturally, cartilaginous tissue would subsequently develop; and + the reason why it is impossible to reduce the cranium into a series of segments would be + self-evident, for even though, as Schimkéwitsch thinks, the plastron may have been originally + segmented, it has long lost all sign of segmentation. The latter would be derived from a second + entosternite of the same nature as the plastron, but especially connected with the auditory + apparatus of the invertebrate ancestor. The following out of these two clues will be the subject + of a future chapter.</p> + + <p class="sp3">In our search, then, for a clue to the origin of the skeletal tissues of the + vertebrate we see again that we are led directly to the palæostracan stock on the invertebrate + side and to the Cyclostomata on that of the vertebrate; for in Limulus, the only living + representative of the Palæostraca, and in Limulus alone, we find a skeleton marvellously similar + to the earliest vertebrate skeleton—that found in Ammocœtes. Later on I shall give + reasons for the belief that the earliest fishes so far found, the Cephalaspidæ, etc., were built + up on the same plan as Ammocœtes, so that, in my opinion, in Limulus and in + Ammocœtes we actually possess living examples allied to the ancient fauna of the Silurian + times.</p> + + <div><span class="pagenum" id="page146">{146}</span></div> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp2"> + <p>The skeleton considered in this chapter is not the notochord, but that composed of cartilage. + The tracing downwards of the vertebrate bony and cartilaginous skeleton to its earliest + beginnings leads straight to the skeleton of the larval lamprey (Ammocœtes), in which + vertebræ are not yet formed, but the cranial and branchial skeleton is well marked.</p> + <p>The embryological and phylogenetic histories are in complete unison to show that the cranial + skeleton is older than the spinal, and this primitive branchial skeleton is also in harmony with + the laws of evolution, in that its structure, even in the adult lamprey (Petromyzon), never gets + beyond the stage characteristic of embryonic cartilage in the higher vertebrates.</p> + <p>The simplest and most primitive skeleton is that found in Ammocœtes and consists of + two parts: (1) a prosomatic, (2) a mesosomatic skeleton.</p> + <p>The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the simplest + kind—the trabeculæ and the parachordals with their attached auditory capsules, just as the + embryology of the higher vertebrates teaches us must be the case. There in the free-living, + still-existent Ammocœtes we find the manifest natural outcome of the embryological + history in the shape of simple trabeculæ and parachordals, from which the whole complicated + basi-cranial skeleton of the higher vertebrates arose.</p> + <p>The mesosomatic skeleton, which is formed before the prosomatic, consisted, in the first + instance, of simple branchial bars segmentally arranged, which were connected together by a + longitudinal subchordal bar, situated laterally on each side of the notochord. These simple + branchial bars later on form the branchial basket-work, which forms an open-work cage within + which the branchiæ are situated.</p> + <p>The cartilages which compose these two skeletons respectively are markedly different in + chemical constitution, in that the first (hard cartilage) is mainly composed of chondro-gelatin, + the second (soft cartilage) of chondro-mucoid material.</p> + <p>The same kind of difference is seen in the two kinds of connective tissue which are the + forerunners of these two kinds of cartilage. Thus, the cranial walls in Ammocœtes are + formed of white fibrous tissue, an essentially gelatin-containing tissue; at transformation + these are invaded by chondro-blasts and the cartilaginous cranium, formed of hard cartilage, + results. On the other hand, the forerunner of the branchial soft cartilage is a very striking + and peculiar kind of connective tissue loaded with mucoid material, to which the name + muco-cartilage has been given.</p> + <p>The enormous interest of this muco-cartilage consists in the fact that it forms very + well-defined plates of tissue, entirely confined to the head-region, which are not found in any + higher vertebrate, not even in the adult form Petromyzon, for every scrap of the tissue as such + disappears at transformation.</p> + <p>It is this evidence of primitive non-vertebrate tissues, which occur in the larval but not in + the adult form, which makes Ammocœtes so valuable for the investigation of the origin of + vertebrates.</p> + <p class="sp0">The evidence, then, is extraordinarily clear as to the beginnings of the + vertebrate skeletal tissues.</p> + </div> + + <div><span class="pagenum" id="page147">{147}</span></div> + + <div class="bq1 smaller sp5"> + <p>In the invertebrate kingdom true cartilage occurs but scantily. There is a cartilaginous + covering of the brain of cephalopods. It is never found in crabs, lobsters, bees, wasps, + centipedes, butterflies, flies, or any of the great group of Arthropoda, except, to a slight + extent, in some members of the scorpion group, and more fully in one single animal, the + King-crab or Limulus: a fact significant of itself, but still more so when the nature of the + cartilage and its position in the animal is taken into consideration, for the identity both in + structure and position of this internal cartilaginous skeleton with that of Ammocœtes is + extraordinarily great.</p> + <p>Here, in Limulus, just as in Ammocœtes, an internal cartilaginous skeleton is found, + composed of two distinct parts: (1) prosomatic, (2) mesosomatic. As in Ammocœtes, the + latter consists of simple branchial bars, segmentally arranged, which are connected together on + each side by a longitudinal ligament containing cartilage—the entapophysial ligament. This + cartilage is identical in structure and in chemical composition with the soft cartilage of + Ammocœtes, and, as in the latter case, arises in a markedly mucoid connective tissue. The + former, as in Ammocœtes, consists of a non-segmental skeleton, the plastron, composed of + a white fibrous connective tissue matrix, an essentially gelatin-containing tissue, in which are + found nests of cartilage cells of the hard cartilage variety.</p> + <p>This remarkable discovery of the branchial cartilaginous bars of Limulus, together with that + of the internal prosomatic plastron, causes the original difficulty of deriving an animal such + as the vertebrate from an animal resembling an arthropod to vanish into thin air, for it shows + that in the past ages when the vertebrates first appeared on the earth, the dominant arthropod + race at that time, the members of which resembled Limulus, had solved the question; for, in + addition to their external chitinous covering, they had manufactured an internal cartilaginous + skeleton. Not only so, but that skeleton had arrived, both in structure and position, exactly at + the stage at which the vertebrate skeleton starts.</p> + <p>What the precise steps are by which chitin-formation gives place to chondrin-formation are + not yet fully known, but Schmiedeberg has shown that a substance, glycosamine, is derivable from + both these skeletal tissues, and he concludes his observations in the following words: "Thus, by + means of glycosamine, the bridge is formed which connects together the chitin of the lower + animals with the cartilage of the more highly organized creations."</p> + <p class="sp0">The evidence of the origin of the cartilaginous skeleton of the vertebrate points + directly to the origin of the vertebrate from the Palæostraca, and is of so strong a character + that, taken alone, it may almost be considered as proof of such origin.</p> + </div> + + <div><span class="pagenum" id="page148">{148}</span></div> + + <p class="ac">CHAPTER IV</p> + + <p class="ac"><i>THE EVIDENCE OF THE RESPIRATORY APPARATUS</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Branchiæ considered as internal branchial appendages.—Innervation of + branchial segments.—Cranial region older than spinal.—Three-root system of cranial + nerves, dorsal, lateral, ventral.—Explanation of van Wijhe's segments.—Lateral mixed + root is appendage-nerve of invertebrate.—The branchial chamber of + Ammocœtes.—The branchial unit, not a pouch but an appendage.—The origin of + the branchial musculature.—The branchial circulation.—The branchial heart of the + vertebrate.—Not homologous with the systemic heart of the arthropod.—Its formation + from two longitudinal venous sinuses.—Summary.</p> + </div> + + <p>The respiratory apparatus in all the terrestrial vertebrates is of the same kind—one + single pair of lungs. These lungs originate as a diverticulum of the alimentary canal. On the + other hand, the aquatic vertebrates breathe by means of a series of branchiæ, or gills, which are + arranged segmentally, being supported by the segmental branchial cartilaginous bars, as already + mentioned in the last chapter.</p> + + <p>The transition from the gill-bearing to the lung-bearing vertebrates is most interesting, for + it has been proved that the lungs are formed by the modification of the swim-bladder of fishes; + and in a group of fishes, the Dipnoi, or lung-fishes, of which three representatives still exist + on the earth, the mode of transition from the fish to the amphibian is plainly visible, for they + possess both lungs and gills, and yet are not amphibians, but true fishes. But for the fortunate + existence of Ceratodus in Australia, Lepidosiren in South America, and Protopterus in Africa, it + would have been impossible from the fossil remains to have asserted that any fish had ever existed + which possessed at the same moment of time the two kinds of respiratory organs, although from our + knowledge of the development of the amphibian we might have felt sure that such a transitional + stage must have existed. Unfortunately, there is at present no likelihood of any corresponding + transitional stage being discovered <span class="pagenum" id="page149">{149}</span>living on the + earth in which both the dorsal arthropod alimentary canal and the ventral vertebrate one should + simultaneously exist in a functional condition; still it seems to me that even if Ceratodus, + Lepidosiren, and Protopterus had ceased to exist on the earth, yet the facts of comparative + anatomy, together with our conception of evolution as portrayed in the theory of natural + selection, would have forced us to conclude rightly that the amphibian stage in the evolution of + the vertebrate phylum was preceded by fishes which possessed simultaneously lungs and gills.</p> + + <p class="sp3">In the preceding chapter the primitive cartilaginous vertebrate skeleton, as found + in Ammocœtes, was shown to correspond in a marvellous manner to the cartilaginous skeleton + of Limulus. In a later chapter I will deal with the formation of the cranium from the prosomatic + skeleton; in this chapter it is the mesosomatic skeleton which is of interest, and the + consideration of the necessary consequences which logically follow upon the supposition that the + branchial cartilaginous bars of Limulus are homologous with the branchial basket-work of + Ammocœtes.</p> + + <p class="ac"><span class="sc">Internal Branchial Appendages.</span></p> + + <p>Seeing that in both cases the cartilaginous bars of Limulus and Ammocœtes are confined + to the branchial region, their homology of necessity implies an homology of the two branchial + regions, and leads directly to the conclusion that the branchiæ of the vertebrate were derived + from the branchiæ of the arthropod, a conclusion which, according to the generally accepted view + of the origin of the respiratory region in the vertebrate, is extremely difficult to accept; for + the branchiæ of Limulus and of the Arthropoda in general are part of the mesosomatic appendages, + while the branchiæ of vertebrates are derived from the anterior part of the alimentary canal. This + conclusion, therefore, implies that the vertebrate has utilized in the formation of the anterior + portion of its new alimentary canal the branchial appendages of the palæostracan ancestor.</p> + + <div><span class="pagenum" id="page150">{150}</span></div> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig062.png" id="fig62"><img style="width:47%" src="images/fig062.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 62.</span>—<i>Eurypterus.</i></p> + <p class="sp0">The segments and appendages on the right are numbered in correspondence with + the cranial system of lateral nerve-roots as found in vertebrates. <i>M.</i>, metastoma. The + surface ornamentation is represented on the first segment posterior to the branchial segments. + The opercular appendage is marked out by dots.</p> + </div> + </div> + + <p>Let us consider dispassionately whether such a suggestion is <i>a priori</i> so impossible as + it at first appears. One of the principles of evolution is that any change which is supposed to + have taken place in the process of formation of one animal or group of animals from a lower group + must be in harmony with changes which are known to have occurred in that lower group. On the + assumption, therefore, that the vertebrate branchiæ represent the branchial portion of the + arthropod mesosomatic appendages which have sunk in and so become internal, we ought to find that + in members of this very group such inclusion of branchial appendages has taken place. This, + indeed, is exactly what we do find, for in all the scorpion tribe, which is acknowledged to be + closely related to Limulus, there are no external mesosomatic appendages, but in all cases these + appendages have sunk into the body, have disappeared as such, and retained only the vital part of + them—the branchiæ. In this way the so-called lung-books of the scorpion are formed, which + are in all respects homologous with the branchiæ or gill-books of Limulus. Now, as already + mentioned, the lords of creation in the palæostracan times were the sea-scorpions, which, as is + seen in Fig. <a href="#fig62">62</a>, resembled the land-scorpions of the present day in the + entire absence of any external appendages on the segments of the mesosomatic region. As they lived + in the sea, they must have breathed with gills, and those branchial appendages must have been + internal, just as in the land-scorpions of the present time. Indeed, markings have been found on + the internal side of the segments 1-5, Fig. <a href="#fig62">62</a>, which are supposed to + indicate branchiæ, and these segments are therefore supposed to have borne the branchiæ. Up to the + present time no indication of gill-slits has been found, and we cannot say with certainty how + these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion + (<i>Palæophonus Hunteri</i>), closely resembling the modern scorpion, has been found, which, as + Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it <span + class="pagenum" id="page151">{151}</span>breathed is unknown; it shows no signs of stigmata, such + as exist in the scorpion of to-day.</p> + + <p>Although we possess as yet no certain knowledge of the position of the gill-openings in these + ancient scorpion-like forms, what we can say with certainty—and that is the important + fact—is, that at the time when the vertebrates appeared, a very large number of the dominant + arthropod race possessed internally-situated branchiæ, which had been directly derived from the + branchiæ-bearing appendages of their Limulus-like kinsfolk.</p> + + <p class="sp3">This abolition of the branchiæ-bearing appendages as external organs of locomotion, + with the retention of the important branchial portion of the appendage as internal branchiæ, is a + very important suggestion in any discussion of the way vertebrates have arisen from arthropods; + for, if the same principle is of universal application, it leads directly to the conclusion that + whenever an appendage possesses an organ of vital importance to the animal, that organ will + remain, even though the appendage as such completely vanishes. Thus, as will be shown later, + special sense-organs such as the olfactory remain, though the animal no longer possesses antennæ; + the important excretory organs, the coxal glands, and important respiratory organs, the branchiæ, + are still present in the vertebrate, although the appendages to which they originally belonged + have dwindled away, or, at all events, are no longer recognizable as arthropod appendages.</p> + + <p class="ac"><span class="sc">Innervation of Branchial Segments.</span></p> + + <p>Passing from <i>a priori</i> considerations to actual facts, it is advisable to commence with + the innervation of the branchial segments; for, seeing that the foundation of the whole of this + comparative study of the vertebrate and the arthropod is based upon the similarity of the two + central nervous systems, it follows that we must look in the first instance to the innervation of + any organ or group of organs in order to find out their relationship in the two groups of + animals.</p> + + <p>The great characteristic of the vertebrate branchial organs is their segmental arrangement and + their innervation by the vagus group of nerves, <i>i.e.</i> by the hindermost group of the cranial + segmental nerves. These cranial nerves are divided by Gegenbaur into two great groups—an + anterior group, the trigeminal, which supplies the muscles of mastication, and a posterior group, + the vagus, which is essentially <span class="pagenum" id="page152">{152}</span>respiratory in + function. Of these two groups, I will consider the latter group first.</p> + + <p>In Limulus the great characteristic of the branchial region is its pronounced segmental + arrangement, each pair of branchial appendages belonging to a separate segment. This group of + segments forms the mesosoma, and these branchial appendages are the mesosomatic appendages. + Anterior to them are the segments of the prosoma, which bear the prosomatic or locomotor + appendages. The latter are provided at their base with gnathites or masticating apparatus, so that + the prosomatic group of nerves, like the trigeminal group in the vertebrate, comprises essentially + the nerves subserving the important function of mastication. As already pointed out, the + brain-region of the vertebrate is comparable to the supra-œsophageal and + infra-œsophageal ganglia of the invertebrate, and it has been shown (p. <a + href="#page54">54</a>) how, by a process of concentration and cephalization, the foremost region + of the infra-œsophageal ganglia becomes the prosomatic region, and is directly comparable + to the trigeminal region in the vertebrate; while the hindermost region is formed from the + concentration of the mesosomatic ganglia, and is directly comparable to the medulla oblongata, + <i>i.e.</i> to the vagus region of the vertebrate brain.</p> + + <p class="sp3">As far, then, as concerns the centres of origin of these two groups of nerves and + their exits from the central nervous system, they are markedly homologous in the two groups of + animals.</p> + + <p class="ac"><span class="sc">Comparison of the Cranial and Spinal Segmental Nerves.</span></p> + + <p>It has often been held that the arrangements of the vertebrate nervous system differ from those + of other segmented animals in one important particular. The characteristic of the vertebrate is + the origin of every segmental nerve from two roots, of which one contains the efferent fibres, + while the other possesses a sensory ganglion, and contains only afferent fibres. This arrangement, + which is found along the whole spinal cord of all vertebrates, is not found in the segmental + nerves of the invertebrates; and as it is supposed that the simpler arrangement of the spinal cord + was the primitive arrangement from which the vertebrate central nervous system was built up, it is + often concluded that the animal from which the vertebrate arose must have possessed a series of + nerve-segments, from each of which there arose bilaterally ventral (efferent) and dorsal + (afferent) roots.</p> + + <div><span class="pagenum" id="page153">{153}</span></div> + + <p>Now, the striking fact of the vertebrate segmental nerves consists in this, that, as far as + their structure and the tissues which they innervate are concerned, the cranial segmental nerves + are built up on the same plan as the spinal; but as far as concerns their exit from the central + nervous system they are markedly different. A large amount of ingenuity, it is true, has been + spent in the endeavour to force the cranial nerves into a series of segmental nerves, which arise + in the same way as the spinal by two roots, of which the ventral series ought to be efferent and + the dorsal series afferent, but without success. We must, therefore, consider the arrangement of + the cranial segmental nerves by itself, separately from that of the spinal nerves, and the problem + of the origin of the vertebrate segmental nerves admits of two solutions—either the cranial + arrangement has arisen from a modification of the spinal, or the spinal from a simplification of + the cranial. The first solution implies that the spinal cord arrangement is older than the + cranial, the second that the cranial is the oldest.</p> + + <p>In my opinion, the evidence of the greater antiquity of the cranial region is overwhelming.</p> + + <p>The evidence of embryology points directly to the greater phylogenetic antiquity of the cranial + region, for we see how, quite early in the development, the head is folded off, and the organs in + that region thereby completed at a time when the spinal region is only at an early stage of + development. We see how the first of the trunk somites is formed just posteriorly to the head + region, and then more and more somites are formed by the addition of fresh segments posteriorly to + the one first formed. We see how, in Ammocœtes, the first formed parts of the skeleton are + the branchial bars and the basi-cranial system, while the rudiments of the vertebræ do not appear + until the Petromyzon stage. We see how, with the elongation of the animal by the later addition of + more and more spinal segments, organs, such as the heart, which were originally in the head, + travel down, and the vagus and lateral-line nerves reach their ultimate destination. Again, we see + that, whereas the cranial nerves, viz. the ocular motor, the trigeminal, facial, auditory, + glossopharyngeal, and vagus nerves, are wonderfully fixed and constant in all vertebrates, the + only shifting being in the spino-occipital region, in fact, at the junction of the cranial and + spinal region, the spinal nerves, on the other hand, are not only remarkably variable in number in + different <span class="pagenum" id="page154">{154}</span>groups of animals, but that even in the + same animal great variations are found, especially in the manner of formation of the + limb-plexuses. Such marked meristic variation in the spinal nerves, in contrast to the fixed + character of the cranial nerves, certainly points to a more recent formation of the former + nerves.</p> + + <p>Also the observations of Assheton on the primitive streak of the rabbit, and on the growth in + length of the frog embryo, have led him to the conclusion that, as in the rabbit so in the frog, + there is evidence to show that the embryo is derived from two definite centres of growth: the + first, phylogenetically the oldest, being a protoplasmic activity, which gives rise to the + anterior end of the embryo; the second, one which gives rise to the growth in length of the + embryo. This secondary area of proliferation coincides with the area of the primitive streak, and + he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated + blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the + anterior end of the primitive streak, and subsequently is found at the level of the most anterior + pair of somites.</p> + + <p>He then goes on to say—</p> + + <p>"From these specimens it seems clear that all those parts in front of the first pair of + mesoblastic somites—that is to say, the heart, the brain and medulla oblongata, the + olfactory, optic, auditory organs and foregut—are developed from that portion of the + unincubated blastoderm which lies anterior to the centre of the blastoderm, and that all the rest + of the embryo is formed by the activity of the primitive streak area."</p> + + <p>In other words, the secondary area of growth, <i>i.e.</i> the primitive streak area, includes + the whole of the spinal cord region, while the older primary centre of growth is coincident with + the cranial region.</p> + + <p>In searching, then, for the origin of the segmental nerves, we must consider the type on which + the cranial nerves are arranged rather than that of the spinal nerves.</p> + + <p>The first striking fact occurs at the spino-occipital region, where the spinal cord merges into + the medulla oblongata, for here in the cervical region we find each spinal segment gives origin to + three distinct roots, not two—a dorsal root, a ventral root, and a lateral root. This third + root gives origin to the spinal accessory nerve, and in the region of the medulla oblongata these + lateral roots merge directly into the roots of the vagus nerve; more anteriorly the same system + <span class="pagenum" id="page155">{155}</span>continues as the roots of the glossopharyngeal + nerve, as the roots of the facial nerve, and as a portion, especially the motor portion, of the + trigeminal nerve. Now, all these nerves belong to a well-defined system of nerves, as Charles + Bell<a id="NtA_1" href="#Nt_1"><sup>[1]</sup></a> pointed out in 1830, a system of nerves + concerned with respiration and allied mechanisms, such as laughing, sneezing, mastication, + deglutition, etc., nerves innervating a set of muscles of very different kind from the ordinary + body-muscles concerned with locomotion and equilibration. Also the centres from which these motor + nerves arise are well defined, and form cell-masses in the central nervous system, quite separate + from those which give origin to somatic muscles.</p> + + <p>This original idea of Charles Bell, after having been ignored for so long a time, is now seen + to be a very right one, and it is an extraordinary thing that his enunciation of the dual nature + of the spinal roots, which was, to his mind, of subordinate importance, should so entirely have + overshadowed his suggestion, that in addition to the dorsal and ventral roots, a lateral system of + nerves existed, which were not exclusively sensory or exclusively motor, but formed a separate + system of respiratory nerves.</p> + + <p>Further, anatomists divide the striated muscles of the body into two great natural groups, + characterized by a difference of origin and largely by a difference of appearance. The one set is + concerned with the movements of internal organs, and is called visceral, the other is derived from + the longitudinal sheet of musculature which forms the myotomes of the fish, and has been called + parietal or somatic. The motor nerves of these two sets of muscles correspond with the lateral or + respiratory and ventral roots respectively.</p> + + <p>Finally, it has been shown that the segments of which a vertebrate is composed are recognizable + in the embryo by the segmented manner in which the musculature is laid down, and van Wijhe has + shown that in the cranial region two sets of muscles are laid down segmentally, thus forming a + dorsal and ventral series of commencing muscular segments. Of these the anterior segments of the + dorsal series give origin to the striated muscles of the eye which are innervated by the IIIrd + (oculomotor), IVth (trochlearis), and VIth (abducens) nerves, while the posterior segments give + origin to the <span class="pagenum" id="page156">{156}</span>muscles from the cranium to the + shoulder-girdle, innervated by the XIIth (hypoglossal) nerve. The ventral series of segments give + origin to the musculature supplied by the trigeminal, facial, glossopharyngeal, and vagus + nerves.</p> + + <p>Also, the afferent or sensory nerves of the skin over the whole of this head-region are + supplied by the trigeminal nerve, while the afferent nerves to the visceral surfaces are supplied + by the vagus, glossopharyngeal and facial nerves.</p> + + <p>In van Wijhe's original paper he arranged the segments belonging to the cranial nerves in the + following table<span class="wnw">:—</span></p> + + <table class="sp2 mc bb nothand" title="Segments belonging to the cranial nerves" + summary="Segments belonging to the cranial nerves"> + <tr> + <th class="vmi smaller bt bb">Segments.</th> + <th class="vmi smaller ba" colspan="2">Ventral nerve-roots and muscles<br/> + derived from myotomes.</th> + <th class="vmi smaller ba">Visceral clefts</th> + <th class="vmi smaller bt bb" colspan="2">Dorsal nerve-roots and muscles.</th> + </tr> + <tr class="br"> + <td class="ac">1</td> + <td class="ac">III.</td> + <td class="al">M. rectus superior,<br/> + m. rectus internus,<br/> + m. rectus inferior,<br/> + m. obliquus inferior</td> + <td></td> + <td class="ac">V. N. opthalmicus<br/> + profundus</td> + <td class="br0"></td> + </tr> + <tr class="br"> + <td class="ac">2</td> + <td class="ac">IV.</td> + <td class="al">M. obliquus superior</td> + <td class="vbm">1st Mandibular</td> + <td class="ac">V.</td> + <td class="ac br0">Masticating<br/> + muscles.</td> + </tr> + <tr class="br"> + <td class="ac">3</td> + <td class="ac">VI.</td> + <td class="al">M. rectus externus</td> + <td></td> + <td class="ac">VII.<sub>1</sub></td> + <td class="ac br0" rowspan="2" style="padding:0"> + <table title="Segments belonging to the cranial nerves" + summary="Segments belonging to the cranial nerves"> + <tr> + <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + <td>Facial muscles<br/> + (VIII. is dorsal<br/> + branch of VII.)</td> + </tr> + </table> + </td> + </tr> + <tr class="br"> + <td class="ac vmi">4</td> + <td class="ac vmi">—</td> + <td class="ac vmi">—</td> + <td class="vmi" style="padding:0"> + <table title="Segments belonging to the cranial nerves" + summary="Segments belonging to the cranial nerves"> + <tr> + <td class="vmi">2nd</td> + <td class="vmi pl0 pr0"><img src="images/lbrace2.png" style="height:4.5ex; width:1em;" + alt="brace" /></td> + <td>Hyoid<sub>1</sub><br/> + Hyoid<sub>2</sub></td> + </tr> + </table> + </td> + <td class="ac vmi">VII.<sub>2</sub></td> + </tr> + <tr class="br"> + <td class="ac">5</td> + <td class="ac">—</td> + <td class="ac">—</td> + <td class="al pr0">3rd 1st Branchial</td> + <td class="ac">IX.</td> + <td class="ac br0" rowspan="5" style="padding:0"> + <table title="Segments belonging to the cranial nerves" + summary="Segments belonging to the cranial nerves"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace5.png" style="height:13.2ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">Branchial and<br/> + visceral muscles.</td> + </tr> + </table> + </td> + </tr> + <tr class="br"> + <td class="ac">6</td> + <td class="ac">—</td> + <td class="ac">—</td> + <td class="al pr0">4th 2nd <span class="hid">Brt</span>"</td> + <td class="ac">X.<sub>1</sub></td> + </tr> + <tr class="br"> + <td class="ac">7</td> + <td class="ac">XII.</td> + <td class="al vmi" rowspan="3" style="padding:0"> + <table title="Segments belonging to the cranial nerves" + summary="Segments belonging to the cranial nerves"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">Muscles from<br/> + cranium to<br/> + shoulder-girdle</td> + <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + </tr> + </table> + </td> + <td class="al pr0">5th 3rd <span class="hid">Bra</span>"</td> + <td class="ac">X.<sub>2</sub></td> + </tr> + <tr class="br"> + <td class="ac">8</td> + <td class="ac">XII.</td> + <td class="al pr0">6th 4th <span class="hid">Bra</span>"</td> + <td class="ac">X.<sub>3</sub></td> + </tr> + <tr class="br"> + <td class="ac">9</td> + <td class="ac">XII.</td> + <td class="al pr0">7th 5th <span class="hid">Bra</span>"</td> + <td class="ac">X.<sub>4</sub></td> + </tr> + </table> + +<!-- Left half for handhelds --> + + <table class="sp2 bb w100 handonly" title="Segments belonging to the cranial nerves + (1 of 2)" summary="Segments belonging to the cranial nerves + (1 of 2)"> + <tr> + <th class="vmi smaller bt bb">Segments.</th> + <th class="vmi smaller ba" colspan="2">Ventral nerve-roots and muscles<br/> + derived from myotomes.</th> + <th class="vmi smaller bt bb">Visceral clefts</th> + </tr> + <tr class="br"> + <td class="ac">1</td> + <td class="ac">III.</td> + <td class="al">M. rectus superior,<br/> + m. rectus internus,<br/> + m. rectus inferior,<br/> + m. obliquus inferior</td> + <td class="br0"></td> + </tr> + <tr class="br"> + <td class="ac">2</td> + <td class="ac">IV.</td> + <td class="al">M. obliquus superior</td> + <td class="br0 vbm">1st Mandibular</td> + </tr> + <tr class="br"> + <td class="ac">3</td> + <td class="ac">VI.</td> + <td class="al">M. rectus externus</td> + <td class="br0"></td> + </tr> + <tr class="br"> + <td class="ac vmi">4</td> + <td class="ac vmi">—</td> + <td class="ac vmi">—</td> + <td class="br0 vmi" style="padding:0"> + <table title="Segments belonging to the cranial nerves + (1 of 2)" summary="Segments belonging to the cranial nerves + (1 of 2)"> + <tr> + <td class="vmi">2nd</td> + <td class="vmi pl0 pr0"><img src="images/lbrace2.png" style="height:4.5ex; width:1em;" + alt="brace" /></td> + <td>Hyoid<sub>1</sub><br/> + Hyoid<sub>2</sub></td> + </tr> + </table> + </td> + </tr> + <tr class="br"> + <td class="ac">5</td> + <td class="ac">—</td> + <td class="ac">—</td> + <td class="al br0 pr0">3rd 1st Branchial</td> + </tr> + <tr class="br"> + <td class="ac">6</td> + <td class="ac">—</td> + <td class="ac">—</td> + <td class="al br0 pr0">4th 2nd <span class="hid">Brt</span>"</td> + </tr> + <tr class="br"> + <td class="ac">7</td> + <td class="ac">XII.</td> + <td class="al vmi" rowspan="3" style="padding:0"> + <table title="Segments belonging to the cranial nerves + (1 of 2)" summary="Segments belonging to the cranial nerves + (1 of 2)"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">Muscles from<br/> + cranium to<br/> + shoulder-girdle</td> + <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + </tr> + </table> + </td> + <td class="al br0 pr0">5th 3rd <span class="hid">Bra</span>"</td> + </tr> + <tr class="br"> + <td class="ac">8</td> + <td class="ac">XII.</td> + <td class="al br0 pr0">6th 4th <span class="hid">Bra</span>"</td> + </tr> + <tr class="br"> + <td class="ac">9</td> + <td class="ac">XII.</td> + <td class="al br0 pr0">7th 5th <span class="hid">Bra</span>"</td> + </tr> + </table> + +<!-- Right half for handhelds --> + + <table class="sp3 bb w100 handonly" title="Segments belonging to the cranial nerves + (2 of 2)" summary="Segments belonging to the cranial nerves + (2 of 2)"> + <tr> + <th class="vmi smaller bt bb br">Segments.</th> + <th class="vmi smaller bt bb" colspan="2">Dorsal nerve-roots and muscles.</th> + </tr> + <tr class="br"> + <td class="ac">1</td> + <td class="ac">V. N. opthalmicus<br/> + profundus</td> + <td class="br0"></td> + </tr> + <tr class="br"> + <td class="ac">2</td> + <td class="ac">V.</td> + <td class="ac br0">Masticating<br/> + muscles.</td> + </tr> + <tr class="br"> + <td class="ac">3</td> + <td class="ac">VII.<sub>1</sub></td> + <td class="ac br0" rowspan="2" style="padding:0"> + <table title="Segments belonging to the cranial nerves + (2 of 2)" summary="Segments belonging to the cranial nerves + (2 of 2)"> + <tr> + <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + <td>Facial muscles<br/> + (VIII. is dorsal<br/> + branch of VII.)</td> + </tr> + </table> + </td> + </tr> + <tr class="br"> + <td class="ac vmi">4</td> + <td class="ac vmi">VII.<sub>2</sub></td> + </tr> + <tr class="br"> + <td class="ac">5</td> + <td class="ac">IX.</td> + <td class="ac br0" rowspan="5" style="padding:0"> + <table title="Segments belonging to the cranial nerves + (2 of 2)" summary="Segments belonging to the cranial nerves + (2 of 2)"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace5.png" style="height:13.2ex; width:1em;" + alt="brace" /></td> + <td class="vmi pl0 pr0">Branchial and<br/> + visceral muscles.</td> + </tr> + </table> + </td> + </tr> + <tr class="br"> + <td class="ac">6</td> + <td class="ac">X.<sub>1</sub></td> + </tr> + <tr class="br"> + <td class="ac">7</td> + <td class="ac">X.<sub>2</sub></td> + </tr> + <tr class="br"> + <td class="ac">8</td> + <td class="ac">X.<sub>3</sub></td> + </tr> + <tr class="br"> + <td class="ac">9</td> + <td class="ac">X.<sub>4</sub></td> + </tr> + </table> + +<!-- End of handheld split --> + + <p>As is seen in the table, van Wijhe attempts to arrange the cranial segmental nerves into dorsal + and ventral roots, in accordance with the arrangement in the spinal region. In order to do this he + calls the Vth, VIIth, IXth, and Xth nerves dorsal roots, although they are not purely sensory + nerves, but contain motor fibres as well.</p> + + <p>It is not accidental that he should have picked out for his dorsal roots the very nerves which + form Charles Bell's lateral series of roots, inasmuch as this system of lateral roots, apart from + dorsal and ventral roots, really is, as Charles Bell thought, an important separate system, + dependent upon a separate segmentation in the embryo of the musculature supplied by these roots. + This segmentation may receive the name of <i>visceral</i> or <i>splanchnic</i> in + contradistinction to <i>somatic</i>, since all the muscles without exception belong to the + visceral group of striated muscles.</p> + + <div><span class="pagenum" id="page157">{157}</span></div> + + <p>These observations of van Wijhe lead directly to the following conclusion. In the cranial + region there is evidence of a double set of segments, which may be called somatic and splanchnic. + The somatic segments, consisting of the outer skin and the body musculature, are <i>doubly</i> + innervated as are those of the spinal cord by a series of ventral motor roots, the oculomotor or + IIIrd nerve, the trochlear or IVth nerve, the abducens or VIth nerve, and the hypoglossal or XIIth + nerve, and by a series of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve. + But the splanchnic segments are innervated by <i>single</i> roots, the vagus or Xth nerve, + glossopharyngeal or IXth nerve, facial or VIIth nerve, and trigeminal or Vth nerve, which are + mixed, containing both sensory and motor fibres, thus differing markedly from the arrangement of + the spinal nerves.</p> + + <p class="sp3">From this sketch it follows that the arrangement seen in the spinal cord, would + result from the cranial arrangement if this third system of lateral roots were left out. Further, + since the cranial system is the oldest, we must search in the invertebrate ancestor for a + tripartite rather than a dual system of nerve-roots for each segment; a system composed of a + dorsal root supplying only the sensory nerves of the skin-surfaces, a lateral mixed root supplying + the system connected with respiration with both sensory and motor fibres, and a ventral root + supplying the motor nerves to the body-musculature.</p> + + <p class="ac"><span class="sc">Comparison of the Appendage Nerves of Limulus and Branchipus to the + Lateral Root System of the Vertebrate.</span></p> + + <p>If the argument used so far is correct, and this tripartite system of nerve-roots, as seen in + the cranial nerves of the vertebrate, really represents the original scheme of innervation in the + palæostracan ancestor, then it follows that each segment of Limulus ought to be supplied by three + nerves—(1), a sensory nerve supplying its own portion of the skin-surface of the prosomatic + and mesosomatic carapaces; (2), a lateral mixed nerve supplying exclusively the appendage of the + segment, for the appendages carry the respiratory organs; and (3), a motor nerve supplying the + body-muscles of the segment.</p> + + <p>It is a striking fact that Milne-Edwards describes the nerve-roots in exactly this manner. The + great characteristic of the nerve-roots <span class="pagenum" id="page158">{158}</span>in Limulus + as in other arthropods is the large appendage-nerve, which is always a mixed nerve; in addition, + there is a system of sensory nerves to the prosomatic and mesosomatic carapaces, called by him the + epimeral nerves, which are purely sensory, and a third set of roots which are motor to the + body-muscles, and possibly also sensory to the ventral surface between the appendages.</p> + + <p>Moreover, just as in the vertebrate central nervous system the centres of origin of the motor + nerves of the branchial segmentation are distinct from those of the somatic segmentation, so we + find, from the researches of Hardy, that a similar well-marked separation exists between the + centres of origin of the motor nerves of the appendages and those of the somatic muscles in the + central nervous system of Branchipus and Astacus.</p> + + <p>In the first place, he points out that the nervous system of Branchipus is of a very primitive + arthropod type; that it is, in fact, as good an example of an ancient type as we are likely to + find in the present day; a matter of some importance in connection with my argument, since the + arthropod ancestor of the vertebrate, such as I am deducing from the study of Ammocœtes, + must undoubtedly have been of an ancient type, more nearly connected with the strange forms of the + trilobite era than with the crabs and spiders of the present day.</p> + + <p>His conclusions with respect to Branchipus may be tabulated as follows<span + class="wnw">:—</span></p> + + <p>1. Each ganglion of the ventral chain is formed mainly for the innervation of the + appendages.</p> + + <p>2. Each ganglion is divided into an anterior and posterior division, which are connected + respectively with the motor and sensory nerves of the appendages.</p> + + <p>3. The motor nerves of the appendages arise as well-defined axis-cylinder processes of + nerve-cells, which are arranged in well-defined groups in the anterior division of the + ganglion.</p> + + <p>4. A separate innervation exists for the muscles and sensory surfaces of the trunk. The + trunk-muscles consist of long bundles, from which slips pass off to the skin in each segment; they + are thus imperfectly segmented. In accordance with this, a diffuse system of nerve-fibres passes + to them from certain cells on the dorsal surface of each lateral half of the ganglion. These + cell-groups are therefore very distinct from those which give origin to the motor <span + class="pagenum" id="page159">{159}</span>appendage-nerves, and, moreover, are not confined to the + ganglion, but extend for some distance into the interganglionic region of the nerve-cords which + connect together the ganglia of the ventral chain.</p> + + <p>Hardy's observations, therefore, combined with those of Milne-Edwards, lead to the conclusion + that in such a primitive arthropod type as my theory postulates, each segment was supplied with + separate sensory and motor somatic nerves, and with a pair of nerves of mixed function, devoted + entirely to the innervation of the pair of appendages; that also, in the central nervous system, + the motor nerve-centres were arranged in accordance with a double set of segmented muscles in two + separate groups of nerve-cells. These nerve-cells in the one case were aggregated into + well-defined groups, which formed the centres for the motor nerves of the markedly segmented + muscles of the appendages, and in the other case formed a system of more diffused cells, less + markedly aggregated into distinct groups, which formed the centres for the imperfectly segmented + somatic muscles.</p> + + <p>Such an arrangement suggests that in the ancient arthropod type a double segmentation existed, + viz. a segmentation of the body, and a segmentation due to the appendages. Undoubtedly, the + segments originally corresponded absolutely as in Branchipus, and every appendage was attached to + a well-defined separate body-segment. In, however, such an ancient type as Limulus, though the + segmentation may be spoken of as twofold, yet the number of segments in the prosomatic and + mesosomatic regions are much more clearly marked out by the appendages than by the divisions of + the soma; for, in the prosomatic region such a fusion of somatic segments to form the tergal + prosomatic carapace has taken place that the segments of which it is composed are visible only in + the young condition, while in the mesosomatic region the separate somatic segments, though fused + to form the mesosomatic carapace, are still indicated by the entapophysial indentations.</p> + + <p>Clearly, then, if the mesosomatic branchial appendages of forms related to Limulus were reduced + to the branchial portion of the appendage, and that branchial portion became internal, just as is + known to be the case in the scorpion group, we should obtain an animal in which the <i>mesosomatic + region</i> would be characterized by a segmentation predominantly branchial, which might be + termed, as in vertebrates, the <i>branchiomeric segmentation</i>, but yet would show <span + class="pagenum" id="page160">{160}</span>indications of a corresponding somatic or <i>mesomeric + segmentation</i>. The nerve supply to these segments would consist of—</p> + + <p>1. The epimeral purely sensory nerves to the somatic surface, equivalent in the vertebrate to + the ascending root of the trigeminal.</p> + + <p>2. The mixed nerves to the internal branchial segments, equivalent in the vertebrate to the + vagus, glossopharyngeal, and facial.</p> + + <p>3. The motor nerves to the somatic muscles, equivalent in the vertebrate to the original + nerve-supply to the somatic muscles belonging to these segments, <i>i.e.</i> to the muscles + derived from van Wijhe's 4th, 5th, and 6th somites.</p> + + <p>Further, the centres of origin of these appendage-nerves would form centres in the central + nervous system separate from the centres of the motor nerves to the somatic muscles, just as the + centres of origin of the motor parts of the facial, vagus, and glossopharyngeal nerves form groups + of cells quite distinct from the centres for the hypoglossal, abducens, trochlear, and oculomotor + nerves.</p> + + <p>In fact, if the vertebrate branchial nerves are looked upon as the descendants of nerves which + originally supplied branchial appendages, then every question connected with the branchial + segmentation, with the origin and distribution of these nerves, receives a simple and adequate + solution—a solution in exact agreement with the conclusion that the vertebrate arose from a + palæostracan ancestor.</p> + + <p>It would, therefore, be natural to expect that the earliest fishes breathed by means of + branchial appendages situated internally, and that the evidence for such appendages would be much + stronger in them than in more recent fishes.</p> + + <p class="sp3">Although we know nothing of the nature of the respiratory apparatus in the extinct + fishes of Silurian times, we have still living, in the shape of Ammocœtes, a possible + representative of such types. If, then, we find, as is the case, that the respiratory apparatus of + Ammocœtes differs markedly from that of the rest of the fishes, and, indeed, from that of + the adult form or Petromyzon, and that that very difference consists in a greater resemblance to + internal branchial appendages in the case of Ammocœtes, then we may feel that the proof of + the origin of the branchial apparatus of the vertebrate from the internal branchial appendages of + the invertebrate has gained enormously.</p> + + <div><span class="pagenum" id="page161">{161}</span></div> + + <p class="ac"><span class="sc">The Respiratory Chamber of Ammocœtes.</span></p> + + <p>In order to make clear the nature of the branchial segments in Ammocœtes, I have divided + the head-part of the animal by means of a longitudinal horizontal section into + halves—ventral and dorsal—as shown in Figs. 63 and 64. These figures are each a + combination of a section and a solid drawing. The animal was slit open by a longitudinal section + in the neighbourhood of the gill-slits, and each half was slightly flattened out, so as to expose + the ventral and dorsal internal surfaces respectively. The structures in the cut surface were + drawn from one of a series of horizontal longitudinal sections taken through the head of the + animal. These figures show that the head-region of Ammocœtes consists of two chambers, the + contents of which are different. In front, an oral or stomodæal chamber, which contains the velum + and tentacles, is enclosed by the upper and lower lips, and was originally separated by a septum + from the larger respiratory chamber, which contains the separate pairs of branchiæ. A glance at + the two drawings shows clearly that Rathke's original description of this chamber is the natural + one, for he at that time, looking upon <i>Ammocœtes branchialis</i> as a separate species, + described the branchial chamber as containing a series of paired gills, with the gill-openings + between consecutive gills. His branchial unit or gill, therefore, was represented by each of the + so-called diaphragms, which, as seen in Figs. 63, 64, are all exactly alike, except the first and + the last. Any one of these is represented in section in Fig. <a href="#fig65">65</a>, and + represents a branchial unit in Rathke's view and in mine. Clearly, it may be described as a + branchial appendage which projects into an open pharyngeal chamber, so that the series of such + appendages divides the chamber into a series of compartments, each of which communicates with the + exterior by means of a gill-slit, and with each other by means of the open space between opposing + appendages.</p> + + <p>Each of these appendages possesses its own cartilaginous bar (<i>Br. cart.</i>), as explained + in Chapter III.; each possesses its own branchial or visceral muscles (coloured blue in Figs. 63 + and 64), separated absolutely from the longitudinal somatic muscles (coloured dark red in Figs. 63 + and 64) by a space (<i>Sp.</i>) containing blood and peculiar fat-cells, etc. Each possesses its + own afferent branchial blood-vessel from the ventral aorta, and its own efferent vessel to the + dorsal aorta (Fig. <a href="#fig65">65</a>, <i>a. br.</i> and <i>v. br.</i>). Each possesses its + own segmental nerve, which supplies its own branchial muscles and no others with motor fibres, and + sends sensory fibres to the general surface of each appendage, as also to the special sense-organs + in the shape of the epithelial pits (<i>S.</i>, Fig. <a href="#fig65">65</a>) arranged along the + free edges of the diaphragms; each of these nerves possesses its own ganglion—the + epibranchial ganglion.</p> + + <div><span class="pagenum" id="page162">{162}</span></div> + + <div class="ac w55 fcenter sp2"> + <a href="images/fig063.jpg" id="fig63"><img style="width:100%" src="images/fig063.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 63.—Ventral half of Head-region of + Ammocœtes.</span></p> + <p class="sp0">Somatic muscles coloured red. Branchial and visceral muscles coloured blue. + Tubular constrictor muscles distinguished from striated constrictor muscles by simple + hatching. <i>Tent.</i>, tentacles; <i>Tent. m.c.</i>, muco-cartilage of tentacles; <i>Vel. + m.c.</i>, muco-cartilage of the velum; <i>Hy. m.c.</i>, muco-cartilage of the hyoid segment; + <i>Ps. br.</i>, pseudo-branchial groove; <i>Br. cart.</i>, branchial cartilages; <i>Sp.</i>, + space between somatic and splanchnic muscles; <i>Th. op.</i>, orifice of thyroid; <i>H.</i>, + heart.</p> + </div> + </div> + + <div><span class="pagenum" id="page163">{163}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig064.jpg" id="fig64"><img style="width:88%" src="images/fig064.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 64.—Dorsal half of Head-region of Ammocœtes.</span></p> + <p class="sp0"><i>Tr.</i>, trabeculæ; <i>Pit.</i>, pituitary space; <i>Inf.</i>, infundibulum; + <i>Ser.</i>, median serrated flange of velar folds.</p> + </div> + </div> + + <div><span class="pagenum" id="page164">{164}</span></div> + + <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:45%;"><a href="images/fig065.png" id="fig65"><img + style="width:100%" src="images/fig065.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:45%;"><a href="images/fig066.png" id="fig66"><img + style="width:100%" src="images/fig066.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 65.—Section through Branchial Appendage of + Ammocœtes.</span></p> + <p class="sp0"><i>br. cart.</i>, branchial cartilage; <i>v. br.</i>, branchial vein; <i>a. + br.</i>, branchial artery; <i>b.s.</i>, blood-spaces; <i>p.</i>, pigment; <i>S.</i>, + sense-organ; <i>c.</i>, ciliated band; <i>E., I.</i>, external and internal borders; <i>m. + add.</i>, adductor muscle; <i>m.c.s.</i>, striated constrictor muscle; <i>m.c.t.</i>, + tubular constrictor muscle; <i>m.</i> and <i>m.v.</i>, muscles of valve.</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 66.—Section through Branchial Appendage of + Limulus.</span></p> + <p class="sp0"><i>br. cart.</i>, branchial cartilage; <i>v.br.</i>, branchial vein; + <i>b.s.</i>, blood-spaces formed by branchial artery; <i>P.</i>, pigment; + <i>m<sub>1</sub></i>, posterior entapophysio-branchial muscle; <i>m<sub>2</sub></i>, + anterior entapophysio-branchial muscle; <i>m<sub>3</sub></i>, external branchial + muscle.</p> + </div> + </td> + </tr> + </table> + + <p>The work of Miss Alcock has shown that the segmental branchial nerve supplies solely and + absolutely such an appendage or branchial <span class="pagenum" id="page165">{165}</span>segment, + and does not supply any portion of the neighbouring branchial segments. The nerve-supply in + Ammocœtes gives no countenance to the view that the original unit was a branchial pouch, + the two sides of which each nerve supplied, but is strong evidence that the original unit was a + branchial appendage, which was supplied by a <i>single</i> nerve with both motor and sensory + fibres.</p> + + <p>Any observer having before him only this picture of the respiratory chamber of + Ammocœtes, upon which to base his view of a vertebrate respiratory chamber, would naturally + look upon the branchial unit of a vertebrate as a gilled appendage projecting into the open cavity + of the anterior part of the alimentary canal or pharynx. This is not, however, the usual + conception. The branchial unit is ordinarily described as a gill-pouch, which possesses two + openings or slits, an internal one into the lumen of the alimentary canal, and an external one + into the surrounding medium. This view is based upon embryological evidence of the following + character<span class="wnw">:—</span></p> + + <p>The alimentary canal of all vertebrates forms a tube stretching the whole length of the animal; + the anterior part of this tube becomes pouched on each side at regular intervals, and the walls of + each pouch becoming folded form the respiratory surfaces or gills. The openings of these separate + pouches into the central lumen of the gut form the internal gill-pouch openings; the other + extremity of the pouch approaches the external surface of the animal, and finally breaks through + to form a series of external gill-pouch openings.</p> + + <p>From the mesoblastic tissue, between each gill-pouch, there is formed a supporting + cartilaginous bar, to which are attached a system of branchial muscles, with their nerves and + blood-vessels. These cartilaginous bars, in all fishes above the Cyclostomata, form a supporting + framework for the internal gill-slit, so that the gills are situated externally to them; the more + primitive arrangement is, as already mentioned, a system of cartilaginous bars, extra-branchial in + position, so that the gills are situated internally to them.</p> + + <p>From this description of the mode of formation of the respiratory apparatus in water-breathing + vertebrates the conception has arisen of the gill-pouch as the branchial unit, a conception which + is absolutely removed from all idea of a branchial unit such as is found in an arthropod, viz. an + appendage.</p> + + <p>This conception of spaces as units pervades the whole of embryology, and is the outcome of the + gastrula theory—a theory which <span class="pagenum" id="page166">{166}</span>teaches that + all animals above the Protozoa are derived from a form which by invagination of its external + surface formed an internal cavity or primitive gut. From pouches of this gut other cavities were + said to be formed, called cœlomic cavities, and thus arose the group of cœlomatous + animals. To speak of the developmental history of animals in terms of spaces; to speak of the + atrophy of a cavity as though such a thing were possible, is, to my mind, the wrong way of looking + at the facts of anatomy. It resembles the description of a net as a number of holes tied together + with string, which is not usually considered the best method of description.</p> + + <p>There are two ways in which a series of pouches can be formed from a simple tube without + folding, either by a thinning at regular intervals of the original tissue surrounding the tube, or + by the ingrowth into the tube of the surrounding tissue at regular intervals, thus—</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig067.png" id="fig67"><img style="width:100%" src="images/fig067.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 67.—Diagrams to show the two methods of + Pouch-formation.</span></p> + <p class="sp0">A, by the thinning of the mesoblast at intervals. B, by the ingrowth of + mesoblast at intervals. <i>Ep.</i>, epiblast; <i>Mes.</i>, mesoblast; <i>Hy.</i>, + hypoblast.</p> + </div> + </div> + + <p>In the first case (A) the formation of a pouch is the significant act, and therefore the + branchial segments might be expressed in terms of pouches. In the second case (B) the formation of + a pouch is <span class="pagenum" id="page167">{167}</span>brought about in consequence of the + ingrowth of the mesoblastic tissues at intervals; here, although the end-result is the same as in + the first case, the pouch-formation is only secondary, the true branchial unit is the mesoblastic + ingrowth.</p> + + <p>The evidence all points directly to the second method of formation. Thus Shipley, in his + description of the development of the lamprey, says—</p> + + <p>"The gill-slits appear to me to be the result of the ventral downgrowth of mesoblast taking + place only at certain places, these forming the gill-bars. Between each downgrowth the hypoblastic + lining of the alimentary canal remains in contact with the epiblast; here the gill-opening + subsequently appears about the twenty-second day."</p> + + <p>Dohrn describes and gives excellent pictures of the growth of the diaphragms, as the + Ammocœtes grows in size, pictures which are distinctly reminiscent of the corresponding + illustrations given by Brauer of the growth of the internal gills in the scorpion embryo.</p> + + <p>Another piece of evidence confirmatory of the view that the branchial segments are really of + the nature of internal appendages, as the result of which gill-pouches are formed, is given by the + presence in each of these branchial bars or diaphragms of a separate cœlomic cavity. From + the walls of this cavity the branchial muscles and cartilaginous bar are formed.</p> + + <p>Now, from an embryological point of view, the vertebrate shows that it is a segmented animal by + the formation of somites, which consist of a series of divisions of the cœlom, of which the + walls form a series of muscular and skeletal segments. In the head-region, as already mentioned, + such cœlomic divisions form two rows—a dorsal and a ventral set. From the walls of + the dorsal set the somatic musculature is formed. From those of the ventral set the branchial + musculature. From the latter also the branchial cartilaginous bars are formed. Thus Shipley, in + his description of the development of the lamprey, says: "The mesoblast between the gills arranges + itself into head-cavities, and the walls of these cavities ultimately form the skeleton of the + gill-arches."</p> + + <p>Similarly, in the arthropod, the segments in the embryo are marked out by a series of + cœlomic cavities and Kishinouye has described in Limulus a separate cœlomic cavity + for every one of the mesosomatic or branchial segments, and he states that in Arachnida <span + class="pagenum" id="page168">{168}</span>the segmental cœlomic cavities extend into the + limbs. These cavities both in the vertebrate and in the arthropod disappear before the adult + condition is reached.</p> + + <p>The whole evidence thus points strongly to the conclusion that the true branchial segmental + units are the branchial bars or diaphragms, not the pouches between them.</p> + + <p>It is possible to understand why such prominence has been given to the conception of the + branchial unit as a gill-pouch rather than as a gill-appendage, when the extraordinary change of + appearance in the respiratory chamber of the lamprey which occurs at transformation, is taken into + consideration. This change is of a very far-reaching character, and consists essentially of the + formation of a new alimentary canal in this region, whereby the pharyngeal chamber of + Ammocœtes is cut off posteriorly from the alimentary canal, and is confined entirely to + respiratory purposes, its original lumen now forming a tube called the bronchus, which opens into + the mouth and into a series of branchial pouches.</p> + + <p>In Fig. <a href="#fig68">68</a> I give diagrammatic illustrations taken from Nestler's paper to + show the striking change which takes place at transformation, (A) representing three branchial + segments of Ammocœtes, and (B) the corresponding three segments of Petromyzon. The + corresponding parts in the two diagrams are shown by the cartilages (<i>br. cart.</i>), the + sense-organs (S), and the branchial veins (<i>V. br.</i>); the corresponding diaphragms are marked + by the figures 1, 2, 3 respectively. As is clearly seen, it is perfectly possible in the latter + case to describe the respiratory chamber, as Nestler has done, as divided into a series of + separate smaller chambers—the gill-pouches—by means of a series of diaphragms or + branchial bars. The surface of these gill-pouches is in part thrown into folds for respiratory + purposes, and each gill-pouch opens, on the one hand, into the bronchus (<i>Bro.</i>), and, on the + other, to the exterior by means of the gill-slit. The branchial unit in Petromyzon is, therefore, + according to Nestler and other morphologists, the folded opposed surfaces of two contiguous + diaphragms, and each one of the diaphragms is intersegmental between two gill-pouches.</p> + + <p>Nestler then goes on to describe the arrangement in Ammocœtes in the same terms, + although there is no bronchus or gill-pouch, but only an open chamber into which these + gill-bearing diaphragms project, which open chamber serves both for the passage of food and <span + class="pagenum" id="page169">{169}</span>of the water for respiration. This is manifestly the + wrong way to look at the matter: the adult form is derived from the larval, not <i>vice versâ</i>, + and the transformation process shows exactly how the gills, in Rathke's sense, come together to + form the bronchus and so make the gill-pouches of Petromyzon.</p> + + <p>When we bear in mind that almost all observers consider that the internal branchiæ of the + scorpion group are directly derived from branchial appendages of a kind similar to those of + Limulus, it is evident that a branchial appendage such as that of Ammocœtes might also have + arisen from such an appendage, because in various respects it is easier to compare the branchial + appendage of Ammocœtes, than that of the scorpion group, with that of Limulus.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig068.png" id="fig68"><img style="width:100%" src="images/fig068.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 68.—Diagram of three Branchial Segments of Ammocœtes (A) + compared with three Branchial Segments after Transformation (B) to show how the Branchial + Appendages of Ammocœtes form the Branchial Pouches of Petromyzon.</span> (After <span + class="sc">Nestler</span>.)</p> + <p class="sp0">In both figures the branchial cartilages (<i>br. cart.</i>), the branchial view + (<i>V. br.</i>), and the sense-organs (<i>S</i>), are marked out in order to show + corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial + segment are not distinguished, being represented a uniform black colour. <i>Bro.</i>, the + bronchus into which each gill-pouch opens.</p> + </div> + </div> + + <p>In the case of the scorpions, various suggestions have been made as to the manner in which such + a conversion may have taken place. The most probable explanation is that given by Macleod, in + which <span class="pagenum" id="page170">{170}</span>each of the branchiæ of the scorpion group is + directly compared with the branchial part of the Limulus appendage which has sunk into and + amalgamated with the ventral surface.</p> + + <p>According to this view, the modification which has taken place in transforming the branchial + Limulus-appendage into the branchial scorpion-appendage is a further stage of the process by which + the Limulus branchial appendage itself has been formed, viz. the getting rid of the free locomotor + segments of the original appendage, thus confining the appendage more and more to the basal + branchial portion. So far has this process been carried in the scorpion that all the free part of + the appendage has disappeared; apparently, also, the intrinsic muscles of the appendage have + vanished, with the possible exception of the post-stigmatic muscle, so that any direct comparison + between the branchial appendages of Limulus and the scorpions is limited to the comparison of + their branchiæ, their nerves, and their afferent and efferent blood-vessels.</p> + + <p>In the case of Ammocœtes the comparison must be made not with air-breathing but with + water-breathing scorpions, such as existed in past ages in the forms of Eurypterus, Pterygotus, + Slimonia, and with the crowd of trilobite and Limulus-like forms which were in past ages so + predominant in the sea; forms in some of which the branchial appendages had already become + internal, but which, from the very fact of these forms being water-breathers, probably resembled, + in respect of their respiratory apparatus, Limulus rather than the present-day scorpion.</p> + + <p class="sp3">On the assumption that the branchial appendages of Ammocœtes, like the + branchial appendages of the scorpion group, are to a certain extent comparable with those of + Limulus, it becomes a matter of great interest to inquire whether the mode in which respiration is + effected in Ammocœtes resembles most that of Limulus or of the scorpion.</p> + + <p class="ac"><span class="sc">The Origin of the Branchial Musculature.</span></p> + + <p>The difference between the movements of respiration in Limulus and those of the scorpions + consists in the fact that, although in both cases respiration is effected mainly by dorso-ventral + muscles, these muscles are not homologous in the two cases: in the former, the dorso-ventral + appendage-muscles are mainly concerned, in the latter, the dorso-ventral somatic muscles.</p> + + <div><span class="pagenum" id="page171">{171}</span></div> + + <p>The paper by Benham gives a full description of the musculature of Limulus, and according to + his arrangement the muscles are divided into two sets, longitudinal and dorso-ventral. Of the + latter, each mesosomatic segment possesses a pair of dorso-ventral muscles, attached to the + mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig. <a href="#fig58">58</a>, + <i>Dv.</i>). These muscles are called by Benham the vertical mesosomatic muscles. I shall call + them the somatic dorso-ventral muscles, in contradistinction to the dorso-ventral muscles of the + branchial appendages. Of the latter, the two chief are the external branchial (Fig. <a + href="#fig66">66</a>, <i>m<sub>3</sub></i>) and the posterior entapophysio-branchial (Fig. <a + href="#fig66">66</a>, <i>m<sub>1</sub></i>); a third muscle is the anterior entapophysio-branchial + (Fig. <a href="#fig66">66</a>, <i>m<sub>2</sub></i>). Of these muscles, the posterior + entapophysio-branchial (<i>m<sub>1</sub></i>) is closely attached along the branchial + cartilaginous bar up to its round-headed termination on the anterior surface of the appendage. The + anterior entapophysio-branchial muscle (<i>m<sub>2</sub></i>) is attached to the branchial + cartilage near the entapophysis.</p> + + <p>In the case of the scorpion, as described by Miss Beck, the branchial appendage has become + reduced to the branchiæ, and the intrinsic appendage-muscles have entirely disappeared, with the + possible exception of the small post-stigmatic muscle; on the other hand, the dorso-ventral + somatic muscles, which are clearly homologous with the corresponding muscles of Limulus, have + remained, and become the essential respiratory muscles.</p> + + <p>Of these two possible types of respiratory movement it is quite conceivable that in the + water-breathing scorpions of olden times and in their allies, the dorso-ventral muscles of their + branchial appendages may have continued their <i>rôle</i> of respiratory muscles, and so have + given origin to the respiratory muscles of the ancestors of Ammocœtes.</p> + + <p>The respiratory muscles of Ammocœtes are three in number, and have been described by + Nestler and Miss Alcock as the adductor muscle, the striated constrictor muscle, and the tubular + constrictor muscle (Fig. <a href="#fig65">65</a>, <i>m. add.</i>, <i>m.c.s.</i>, and + <i>m.c.t.</i>). Of these, the constrictor muscle (Fig. <a href="#fig71">71</a>, <i>m. con. + str.</i>) is in close contact with its cartilaginous bar, while the adductor (Fig. <a + href="#fig71">71</a>, <i>m. add.</i>) is attached to the cartilage only at its origin and + insertion, and the tubular muscles (Fig. <a href="#fig71">71</a>, <i>m. con. tub.</i>) have + nothing whatever to do with the cartilage at all, being attached ventrally to the connective + tissue in the neighbourhood <span class="pagenum" id="page172">{172}</span>of the ventral aorta + (<i>V.A.</i>), and dorsally to the mid-line between the dorsal aorta (<i>D.A.</i>) and the + notochord.</p> + + <p>The close relationship of the constrictor muscle to the cartilaginous branchial bar does not + favour the surmise that this muscle is homologous with the dorso-ventral somatic muscle of the + scorpion. It is, however, directly in accordance with the view that this muscle is homologous with + one of the dorso-ventral appendage-muscles, such as the posterior entapophysio-branchial muscle + (<i>m<sub>1</sub></i>, Fig. <a href="#fig66">66</a>) of the Limulus appendage, especially when the + homology of the Ammocœtes branchial cartilage with the Limulus branchial cartilage is borne + in mind. I am, therefore, inclined to look upon the constrictor and adductor muscles of the + Ammocœtes branchial segment as more likely to have been derived from dorso-ventral muscles + which belonged originally to a branchial appendage, such as we see in Limulus, than from + dorso-ventral somatic muscles, such as the vertical mesosomatic muscles which are found both in + Limulus and scorpion. In other words, I am inclined to hold the view that the somatic + dorso-ventral muscles have disappeared in this region in Ammocœtes, while dorso-ventral + appendage-muscles have been retained, <i>i.e.</i> the exact reverse to what has taken place in the + air-breathing scorpion.</p> + + <p>I am especially inclined to this view because of the manner in which it fits in with and + explains van Wijhe's results. Ever since Schneider divided the striated muscles of vertebrates + into parietal and visceral, such a division has received general acceptance and, as far as the + head-region is concerned, has received an explanation in van Wijhe's work; for Schneider's + grouping corresponds exactly to the two segmentations of the head-mesoblast, discovered by van + Wijhe, <i>i.e.</i> to the somatic and splanchnic striated muscles according to my nomenclature. Of + these two groups the splanchnic or visceral striated musculature, innervated by the Vth, VIIth, + IXth, and Xth nerves, which ought on this theory to be derived from the musculature of the + corresponding appendages, is, speaking generally, dorso-ventral in direction in Ammocœtes + and of the same character throughout; the somatic musculature, on the other hand, is clearly + divisible, in the head region, into two sets—a spinal and a cranial set. The somatic muscles + innervated by the spinal set of nerves, including in this term the spino-occipital or so-called + hypoglossal nerves, are in Ammocœtes most sharply defined from all the other muscles of the + body. They form the great dorsal and ventral longitudinal <span class="pagenum" + id="page173">{173}</span>body-muscles, which extend dorsally as far forward as the nose and are + developed embryologically quite distinctly from the others, being formed as muscle-plates + (Kästchen). On the other hand, the cranial somatic muscles are the eye-muscles, the formation of + which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is + not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic + trunk-muscles, and must, therefore, form a separate group to themselves. Thus the striated + musculature of the Ammocœtes must be divided into (1) the visceral muscles; (2) the + longitudinal somatic muscles; and (3) the dorso-ventral somatic muscles. Of these the 1st, on the + view just stated, represent the original appendage-muscles; the 2nd belong to the spinal region, + and will be considered with that region; the 3rd represent the original segmental dorso-ventral + somatic muscles, which are so conspicuous in the musculature of the Limulus and the scorpion + group.</p> + + <p>The discussion of this last statement will be given when I come to deal with the prosomatic + segments of Ammocœtes. I wish, here, simply to point out that van Wijhe has shown that the + eye-muscles develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and therefore + represent the somatic muscles belonging to those segments, while no development of any + corresponding muscles takes place in the 4th, 5th, and 6th segments; so that if the eye-muscles + represent a group of dorso-ventral somatic muscles, such muscles have been lost in the 4th, 5th, + and 6th segments. The latter segments are, however, the glossopharyngeal and vagus segments, the + branchial musculature of which is derived from the ventral segments of the mesoderm. In other + words, van Wijhe's observations mean that the dorso-ventral somatic musculature has been lost in + the branchial or mesosomatic region, while the dorso-ventral appendage musculature has been + retained, and that, therefore, the mode of respiration in Ammocœtes more closely resembles + that of Limulus than of Scorpio.</p> + + <p>In addition to these branchial muscles, another and very striking set of muscles is found in + the respiratory region of Ammocœtes—the so-called tubular muscles. These muscles are + of great interest, but as they are especially connected with the VIIth nerve, their consideration + is best postponed to the chapter dealing with that nerve.</p> + + <p class="sp3">Also, in connection with the vagus group of nerves, special sense-organs are found + in the skin covering this mesosomatic region, the so-called epithelial pit-organs (<i>Ep. + pit.</i>, Fig. <a href="#fig71">71</a>). They, too, are of <span class="pagenum" + id="page174">{174}</span>great interest, but their consideration may also better be deferred to + the chapter dealing with those special sense-systems known as the lateral line and auditory + systems.</p> + + <p class="ac"><span class="sc">Comparison of the Branchial Circulation in Ammocœtes and + Limulus.</span></p> + + <p>Closely bound up with the respiratory system is the nature of the circulation of blood through + the gills. Before, therefore, proceeding to the consideration of the segments in front of those + which carry branchiæ, it is worth while to compare the circulation of the blood in the gills of + Limulus and of Ammocœtes respectively.</p> + + <p>In all the higher vertebrates the blood circulates in a closed system of capillaries, which + unite the arterial with the venous systems. In all the higher invertebrates this capillary system + can hardly be said to exist; the blood is pumped from the arterial system into blood spaces or + lacunæ, and thus comes into immediate contact with the tissues. From these it is collected into + veins, and so returned to the heart. There is, in fact, no separate lymph-system in the higher + invertebrates; the blood-system and lymph-system are not yet differentiated from each other. This + also is the case in Ammocœtes; here, too, in many places the blood is poured into a lacunar + space, and collected thence by the venous system; a capillary system is only in its commencement + and a lymph-system does not yet exist. In this part of its vascular system Ammocœtes again + resembles the higher invertebrates more than the higher vertebrates.</p> + + <p>This resemblance is still more striking when the circulation in the respiratory organs of the + two animals is compared. A branchial appendage is essentially an appendage whose vascular system + is arranged for the special purpose of aerating blood. In the higher vertebrates such a purpose is + attained by the pulmonary capillaries, in Limulus by the division of the posterior surface of the + basal part of the appendage into thin lamellar plates, the interior of each of which is filled + with blood. The two surfaces of each lamella are kept parallel to each other by means of fibrous + or cellular strands forming little pillars at intervals, called by Macleod "colonettes." A + precisely similar arrangement is found in the scorpion gill-lamella, as seen in Fig. <a + href="#fig69">69</a>, A, taken from Macleod. In Ammocœtes there are no well-defined + branchial capillaries, but the blood circulates, as in <span class="pagenum" + id="page175">{175}</span>the invertebrate gill, in a lamellar space; here, also, as Nestler has + shown, the opposing walls of the gill-lamella are held in position by little pillar-like cells, as + seen in Fig. <a href="#fig69">69</a>, B, taken from his paper.</p> + + <p>In this representative of the earliest vertebrates the method of manufacturing an efficient + gill out of a lacunar blood-space is precisely the same as that which existed in Limulus and the + scorpion, and, therefore, as that which existed in the dominant invertebrate group at the time + when vertebrates first appeared. This similarity indicates a close resemblance between the + circulatory systems of the two groups of animals, and therefore, to the superficial inquirer, + would indicate an homology between the heart of the vertebrate and the heart of the higher + invertebrate; but the former is situated ventrally to the gut and the nervous system, while the + latter is composed of a long vessel which lies in the mid-dorsal line immediately under the + external dorsal covering. Indeed, this ventral position of the heart in the one group of animals + and its dorsal position in the other, combined with the corresponding positions of the central + nervous system, is one of the principal reasons why all the advocates of the origin of vertebrates + from the Appendiculata, with the single exception of myself, feel compelled to reverse the dorsal + and ventral surfaces in deriving the vertebrate from the invertebrate. But there is one most + important fact which ought to make us hesitate before accepting the homology of the dorsal heart + of the arthropod with the ventral heart of the vertebrate—The heart in all invertebrates is + a systemic heart, <i>i.e.</i> drives the arterial blood to the different organs of the body, and + then the veins carry it back to the respiratory organ, from whence it passes to the heart.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig069.png" id="fig69"><img style="width:100%" src="images/fig069.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 69.—Comparison of Branchial Lamellæ of Limulus and Scorpio with + Branchial Lamellæ of Ammocœtes.</span></p> + <p class="sp0">A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of + Ammocœtes (after Nestler).</p> + </div> + </div> + + <p>The only exception to this scheme is found in the vertebrate where the heart is essentially a + branchial heart, the blood being <span class="pagenum" id="page176">{176}</span>driven from the + heart to the ventral aorta, from which by the branchial arteries it is carried to the gills, and + then, after aeration, is collected into the dorsal aorta, whence it is distributed over the body. + The distributing systemic vessel is the dorsal aorta, not the heart which belongs essentially to + the ventral venous system. This constitutes a very strong reason for believing that the systemic + heart of the invertebrate is not homologous with the heart of the vertebrate. How, then, did the + vertebrate heart arise?</p> + + <p>Let us first see how the blood is supplied to the gills in Limulus.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig070.png" id="fig70"><img style="width:100%" src="images/fig070.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 70.—Longitudinal Diagrammatic Section through the Mesosomatic + Region of Limulus, to show the origin of the Branchial Arteries.</span> (After <span + class="sc">Benham</span>.)</p> + <p class="sp0 ac"><i>L.V.S.</i>, longitudinal venous sinus, or collecting sinus; <i>a. + br.</i>, branchial arteries; <i>V.p.</i>, veno-pericardial muscles; <i>P.</i>, + pericardium.</p> + </div> + </div> + + <p>In Limulus the blood flows into the lamellæ from sinuses or blood-spaces (<i>b.s.</i>, Fig. <a + href="#fig66">66</a>) at the base of each of the lamellæ, which sinuses are filled by a vessel + which may be called the branchial artery, since it is the afferent branchial vessel. On each side + of the middle line of the ventral surface of the body a large longitudinal venous sinus exists, + called by Milne-Edwards the venous collecting sinus, <i>L.V.S.</i>, (Fig. <a href="#fig70">70</a> + and Fig. <a href="#fig58">58</a>), which gives off to each of the branchial appendages on that + side a well-defined afferent branchial vessel—the branchial artery (<i>a. br.</i>). The + blood of the branchial artery flows into the blood-spaces between the anterior and posterior + laminæ of the appendage and thence into the gill-lamellæ, from which it is collected into an + efferent vessel or branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which + carries it back to the dorsal heart. The position of the branchial artery and vein is shown in + Fig. <a href="#fig66">66</a>, which represents a section through the branchial appendage of + Limulus at right angles to the cartilaginous branchial bar (<i>br. cart.</i>), just as Fig. <a + href="#fig65">65</a> represents a section through the <span class="pagenum" + id="page177">{177}</span>branchial appendage of Ammocœtes at right angles to the + cartilaginous branchial bar.</p> + + <p>Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and Benham concur in + showing that in both Limulus and the scorpion group a striking and most useful connection exists + between the heart and these two collecting venous sinuses, in the shape of a segmentally arranged + series of muscular bands (<i>V.p.</i>, Fig. 70 and Fig. <a href="#fig58">58</a>), attached, on the + one hand, to the pericardium, and on the other to the venous collecting sinus on each side. These + muscular bands, to which Lankester and Benham have given the name of 'veno-pericardial muscles,' + are so different in appearance from the rest of the muscular substance, that Milne-Edwards did not + recognize them as muscular, but called them 'brides transparentes.' Blanchard speaks of them in + the scorpion as 'ligaments contractiles,' and considers that they play an important part in + assisting the pulmonary circulation; for, he says, "en mettant a nu une portion du cœur, on + remarque que ces battements se font sentir sur les ligaments contractiles, et determinent sur les + poches pulmonaires une pression qui fait aussitot refluer et remonter le sang dans les vaisseaux + pneumocardiaques." Lankester, in discussing the veno-pericardial muscles of Limulus and of the + scorpions, says that these muscles probably contract simultaneously with the heart and are of + great importance in assisting the flow through the pulmonary system. More recently Carlson has + investigated the action of these muscles in the living Limulus and found that they act + simultaneously with the muscles of respiration.</p> + + <p>Precisely the same arrangement of veno-pericardial muscles and of longitudinal venous + collecting sinuses occurs in the scorpions. It is one of the fundamental characters of the group, + and we may fairly assume that a similar arrangement existed in the extinct forms from which I + imagine the vertebrate to have arisen. The further consideration of this group of muscles will be + given in Chapter IX.</p> + + <p>Passing now to the condition of the branchial blood-vessels of Ammocœtes, we see that + the blood passes into the gill-lamellæ from a blood-space in the appendage, which can hardly be + dignified by the name of a blood-vessel. This blood-space is supplied by the branchial artery + which arises segmentally from the ventral aorta (<i>V.A.</i>), as seen in Fig. <a + href="#fig71">71</a> (taken from Miss Alcock's paper). From the gill-lamellæ the blood is + collected into an efferent or branchial vein (<i>v. br.</i>), which <span class="pagenum" + id="page178">{178}</span>runs, as seen in Fig. <a href="#fig65">65</a>, along the free edge of the + diaphragm, and terminates in the dorsal aorta.</p> + + <p>The ventral aorta is a single vessel near the heart, but at the commencement of the thyroid it + divides into two, and so forms two ventral longitudinal vessels, from which the branchial arteries + arise segmentally.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig071.jpg" id="fig71"><img style="width:72%" src="images/fig071.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 71.—Diagram constructed from a series of Transverse Sections + through a Branchial Segment, showing the arrangement and relative positions of the Cartilage, + Muscles, Nerves, and Blood-Vessels.</span></p> + <p class="sp0">Nerves coloured red are the motor nerves to the branchial muscles. Nerves + coloured blue are the internal sensory nerves to the diaphragms and the external sensory + nerves to the sense-organs of the lateral line system. <i>Br. cart.</i>, branchial cartilage; + <i>M. con. str.</i>, striated constrictor muscles; <i>M. con. tub.</i>, tubular constrictor + muscles; <i>M. add.</i>, adductor muscle; <i>D.A.</i>, dorsal aorta; <i>V.A.</i>, ventral + aorta; <i>S.</i>, sense-organs on diaphragm; <i>n. Lat.</i>, lateral line nerve; <i>X.</i>, + epibranchial ganglia of vagus; <i>R. br. prof. VII.</i>, <i>ramus branchialis profundus</i> of + facial; <i>J.v.</i>, jugular vein; <i>Ep. pit.</i>, epithelial pit.</p> + </div> + </div> + + <p>From this description it is clear that the vascular supply of the branchial segment of + Ammocœtes would resemble most closely the vascular supply of the Limulus branchial + appendage, if the ventral aorta of the former was derived from two longitudinal veins, homologous + with the paired longitudinal venous sinuses of the latter.</p> + + <div><span class="pagenum" id="page179">{179}</span></div> + + <p class="sp3"><i>A priori</i>, such a derivation seems highly improbable; and yet it is precisely + the manner in which embryology teaches us that the heart and ventral aorta of the vertebrate have + arisen.</p> + + <p class="ac"><span class="sc">The Origin of the Invertebrate Heart and the Origin of the + Vertebrate Heart.</span></p> + + <p>Not only does the vertebrate heart differ from that of the invertebrate, in that it is + branchial while the latter is systemic, but also it is unique in its mode of formation in the + embryo. In the Appendiculata the heart is formed as a single organ in the mid-dorsal line by the + growth of the two lateral plates of mesoblast dorsalwards, the heart being formed where they meet. + In Mammalia and Aves, the heart and ventral aorta commence as a pair of longitudinal veins, one on + each side of the commencing notochord.</p> + + <p>If the embryo be removed from the yolk, the surface of the embryo covering these two venous + trunks can be spoken of as the ventral surface of the embryo at that stage, and indeed we find + that in the present day there is an increasing tendency to speak of this surface as the ventral + surface of the embryo. Thus, Mitsukuri, in his studies of chelonian embryos, lays great stress on + the importance of surface views and when the embryo has been removed from the yolk, figures and + speaks of its ventral surface. So, also, Locy and Neal find that the best method of seeing the + early segments of the embryo is to remove the embryo from the yolk, and examine what they speak of + as a ventral view. At the period, then, before the formation of the throat, we may say that on the + ventral surface of the embryo a pair of longitudinal venous sinuses are found, one on each side of + the mid-ventral line, which are in the same position with respect to the mid-axis of the embryo as + are the longitudinal venous sinuses in Limulus.</p> + + <p>The next step is the formation of the throat by the extension of the layers of the embryo + laterally to meet in the mid-line and so form the pharynx, with the consequence that a new ventral + surface is formed; these two veins, as is well known, travel round also, and, meeting together in + the new mid-ventral line, form the subintestinal vein, the heart, and the ventral aorta.</p> + + <p>What is true of Mammalia and Aves, has been shown by P. Mayer to be true universally among + vertebrates, so that in all cases the heart and ventral aorta have arisen by the coalescence in + the new mid-ventral <span class="pagenum" id="page180">{180}</span>line of two longitudinal venous + channels, which were originally situated one on each side of the notochord, in what was then the + ventral surface of this part of the embryo. This history is especially instructive in showing how + the pharyngeal region is formed by the growing round of the lateral mesoblast, <i>i.e.</i> the + muscular and other mesoblastic tissues of the branchial segments, and how the two longitudinal + veins take part in this process. The phylogenetic interpretation of this embryological fact seems + to be, that the new ventral surface of the vertebrate in this region is formed, not only by the + branchial appendages, but also by the growth ventrally of that part of the original ventral + surface which covered each longitudinal venous sinus.</p> + + <p>The following out of the consecutive clues, which one after the other arise in harmonious + succession as the necessary sequence of the original working hypothesis, brings even now into view + the manner in which the respiratory portion of the alimentary canal arose, and gives strong hints + as to the position of that part of the arthropod which gave origin to the notochord. Here I will + say no more at present, for the origin of the new alimentary canal of the vertebrate and of the + notochord will be more fittingly discussed as a whole, after all the other organs of the + vertebrate have been compared with the corresponding organs of the arthropod.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig072.png" id="fig72"><img style="width:44%" src="images/fig072.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 72.—Diagram (Upper Half of Figure) of the Original Position of + Veins (H) which come together to form the Heart of a Vertebrate.</span></p> + <p><i>C.N.S.</i>, central nervous system; <i>nc.</i>, notochord; <i>m.</i>, myotome.</p> + <p class="sp0">The lower half of figure shows comparative position of the longitudinal venous + sinus (<i>L.V.S.</i>) in Limulus. <i>C.N.S.</i>, central nervous system; <i>Al.</i>, + alimentary canal; <i>H.</i>, heart; <i>m.</i>, body-muscles.</p> + </div> + </div> + + <p>The strong evidence that the vertebrate heart was formed from a pair of longitudinal venous + sinuses on the ventral side of the central canal, carries with it the conclusion that the original + single median dorsal heart of the arthropod is not represented in the vertebrate, <span + class="pagenum" id="page181">{181}</span>for the dorsal aorta cannot by any possibility represent + that heart.</p> + + <p>Although it is not now functional the original existence of so important an organ as a dorsal + heart may have left traces of its former presence; if so, such traces would be most likely to be + visible in the lowest vertebrates, just as the median eyes are much more evident in them than in + the higher forms. In Fig. <a href="#fig58">58</a> the position of the dorsal heart is shown in + Limulus, and in Fig. <a href="#fig70">70</a> the shape and extent of this dorsal heart is shown. + It extends slightly into the prosomatic region, and thins down to a point there, runs along the + length of the animal and finally thins down to a point at the caudal end.</p> + + <p>The heart is surrounded by a pericardium, from which at regular intervals a number of + dorso-ventral muscles pass, to be inserted into the longitudinal venous sinus on each side. These + veno-pericardial muscles are absolutely segmental with the mesosomatic segments, and are confined + to that region, with the exception of two pairs in the prosomatic region. Their homologies will be + discussed later.</p> + + <p>Any trace of a heart such as we have just described must be sought for in Ammocœtes + between the central nervous system and the mid-line dorsally. Now, in this very position a large + striking mass of tissue is found, represented in section in Fig. <a href="#fig73">73</a>, + <i>f</i>. It forms a column of similar tissue along the whole mid-dorsal region, except at the two + extremities; it tapers away in the caudal region, and headwards grows thinner and thinner, so that + no trace of it is seen anterior to the commencement of the branchial region. It resembles in its + dorsal position, in its shape, and in its size a dorsal heart-tube such as is seen in Limulus and + elsewhere, but it differs from such a tube in its extension headwards. The heart-tube of Limulus + ceases at the anterior end of the mesosomatic region, this fat-column of Ammocœtes at the + posterior end. In its structure there is not the slightest sign of anything of the nature of a + heart; it is a solid mass of closely compacted cells, and the cells are all very full of fat, + staining intensely black with osmic acid. Nowhere else in the whole body of Ammocœtes is + such a column of fat to be found. It is not skeletogenous tissue with cells of the nature of + cartilage-cells, as Gegenbaur thought and as Balfour has depicted (Vol. II., Fig. 315) in his + 'Comparative Embryology,' as though this tissue were a part of the vertebral column, but is simply + fat-cells, such as might easily have taken the place of some other previously existing organ.</p> + + <div><span class="pagenum" id="page182">{182}</span></div> + + <p>I do not know how to decide the question which thus arises. Supposing, for the sake of + argument, that this column of fat-cells has really taken the place of the original dorsal heart, + what criterion would there be as to this? The heart <i>ex hypothesi</i> having ceased to function, + the muscular tissue would not remain, and the space would be filled up, presumably with some form + of connective tissue. As likely as not, the connective tissue might take the form of fatty tissue, + the storage of fat being a physiological necessity to an animal, while at the same time no special + organ has been developed for such a purpose, but fat is being laid down in all manner of places in + the body.</p> + + <p>This dorsal fat-column, as it is seen in Ammocœtes, is not found in the higher + vertebrates, so that it possesses, at all events, the significance of being a peculiarity of + ancient times before the vertebrate skeletal column was formed.</p> + + <p>I mention it here in connection with my view as to the origin of vertebrates, because there it + is, in the very place where the dorsal heart ought to have been. For my own part, I should not + have expected that a muscular organ such as the heart would leave any trace of itself if it + disappeared, so that its absence in the dorsal region of the vertebrate does not seem to me in the + slightest degree to invalidate my theory.</p> + + <div class="ac w30 fcenter sp3"> + <a href="images/fig073.png" id="fig73"><img style="width:100%" src="images/fig073.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 73.—Section through the Notochord (<i>nc.</i>), the Spinal + Canal and the Fat-column (<i>f.</i>), of Ammocœtes, drawn from an Osmic + Preparation.</span></p> + <p class="sp0"><i>sp. c.</i>, spinal cord; <i>gl.</i>, glandular tissue filling the spinal + canal; <i>sk.</i>, Gegenbaur's skeletogenous cells; <i>p.</i>, pigment.</p> + </div> + </div> + + <div><span class="pagenum" id="page183">{183}</span></div> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>From the close similarity of structure and position between the branchial skeleton of Limulus + and of Ammocœtes, as given in the preceding chapter, it logically follows that the + branchiæ of Ammocœtes must be homologous with the branchiæ of Limulus. But the + respiratory apparatus of Limulus consists of branchial appendages. It follows, therefore, that + the branchiæ of Ammocœtes, and consequently of the vertebrates, must have been derived + from branchial appendages, and as they are internal, not external, such branchial appendages + must have been of the nature of 'sunk-in' branchial appendages. Such internal appendages are + characteristic of the scorpion tribe, and of, perhaps, the majority of the Palæostraca, for no + external respiratory appendages have been discovered in any of the sea-scorpions.</p> + <p>In the vertebrates—and it is especially well shown in Ammocœtes—a double + segmentation exists in the head-region, a body or somatic segmentation, and a branchial or + splanchnic segmentation, respectively expressed by the terms mesomeric and branchiomeric + segmentations. The nerves which supply the latter segments form a very well-marked group + (Charles Bell's system of lateral or respiratory nerves) which do not conform to the system of + spinal nerves, for they do not arise from separate motor and sensory roots, but are mixed nerves + from the very beginning.</p> + <p>The system of cranial segmental nerves is older than the spinal system, and cannot, + therefore, be derived from it, but can be arranged as a system supplying two segments, somatic + and splanchnic, which differ in the following way: Each somatic segment is supplied by two + roots, motor and sensory respectively, as in the spinal cord segments, while each splanchnic + segment possesses only one root, which is mixed in function.</p> + <p>The peculiarities of the grouping of the cranial segmental nerves, which have hitherto been + unexplained, immediately receive a straightforward and satisfactory explanation if the + splanchnic or branchiomeric segments owe their origin to a system of appendages after the style + of those of Limulus.</p> + <p>In Limulus and all the Arthropoda, the segmentation is double, being composed of (1) somatic + or body-segments, constituting the mesomeric segmentation; (2) appendage-segments, which, seeing + that they carry the branchiæ, constitute a branchiomeric segmentation. Similarly to the cranial + region of the vertebrate, the nerves which supply the somatic segments arise from separate + sensory and motor roots, while the single nerve which supplies each appendage contains all the + fibres for the appendage, both motor and sensory.</p> + <p>It follows from this that the branchial segments supplied by the vagus and glossopharyngeal + nerves ought to have arisen from appendages bearing branchiæ.</p> + <p>Although the evidence of such appendages has entirely disappeared in the higher vertebrates, + together with the disappearance of branchiæ, and is not strikingly apparent in the higher + gill-bearing fishes, yet in Ammocœtes, so great is the difference here from all other + fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which + a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. Each of + these appendages possesses its own mixed nerve, glossopharyngeal or vagus, <span class="pagenum" + id="page184">{184}</span>its own cartilage, its own set of visceral muscles, its own + sense-organs, just as do the respiratory appendages of Limulus.</p> + <p>The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or + appendage between the pouches. Embryology shows how each such appendage grows inwards, how a + cœlomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in + scorpions.</p> + <p>We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the + present day somewhat in form, but they are in many respects closely allied to Limulus. The + present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral + somatic muscles, while those of Limulus are the appendage-muscles.</p> + <p>The old sea-scorpions very probably used their appendage-muscles after the Limulus fashion, + being water-breathers, even although their respiratory appendages were no longer free but sunk + in below the surface of the body. The probability that such was the case is increased after + consideration of the method of breathing in Ammocœtes, for the respiratory muscles of the + latter animal are directly comparable with the muscles of the respiratory appendages of Limulus, + and are not somatic. Even the gills themselves of Ammocœtes are built up in the same + fashion as are those of Limulus and the scorpions. The conception of the branchial unit as a + gill-bearing appendage, not a gill-pouch, immediately explains the formation of the vertebrate + heart, which is so strikingly different from that of all invertebrate hearts, in that it + originates as a branchial and not as a systemic heart, and is formed by the coalescence of two + longitudinal veins.</p> + <p class="sp0">The origin of these two longitudinal veins is immediately apparent if the + vertebrate arose from a palæostracan, for in Limulus and the whole scorpion tribe, in which the + heart is a systemic heart, the branchiæ are supplied with blood from two large longitudinal + venous sinuses, situated on each side of the middle line of the animal in an exactly + corresponding position to that of the two longitudinal veins, which come together to form the + heart and ventral aorta of the vertebrate. The consideration of the respiratory apparatus and of + its blood-supply in the vertebrate still further points to the origin of vertebrates from the + Palæostraca.</p> + </div> + + <div><span class="pagenum" id="page185">{185}</span></div> + + <p class="ac">CHAPTER V</p> + + <p class="ac"><i>THE EVIDENCE OF THE THYROID GLAND</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">The value of the appendage-unit in non-branchial segments.—The double + nature of the hyoid segment.—Its branchial part.—Its thyroid part.—The double + nature of the opercular appendage.—Its branchial part.—Its genital + part.—Unique character of the thyroid gland of Ammocœtes—Its + structure.—Its openings.—The nature of the thyroid segment.—The uterus of the + scorpion.—Its glands.—Comparison with the thyroid gland of + Ammocœtes.—Cephalic genital glands of Limulus.—Interpretation of glandular + tissue filling up the brain-case of Ammocœtes.—Function of thyroid + gland.—Relation of thyroid gland to sexual functions.—Summary.</p> + </div> + + <p>I have now given my reasons why I consider that the glossopharyngeal and vagus nerves were + originally the nerves belonging to a series of mesosomatic branchial appendages, each of which is + still traceable in the respiratory chamber of Ammocœtes, and gives the type-form from which + to search for other serially homologous, although it may be specially modified, segments.</p> + + <p class="sp3">As long as the branchial unit consisted of the gill-pouch the segments of the + head-region were always referred to such units, hence we find Dohrn and Marshall picturing to + themselves the ancestor of vertebrates as possessing a series of branchial pouches right up to the + anterior end of the body. Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of + the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying + modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on + this earth, or could be reasonably imagined to have lived on it. Yet Dohrn's conceptions of the + segmentation were sound, his interpretation only was in fault, because he was obliged to express + his segments in terms of the gill-pouch unit. Once abandon that point of view and take as the unit + a branchial appendage, then immediately we see that in the region in front of the branchiæ we may + still have segments <span class="pagenum" id="page186">{186}</span>homologous to the branchial + segments, originally characterized by the presence of appendages, but that such appendages need + never have carried branchiæ. The new mouth may have been formed by such appendages, which would + express Dohrn's suggestion of its formation by coalesced gill-slits; the olfactory organ may have + been the sense-organ belonging to an antennal appendage, which would be what Marshall really meant + in calling it a branchial sense-organ.</p> + + <p class="ac"><span class="sc">The Facial Nerve and the Foremost Respiratory Segment.</span></p> + + <p>This simple alteration of the branchiomeric unit from a gill-pouch to an appendage, which may + or may not bear branchiæ, immediately sheds a flood of light on the segmentation of the + head-region, and brings to harmony the chaos previously existing. Let us, then, follow out its + further teachings. Next anteriorly to the glossopharyngeal and vagus nerves comes the facial + nerve; a nerve which supplies the hyoid segment, or, rather, according to van Wijhe the two hyoid + segments, for embryologically there is evidence of two segments. As already mentioned, the facial + nerve is usually included in the trigeminal or pro-otic group of nerves, the opisthotic group + being confined to the glossopharyngeal and vagus. This inclusion of the facial nerve into the + pro-otic group of nerves forms one of the main reasons why this group has been supposed to have + originally supplied gill-pouch segments, for the hyoid segment is clearly associated with + branchiæ.</p> + + <p>When, however, we examine Ammocœtes (<i>cf.</i> Figs. 63 and 64) it is clear that the + foremost of the segments forming the respiratory chamber, which must be classed with the rest of + the mesosomatic or opisthotic segments, is that supplied by the facial nerves.</p> + + <p>An examination of this respiratory chamber shows clearly that there are six pairs of branchial + appendages or diaphragms, which are all exactly similar to each other. These are those already + considered, the foremost of which are supplied by the IXth or glossopharyngeal nerves. Immediately + anterior to this glossopharyngeal segment is seen in the figures the segment supplied by the VIIth + or facial nerves. It is so much like the segments belonging to the glossopharyngeal and vagus + nerves as to make it certain that we are dealing here with a branchial segment, composed of a pair + of branchial appendages similar to those in the other cases, except that the cartilaginous bar is + here replaced by a bar of muco-cartilage and the branchiæ are confined to the posterior part of + each appendage. The anterior portion is, as is seen in Fig. <a href="#fig74">74</a>, largely + occupied by blood-spaces, but in addition carries the ciliated groove (<i>ps. br.</i>) called by + Dohrn 'pseudo-branchiale Rinne.' This groove leads directly into the thyroid gland, which is a + large bilateral organ situated in the middle line, as seen in Fig. <a href="#fig80">80</a> and + Fig. <a href="#fig85">85</a>. As shown by Miss Alcock, the facial nerve supplies this thyroid + gland, as well as the posterior hyoid branchial segment, and, as pointed out by Dohrn, there is + every reason to consider this thyroid gland as indicative of a separate segment, especially when + van Wijhe's statement that the hyoid segment is in reality double is taken into account.</p> + + <div><span class="pagenum" id="page187">{187}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig074.jpg" id="fig74"><img style="width:88%" src="images/fig074.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 74.—Ventral half of Head-region of + Ammocœtes.</span></p> + <p class="sp0">Somatic muscles coloured red. Branchial and visceral muscles coloured blue. + Tubular constrictor muscles distinguished from striated constrictor muscles by simple + hatching. <i>Tent.</i>, tentacles; <i>Tent. m.c.</i>, muco-cartilage of tentacles; <i>Vel. + m.c.</i>, muco-cartilage of the velum; <i>Hy. m.c.</i>, muco-cartilage of the hyoid segment; + <i>Ps. br.</i>, pseudo-branchial groove; <i>Br. cart.</i>, branchial cartilages; <i>Sp.</i>, + space between somatic and splanchnic muscles; <i>Th. op.</i>, orifice of thyroid; <i>H.</i>, + heart.</p> + </div> + </div> + + <div><span class="pagenum" id="page188">{188}</span></div> + + <p class="sp3">The evidence, then, of Ammocœtes points directly to this conclusion: The + facial nerves represent the foremost of the mesosomatic group of nerves, and supply two segments, + which have amalgamated with each other. The most posterior of these, the hyoid segment, is a + branchial segment of the same character as those supplied by the vagus and glossopharyngeal + nerves; represents, therefore, the foremost pair of branchial appendages. The anterior or thyroid + segment, on the other hand, differs from the rest in that, instead of branchiæ, it carries the + thyroid gland with its two ciliated grooves. If this segment, which is the foremost of the + mesosomatic segments, also indicates a pair of appendages which carry the thyroid gland instead of + branchiæ, then it follows that this pair of appendages has joined together in the mid-line + ventrally and thus formed a single median organ—the thyroid gland. If, then, we find that + the foremost of the mesosomatic appendages in the Palæostraca was really composed of two pairs of + appendages, of which the most posterior carried branchiæ, while the anterior pair had amalgamated + in the mid-line ventrally, and carried some special organ instead of branchiæ, then the + accumulation of coincidences is becoming so strong as to amount to proof of the correctness of our + line of investigation.</p> + + <p class="ac"><span class="sc">The First Mesosomatic Segment in Limulus and its Allies.</span></p> + + <p>What, then, is the nature of the foremost pair of mesosomatic appendages in Limulus. They + differ from the rest of the mesosomatic appendages in that they do not carry branchiæ, and instead + of being <span class="pagenum" id="page189">{189}</span>separate are joined together in the + mid-line ventrally to form a single terminal plate-like appendage known as the operculum. On its + posterior surface the operculum carries the genital duct on each side.</p> + + <p>So also in the scorpion group, the operculum is always found and always carries the genital + ducts.</p> + + <p>A survey of the nature of the opercular appendage demonstrates the existence of three different + types—</p> + + <p>1. That of Limulus, in which the operculum is free, and carries only the terminations of the + genital ducts. In this type the duct on each side opens to the exterior separately (Fig. <a + href="#fig75">75</a>).</p> + + <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:45%;"><a href="images/fig075.png" id="fig75"><img + style="width:100%" src="images/fig075.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:45%;"><a href="images/fig076.png" id="fig76"><img + style="width:100%" src="images/fig076.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 75.—operculum of Limulus To Show the two + separate Genital Ducts.</span></p> + </div> + </td> + <td> + <div class="smaller ac"> + <p><span class="sc">Fig. 76.—Operculum of Male Scorpion.</span></p> + <p class="sp0"><i>Ut.</i>, terminal chamber, or uterus.</p> + </div> + </td> + </tr> + </table> + + <p>2. The type of Scorpio, Androctonus, Buthus, etc., in which the operculum is not free, but + forms part of the ventral surface of the body-wall, but, like Limulus, carries only the + terminations of the genital ducts. In this type the duct on each side terminates in a common + chamber (vagina or uterus), which communicates with the exterior by a single external median + opening. This common chamber, or uterus (<i>Ut.</i>), extends the whole breadth of the operculum + (as seen in Fig. <a href="#fig76">76</a>), and is limited to that segment.</p> + + <p>3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the Pedipalpi, in which + the operculum forms a part of the ventral surface of the body wall, but no longer covers only the + termination of the genital apparatus. It really consists of two parts, a median anterior, which + covers the terminal genital apparatus, <span class="pagenum" id="page190">{190}</span>and a + lateral posterior, which covers the first pair of gills, or lung-books, as they are called. In + this type (Fig. <a href="#fig77">77</a>) the genital ducts terminate in a common chamber or + uterus, the nature of which will be further considered.</p> + + <p>As has been pointed out by Blanchard, the terminal genital organs of the scorpions and the + Pedipalpi vary considerably in the different genera, especially the male genital organs. The + general type of structure is the same, and consists in both male and female of vasa deferentia, + which come together to form a common chamber before the actual opening to the exterior. This + common chamber has been called in the female scorpion the vagina, or in Thelyphonus the uterus. I + shall use the latter term, in accordance with Tarnani's work, and the corresponding chamber in the + male will be the <i>uterus masculinus</i>.</p> + + <p>A considerable discussion has taken place about the method of action of the external genital + organs in the members of the scorpion tribe, into which it is hardly necessary to enter here. The + evidence points to the conclusion that in all these forms the operculum covers a median single + chamber or uterus, into which the genital ducts open on each side, the main channels of emission + being provided with a massive chitinous internal framework. We may feel certain that in the old + extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed, and that therefore in them + also the median portion of the operculum covered a median chamber or uterus composed of the + amalgamation of the terminations of the two genital ducts, which were originally separate, as in + Limulus.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig077.png" id="fig77"><img style="width:62%" src="images/fig077.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 77.—Operculum and Following Segments Of Male + Thelyphonus.</span></p> + <p class="sp0">Opercular segment is marked out by thick black line. <i>Ut. Masc.</i>, uterus + masculinus; <i>Int. Op.</i>, internal opening of uterus into genital chamber; <i>Ext. Op.</i>, + common external opening to genital chamber (<i>Gen. Ch.</i>) and pulmonary chamber.</p> + </div> + </div> + + <p>The observations of Schmidt, Zittel, and others show that the <span class="pagenum" + id="page191">{191}</span>operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, etc., + belonged to the type of Thelyphonus, rather than to that of Limulus or Scorpio. In Fig. <a + href="#fig78">78</a> I give a picture from Schmidt of the ventral aspect of Eurypterus, and by the + side of it a picture of the isolated operculum. Schmidt considers that there were five + branchiæ-bearing segments constituting the mesosoma, the foremost of which formed the operculum. + Such operculum is often found isolated, and is clearly composed of two lateral appendages fused + together in the middle line, of such a nature as to form a median elongated tongue, which lies + between and separates the first three pairs of branchial segments. This median tongue, together + with the anterior and median portion of the operculum, concealed, in all probability, according to + Schmidt, the terminal parts of the genital organs, just as the median part of the operculum in + Phrynus and Thelyphonus conceals the complicated terminal portions of the genital organs. The + posterior part of the operculum, like that of Phrynus and Thelyphonus, carried the first pair of + branchiæ, so Schmidt thinks from the evidence of markings on some specimens.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig078.png" id="fig78"><img style="width:50%" src="images/fig078.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 78.</span>—<i>Eurypterus.</i></p> + <p class="sp0">The segments and appendages on the right are numbered in correspondence with + the cranial system of lateral nerve-roots as found in vertebrates. <i>M.</i>, metastoma. The + surface ornamentation is represented on the first segment posterior to the branchial segments. + The opercular appendage is marked out by dots.</p> + </div> + </div> + + <p>Apparently an opercular appendage of this kind is in reality the result of a fusion of the + genital operculum with the first branchial appendage in forms such as the scorpion; for, in order + that the tergal plates may correspond in number with the sternal in Eurypterus, etc., it is + necessary to consider that the operculum is composed of two sternites joined together. Similarly + in Thelyphonus, Phrynus, etc., this numerical correspondence is only observed if the operculum is + looked upon as double.</p> + + <p>A restoration of the mesosomatic region of Eurypterus, viewed <span class="pagenum" + id="page192">{192}</span>from the internal surface, might be represented by Fig. <a + href="#fig79">79</a>, in which the thick line represents the outline of the opercular segment, and + the fainter lines the succeeding branchial segments. The middle and anterior part of the opercular + segment carried the terminations of the genital organs; these I have represented, in accordance + with our knowledge of the nature of these organs in the present-day scorpions, as a median + elongated uterus, bilaterally formed, from which the genital ducts passed, probably as in Limulus, + towards a mass of generative gland in the cephalic region, and not as in Scorpio or Thelyphonus, + tailwards to the abdominal region.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig079.png" id="fig79"><img style="width:52%" src="images/fig079.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 79.—Diagram To indicate the probable nature of the Mesosomatic + Segments of Eurypterus.</span></p> + <p class="sp0">The opercular segment is marked out by the thick black line. The segments + <i>II.-VI.</i> bear branchiæ, and segment <i>I.</i> is supposed in the male to carry the + uterus masculinus (<i>Ut. Masc.</i>) and the genital ducts.</p> + </div> + </div> + + <p class="sp3">It is possible that in Holm's representation of Eurypterus, Fig. 104, the genital + duct on each side is indicated.</p> + + <p class="ac"><span class="sc">The Thyroid Gland of Ammocœtes.</span></p> + + <p>If we compare this mesosomatic region of Eurypterus with that of Ammocœtes, the + resemblance is most striking, and gives a meaning to the facial nerve which is in absolute + accordance with the interpretation already given of the glossopharyngeal and vagus nerves. In both + cases the foremost respiratory or mesosomatic segment is double, the posterior lateral part alone + bearing the branchiæ, while the median and anterior part bore in the one animal the uterus and + genital ducts, in the other the thyroid gland and ciliated grooves. We are driven, therefore, to + the conclusion that this extraordinary and unique organ, the so-called thyroid gland of + Ammocœtes, which exists only in the larval condition and is got rid of as soon as the adult + sexual organs are formed, shows the very form and position of the uterus of this invertebrate + ancestor of Ammocœtes. What, then, is the nature of the thyroid gland in + Ammocœtes?</p> + + <div><span class="pagenum" id="page193">{193}</span></div> + + <p>Throughout the vertebrate kingdom it is possible to compare the thyroid gland of one group of + animals with that of another without coming across any very marked difference of structure right + down to and including Petromyzon. When, however, we examine Ammocœtes, we find that the + thyroid has suddenly become an organ of much more complicated structure, covering a much larger + space, and bearing no resemblance to the thyroid glands of the higher forms. At transformation the + thyroid of Ammocœtes is largely destroyed, and what remains of the gland in Petromyzon + becomes limited to a few follicles resembling those of other fishes. The structure and position of + this gland in Ammocœtes is so well known that it is unnecessary to describe it in detail. + For the purpose, however, of making my points clear, I give in Fig. <a href="#fig80">80</a> the + position and appearance of the thyroid gland (<i>Th.</i>) when the skin and underlying laminated + layer has been removed by the action of hypochlorite of soda. On the one side the ventral somatic + muscles have been removed to show the branchial cartilaginous basket-work.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig080.png" id="fig80"><img style="width:45%" src="images/fig080.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 80.—Ventral View of Head Region of Ammocœtes.</span></p> + <p class="sp0"><i>Th.</i>, thyroid gland; <i>M.</i>, lower lip, with its muscles.</p> + </div> + </div> + + <p>The series of transverse sections in Fig. <a href="#fig81">81</a> represents the nature of the + organ at different levels in front of and behind the opening into the respiratory chamber; and in + Fig. <a href="#fig82">82</a> I have sketched the appearance of the whole gland, viewed so as to + show its opening into the respiratory chamber and its posterior curled-up termination.</p> + + <div><span class="pagenum" id="page194">{194}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig081.png" id="fig81"><img style="width:100%" src="images/fig081.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 81.—Samples from a Complete Series of Transverse + Sections through the Thyroid Gland of Ammocœtes.</span></p> + <p class="sp0">Sections 1 and 2 are anterior to the thyroid opening, <i>Th. o.</i>; sections + 3, 4, and 5 are through the thyroid opening; and section 6 is posterior to the thyroid opening + before the commencement of the curled portion.</p> + </div> + </div> + + <div><span class="pagenum" id="page195">{195}</span></div> + + <p>The series of transverse sections (1-6, Fig. <a href="#fig81">81</a>) show that we are dealing + here with a central glandular chamber, C (Fig. <a href="#fig81">81</a> (6) and Fig. <a + href="#fig82">82</a>), which opens by the thyroid duct (<i>Th. o.</i>) into the pharyngeal + chamber, and is curled upon itself in its more posterior part. This central chamber divides, + anteriorly to the thyroid orifice, into two portions, A, A′ (Fig. <a href="#fig82">82</a>), + giving origin to two tubes, B, B′, which lie close alongside of, and extend further back + than, the posterior limit of the curled portion of the central chamber, C. The structure of the + central chamber, C, and, therefore, of the separate coils, is given in both Schneider's and + Dohrn's pictures, and is represented in Fig. <a href="#fig81">81</a> (6), which shows the peculiar + arrangement and character of the glandular cells typical of this organ, and also the nature of the + central cavity, with the arrangement of the ciliated epithelium. The structure of each of the + lateral tubes, B, is different from that of the central chamber, in that only half the central + chamber is present in them, as is seen by the comparison of the tube B with the tube C in Fig. <a + href="#fig81">81</a> (5 and 6), so that we may look upon the central chamber, C, as formed of two + tubes, similar in structure to the tubes B, which have come together to form a single chamber by + the partial absorption of their walls, the remains of the wall being still visible as the septum, + which partially divides the chamber, C, into halves.</p> + + <p>In the walls of each of these tubes is situated a continuous glandular line, the structure of + the glandular elements being specially characterized by the length of the cells, by the large + spherical nucleus situated at the very base of each cell, and by the way in which the cells form a + wedge-shaped group, the thin points of all the wedge-shaped cells coming together so as to form a + continuous line along the chamber wall. This free termination of the cells of the gland in the + lumen of the chamber constitutes the whole method for the secretion of the gland; there is no + duct, no alveolus, nothing but this free termination of the cells.</p> + + <p>Moreover, sections through the portion A, A′ (Fig. <a href="#fig82">82</a>) show that + here, as in the central chamber, C, four of these glandular lines open into a common chamber, but + they are not the same four as in the case of the central chamber, for if we name these glandular + lines on the left side <i>a b, a′ b′</i> (Fig. <a href="#fig81">81</a>), and on the + right side <i>c d, c′ d′</i>, then the central chamber has opening into it the glands + <i>a b, c d</i>, while the chambers of A and A′ have opening into them respectively <i>a b, + a′ b′</i>, and <i>c d, c′ d′</i>. Further, the same series of sections + shows that the glands <i>a</i> and <i>b</i> are continuous with the glands <i>a′</i> and + <i>b′</i> respectively across the apex of A, and similarly on the other side, so that the + two glandular rows <i>a b</i> are continuous with the two glandular rows <i>a′ b′</i>, + and we see that the <span class="pagenum" id="page196">{196}</span>cavity of the portion A or + A′ is formed by the bending over of the tube or horn, B or B′, with the partial + absorption of the septum so formed between the tube and its bent-over part. If, then, we uncoil + the curled-up part of C, and separate the portion, B, on each side from the chamber, C, we see + that the so-called thyroid of Ammocœtes may be represented as in Fig. <a + href="#fig83">83</a>, <i>i.e.</i> it consists of a long, common chamber, C, which, for reasons + apparent afterwards, I will call the <i>palæo-hysteron</i>, which opens, by means of a large + orifice, into the respiratory or pharyngeal chamber. The anterior end of this chamber terminates + in two tubes, or horns, B, B′, the structure of which shows that the median chamber, C, is + the result of the amalgamation of two such tubes, and consequently in this chamber, or + <i>palæo-hysteron</i>, the glandular lines are symmetrically situated on each side.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig082.png" id="fig82"><img style="width:100%" src="images/fig082.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 82.—Diagrammatic Representation of the so-called + Thyroid Gland of Ammocœtes.</span></p> + <p class="sp0"><i>C</i>, central chamber; <i>A, A′</i>, anterior extremity; <i>B, + B′</i>, posterior extremity; <i>Th. o.</i>, thyroid opening into respiratory chamber; + <i>Ps. br., Ps. br′.</i>, ciliated grooves, Dohrn's pseudo-branchial grooves.</p> + </div> + </div> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig083.png" id="fig83"><img style="width:100%" src="images/fig083.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 83.—Thyroid Gland as it would appear if the Central + Chamber were Uncurled and the Two Horns,</span> <i>B</i>, <i>B′</i>, <span + class="sc">separated from the Central Chamber.</span></p> + </div> + </div> + + <p>Any explanation, then, of the thyroid gland of Ammocœtes, must <span class="pagenum" + id="page197">{197}</span>take into account the clear evidence that it is composed of two tubes, + which have in part fused together to form an elongated central chamber, in part remain as horns to + that chamber, and that in its walls there exist lines of gland-cells of a striking and + characteristic nature.</p> + + <p>Further, this central chamber, with its horns, is not a closed chamber, but is in communication + with the pharyngeal or respiratory chamber by three ways. In the first place, the central chamber, + as is well known, opens into the respiratory chamber by a funnel-shaped opening—the + so-called thyroid duct (<i>Th. o.</i>). In the second place, there exist two ciliated grooves + (<i>Ps. br.</i>, <i>Ps. br′.</i>), the pseudo-branchial grooves of Dohrn, which have direct + communication with the thyroid chamber. The manner in which these grooves communicate with the + thyroid chamber has never, to my knowledge, been described previously to my description in the + <i>Journal of Physiology and Anatomy</i>; it is very instructive, for, as I have there shown, each + groove enters into the corresponding lateral horn, so that, in reality, there are three openings + into the thyroid chamber or palæo-hysteron—a median opening into the central chamber, and a + separate opening into each lateral horn.</p> + + <p>The system of ciliated grooves on the inner ventral surface of the respiratory chamber of + Ammocœtes was originally described by Schneider as consisting of a single median groove, + which extends from the opening of the thyroid to the posterior extremity of the branchial chamber, + and a pair of grooves, or semi-canals, which, starting from the region of the thyroid orifice, run + headwards and diverge from each other, becoming more and more lateral, and more and more dorsal, + till they come together in the mid-dorsal pharyngeal line below the auditory capsules. The latter + are the pseudo-branchial grooves of Dohrn, of which I have already spoken. Schneider looked upon + the whole of this system as a single system, for he speaks of "a ciliated groove, which extends + from the orifice of the stomach (<i>i.e.</i> anterior intestine) to the orifice of the thyroid, + then divides into two, and runs forward right and left of the median ridge, etc." Dohrn rightly + separates the median ciliated groove posterior to the thyroid orifice (seen in Fig. <a + href="#fig81">81</a> (6)) from the paired pseudo-branchial grooves; the former is a shallow + depression which opens into the rim of the thyroid orifice, while the latter has a much more + intimate connection with the thyroid gland itself.</p> + + <div><span class="pagenum" id="page198">{198}</span></div> + + <p>A series of sections, such as is given in Fig. <a href="#fig81">81</a>, shows the relation of + this pair of ciliated grooves to the thyroid better than any elaborate description. In the first + place, it is clear that they remain separate up to their termination—they do not join in the + middle line to open into the thyroid duct; in the second place, they are separate from the thyroid + orifice—they do not terminate at the rim of the orifice, as is the case with the median + groove just mentioned, but continue on each side on the wall of the thyroid duct (Fig. <a + href="#fig81">81</a> (2)), gradually moving further and further away from the actual opening of + the duct into the pharyngeal chamber. During the whole of their course on the wall of the + funnel-shaped duct they retain the character of grooves, and are therefore open to the lumen of + the duct. The direction of the groove (<i>Ps. br.</i>) shifts as it passes deeper and deeper + towards the thyroid, until at last, as seen in Fig. <a href="#fig81">81</a> (3 and 4), it is + continuous with the narrow diverticulum of the turned-down single part of the thyroid (B), or + turned-down horn, as I have called it. In other words, the median chamber opens into the + pharyngeal or respiratory chamber by a single large, funnel-shaped opening, and, in addition, the + two ciliated grooves terminate in the lateral horns on each side, and only indirectly into the + central chamber, owing to their being semi-canals, and not complete canals. If they were + originally canals, and not grooves, then the thyroid of Ammocœtes would be derived from an + organ composed of a large, common glandular chamber, which opened into the respiratory chamber by + means of an extensive median orifice, and possessed anteriorly two horns, from each of which a + canal or duct passed headwards to terminate somewhere in the region of the auditory capsule.</p> + + <p>Dohrn has pointed out that a somewhat similar structure and topographical arrangement is found + in Amphioxus and the Tunicata, the gland-cells being here arranged along the hypobranchial groove + to form the endostyle and not shut off to form a closed organ, as in the thyroid of + Ammocœtes. Dohrn concludes, in my opinion rightly, that the endostyle in the Tunicata and + in Amphioxus represents the remnants of the more elaborate organ in Ammocœtes, and that, + therefore, in order to explain the meaning of these organs in the former animals, we must first + find out their meaning in Ammocœtes. Dohrn, however, goes further than this; for just as he + considers Amphioxus and the Tunicata to have arisen by degeneration from an Ammocœtes-like + form, so he considers Ammocœtes to have arisen <span class="pagenum" + id="page199">{199}</span>from a degenerated Selachian; therefore, in order to be logical, he ought + to show that the thyroid of Ammocœtes is an intermediate downward step between the thyroid + of Selachians and that of Amphioxus and the Tunicates. Here, it seems to me, his argument utterly + breaks down; it is so clear that the thyroid of Petromyzon links on to that of the higher fishes, + and that the Ammocœtes thyroid is so immeasurably more complicated and elaborate a + structure than is that of Petromyzon, as to make it impossible to believe that the + Ammocœtes thyroid has been derived by a process of degeneration from that of the Selachian. + On the contrary, the manner in which it is eaten up at transformation and absolutely disappears in + its original form is, like the other instances mentioned, strong evidence that we are dealing here + with an ancestral organ, which is confined to the larval form, and disappears when the change to + the higher adult condition takes place. Dohrn's evidence, then, points strongly to the conclusion + that the starting-point of the thyroid gland in the vertebrate series is to be found in the + thyroid of Ammocœtes, which has given rise, on the one hand, to the endostyle of Amphioxus + and the Tunicata, and on the other, to the thyroid gland of Petromyzon and the rest of the + Vertebrata.</p> + + <p>The evidence which I have just given of the intimate connection of the two pseudo-branchial + grooves with the thyroid chamber shows, to my mind, clearly that Dohrn is right in supposing that + morphologically these two grooves and the thyroid must be considered together. His explanation is + that the whole system represents a modified pair of branchial segments distinct from those + belonging to the VIIth and IXth nerves. The cavity of the thyroid and the pseudo-branchial grooves + are, therefore, according to him, the remains of the gill-pouches of this fused pair of branchial + segments, which no longer open to the surface, and the glandular tissue of the thyroid is derived + from the modified gill-epithelium. This view of Dohrn's, which he has urged most strongly in + various papers, is, I think, right in so far as the separateness of the thyroid segment is + concerned, but is not right, and is not proven, in so far as concerns the view that the thyroid + gland is a modified pair of gills.</p> + + <p>We may distinctly, on my view, look upon the thyroid segment, with its ciliated grooves and its + covering plate of muco-cartilage, as a distinct paired segment, homologous with the branchial + segments, without any necessity of deriving the thyroid gland from a pair of gills.</p> + + <div><span class="pagenum" id="page200">{200}</span></div> + + <p>The evidence that such a median segment has been interpolated ventrally between the foremost + pairs of branchial segments is remarkably clear, for the limits ventrally of the branchial + segments are marked out on each side by the ventral border of the cartilaginous basket-work; and + it is well known, as seen in Fig. <a href="#fig80">80</a>, that whereas this cartilaginous + framework on the two sides meets together in the middle ventral line in the posterior branchial + region, it diverges in the anterior region so as to form a tongue-shaped space between the + branchial segments on the two sides. This space is covered over with a plate of muco-cartilage + which bears on its inner surface the thyroid gland.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig084.png" id="fig84"><img style="width:100%" src="images/fig084.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 84.—Diagram of (A) Ventral Surface and (B) Lateral + Surface of Ammocœtes, showing the arrangement of the Epithelial Pits on the Branchial + Region, and their innervation by <i>VII.</i>, the Facial, <i>IX.</i>, the Glossopharyngeal, + and <i>X<sup>1</sup></i>-<i>X<sup>6</sup></i>, the Vagus Nerves.</span></p> + </div> + </div> + + <p>In addition to this evidence that we are dealing here with a ventral tongue-like segment + belonging to the facial nerve which is interpolated between the foremost branchial segments, we + find the most striking fact that at transformation the whole of this muco-cartilaginous plate + disappears, the remarkable thyroid gland of the <span class="pagenum" + id="page201">{201}</span>Ammocœtes is eaten up, and nothing is left except a small, totally + different glandular mass; and now the cartilaginous basket-work meets together in the middle line + in this region as well as in the more posterior region. In other words, the striking + characteristic of transformation here is the destruction of this interpolated segment, and the + resulting necessary drawing together ventrally of the branchial segments on each side.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig085.png" id="fig85"><img style="width:100%" src="images/fig085.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 85.—Facial Segment of Ammocœtes marked out by + Shading.</span></p> + <p class="sp0"><i>VII.</i> 1, thyroid part of segment; <i>VII.</i> 2, hyoid or branchial part; + 3-9, succeeding branchial segments belonging to IXth and Xth nerves; <i>V</i>, the velar + folds; <i>Ps. br.</i>, Dohrn's pseudo-branchial groove; <i>Th. o.</i>, thyroid opening; + <i>C</i>, curled portion of thyroid.</p> + </div> + </div> + + <p>Moreover, another most instructive piece of evidence pointing in the same direction is afforded + by the behaviour of the ventral epithelial <span class="pagenum" id="page202">{202}</span>pits, as + determined by Miss Alcock. Although there is no indication on the ventral surface of the skin of + any difference between the anterior and posterior portions of the respiratory region, yet when the + ventral rows of the epithelial pits supplied by each branchial nerve are mapped out, we see how + the most anterior ones diverge more and more from the mid-ventral line, following out exactly the + limits of the underlying muco-cartilaginous thyroid plate (Fig. <a href="#fig84">84</a>).</p> + + <p class="sp3">The whole evidence strongly leads to the conclusion that the thyroid portion of the + facial segment was inserted as a median tongue between the foremost branchial segments on each + side, and that, therefore, the whole facial segment, consisting as it does of a thyroid part and a + hyoid or branchial part, may be represented as in Fig. 85, which is obtained by splitting an + Ammocœtes longitudinally along the mid-dorsal line, so as to open out the pharyngeal + chamber and expose the whole internal surface. The facial segment is marked out by shading lines, + the glosso-pharyngeal and vagus segments and the last of the trigeminal segments being indicated + faintly. The position of the thyroid gland is indicated by oblique lines, C being the curled + portion.</p> + + <p class="ac"><span class="sc">The Uterus of the Scorpion Group.</span></p> + + <p>Seeing how striking is the arrangement and the structure of the glandular tissue of this + thyroid, how large the organ is and how absolutely it is confined to Ammocœtes, + disappearing entirely as such at transformation, we may feel perfectly certain that a + corresponding, probably very similar, organ existed in the invertebrate ancestor of the + vertebrate; for the transformation process consists essentially of the discarding of invertebrate + characteristics and the putting on of more vertebrate characters; also, so elaborate an organ + cannot possibly have been evolved as a larval adaptation during the life of Ammocœtes. We + may therefore assert with considerable confidence that the thyroid gland was the + <i>palæo-hysteron</i>, and was derived from the uterus of the ancient palæostracan forms. If, + then, it be found that a glandular organ of this very peculiar structure and arrangement is + characteristic of the uterus of any living member of the scorpion group, then the confidence of + this assertion is greatly increased.</p> + + <p>In Limulus, as already stated, the genital ducts open separately <span class="pagenum" + id="page203">{203}</span>on each side of the operculum, and do not combine to form a uterus; I + have examined them and was unable to find any glandular structure at all resembling that of the + thyroid gland of Ammocœtes. I then turned my attention to the organs of the scorpion, in + which the two ducts have fused to form a single uterus.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig086.png" id="fig86"><img style="width:100%" src="images/fig086.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 86.—Section through the Terminal Chamber or Uterus of the Male + Scorpion.</span></p> + <p class="sp0"><i>C</i>, cavity of chamber. A portion of the epithelial lining of the channels + of emission is drawn above the section of the uterus.</p> + </div> + </div> + + <div><span class="pagenum" id="page204">{204}</span></div> + + <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:42%;"><a href="images/fig087.png" id="fig87"><img + style="width:100%" src="images/fig087.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:42%;"><a href="images/fig088.png" id="fig88"><img + style="width:100%" src="images/fig088.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 87.—Longitudinal Section through three of the + Cones of the Uterine Glands of the Scorpion.</span></p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 88.—Sagittal Section through the Uterine Gland + of Scorpion, showing the Internal Chitinous Surface</span> (<i>b</i>) <span class="sc">and + the Glandular Cones</span> (<i>a</i>) <span class="sc">cut through at various distances + from the Internal Surface</span>.</p> + </div> + </td> + </tr> + </table> + + <p>I there found that both in the male and in the female the genital ducts on each side terminate + in a common chamber or uterus, which underlies the whole length of the operculum, and opens to the + exterior in the middle line, as shown in Fig. <a href="#fig76">76</a>. In transverse section, this + uterus has the appearance shown in Fig. <a href="#fig86">86</a>, <i>i.e.</i> it is a large tube, + evidently expansible, lined with a chitinous layer and epithelial cells belonging to the + chitinogenous layer, except in two symmetrical places, where the uniformity of the uterine wall is + interrupted by two large, remarkable glandular structures. The structure of these glands is better + shown by means of sagittal sections. They are composed of very long, wedge-shaped cells, each of + which possesses a large, round nucleus at the basal end of the cell (Fig. <a + href="#fig87">87</a>). These cells are arranged in bundles of about eight to ten, which are + separated from each other by connective tissue, the apex of each conical bundle being directed + into the cavity of the uterus; where this brush-like termination of the cells reaches the surface, + the chitinous layer is absent, so that this layer is, on surface view, seen (Fig. <a + href="#fig88">88</a> (<i>b</i>)) to be pitted with round holes over that part of the internal + surface of the uterus where these glands are situated. Each of these holes represents the + termination of one of these cone-shaped wedges of cells. If the section is cut across at right + angles to the axis of these cones, then its appearance is represented in Fig. <a + href="#fig88">88</a> (<i>a</i>), and shows well the arrangement of the blocks of cells, separated + from each other by connective tissue. When the section passes through the basal part of the cones, + and only in that case, then the nuclei of the cells appear, often in considerable numbers in one + section, as <span class="pagenum" id="page205">{205}</span>is seen in Fig. <a + href="#fig89">89</a>. In Fig. <a href="#fig88">88</a> the section shows at <i>b</i> the holes in + the chitin in which the cones terminate, and then a series of layers of sections through the cones + further and further away from their apices.</p> + + <p>These conical groups of long cells, represented in Fig. <a href="#fig87">87</a>, form on each + side of the uterus a gland, which is continuous along its whole length, and thus forms a line of + secreting surface on each side, just as in the corresponding arrangement of the glandular + structures in the thyroid of Ammocœtes. This uterus and glandular arrangement is found in + both sexes; the gland is, however, more developed in the male than in the female scorpion.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig089.png" id="fig89"><img style="width:35%" src="images/fig089.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 89.—Transverse Section through the Basal Part of + the Uterine Glands of the Scorpion.</span></p> + </div> + </div> + + <p>The resemblance between the structure of the thyroid of Ammocœtes and the uterus of the + scorpion is most striking, except in two respects, viz. the nature of the lining of the + non-glandular part of the cavity—in the one case ciliated, in the other chitinous—and + the place of exit of the cavity, the thyroid of Ammocœtes opening into the respiratory + chamber, while the uterus of Scorpio opens direct to the exterior.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig090.png" id="fig90"><img style="width:100%" src="images/fig090.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 90.—Section of Central Chamber of Thyroid of + Ammocœtes and Section of Uterus of Scorpion.</span></p> + </div> + </div> + + <p>With respect to the first difference, the same difficulty is met <span class="pagenum" + id="page206">{206}</span>with in the comparison of the ciliated lining of the tube in the central + nervous system of vertebrates with the chitinous lining of the intestine in the arthropod. Such a + difference does not seem to me either unlikely or unreasonable, seeing that cilia are found + instead of chitin in the intestine of the primitive arthropod Peripatus. Also the worm-like + ancestors of the arthropods almost certainly possessed a ciliated intestine. Finally, the + researches of Hardy and McDougall on the intestine of Daphnia point directly to the presence of a + ciliated rather than a chitinous epithelial lining of the intestine in this animal—all + evidence pointing to the probability that in the ancient arthropod forms, derived as they were + from the annelids, the intestine was originally ciliated and not chitinous. It is from such forms + that I suppose vertebrates to have sprung, and not from forms like the living king-crabs, + scorpions, Apus, Branchipus, etc. I only use them as illustrations, because they are the only + living representatives of the great archaic group, from which the Crustacea, Arachnida, and + Vertebrata all took origin.</p> + + <p>The second difference is more important, and is at first sight fatal to any comparison between + the two organs. How is it possible to compare the uterus of the scorpion, which opens on the + surface by an <i>external</i> genital opening, with the thyroid of Ammocœtes, which opens + by an <i>internal</i> opening into the respiratory chamber? However close may be the histological + resemblance of structure in the two cases, surely such a difference is too great to be accounted + for.</p> + + <p>It is, however, to be remembered that the operculum of Scorpio covers only the terminal genital + apparatus, and does not, therefore, resemble the operculum of the presumed ancestor of + Ammocœtes, which, as already argued, must have resembled the operculum of Thelyphonus with + its conjoint branchial and genital apparatus, rather than that of Scorpio. Before, therefore, + making too sure of the insuperable character of this difficulty, we must examine the uterus of the + Pedipalpi, and see the nature of its opening.</p> + + <p>The nature of the terminal genital organs in Thelyphonus has been described to some extent by + Blanchard, and more recently by Tarnani. The ducts of the generative organs terminate, according + to the latter observer, in the large uterus, which is found both in the male and female; he + describes the walls of the uterus in the female as formed of elongated glandular epithelium, with + a strongly-developed porous, chitinized intima. In the male, he says that the <span + class="pagenum" id="page207">{207}</span>epithelium of the uterus masculinus and its processes is + extraordinarily elongated, the chitin covering being thick. In these animals, then, the common + chamber or uterus into which the genital ducts empty, which, like the corresponding chamber in the + scorpion, occupies the middle region of the operculum, is a large and conspicuous organ. Further, + and this is a most striking fact, the <i>uterus masculinus</i> does not open direct to the + exterior, but into the genital cavity, "which lies above the uterus, so that the latter is + situated between the lower wall of the genital cavity and the outer integument." The opening, + therefore, of the uterus is not external but <i>internal</i>, into the large internal space known + as the genital cavity. The arrangement is shown in Fig. 91, taken from Tarnani's paper, which + represents a diagrammatic sagittal section through the exit of the male genital duct. Yet another + most striking fact is described by Tarnani. This genital cavity is continuous with the pulmonary + or gill cavities on each side, so that instead of a single opening for the genital products and + one on each side for each gill-pouch, as would be the case if the arrangement was of the same kind + as in the scorpion, there is a single large chamber, the genital chamber, common to both + respiratory and genital organs.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig091.png" id="fig91"><img style="width:62%" src="images/fig091.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 91.—Sagittal Median Diagrammatic Section through the Operculum + of the Male Thelyphonus.</span> (From <span class="sc">Tarnani</span>.)</p> + <p class="sp0">The thick line is the operculum, composed of two segments, <i>I.</i> and + <i>II.</i> <i>Ut. Masc.</i>, uterus masculinus; <i>Gen. Ch.</i>, genital chamber; <i>Int. + Op.</i>, internal opening; <i>Ext. Op.</i>, external opening common to the genital and + respiratory organs.</p> + </div> + </div> + + <p>This genital chamber, according to Tarnani, opens to the exterior by a single median opening + between the operculum and the succeeding segment; similarly, a communication from side to side + exists between the second pair of gill-pouches. I have been able to examine <i>Hypoctonus + formosus</i> and <i>Thelyphonus caudatus</i>, and in both cases, in both male and female, the + opening to the exterior of the common chamber for respiration and for the genital products was + <span class="pagenum" id="page208">{208}</span>not a single opening, as described by Tarnani in + <i>Thelyphonus asperatus</i>, but on each side of the middle line, a round orifice closed by a + lid, like the nest of the trapdoor spider, led into the common genital chamber (<i>Gen. Ch.</i>) + into which both uterus and gills opened. In Fig. <a href="#fig77">77</a> I have endeavoured to + represent the arrangement of the genital and respiratory organs in the male Thelyphonus according + to Tarnani's and my own observations.</p> + + <p>If we may take Thelyphonus as a sample of the arrangement in those scorpions in which the + operculum was fused with the first branchial appendage, among which must be included the old + sea-scorpions, then it is most significant that their uterus should open internally into a cavity + which was continuous with the respiratory cavity. Thus not only the structure of the gland, but + also the arrangement of the internal opening into the respiratory, or, as it became later, the + pharyngeal cavity, is in accordance with the suggestion that the thyroid of Ammocœtes + represents the uterus of the extinct Eurypterus-like ancestor.</p> + + <p>Into this uterus the products of the generative organs were poured by means of the <i>vasa + deferentia</i>, so that there was not a single median opening or duct in connection with it, but + also two side openings, the terminations of the <i>vasa deferentia</i>. These are described by + Tarnani in Thelyphonus as opening into the two horns of the uterus, which thus shows its bilateral + character, although the body of the organ is median and single; these ducts then pass within the + body of the animal, dorsal to the uterus, towards the testes or ovaries as the case may be, organs + which are situated in these animals, as in other scorpions, in the abdomen, so that the direction + of the ducts from the generative glands to the uterus is headwards. If, however, we examine the + condition of affairs in Limulus, we find that the main mass of the generative material is + cephalic, forming with the liver that dense glandular mass which is packed round the + supra-œsophageal and prosomatic ganglia, and round the stomach and muscles of the + head-region. From this cephalic region the duct passes out on each side at the junction of the + prosomatic and mesosomatic carapace to open separately on the posterior surface of the operculum, + near the middle line, as is indicated in Fig. <a href="#fig75">75</a>.</p> + + <p class="sp3">We have, therefore, two distinct possible positions for the genital ducts among the + group of extinct scorpion-like animals, the one from the cephalic region to the operculum, and the + other from the abdominal region to the operculum.</p> + + <div><span class="pagenum" id="page209">{209}</span></div> + + <p class="ac"><span class="sc">The Generative Glands of Limulus and its Allies.</span></p> + + <p>The whole argument, so far, has in every case ended with the conclusion that the original + scorpion-like form with which I have been comparing Ammocœtes resembled in many respects + Limulus rather than the present-day scorpions, and therefore in the case also of the generative + organs, with which the thyroid gland or palæo-hysteron was in connection, it is more probable that + they were cephalic in position rather than abdominal. If this were so, then the duct on each side, + starting from the median ventral uterus, would take a lateral and dorsal course to reach the huge + mass of generative gland lying within the prosomatic carapace, just as I have represented in the + figure of Eurypterus (Fig. <a href="#fig79">79</a>), a course which would take much the same + direction as the ciliated groove in Ammocœtes.</p> + + <p>We ought, therefore, on this supposition, to expect to find the remains of the invertebrate + generative tissue, the ducts of which terminated in the thyroid, in the head-region, and not in + the abdomen.</p> + + <p>Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is + seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is + composed of two separate glands, the generative glands and the hepatico-pancreatic + glands—the so-called liver—and surrounds closely the central nervous system and the + alimentary canal. From the generative glands proceed the genital ducts to terminate on the + posterior surface of the operculum. From the liver ducts pass to the pyloric end of the cephalic + stomach, and carry the fluid by means of which the food is digested, for, in all these animals, + the active digesting juices are formed in the so-called liver, and not in the cells of the stomach + or intestine.</p> + + <p>It is a very striking fact that the brain of Ammocœtes is much too small for the + brain-case, and that the space between brain and brain-case is filled up with a very peculiar + glandular-looking tissue, which is found in Ammocœtes and not elsewhere. Further, it is + also striking that in the brain of Ammocœtes there should still exist the remains of a tube + extending from the IVth ventricle to the surface at the <i>conus post-commissuralis</i>, which can + actually be traced right into this tissue on the outside of the brain (see Fig. <a + href="#fig13">13</a>, <i>a-e</i>, Pl. XXVI., in my paper in the <i>Quarterly Journal of + Microscopical Science</i>). <span class="pagenum" id="page210">{210}</span>This, in my opinion, is + the last remnant of one of the old liver-ducts which extended from the original stomach and + intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the + pineal eye and its nerve, in Fig. <a href="#fig31">31</a>, also in Fig. <a href="#fig22">22</a>, + and separately in Fig. <a href="#fig92">92</a>. It is composed of large cells, with a badly + staining nucleus, closely packed together with lines of pigment here and there between the cells; + this pigment is especially congregated at the spot where the so-called liver-duct loses itself in + this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no + sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is + not true; peculiar it certainly is, but fatty it is not.</p> + + <div class="ac w20 fcenter sp2"> + <a href="images/fig092.png" id="fig92"><img style="width:100%" src="images/fig092.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 92.—Drawing of the Tissue which surrounds the Brain + of Ammocœtes.</span></p> + </div> + </div> + + <p>This tissue has been largely described as a peculiar kind of connective tissue, which is there + as packing material, for the purpose of steadying a brain too small for its case. On the face of + it such an explanation is unscientific; certainly for all those who really believe in evolution, + it is out of the question to suppose that a brain-case has been laid down in the first instance + too large for the brain, in order to provide room for a subsequent increase of brain; just as it + is out of the question to suppose that the nervous system was laid down originally as an + epithelial tube in order to provide for the further development of the nervous system by the + conversion of more and more of that tube into nervous matter. Yet this latter proposition has been + seriously put forward by professed believers in evolution and in natural selection.</p> + + <p>This tissue bears no resemblance whatever to any form of connective tissue, either fatty or + otherwise. By every test this tissue tells as plainly as possible that it is a vestige of some + former organ, presumably glandular, which existed in that position; that it is not there as + packing material because the brain happened to be too small for its case, but that, on the + contrary, the brain is too small for its case, because the case, when it was formed, included this + organ as well as the brain; in other words, this tissue <span class="pagenum" + id="page211">{211}</span>is there because it is the remnant of the great glandular mass which so + closely surrounds the brain and alimentary canal in animals such as Limulus. In my paper in the + <i>Quarterly Journal of Microscopical Science</i>, in which I was comparing the tube of the + vertebrate nervous system with the alimentary canal of the invertebrate, I spoke of this tissue as + being the remnant of the invertebrate liver. At the same time the whole point of my argument was + that the glandular material surrounding the brain of Limulus was made up of two glands—liver + and generative gland—so that this tissue might be the remnant of either one or the other, or + both. All I desired, at that time, was to point out the glandular appearance of this so-called + packing tissue, which surrounded the brain-region of Ammocœtes, in connection with the fact + that the brain and alimentary canal of Limulus were closely surrounded with a glandular mass + composed partly of liver, partly of the generative gland. At present, I think these large cells + found round the brain in Ammocœtes are much more likely to be the remnant of the generative + gland than of the liver; the size of the cells and their arrangement recalls Owen's picture of the + generative gland in Limulus, and seeing how important all generative glands are in their capacity + of internal secreting glands, apart entirely from the extrusion of the ripe generative products, + and how unimportant is an hepato-pancreas when the alimentary canal is closed, it is much more + likely that of the two glands the former would persist longer than the latter. It may be that all + that is left of the old hepato-pancreas consists of the pigment so markedly found in between these + cells, especially at the place where the old liver-duct reaches the surface of the brain; just as + the only remnant of the two pineal eyes in the higher vertebrates is the remains of the pigment, + known as brain-sand, which still exists in the pineal gland of even the highest vertebrate. This, + however, is a mere speculation of no importance. What is important is the recognition of this + tissue round the brain as the remnant of the glandular mass round the brain of animals such as + Limulus. Still further confirmation of the truth of this comparison will be given when the origin + of the auditory organ comes up for discussion.</p> + + <p>I conclude, therefore, from the evidence of Ammocœtes, that the generative glands in the + ancestral form were situated largely in the cephalic region, and suggest that the course and + direction of the ciliated pseudo-branchial grooves on each side indicate the direction of the + <span class="pagenum" id="page212">{212}</span>original opercular ducts by which the generative + products were conveyed to the uterine chamber, i.e. to the chamber of the thyroid gland, and + thence to the common genital and respiratory cavity, and so to the exterior.</p> + + <p>It is easy to picture the sequence of events. First, the generative glands, chiefly confined to + the cephalic region, communicating with the exterior by separate ducts on the inner surface of the + operculum as in Limulus. Then, in connection with the viviparous habit, these two oviducts fused + together to form a single chamber, covered by the operculum, which opened out to the exterior by a + single opening as in Scorpio: or, in forms such as Eurypterus, in which the operculum had + amalgamated with the first branchial appendage and possessed a long, tongue-like ventral + projection, the amalgamated ducts formed a long uterine chamber which opened internally into the + genital chamber—a chamber which, as in Thelyphonus, was common with that of the two + gill-chambers, while at the same time the genital ducts from the cephalic generative material + opened into two uterine horns which arose from the anterior part of the uterus, as in + Thelyphonus.</p> + + <p>Such an arrangement would lead directly to the condition found in Ammocœtes, if the + generative material around the brain lost its function, owing to a new exit for generative + products being formed in the posterior part of the body. The connection of the genital duct with + this cephalic gland being then closed and cut off by the brain-case, the position of the oviducts + would still be shown by the ciliated grooves opening into the folded-down thyroid tube, + <i>i.e.</i> the folded-down horns of the uterus; the uterus itself would remain as the main body + of the thyroid and still open by a conspicuous orifice into the common respiratory chamber. Next, + in the degeneration process, we may suppose that not only the oviducts opened out to form the + ciliated groove, but that the uterine chamber itself also opened out, and thus formed the + endostyle of Amphioxus and of the Tunicata.</p> + + <p>It might seem at first sight improbable that a closed tube should become an open groove, + although the reverse phenomenon is common enough; the difficulty, however, is clearly not + considered great, for it is precisely what Dohrn imagines to have taken place in the conversion of + the thyroid of Ammocœtes into the endostyle of Amphioxus and the Tunicata; it is only + carrying on the same idea a stage further to see in the open, ciliated groove of Ammocœtes + the remains of the closed genital duct of Limulus and its allies.</p> + + <div><span class="pagenum" id="page213">{213}</span></div> + + <p class="sp3">Such is the conclusion to which the study of the thyroid gland in Ammocœtes + seems to me to lead, and one cannot help wondering why such an unused and rudimentary organ should + have remained after its original function had gone. Is it possible to find out its function in + Ammocœtes?</p> + + <p class="ac"><span class="sc">The Function of the Thyroid Gland in Ammocœtes.</span></p> + + <p>The thyroid gland has been supposed to secrete mucus into the respiratory chamber for the + purpose of entangling the particles of food, and so aiding in digestion. I see no sign of any such + function; neither by the thionin method, nor by any other test, have Miss Alcock and myself ever + been able to see any trace of mucous secretion in the thyroid, and, indeed, the thyroid duct is + always remarkably free from any sign of any secretion whatever. Not only is there no evidence of + any mucous secretion in the thyroid of the fully developed Ammocœtes, but also no necessity + for such secretion from Dohrn's point of view, for so copious a supply of mucus is poured out by + the glands of the branchiæ, along the whole pharyngeal tract, especially from the cells of the + foremost or hyoid gills, as to mix up with the food as thoroughly as can possibly be needed. + Further, too, the ciliated pharyngeal bands described by Schneider are amply sufficient to move + this mixed mass along in the way required by Dohrn. Finally, the evidence given by Miss Alcock is + absolutely against the view that the thyroid takes any part in the process of digestion, while, on + the other hand, her evidence directly favours the view that these glandular <i>branchial</i> + mucus-secreting cells play a most important part in the digestive process.</p> + + <p>In Fig. <a href="#fig93">93</a>, A is a representation of the respiratory tissue of a normal + gill; B is the corresponding portion of the first or hyoid gill, in which, as is seen, the whole + of the respiratory epithelium is converted into gland-tissue of the nature of mucous cells.</p> + + <p>To sum up, the evidence is clear and conclusive that the Ammocœtes possesses in its + pharyngeal chamber mucus-secreting glands, which take an active part in the digestive process, + which do not in the least resemble either in structure or arrangement the remarkable cells of the + thyroid gland, and that the experimental evidence that the latter cells either secrete mucus or + take any part in digestion is so far absolutely negative. It is, of course, possible, that they + <span class="pagenum" id="page214">{214}</span>may contain mucin in the younger developmental + stages, and therefore possible that they might at that stage secrete it; they certainly, however, + show no sign of doing so in their more adult condition, and cannot be compared in the very + faintest degree to the glandular cells of the pharyngeal region. It is also perfectly possible for + gland-cells belonging to a retrograde organ to become mucus-secreting, and so to give rise to the + cells of Amphioxus and the Tunicata.</p> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig093.png" id="fig93"><img style="width:100%" src="images/fig093.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0">Fig. 93.—<span class="sc">A, Portion of a Gill of Ammocœtes with + ordinary Respiratory Epithelium; B, Corresponding Portion of the First or Hyoid + Gill.</span></p> + </div> + </div> + + <p>If, then, these cells were not retained for digestive purposes, what was their function? To + answer this question we must first know the function of the corresponding gland-cells in the + uterus of the scorpion, which undoubtedly secreted into the cavity of the uterus and took some + part in connection with the generative act, and certainly not with digestion. What the function of + these cells is or in what way they act I am unable at present to say. I can only suppose that the + reason why the thyroid gland has persisted throughout the vertebrate kingdom, after the generative + tissues had found a new outlet for their products in the body-cavity of the posterior region, is + because it possessed some important function in addition to that connected with the exit of the + products of the generative organs; a function which was essential to the well-being, or even to + the life of the animal. We do not know its function in the scorpion, or the nature of its + secretion in that animal. We know only that physiology at the present day has demonstrated clearly + that the actual external secretion of a gland may be by no means its most important function; in + addition, glands possess what is called an internal secretion, viz. a <span class="pagenum" + id="page215">{215}</span>secretion into the blood and lymph, and this latter secretion may be of + the most vital importance. Now, the striking fact forces itself prominently forward, that the + thyroid gland of the higher vertebrates is the most conspicuous example of the importance of such + internal secretion. Here, although ductless, we have a gland which cannot be removed without fatal + consequences. Here, in the importance of its internal secretion, we have a reason for the + continued existence of this organ; an organ which remains much the same throughout the Vertebrata + down to and including Petromyzon, but, as is seen at transformation, is all that remains of the + more elaborate, more extensive organ of Ammocœtes. Surely we may argue that it is this + second function which has led to the persistence of the thyroid, and that its original form, + without its original function, is seen in Ammocœtes, because that is a larval form, and not + a fully-developed animal. As soon as the generative organs of Petromyzon are developed at + transformation, all trace of its connection with a genital duct vanishes, and presumably its + internal secretory function alone remains.</p> + + <p>Yet, strange to say, a mysterious connection continues to exist between the thyroid gland and + the generative organs, even up to the highest vertebrate. That the thyroid gland, situated as it + is in the neck, should have any sympathy with sexual functions if it was originally a gland + concerned with digestion is, to say the least of it, extremely unlikely, but, on the contrary, + likely enough if it originated from a glandular organ in connection with the sexual organs of the + palæostracan ancestor of the vertebrate.</p> + + <p class="sp3">Freund has shown, and shown conclusively, that there is an intimate connection + between the condition of the thyroid gland and the state of the sexual organs, not only in human + beings, but also in numerous animals, such as dogs, sheep, goats, pigs, and deer. He points out + that the swelling of the gland, which occurs in consequence of sexual excitement (a fact mentioned + both in folk-lore tales and in poetical literature), and also the swelling at the time of puberty, + may both lead to a true goitrous enlargement; that most of the permanent goitres commence during a + menstrual period; that during pregnancy swelling of the thyroid is almost universal, and may + become so extreme as to threaten suffocation, or even cause death; that the period of puberty and + the climacteric period are the two maximal periods for the onset of goitre, and that exophthalmic + goitre especially is associated with a special disease connected with the uterus.</p> + + <div><span class="pagenum" id="page216">{216}</span></div> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>Step by step in the preceding chapters the evidence is accumulating in favour of the origin + of vertebrates from a member of the palæostracan group. In a continuously complete and + harmonious manner the evidence has throughout been most convincing when the vertebrate chosen + for the purpose of my arguments has been Ammocœtes.</p> + <p>So many fixed points have been firmly established as to enable us to proceed further with + very great confidence, in the full expectation of being able ultimately to homologize the + Vertebrata with the Palæostraca even to minute details.</p> + <p>Perhaps the most striking and unexpected result of such a comparison is the discovery that + the thyroid gland is derived from the uterus of the palæostracan ancestor. Yet so clear is the + evidence that it is difficult to see how the homology can be denied.</p> + <p>In the one animal (Palæostraca) the foremost pair of mesosomatic appendages forms the + operculum, which always bears the terminal generative organs and is fused in the middle line. In + many forms, essentially in Eurypterus and the ancient sea-scorpions, the operculum was composed + of two segments fused together: an anterior one which carried the uterus, and a posterior one + which carried the first pair of branchiæ.</p> + <p>In the other animal (Ammocœtes) the foremost segments of the mesosomatic or + respiratory region, immediately in front of the glossopharyngeal segments, are supplied by the + facial nerve, and are markedly different from those supplied by the vagus and glossopharyngeal, + for the facial supplies two segments fused together; the anterior one, the thyroid segment, + carrying the thyroid gland, the posterior one, the hyoid segment, carrying the first pair of + branchiæ.</p> + <p>Just as in Eurypterus the fused segment, carrying the uterus on its internal surface, forms a + long median tongue which separates the most anterior branchial segments on each side, so also + the fused segment carrying the thyroid forms in Ammocœtes a long median tongue, which + separates the most anterior branchial segments on each side.</p> + <p>Finally, and this is the most conclusive evidence of all, this thyroid gland of + Ammocœtes is totally unlike that of any of the higher vertebrates, and, indeed, of the + adult form Petromyzon itself, but it forms an elaborate complicated organ, which is directly + comparable with the uterus and genital ducts of animals such as scorpions. Not only is such a + comparison valid with respect to its shape, but also with respect to its structure, which is + absolutely unique among vertebrates, and very different to that of any other vertebrate gland, + but resembles in a striking manner a glandular structure found in the uterus, both of male and + female scorpions.</p> + <p>The generative glands in Limulus, together with the liver-glands, form a large glandular + mass, situated in the head-region closely surrounding the central nervous system, so that the + genital ducts pass from the head-region tailwards to the operculum. In the scorpion they lie in + the abdominal region, so that their ducts pass headwards to the operculum.</p> + <p>Probably in the Palæostraca the generative mass was situated in the cephalic region as in + Limulus, and it is probable that the remnant of it still exists in <span class="pagenum" + id="page217">{217}</span>Ammocœtes in the shape of the peculiar large cells packed + together, with pigment masses in between them, which form such a characteristic feature of the + glandular-looking material, which fills up the space between the cranial walls and the central + nervous system.</p> + <p>Finally, the relationship which has been known from time immemorial to exist between the + sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without + any explanation, may possibly be the last reminiscence of a time when the thyroid glands were + the uterine glands of the palæostracan ancestor.</p> + <p class="sp0">The consideration of the facial nerve, and the segments it supplies, still + further points to the origin of the Vertebrata from the Palæostraca.</p> + </div> + + <div><span class="pagenum" id="page218">{218}</span></div> + + <p class="ac">CHAPTER VI</p> + + <p class="ac"><i>THE EVIDENCE OF THE OLFACTORY APPARATUS</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Fishes divided into Amphirhinæ and Monorhinæ.—Nasal tube of the + lamprey.—Its termination at the infundibulum.—The olfactory organs of the scorpion + group.—The camerostome.—Its formation as a tube.—Its derivation from a pair of + antennæ.—Its termination at the true mouth.—Comparison with the olfactory tube of + Ammocœtes.—Origin of the nasal tube of Ammocœtes from the tube of the + hypophysis.—Direct comparison of the hypophysial tube with the olfactory tube of the + scorpion group—Summary.</p> + </div> + + <p>In the last chapter I finished the evidence given by the consideration of the mesosomatic or + opisthotic nerves, and the segments they supplied. The evidence is strongly in accordance with + that of previous chapters, and not only confirms the conclusion that vertebrates arose from some + member of the Palæostraca, but helps still further to delimit the nature of that member. It is + almost startling to see how the hypothesis put forward in the second chapter, suggested by the + consideration of the nature of the vertebrate central nervous system and of the geological record, + has received stronger and stronger confirmation from the consideration of the vertebrate optic + apparatus, the vertebrate skeleton, the respiratory apparatus, and, finally, the thyroid gland. + All fit naturally into a harmonious whole, and give a feeling of confidence that a similar harmony + will be found upon consideration of the rest of the vertebrate organs.</p> + + <p>Following naturally upon the segments supplied by the opisthotic (mesosomatic) cranial nerves, + we ought to consider now the segments supplied by the pro-otic (prosomatic) cranial nerves, i.e. + the segments belonging to the trigeminal nerve-group in the vertebrate, and in the invertebrate + the segments of the prosoma with their characteristic appendages. There are, however, in all + vertebrates in this foremost cranial region, in addition to the optic nerves, two other + well-marked nerves of special sense, the olfactory and the auditory. Of these, the former are in + the same class as the optic nerves, for they arise <span class="pagenum" + id="page219">{219}</span>in the vertebrate from the supra-infundibular nerve-mass, and in the + invertebrate from the supra-œsophageal ganglia. The latter arise in the vertebrate from the + infra-infundibular nerve-mass, and, as the name implies, are situated in the region where the + pro-otic nerves are contiguous to the opisthotic, <i>i.e.</i> at the junction of the prosomatic + and mesosomatic nerve-regions.</p> + + <p>The chapter dealing with the evidence given by the olfactory nerves and the olfactory apparatus + ought logically to have followed immediately upon the one dealing with the optic apparatus, seeing + that both these special sense-nerves belong to the supra-infundibular segments in the vertebrate + and to the supra-œsophageal in the invertebrate.</p> + + <p class="sp5">I did not deal with them in that logical sequence because it was necessary for + their understanding to introduce first the conception of modified appendages as important factors + in any consideration of vertebrate segments; a conception which followed naturally after the + evidence afforded by the skeleton in Chapter III., and by the branchial segments in Chapter IV. + So, too, now, although the discussion of the prosomatic segmentation ought logically to follow + immediately on that of the mesosomatic segmentation, I have determined to devote this chapter to + the evidence of the olfactory organs, because the arguments as to the segments belonging to the + trigeminal nerve-group are so much easier to understand if the position of the olfactory apparatus + is first made clear.</p> + + <p>In all vertebrates the nose is double and opens into the pharynx, until we descend to the + fishes, where the whole group Pisces has been divided into two subsidiary groups, Monorhinæ and + Amphirhinæ, according as they possess a median unpaired olfactory opening, or a paired opening. + The Monorhinæ include only the Cyclostomata—the lampreys and hag-fishes.</p> + + <p>In the lampreys the single olfactory tube ends blindly, while in the hag-fishes it opens into + the pharynx. In the lamprey, both in Petromyzon and Ammocœtes, the opening of this nasal + tube is a conspicuous object on the dorsal surface of the head in front of the transparent spot + which indicates the position of the right median eye. It is especially significant, as showing the + primitive nature of this median olfactory passage, that a perfectly similar opening in the <span + class="pagenum" id="page220">{220}</span>same position is always found in the dorsal head-shields + of all the Cephalaspidæ and Tremataspidæ, as will be explained more fully in Chapter X.</p> + + <p>All the evidence points to the conclusion that the olfactory apparatus of the vertebrate + originated as a single median tube, containing the special olfactory sense-epithelium, which, + although median and single, was innervated by the olfactory nerve of each side. The external + opening of this tube in the lamprey is dorsal. How does it terminate ventrally?</p> + + <p>The ventral termination of this tube is most instructive and suggestive. It terminates blindly + at the very spot where the infundibular tube terminates blindly and the notochord ends. After + transformation, when the Ammocœte becomes the Petromyzon, the tube still ends blindly, and + does not open into the pharynx as in Myxine; it, however, no longer terminates at the + infundibulum, but extends beyond it towards the pharynx.</p> + + <p>This position of the nasal tube suggests that it may originally have opened into the tube of + the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in + value by the fact that in the larval Amphioxus the tube of the central nervous system is open to + the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is + situated at the base of a pit, known as the olfactory pit because it is supposed to represent the + olfactory organ of other fishes.</p> + + <p class="sp3">Following the same lines of argument as in previous chapters, this suggestion + indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates + consisted of a single median olfactory tube or passage, which led directly into the + œsophagus and was innervated, though single and median, by a pair of olfactory nerves which + arose from the supra-œsophageal ganglia. Let us see what is the nature of the olfactory + organs among arthropods, and whether such a suggestion possesses any probability.</p> + + <p class="ac"><span class="sc">The Olfactory Organs of the Scorpion Group.</span></p> + + <p>At first sight the answer appears to be distinctly adverse, for it is well known that in all + the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often + called the antennules, are olfactory in function, and these are free-moving, bilaterally <span + class="pagenum" id="page221">{221}</span>situated, independent appendages. Still, even here there + is the striking fact that the nerves of these olfactory organs always arise from the + supra-œsophageal ganglia, although those to the second pair of antennæ arise from the + infra-œsophageal ganglia, just as the olfactory nerves of the vertebrate arise from the + supra-infundibular brain-mass. Not only is there this similarity of position, but also a + similarity of structure in the olfactive lobes of the brain itself of so striking a character as + to cause Bellonci to sum up his investigations as follows<span class="wnw">:—</span></p> + + <p>"The structure and connections of the olfactive lobes present the same fundamental plan in the + higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form, + with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does + of separate groups, may each one be called an olfactory glomerulus."</p> + + <p>He attributes this remarkable resemblance to a physiological necessity that similarity of + function necessitates similarity of structure, for he considers it out of the question to suppose + any near relationship between arthropods and vertebrates.</p> + + <p>Truly an interesting remark, with the one fallacy that relationship is out of the question.</p> + + <p>The evidence so far has consistently pointed to some member of the palæostracan group as the + ancestor of the vertebrates—a group which had affinities both to the crustaceans and the + arachnids; indeed, many of its members resembled scorpions much more than they resemble + crustaceans. The olfactory organs of the scorpions and their allies are, therefore, more likely + than any others to give a clue to the position of the desired olfactory organs. In these animals + and their allies paired olfactory antennæ are not present, either in the living land-forms or the + extinct sea-scorpions, for all the antennæ-like, frequently chelate, appendages seen in + Pterygotus, etc. (Fig. <a href="#fig8">8</a>), represent the cheliceræ, and correspond, therefore, + to the second pair of antennæ in the crustaceans.</p> + + <p>What, then, represents the olfactory antennæ in the scorpions? The answer to this question has + been given by Croneberg, and very striking it is. The two olfactory antennæ of the crustacean have + combined together to form a hollow tube at the base of which the mouth of the animal is situated, + so that the food passes along this olfactory passage before it reaches the mouth. This organ is + often called after Latreille, the camerostome, sometimes the rostrum; it is naturally median in + position and appears, therefore, to be an unpaired organ; its paired <span class="pagenum" + id="page222">{222}</span>character is, of course, evident enough, for it is innervated by a pair + of nerves, and these nerves, as ought to be the case, arise from the supra-œsophageal + ganglia. In Galeodes it is a conspicuously paired antennæ-like organ (Fig. <a + href="#fig94">94</a>).</p> + + <p>Croneberg has also shown that this rostrum, or camerostome, arises embryologically as a pair of + appendages similar to the other appendages. This last observation of Croneberg has been confirmed + by Brauer in 1894, who describes the origin of the upper lip, as he calls it, in very similar + terms, without, however, referring to Croneberg's paper. Croneberg further shows that this + foremost pair of antennæ not only forms the so-called upper lip or camerostome, but also a lower + lip, for from the basal part of the camerostome there projects on each side of the pharynx a + dependent accessory portion, which in some cases fuses in the middle line, and forms, as it were, + a lower lip. The entosclerite belonging to this dependent portion is apparently the post-oral + entosclerite of Lankester and Miss Beck.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig094.png" id="fig94"><img style="width:62%" src="images/fig094.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 94.—Dorsal View of Brain and Camerostome of + Galeodes.</span></p> + <p class="sp0"><i>cam.</i>, camerostome; <i>pr. ent.</i>, pre-oral entosclerite; <i>l.l.</i>, + dependent portion of camerostome; <i>ph.</i>, pharynx; <i>al.</i>, alimentary canal; <i>n. + op.</i>, median optic nerves; <i>pl.</i>, plastron; <i>v.c.</i>, ventral nerve chain; 2, 3, + second and third appendages.</p> + </div> + </div> + + <p>At the base of the tubular passage formed by this modified first pair of antennæ the true mouth + is found opening directly into the dilated pharynx, the muscles of which enable the act of suction + to be carried out. The narrow œsophagus leads out from the pharynx and is completely + surrounded by the supra- and infra-œsophageal nerve masses.</p> + + <p>Huxley also describes the mouth of the scorpion in precisely the same position (<i>cf. o</i>, + Fig. <a href="#fig96">96</a>).</p> + + <div><span class="pagenum" id="page223">{223}</span></div> + + <p>In order to convey to my readers the antennæ-like character of the camerostome in Galeodes + (Fig. <a href="#fig101">101</a>), and its position, I give a figure (Fig. <a href="#fig94">94</a>) + of the organ from its dorsal aspect, after removal of the cheliceræ and their muscles. A side view + of the same organ is given in Fig. <a href="#fig95">95</a> to show the feathered termination of + the camerostome, and the position of the dependent accessory portion (<i>l.l.</i>) (Croneberg's + 'untere Anhang') with its single long antenna-like feather. In both figures the alimentary canal + (<i>al.</i>) is seen issuing from the conjoined supra- and infra-œsophageal mass.</p> + + <p>As is seen in the figures, the bilateral character of the rostrum, as Croneberg calls it, is + apparent not only in its feathered extremity but also in its chitinous covering, the softer median + dorsal part (left white in figure) being bounded by two lateral plates of hard chitin, which meet + in the middle line near the extremity of the organ. In all the members of the scorpion group, as + is clearly shown in Croneberg's figures, the rostrum or camerostome is built up on the same plan + as in Galeodes, though the antenna-like character may not be so evident.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig095.png" id="fig95"><img style="width:100%" src="images/fig095.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 95.—Lateral View of Brain and Camerostome of + Galeodes.</span></p> + <p class="sp0"><i>gl. supr. œs.</i>, supra-œsophageal ganglion; <i>gl. infr. + œs.</i>, infra-œsophageal ganglion. The rest of the lettering same as in Fig. <a + href="#fig94">94</a>.</p> + </div> + </div> + + <p>When we consider that the first pair of antennæ in the crustaceans are olfactory in function, + Croneberg's observations amount to this—</p> + + <p>In the arachnids and their allies the first pair of antennæ form a pre-oral passage or tube, + olfactory in function; the small mouth, which opens directly into the pharynx, being situated at + the end of this olfactory passage.</p> + + <div><span class="pagenum" id="page224">{224}</span></div> + + <p>Croneberg's observations and conclusions are distinctly of very great importance in bringing + the arachnids into line with the crustaceans, and it is therefore most surprising that they are + absolutely ignored by Lankester and Miss Beck in their paper published in 1883, in which Latreille + only is mentioned with respect to this organ, and his term "camerostome," or upper lip, is used + throughout, in accordance with the terminology in Lankester's previous paper. That this organ is + not only a movable lip or tongue, but essentially a sense-organ, almost certainly of smell and + taste, as follows from Croneberg's conclusions, is shown by the series of sections which I have + made through a number of young Thelyphonus (Fig. <a href="#fig102">102</a>).</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig096.png" id="fig96"><img style="width:100%" src="images/fig096.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 96.—Median Sagittal Section through a Young + Thelyphonus.</span></p> + </div> + </div> + + <p>I give in Fig. <a href="#fig96">96</a> a sagittal median section through the head-end of the + animal, which shows clearly the nature of Croneberg's conception. At the front end of the body is + seen the median eye (<i>ce.</i>), <i>o</i> is the mouth, <i>Ph.</i> the pharynx, <i>œs.</i> + the narrow œsophagus, compressed between the supra-œsophageal (<i>supr. + œs.</i>) and infra-œsophageal (<i>infr. œs.</i>) brain mass, which opens into + the large alimentary canal (<i>Al.</i>); <i>Olf. pass.</i> is the olfactory passage to the mouth, + lined with thick-set, very fine hairs, which spring from the hypostome (<i>Hyp.</i>) as well as + from the large conspicuous camerostome (<i>Cam.</i>), which limits this tube anteriorly. The space + between the camerostome and the median eye is filled up by the massive cheliceræ, which are not + shown in this section, as they begin to appear in the <span class="pagenum" + id="page225">{225}</span>sections on each side of the median one. The muscles of the pharynx and + the muscles of the camerostome are attached to the pre-oral entosclerite (<i>pr. ent.</i>). The + post-oral entosclerite is shown in section as <i>post. ent.</i> The dorsal blood-vessel, or heart, + is indicated at <i>H.</i></p> + + <p>In Fig. <a href="#fig97">97</a> I give a transverse section through another specimen of the + same litter, to show the nature of this olfactory tube when cut across. Both sections show most + clearly that we are dealing here with an elaborate sense-organ, the surface of which is partly + covered with very fine long hairs, partly, as is seen in the figure, is composed of long, + separate, closely-set sense-rods (<i>bat.</i>), well protected by the long hairs which project on + every side in front of them, which recall to mind Bellonci's figure of the 'batonnets olfactives' + on the antennæ of Sphæroma. Finally, we have the observation of Blanchard quoted by Huxley, to the + effect that this camerostome is innervated by nerves from the supra-œsophageal ganglia + which are clearly bilateral, seeing that they arise from the ganglion on each side and then unite + to pass into the camerostome; in other words, paired olfactory nerves from the + supra-œsophageal ganglia.</p> + + <p>These facts demonstrate with wonderful clearness that in one group of the Arthropoda the + olfactory antennæ have been so modified as to form an olfactory tube or passage, which leads + directly into the mouth and so to the œsophagus of the animal, and, strikingly enough, this + group, the Arachnida, is the very one to which the scorpions belong.</p> + + <p class="sp3">If for any cause the mouth <i>o</i> in Fig. <a href="#fig96">96</a> were to be + closed, then the olfactory tube (<i>olf. pass.</i>) might still remain, owing to its importance as + the organ of smell, and the olfactory tube would terminate blindly at the very spot where the + corresponding tube does terminate in the vertebrate, according to the theory put forward in this + book.</p> + + <p class="ac"><span class="sc">The Olfactory Tube of Ammocœtes.</span></p> + + <p>In all cases where there is similarity of topographical position in the organs of the + vertebrate and arthropod we may expect also to find similarity of structure. At first sight it + would appear as though such similarity fails us here, for a cross-section of the olfactory tube in + Petromyzon represents an elaborate organ such as is shown in Fig. 98, very different in appearance + to the section across the olfactory passage of a young Thelyphonus given in Fig. <a + href="#fig97">97</a>.</p> + + <div><span class="pagenum" id="page226">{226}</span></div> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig097.png" id="fig97"><img style="width:100%" src="images/fig097.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 97.—Transverse Section through the Olfactory Passage of a Young + Thelyphonus.</span></p> + <p class="sp0">1 and 2, sections of first and second appendages.</p> + </div> + </div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig098.png" id="fig98"><img style="width:100%" src="images/fig098.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 98.—Transverse Section through the Olfactory Passage of + Petromyzon.</span></p> + <p class="sp0"><i>cart.</i>, nasal cartilage.</p> + </div> + </div> + + <div><span class="pagenum" id="page227">{227}</span></div> + + <p>As is seen, it is difficult to see any connection between these folds of olfactory epithelium + and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of + the lamprey, remarkable changes take place at transformation, and examination of the same tube in + Ammocœtes demonstrates that the elaborate structure of the adult olfactory organ is + actually derived from a much simpler form of organ, represented in Fig. <a href="#fig99">99</a>. + Here, in Ammocœtes, the section is no longer strikingly different from that of the + Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this + larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the + rest of the vertebrates put together.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig099.png" id="fig99"><img style="width:100%" src="images/fig099.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 99.—Transverse Section through the Olfactory Passage of + Ammocœtes.</span></p> + <p class="sp0"><i>cart.</i>, nasal cartilage.</p> + </div> + </div> + + <p>Still, even now the similarity between the two organs is not complete, for the tube in the + lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it + is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with + this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or + of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids, + and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does + the development of the individual recapitulate the development of the race, that this very + discrepancy becomes converted into a triumphant vindication of the <span class="pagenum" + id="page228">{228}</span>correctness of the theory advocated in this book, as soon as we turn our + attention to the development of this nasal tube in the lamprey.</p> + + <p>We must always remember not only the great importance of a larval stage for the unriddling of + problems of ancestry, but also the great advantage of being able to follow more favourably any + clues as to past history afforded by the development of the larva itself, owing to the greater + slowness in the development of the larva than of the embryo. Such a clue is especially well marked + in the course of development of Ammocœtes according to Kupffer's researches, for he finds + that when the young Ammocœtes is from 5 to 7 mm. in length, some time after it has left the + egg, when it is living a free larval life, a remarkable series of changes takes place with + considerable rapidity, so that we may regard the transformation which takes place at this stage, + as in some degree comparable with the great transformation which occurs when the Ammocœtes + becomes a Petromyzon.</p> + + <p>All the evidence emphasizes the fact that the latter transformation indicates the passage from + a lower into a higher form of vertebrate, and is to be interpreted phylogenetically as an + indication of the passage from the Cephalaspidian towards the Dipnoan style of fish. If, then, the + former transformation is of the same character, it would indicate the passage from the + Palæostracan to the Cephalaspid.</p> + + <p>What is the nature of this transformation process as described by Kupffer?</p> + + <p>It is characterized by two most important events. In the first place, up to this time the oral + chamber has been cut off from the respiratory chamber by a septum—the velum—so that no + food could pass from the mouth to the alimentary canal. At this stage the septum is broken + through, the oral chamber communicates with the respiratory chamber, and the velar folds of the + more adult Ammocœtes are left as the remains of the original septum. The other striking + change is the growth of the upper lip, by which the orifice of the nasal tube is transferred from + a ventral to a dorsal position. Fig. <a href="#fig100">100</a>, taken from Kupffer's paper, + represents a sagittal section through an Ammocœtes 4 mm. long; <i>l.l.</i> is the lower + lip, <i>u.l.</i> the upper lip, and, as is seen, the short oral chamber is closed by the septum, + <i>vel.</i> Opening ventrally is a tube called the tube of the hypophysis, <i>Hy.</i>, which + extends close up to the termination of the infundibulum. On the anterior surface of this tube is + the projection called by Kupffer the olfactory plakode. At this stage the upper lip grows with + great <span class="pagenum" id="page229">{229}</span>rapidity and thickens considerably, thus + forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the + full-grown Ammocœtes, it becomes the dorsal opening of the nasal tube, as already + described. Here, then, in the hypophysial tube we have the original position of the olfactory tube + of the vertebrate ancestor, and it is significant, as showing the importance of this organ, to + find that such a hypophysial tube is characteristic of the embryological development of every + vertebrate, whatever may be the ultimate form of the external nasal orifices.</p> + + <p>The single median position of the olfactory organ in the Cyclostomata, in contradistinction to + its paired character in the rest of the vertebrates, has always been a stumbling-block in the way + of those who desired to consider the Cyclostomata as degenerated Selachians, for the origin of the + olfactory protuberance, as a single median plakode, seemed to indicate that the nose arose as a + single organ and not as a paired organ.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig100.png" id="fig100"><img style="width:100%" src="images/fig100.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 100.—Ganglia of the Cranial Nerves of an + Ammocœtes, 4 mm. in length, projected on to the Median Plane.</span> (After <span + class="sc">Kupffer</span>.)</p> + <p class="sp0"><i>A-B</i>, the line of epibranchial ganglia; <i>au.</i>, auditory capsule; + <i>nc.</i>, notochord; <i>Hy.</i>, tube of hypophysis; <i>Or.</i>, oral cavity; <i>u.l.</i>, + upper lip; <i>l.l.</i>, lower lip; <i>vel.</i>, septum between oral and respiratory cavities; + <i>V.</i>, <i>VII.</i>, <i>IX.</i>, <i>X.</i>, cranial nerves; <i>x.</i>, nerve with four + epibranchial ganglia.</p> + </div> + </div> + + <p>On the other hand, the two olfactory nerves of Ammocœtes compare absolutely with the + olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of + Ammocœtes arose from a pair of bilateral organs, which have fused in the middle line.</p> + + <div><span class="pagenum" id="page230">{230}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig101.jpg" id="fig101"><img style="width:100%" src="images/fig101.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 101.</span>—<i>Galeodes.</i> (From the Royal + Natural History.)</p> + </div> + </div> + + <div><span class="pagenum" id="page231">{231}</span></div> + + <p>The comparison of this olfactory organ with the camerostome gives a satisfactory reason for its + appearance in the lowest vertebrates as an unpaired median organ; equally so, the history of the + camerostome itself supplies the reason why the olfactory nerves are double, why the organ is in + reality a paired organ and not a single median one. Thus, in a sense, the grouping of the fishes + into Monorhinæ and Amphirhinæ has not much meaning, seeing that the olfactory organ is in all + cases double.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig102.png" id="fig102"><img style="width:100%" src="images/fig102.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 102.</span>—<i>Thelyphonus.</i> (From the Royal + Natural History.)</p> + </div> + </div> + + <p class="sp3">The evidence of the olfactory organs in the vertebrate not only confirms, in a most + striking manner, the theory of the origin of the <span class="pagenum" + id="page232">{232}</span>vertebrate from the Palæostracan, but points indubitably to an origin + from a scorpion-like rather than a crustacean-like stock. To complete the evidence, it ought to be + shown that the ancient sea-scorpions did possess an olfactory passage similar to the modern + land-scorpions. The evidence on this question will come best in the next chapter, where I propose + to deal with the prosomatic appendages of the Palæostracan group.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The vertebrate olfactory apparatus commences as a single median tube which terminates + dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the + supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and + terminates blindly at the infundibulum in Ammocœtes. The dorsal position of the nasal + opening is not the original one, but is brought about by the growth of the upper lip. The nasal + tube originally opened ventrally, and was at that period of development known as the tube of the + hypophysis.</p> + <p>The evidence of Ammocœtes thus goes to show that the olfactory apparatus started as an + olfactory tube on the ventral side of the animal, which led directly up to, and probably into, + the œsophagus of the original alimentary canal of the palæostracan ancestor.</p> + <p>Strikingly enough, although in the crustaceans the first pair of antennæ form the olfactory + organs, no such free antennæ are found in the arachnids, but they have amalgamated to form a + tube or olfactory passage, which leads directly into the mouth and œsophagus of the + animal.</p> + <p>This olfactory passage is very conspicuous in all members of the scorpion group, and, like + the olfactory tube of the vertebrate, is innervated by a pair of nerves, which resemble those + supplying the first pair of antennæ in crustaceans as to their origin from the + supra-œsophageal ganglia.</p> + <p>This nasal passage, or tube of the hypophysis, corresponds in structure and in position most + closely with the olfactory tube of the scorpion group, the only difference being that in the + latter case it opens directly into the œsophagus, while in the former, owing to the + closure of the old mouth, it cannot open into the infundibulum.</p> + <p>The evidence of the olfactory apparatus, combined with that of the optic apparatus, is most + interesting, for, whereas the former points indubitably to an ancestor having scorpion-like + affinities, the structure of the lateral eyes points distinctly to crustacean, as well as + arachnid, affinities.</p> + <p class="sp0">Taking the two together the evidence is extraordinarily strong that the + vertebrate arose from a member of the palæostracan group with marked scorpion-like + affinities.</p> + </div> + + <div><span class="pagenum" id="page233">{233}</span></div> + + <p class="ac">CHAPTER VII</p> + + <p class="ac"><i>THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Comparison of the trigeminal with the prosomatic region.—The prosomatic + appendages of the Gigantostraca.—Their number and nature.—Endognaths and + ectognath.—The metastoma.—The coxal glands.—Prosomatic region of Eurypterus + compared with that of Ammocœtes.—Prosomatic segmentation shown by muscular markings + on carapace.—Evidence of cœlomic cavities in Limulus.—Summary.</p> + </div> + + <p>The derivation of the olfactory organs of the vertebrate from the olfactory antennæ of the + arthropod in the last chapter is confirmatory proof of the soundness of the proposition put + forward in Chapter IV., that the segmentation in the cranial region of the vertebrate was derived + from that of the prosomatic and mesosomatic regions of the palæostracan ancestor. Such a + segmentation implies a definite series of body-segments, corresponding to the mesomeric + segmentation of the vertebrate, and a definite series of appendages corresponding to the + splanchnic segmentation of the vertebrate.</p> + + <p>Of the foremost segments belonging to the supra-œsophageal region characterized by the + presence of the median eyes, of the lateral eyes, and of the olfactory organs, a wonderfully exact + replica has been shown to exist in the pineal eyes, the lateral eyes, and the olfactory organ of + the vertebrate, belonging, as they all do, to the supra-infundibular region.</p> + + <p>Of the infra-œsophageal segments belonging to the prosoma and mesosoma respectively, the + correspondence between the mesosomatic segments carrying the branchial appendages and the uterus, + with those in the vertebrate carrying the branchiæ and the thyroid gland respectively, has been + fully proved in previous chapters.</p> + + <p>There remain, then, only the segments of the prosomatic region to be considered, a region + which, both in the vertebrate and invertebrate, is never respiratory in function but always + masticatory, such <span class="pagenum" id="page234">{234}</span>mastication being performed in + Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of + the prosomatic appendages specially modified for that purpose, and in the vertebrates by the + masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This + comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic + appendages.</p> + + <p>The investigations of van Wijhe and of all observers since the publication of his paper prove + that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the + ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied + by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic + segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear + nerves—nerves which supply muscles moving the lateral eyes.</p> + + <p>In accordance, then, with the evidence afforded by the nerves of the branchial segments, it + follows that the muscles supplied by the motor part of the trigeminal ought originally to have + moved the appendages belonging to a series of prosomatic segments. On the other hand, the + eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore + have been grouped originally in a segmental series corresponding to the prosomatic appendages.</p> + + <p>The evidence for and against this conclusion will be the subject of consideration in this and + the succeeding chapters. At the outset it is evident that any such comparison necessitates an + accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature + of the corresponding appendages.</p> + + <p>In all this group of animals, the evidence as to the number of segments in either the + prosomatic or mesosomatic regions is given by—</p> + + <p>1. The number of appendages.</p> + + <p>2. The segmental arrangement of the muscles of the prosoma or mesosoma respectively.</p> + + <p>3. The segmental arrangement of the cœlomic or head-cavities.</p> + + <p>4. The divisions of the central nervous system, or neuromeres, together with their outgoing + segmental nerves.</p> + + <p>It follows, therefore, that if from any cause the appendages are not apparent, as is the case + in many fossil remains, or have dwindled <span class="pagenum" id="page235">{235}</span>away and + become insignificant, we still have the muscular, cœlomic, and nervous arrangements left to + us as evidence of segmentation in these animals, just as in vertebrates.</p> + + <p>In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in + the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve; + (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral + somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres. + The main difference between these two regions in Limulus consists in the closer aggregation of the + prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the + prosomatic region of the brain.</p> + + <p>The number of prosomatic segments in Limulus is not evident by examination of the prosomatic + carapace, so that the most reliable guide to the segmentation of this region is given by the + appendages, of which one pair corresponds to each prosomatic segment.</p> + + <p>The number of such segments, according to present opinion, is seven, viz.<span + class="wnw">:—</span></p> + + <p>(1) The foremost segment, which bears the cheliceræ.</p> + + <p>(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor appendages; and</p> + + <p>(7) The last segment, to which belongs a small abortive pair of appendages, known by the name + of the chilaria, situated between the last pair of locomotor appendages and the operculum or first + pair of mesosomatic appendages. These appendages are numbered from 1-7 in the accompanying drawing + (Fig. <a href="#fig103">103</a>).</p> + + <p>Of these seven pairs of appendages, the significance of the first and the last has been matter + of dispute. With respect to the first pair, or the cheliceræ, the question has arisen whether + their nerves belong to the infra-œsophageal group, or are in reality + supra-œsophageal.</p> + + <p>It is instructive to observe the nature and the anterior position of this pair of appendages in + the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in + these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to + be represented by the small non-chelate antennæ seen in Fig. <a href="#fig8">8</a>, B and C (p. <a + href="#page27">27</a>), taken from Woodward. If such is the case, then these figures show that a + pair of appendages is missing in each <span class="pagenum" id="page236">{236}</span>of these + forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in + Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his + restorations were throughout consistent. Schmidt, in <i>Pterygotus osiliensis</i> has shown that + the true number was six, not five, as seen in his restoration given in Fig. <a href="#fig8">8</a>, + A (p. <a href="#page27">27</a>).</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig103.png" id="fig103"><img style="width:100%" src="images/fig103.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 103.—Ventral Surface of Limulus.</span> (Taken from <span + class="sc">Kishinouye</span>.)</p> + <p class="sp0">The gnathic bases of the appendages have been separated from those of the other + side to show the promesosternite or endostoma (<i>End.</i>).</p> + </div> + </div> + + <p>With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the + coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm, + however, in his recent beautiful preparations from Schmidt's specimens and others collected at + Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as + those of Limulus. I reproduce his figure (Fig. <a href="#fig104">104</a>) showing the small + chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.</p> + + <div><span class="pagenum" id="page237">{237}</span></div> + + <p>So, also, since Woodward's monograph, Laurie has discovered in <i>Slimonia acuminata</i> a + small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of + Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in + Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions + were in the same position and of the same character as the cheliceræ of the scorpions.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig104.png" id="fig104"><img style="width:100%" src="images/fig104.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 104.</span>—<i>Eurypterus Fischeri.</i> (From <span + class="sc">Holm</span>.)</p> + </div> + </div> + + <p>In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise + from the foremost prosomatic ganglia, and the reason why they appear to belong to the + supra-œsophageal brain-mass has been made clear by Brauer's investigations on the + embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the + dorsal side of the œsophagus during development, thus becoming + pseudo-supra-œsophageal, though in reality belonging to the infra-œsophageal + ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second + pair of antennæ in the crustacea.</p> + + <div><span class="pagenum" id="page238">{238}</span></div> + + <p>I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that + they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as + they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are + also exceptional in position.</p> + + <p>Next for consideration come the chilaria (7 in Fig. <a href="#fig103">103</a>), which Lankester + did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their + very appearance, with their spinous hairs corresponding to those of the other gnathites and their + separate nerve-supply, all point distinctly to their being a modified pair of appendages, and, + indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found + embryologically that they arise in the same way as the rest of the prosomatic appendages, and + belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer + has found that in the scorpion there is in the embryo a segment, whose appendages degenerate, + which is situated between the segment bearing the last pair of thoracic appendages and the genital + operculum—a segment, therefore, comparable in position to the chilarial segment of + Limulus.</p> + + <p>Coming now to the five locomotor appendages, we find that they resemble each other to a + considerable extent in most cases, with, however, certain striking differences. Thus in Limulus + they are chelate, with their basal joints formed as gnathites, except in the case of the fifth + appendage, in which the extremity is modified for the purpose of digging in the sand. In + Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are + called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal + process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no + prehensile power, and in most cases could have had no appreciable share in locomotion, called by + Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being + withdrawn entirely under the hood. The fifth appendage is usually different, being a large + swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the + ectognath.</p> + + <p>Finally, in <i>Drepanopterus Bembycoides</i>, as stated by Laurie, all five locomotor + appendages are built up after the same fashion, the last one not being formed as a paddle-shaped + organ or elongated as <span class="pagenum" id="page239">{239}</span>in Stylonurus, but all five + possess no special locomotor or prehensile power. According to Laurie this is a specially + primitive form of the group.</p> + + <p>It is significant to notice from this sketch that with the absence of special prehensile + terminations such as chelæ, or the absence of special locomotor functions such as walking or + swimming, these appendages tend to dwindle and become insignificant, taking up the position of + mere feelers round the mouth, and at the same time are concentrated and pressed closely together, + so that their appendage-nerves must also be close together.</p> + + <p>This sketch therefore shows us that—</p> + + <p>Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the + time when the vertebrates first appeared, in very many cases dwindling away; the latter especially + no longer functioned as locomotor appendages, but were becoming more and more mere palps or + tentacles situated round the mouth, which could by no possibility afford any help to + locomotion.</p> + + <p>On the contrary, the sixth pair of appendages—the ectognaths—remained powerful, + being modified in many cases into large oar-like limbs by which the animal propelled itself + through the water.</p> + + <p>It is a striking coincidence that those ancient fishes, <span class="correction" + title="Original reads 'Ptericthys'.">Pterichthys</span> and Bothriolepis, should have possessed a + pair of large oar-like appendages.</p> + + <p>At this time, then, in strong contrast to the endognaths, the ectognaths, or sixth pair of + appendages, remained strong and vigorous. What about the seventh pair, the chilaria of + Limulus?</p> + + <p>Of all the prosomatic appendages these are the most interesting from the point of view of my + theory, for whereas in the scorpion of the present day they have dwindled away and left no trace + except in the embryo, in the sea-scorpions of old, far from dwindling, they had developed and + become a much more important organ than the chilaria of Limulus.</p> + + <p>In all these animals a peculiarly striking and unique structure is found in this region known + by the name of the metastoma, or lip-plate (Figs. 8 and 104 (7)); it is universally considered to + be formed by the fusion of the two chilarial appendages.</p> + + <p>All observers are agreed that this lip-plate was freely movable. Nieskowski considers that the + movement of the metastoma was entirely in a vertical direction, whereby the cleft which is seen + <span class="pagenum" id="page240">{240}</span>between the basal joints of all the pairs of + locomotor appendages could be closed from behind. Woodward says it no doubt represents the labium, + and served more effectually to enclose the posterior part of the buccal orifice, being found + exteriorly to the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with Nieskowski, + and looks on the mestasoma as forming a lower lip within which the bases of the ectognaths + worked.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig105.png" id="fig105"><img style="width:100%" src="images/fig105.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 105.—Diagram of Sagittal Median Section through A, + Limulus, B, Eurypterus.</span></p> + </div> + </div> + + <p>Quite recently Gerhard Holm has worked over again the very numerous specimens of <i>Eurypterus + Fischeri</i>, which are obtainable at Rootziküll, and has thrown new light on the relation of the + metastoma to the mouth-parts. His preparations show clearly that the true lower lip of Eurypterus + was not the metastoma, for when the metastoma is removed another plate (<i>End.</i>, Fig. <a + href="#fig105">105</a>, B) situated <span class="pagenum" id="page241">{241}</span>internally to + it is disclosed, which, in his view, corresponds to the sternite between the bases of the + pro-somatic appendages in Limulus, <i>i.e.</i> to the sternite called by Lankester, the + pro-mesosternite (<i>End.</i>, Fig. <a href="#fig103">103</a>). This inner plate formed with the + metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed posteriorly, within which the + bases of the ectognaths worked. In other words, the removal of the metastoma discloses in + Eurypterus the true anterior ventral surface of the animal which corresponds to that of Limulus, + or of the scorpion group, with its pro-mesosternite and laterally attached gnathites or + sterno-coxal processes. To this inner plate or pro-mesosternite Holm gives the name of + <i>endostoma</i>.</p> + + <p>To the anterior edge of the endostoma a thinner membrane is attached which passes inwards in + the direction of the throat, and forms, therefore, the lower lip (<i>Hyp.</i>, Fig. <a + href="#fig105">105</a>, B) of the passage of the mouth (<i>olf. p.</i>). This membrane bears upon + its surface a tuft of hairs, which he thought were probably olfactory in function. Consequently, + in his preliminary communication, he describes this lower lip as forming, in all probability, an + olfactory organ; in his full communication he repudiates this suggestion, because he thinks it + unlikely that such an organ would be situated within the mouth. I feel sure that if Holm had + referred to Croneberg's paper, and seen how the true mouth in all the scorpion group is situated + at the base of an olfactory passage, he would have recognized that his first suggestion is in + striking accordance with the nature of the entrance to the mouth in other scorpions.</p> + + <p>That Eurypterus also possessed a camerostome (<i>cam.</i>) seems to follow of necessity from + its evident affinities both with Limulus and the scorpions. We see, in fact, that the mouth of + these old sea-scorpions was formed after the fashion of Limulus, surrounded by masticatory organs + in the shape of foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal sagittal + section of one of these old palæostracan forms would be obtained by the combination of actual + sagittal sections through Limulus and a member of the scorpion group, with, at the same time, a + due recognition of Holm's researches. Such a section is represented in Fig. <a + href="#fig105">105</a>, B, in which I have drawn the central nervous system and its nerves, the + median eyes (<i>C.E.</i>), the olfactory organs (<i>Cam.</i>), the pharynx (<i>Ph.</i>), + œsophagus (<i>œs.</i>), and alimentary canal (<i>Al.</i>), but have not tried to + indicate the lateral eyes. I have represented the prosomatic appendages by numbers (1-7), and + <span class="pagenum" id="page242">{242}</span>the foremost mesosomatic segments by numbers + (8-13). I have placed the four endognaths and the nerves going to them close together, and made + them small, mere tentacles, in recognition of the character of these appendages in Eurypterus, and + have indicated the position and size of the large ectognath, with its separate nerve, by (6). If + among the ancient Eurypterus-like forms, which were living at the time when vertebrates first + appeared, there were some in which the ectognaths also had dwindled to a pair of tentacles, then + such animals would possess a prosomatic chamber formed by a metastoma or accessory lip, within + which were situated five pairs of short tactile appendages or tentacles. If the vertebrate were + derived from such an animal, then the trigeminal nerve, as the representative of these prosomatic + appendage-nerves, ought to be found to supply the muscles of this accessory lip and of these five + pairs of tentacles in the lowest vertebrate.</p> + + <p class="sp3">This prosomatic or oral chamber, as it might be called, was limited posteriorly by + the fused metastoma (7) and operculum (8), so that if in the same imaginary animal one imagines + that the gill-chambers, instead of being separate, are united to form one large respiratory + chamber, then, in such an animal, a prosomatic oral chamber, in which the prosomatic appendages + worked, would be separated from a mesosomatic respiratory chamber by a septum composed of the + conjoined basal portions of the mesosomatic operculum and the prosomatic metastoma, as indicated + in the diagram. In this septum the nerves to the last prosomatic appendage (equivalent to the last + part of the trigeminal in the vertebrate) and to the first mesosomatic (equivalent to the thyroid + part of the facial) would run, as shown in the figure, close together in the first part of their + course, and would separate when the ventral surface was reached, to pass headwards and tailwards + respectively.</p> + + <p class="ac"><span class="sc">The Coxal Glands.</span></p> + + <p>One more characteristic of these appendages requires mention, and that is the excretory glands + situated at the base of the four endognaths known as the coxal glands. These glands are the main + excretory organs in Limulus and the scorpions, and extend into the basal segments or coxæ of the + four endognaths, not into those of the ectognaths or the chilaria (or metastoma). Hence their + name, coxal <span class="pagenum" id="page243">{243}</span>glands; and, seeing the importance of + the excretory function, it is likely enough that they would remain, even when the appendages + themselves had dwindled away. With the concentration and dwindling of the endognaths these coxal + glands would also be concentrated, so that in the diagram (Fig. <a href="#fig105">105</a>) they + would rightly be grouped together in the position indicated (<i>cox. gl.</i>).</p> + + <p class="sp3">Such a diagram indicates the position of all the important organs of the + head-region except the special organs for taste and hearing. These, for the sake of convenience, I + propose to take separately, in order at this stage of my argument not to overburden the simplicity + of the comparison I desire to make with too much unavoidable detail.</p> + + <p class="ac"><span class="sc">The Prosomatic Region of Ammocœtes.</span></p> + + <p>Let us now compare this diagram with that of the corresponding region in Ammocœtes and + see whether or no any points of similarity exist.</p> + + <p>With respect to this region, as in so many other instances already mentioned, Ammocœtes + occupies an almost unique position among vertebrates, for the region supplied by the trigeminal + nerve—the prosomatic region—consists of a large oral chamber which was separated from + the respiratory chamber in the very young stage by a septum which is subsequently broken through, + and so the two chambers communicate.</p> + + <p>This chamber is bounded by the lower lip ventrally, the upper lip and trabecular region + dorsally, and the remains of the septum or velum laterally and posteriorly. It contains a number + of tentacles arranged in pairs within the chamber so as to form a sieve-like fringe inside the + circular mouth; of these, the ventral pair are large, fused together, and attached to the lower + lip.</p> + + <p>All the muscles belonging to this oral chamber are of the visceral type, and are innervated by + the trigeminal nerve. In accordance with the evidence obtained up to this point this means that + such an oral chamber was formed by the prosomatic appendages of the invertebrate ancestor, + similarly to the oral chamber just figured for Eurypterus.</p> + + <p>This chamber in the full-grown Ammocœtes is not only open to the respiratory chamber, + but is bounded by the large upper lip (<i>U.L.</i>, Fig. <a href="#fig106">106</a>, D). On the + dorsal surface of this region, in front of the <span class="pagenum" + id="page244">{244}</span>pineal eye (<i>C.E.</i>), is the most conspicuous opening of the + olfactory tube (<i>Na.</i>), which olfactory tube passes from the dorsal region to the ventral + side to terminate blindly at the very spot where the infundibulum comes to the surface of the + brain. Here, also, is situated that extraordinary glandular organ known as the pituitary body + (<i>Pit.</i>). A sagittal section, then, in diagram form, of the position of parts in the + full-grown Ammocœtes, would be represented as in Fig. <a href="#fig106">106</a>, D.</p> + + <p>But, as argued out in the last chapter, the diagram of the adult Ammocœtes must be + compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with + the larval condition of Ammocœtes. In other words, Fig. <a href="#fig106">106</a>, B, must + be compared with Fig. <a href="#fig106">106</a>, C, which represents a section through the larval + Ammocœtes as it would appear if it reached the adult condition without any forward growth + of the upper lip or any breaking through of the septum between the oral and respiratory chambers. + The striking similarity between this diagram and that of Eurypterus becomes immediately manifest + even to the smallest details. The only difference between the two, except, of course, the + notochord, consists in the closure of the mouth opening (<i>o</i>), in Fig. <a + href="#fig106">106</a>, B, by which the olfactory passage (<i>olf. p.</i>) of the scorpion becomes + converted into the hypophysial tube (<i>Hy.</i>), Fig. <a href="#fig106">106</a>, C, and later + into the nasal tube (<i>Na.</i>), Fig. <a href="#fig106">106</a>, D, of the full-grown + Ammocœtes. That single closure of the old mouth is absolutely all that is required to + convert the Eurypterus diagram into the Ammocœtes diagram.</p> + + <p>Such a comparison immediately explains in the simplest manner a number of anatomical + peculiarities which have hitherto been among the great mysteries of the vertebrate organization. + For not only do the median eyes (<i>C.E.</i>) correspond in position in the two diagrams, and the + infundibular tube (<i>Inf.</i>) and the ventricles of the brain (<i>C.C.</i>) correspond to the + œsophagus (<i>œs.</i>) and the cephalic stomach (<i>Al.</i>), as already fully + discussed; but even in the very place where the narrow œsophagus opened into the wider + chamber of the pharynx (<i>Ph.</i>), there, in all the lower vertebrates, the narrow infundibular + tube opens into the wider chamber of the membranous <i>saccus vasculosus</i> (<i>sac. vasc.</i>). + This is the last portion of the membranous part of the tube of the central nervous system which + has not received explanation in the previous chapters, and now it is seen how simple its + explanation is, how natural its presence—it represents the old pharyngeal chamber of the + palæostracan ancestor.</p> + + <div><span class="pagenum" id="page245">{245}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig106.png" id="fig106"><img style="width:100%" src="images/fig106.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 106.—Diagram of Sagittal Median Section through B, + Eurypterus; C, Larval Ammocœtes; D, Full-grown Ammocœtes.</span></p> + </div> + </div> + + <div><span class="pagenum" id="page246">{246}</span></div> + + <p>Next among the mysteries requiring explanation is the pituitary body, that strange glandular + organ always found so closely attached to the brain in the infundibular region that when it is + detached in taking out the brain it leaves the infundibular canal patent right into the IIIrd + ventricle. A comparison of the two diagrams indicates that such a glandular organ (<i>Pit.</i>), + Fig. <a href="#fig106">106</a>, C, was there because the coxal excretory glands (<i>cox. gl.</i>), + Fig. <a href="#fig106">106</a>, B, were in a similar position in the palæostracan + ancestor—that, indeed, the pituitary body is the descendant of the coxal glands.</p> + + <p>Finally, the diagrams not only indicate how the mesosomatic appendage-nerves supplying in the + one case the operculum and the respiratory appendages correspond to the respiratory group of + nerves, VII., IX., X., supplying in the other case the thyroid, hyoid, and branchial segments, but + also that a similar correspondence exists between the prosomatic appendage-nerves in the one case + and the trigeminal nerve in the other; a correspondence which supplies the reason why in the + vertebrate a septum originally existed between an oral and respiratory chamber.</p> + + <p>Such a comparison, then, leads directly to the suggestion that the trigeminal nerve originally + supplied the prosomatic appendages, such appendages being: 1. The metastoma, which has become in + Ammocœtes the lower lip supplied by the velar or mandibular branch of the trigeminal nerve + (7); 2. The ectognath, which has become the large median ventral tentacle, called by Rathke the + tongue, supplied by the tongue nerve (6); 3. The endognaths, which have been reduced to tentacles + and are supplied by the tentacular branch of the trigeminal nerve (2, 3, 4, 5).</p> + + <p>I have purposely put these two diagrams of the larval Ammocœtes and of Eurypterus before + the minds of my readers at this early stage of my argument, so as to make what follows more + understandable. I propose now to consider fully each one of these suggestive comparisons, and to + see whether or no they are in accordance with the results of modern research.</p> + + <p>In the first instance, the diagrams suggest that the trigeminal nerve originally supplied the + prosomatic appendages of the palæostracan ancestor, while the eye-muscle nerves supplied the + body-muscles of the prosoma.</p> + + <div><span class="pagenum" id="page247">{247}</span></div> + + <p>As these appendages did not carry any vital organs such as branchiæ, but were mainly locomotor + and masticatory in function, it follows that their disappearance as such would be much more + complete than that of the mesosomatic branchial appendages. Most probably, then, in the higher + vertebrates no trace of such appendages might be left; consequently the segmentation due to their + presence would be very obscure, so that in this region the very reverse of what is found in the + region of the vagus nerve would be the rule. There branchiomeric segmentation is especially + evident, owing to the persistence of the branchial part of the branchial appendages; here, owing + to the disappearance of the appendages, the segmentation is no longer branchiomeric, but + essentially mesomeric in consequence of the persistence of the somatic eye-muscles.</p> + + <p class="sp3">In addition to the evidence of the appendages themselves, the number of prosomatic + segments is well marked out in all the members of the scorpion group by the divisions of the + central nervous system into well-defined neuromeres in accordance with the appendages, a + segmentation the reminiscence of which may still persist after the appendages themselves have + dwindled or disappeared. In accordance with this possibility we see that one of the most recent + discoveries in favour of a number of segments in the head-region of the vertebrate is the + discovery in the early embryo of a number of partial divisions in the brain-mass, forming a system + of cephalic neuromeres which may well be the rudiments of the well-defined cephalic neuromeres of + animals such as the scorpion.</p> + + <p class="ac"><span class="sc">The Evidence of the Prosomatic Musculature.</span></p> + + <p>Even if the appendages as such become obscure, yet their muscles might remain and show evidence + of their presence. The most persistent of all the appendage-muscles are the basal muscles which + pass from coxa to carapace and are known by the name of tergo-coxal muscles. They are large, well + marked, segmentally arranged muscles, dorso-ventral in direction, and, owing to their connecting + the limb with the carapace, are likely to be retained even if the appendage dwindles away.</p> + + <p>The muscular system of Limulus and Scorpio has been investigated by Benham and Miss Beck under + Lankester's direction, and the conclusions to which Lankester comes are these—</p> + + <div><span class="pagenum" id="page248">{248}</span></div> + + <p>The simple musculature of the primitive animal from which both Limulus and the scorpions arose + consisted of—</p> + + <div class="bq1 sp2"> + <p>1. A series of paired longitudinal dorsal muscles passing from tergite to tergite of each + successive segment.</p> + <p>2. A similar series of paired longitudinal ventral muscles.</p> + <p>3. A pair of dorso-ventral muscles passing from tergite to sternite in each segment.</p> + <p>4. A set of dorso-ventral muscles moving the coxa of each limb in its socket.</p> + <p class="sp0">5. A pair of veno-pericardial muscles in each segment.</p> + </div> + + <p>Of these groups of muscles, any one of which would indicate the number of segments, Groups 1 + and 2 do not extend into the prosomatic region, and Group 5 extends only as far as the heart + extends in the case of both Limulus and the Scorpion group; so that we may safely conclude that in + the Palæostraca the evidence of somatic segmentation in the prosomatic region would be given, as + far as the musculature is concerned, by the dorso-ventral somatic muscles (Group 3), and of + segmentation due to the appendages by the dorso-ventral appendage musculature (Group 4).</p> + + <p>Therefore, if, as the evidence so far indicates, the vertebrate has arisen from a palæostracan + stock, we should expect to find that the musculature of the somatic segments in the region of the + trigeminal nerve did not resemble the segmental muscles of the spinal region, was not, therefore, + the continuation of the longitudinal musculature of the body, but was dorso-ventral in position, + and that the musculature of the splanchic segments resembled that of the vagus region, where, as + pointed out in Chapter IV., the respiratory muscles arose from the dorso-ventral muscles of the + mesosomatic appendages. This is, of course, exactly what is found for the muscles which move the + lateral eyes of the vertebrate; these muscles, innervated by the IIIrd, IVth, and VIth nerves, + afford one of the main evidences of segmentation in this region, are always grouped in line with + the somatic muscles of spinal segments, and yet cannot be classed as longitudinal muscles. They + are dorso-ventral in direction, and yet belong to the somatic system; they are exactly what one + ought to find if they represent Group 3—the dorso-ventral body-muscles of the prosomatic + segments of the invertebrate ancestor.</p> + + <p>The interpretation of these muscles will be given immediately; at present I want to pass in + review all the different kinds of evidence <span class="pagenum" id="page249">{249}</span>of + segmentation in this region afforded by the examination of the invertebrate, whether living or + fossil, so as to see what clues are left if the evidence of appendages fails us. I will take in + the first instance the evidence of segmentation afforded by the presence of the musculature of + Group 4, even when, as in the case of many fossils, no appendages have yet been found. In such + animals as Mygale and Phrynus the prosomatic carapace is seen to be marked out into a series of + elevations and depressions, and upon removing the carapace we see that these elevations correspond + with and are due to the large tergo-coxal muscles of the appendages; so that if such carapace + alone were found fossilized we could say with certainty: this animal possessed prosomatic + appendages the number of which can be guessed with more or less certainty by these indications of + segments on the carapace.</p> + + <p>In those forms, then, which are only known to us in the fossil condition, in which no + prosomatic appendages have been found, but which possess, more or less clearly, radial markings on + the prosomatic carapace resembling those of Phrynus or Mygale, such radial markings may be + interpreted as due to the presence of prosomatic appendages, which are either entirely concealed + by the prosomatic carapace or dorsal head-plate, or were of such a nature as not to have been + capable of fossilization.</p> + + <p>The group of animals in question forms the great group of animals, chiefly extinct, classified + by H. Woodward under the order of Merostomata. They are divided by him into the sub-order of + Eurypteridæ, which includes—(1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, + (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9) Exapinurus, (10) + Pseudoniscus; and the sub-order Xiphosura, which includes—(1) Belinurus, (2) Prestwichia, + (3) Limulus.</p> + + <div><span class="pagenum" id="page250">{250}</span></div> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig107.png" id="fig107"><img style="width:100%" src="images/fig107.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 107.</span>—<i>Phrynus Margine-Maculata.</i></p> + <p class="sp0"><i>Ce.</i>, median eyes; <i>le.</i>, lateral eyes; <i>glab.</i>, median plate + over brain; <i>Fo.</i>, fovea.</p> + </div> + </div> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig108.png" id="fig108"><img style="width:100%" src="images/fig108.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 108.</span>—<i>Phrynus sp.</i> (?). <span class="sc">Carapace + removed.</span></p> + <p class="sp0"><i>cam.</i>, camerostome; <i>pl.</i>, plastron.</p> + </div> + </div> + + <div><span class="pagenum" id="page251">{251}</span></div> + + <p>The evidence of the Xiphosura and of the Hemiaspidæ conclusively shows, in Woodward's opinion, + that the Merostomata are closely related to the Trilobita, and the Hemiaspidæ especially are + supposed to be intermediate between the trilobites and the king-crabs. They are characterized, as + also Belinurus and Prestwichia, by the absence of any prosomatic appendages, so that in these + cases, as is seen in Fig. <a href="#fig12">12</a> (p. <a href="#page30">30</a>), representing + <i>Bunodes lunula</i>, found in the Eurypterus layer at Rootziküll, we have an animal somewhat + resembling Limulus in which the prosomatic appendages have either dwindled away and are completely + hidden by the prosomatic carapace, or became so soft as not to be preserved in the fossilized + condition. The appearance of the prosomatic carapace is, to my mind, suggestive of the presence of + such appendages, for it is marked out radially, as is seen in the figure, in a manner resembling + somewhat the markings on the prosomatic carapace of Mygale or Phrynus; the latter markings, as + already mentioned, are due to the aponeuroses between the tergo-coxal muscles of the prosomatic + appendages which lie underneath and are attached to the carapace.</p> + + <p>A very similar radial marking is shown by Woodward in his picture of <i>Hemiaspis + limuloides</i>, reproduced in Fig. 109, found in the Lower Ludlow beds at Leintwardine. This + species has yielded the most perfect specimens of the genus Hemiaspis, which is recognized as + differing from Bunodes by the possession of a telson.</p> + + <p>It is striking to find that similar indications of segments have been found on the dorsal + surface of the head-region in many of the most ancient extinct fishes, as will be fully discussed + later on.</p> + + <div class="ac w20 fcenter sp3"> + <a href="images/fig109.jpg" id="fig109"><img style="width:100%" src="images/fig109.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig.109.</span>—<i>Hemiaspis limuloides.</i> (From <span + class="sc">Woodward</span>.)</p> + <p class="sp0"><i>gl.</i>, glabellum.</p> + </div> + </div> + + <p class="ac"><span class="sc">The Evidence of Cœlomic Cavities.</span></p> + + <p>In the head-region of the vertebrate, morphologists depend largely upon the embryonic divisions + of the mesoderm for the estimation of the number of segments, and, therefore, upon the number of + cœlomic cavities in this region, the walls of which give origin to the striated muscles of + the head, so that the question of the number of segments depends very largely upon the origin of + the muscles from the walls of these head-cavities. It is therefore interesting to examine whether + a similar criterion of segmentation holds good in such a segmented <span class="pagenum" + id="page252">{252}</span>animal as Limulus, or in the members of the scorpion group, in which the + number of segments are known definitely by the presence of the appendages. In Limulus we know, + from the observations of Kishinouye, that a series of cœlomic cavities are formed + embryologically in the various segments of the mesosoma and prosoma, in a manner exceedingly + similar to their mode of formation in the head-region of the vertebrate, and he has shown that in + the mesosoma a separate cœlomic cavity exists for each segment, so that just as the + dorso-ventral somatic muscles are regularly segmentally arranged in this region, so are the + cœlomic cavities, and we should be right in our estimation of the number of segments in + this region by the consideration of the numerical correspondence of these cavities with the + mesomatic appendages. Similarly, in the vertebrate, we find every reason to believe that a single, + separate head-cavity corresponds to each of the branchial segments in the opisthotic region, and + therefore we should estimate rightly the number of segments by the division of the mesoderm in + this region.</p> + + <p>In the prosomatic region of Limulus, the dorso-ventral muscles are not arranged with such + absolute segmental regularity as in the mesosomatic region, and Kishinouye's observations show + that the cœlomic cavities in this region do not correspond absolutely with the number of + prosomatic appendages. His words are<span class="wnw">:—</span></p> + + <p>A pair of cœlomic cavities appears in every segment except the segments of the 2nd, 3rd, + and 4th appendages, in which cœlomic cavities do not appear at all. At least eleven pairs + of these cavities are produced. The eleventh pair belongs to the seventh abdominal segment.</p> + + <p>The first pair of cœlomic cavities is common to the cephalic lobe and the segment of the + first appendage (<i>i.e.</i> the cheliceræ).</p> + + <p>The second cœlomic cavity belongs to the segment of the fifth appendage. It is well + developed.</p> + + <p>The ventral portion of the second cœlomic cavity remains as the coxal gland.</p> + +<hr style="width:6em"/> + + <p>Consequently, if we were to estimate the number of segments in this region by the number of + cœlomic cavities we should not judge rightly, for we should find only four cavities and + seven appendages, as is seen in the following table<span class="wnw">:—</span></p> + + <div><span class="pagenum" id="page253">{253}</span></div> + + <table class="sp2 mc bt bb nothand" title="Cœlomic Cavities" + summary="Cœlomic Cavities"> + <tr class="bb"> + <th colspan="9" class="vmi smaller br">LIMULUS.</th> + <th colspan="2" class="vmi smaller">VERTEBRATE.</th> + </tr> + <tr class="bb"> + <th colspan="3" class="vmi smaller">Segments.</th> + <th class="vmi smaller ba">Appendages.</th> + <th colspan="4" class="vmi smaller ba">Eurypterid appendages.</th> + <th class="vmi smaller ba">Cœlomic<br/> + cavities.</th> + <th colspan="2" class="vmi smaller">Cœlomic cavities.</th> + </tr> + <tr class="br"> + <td rowspan="7" class="vmi br0 pr0"><img src="images/prosomatic.png" style="width:1.25em" + alt="Prosomatic"/></td> + <td rowspan="7" class="vmi pl0 pr0"><img src="images/lbrace7.png" style="height:19.0ex; + width:1em;" alt="brace" /></td> + <td class="ac">1</td> + <td class="al">Cheliceræ or 1st locomotor.</td> + <td class="br0"></td> + <td colspan="3" class="al">Cheliceræ</td> + <td class="ac">1</td> + <td rowspan="5" class="br0"></td> + <td class="al br0">Anterior</td> + </tr> + <tr class="br"> + <td class="ac">2</td> + <td class="al">2nd locomotor</td> + <td rowspan="4" class="br0 vmi pl0 pr0"><img src="images/rbrace4.png" style="height:10.3ex; + width:1em;" alt="brace" /></td> + <td colspan="3" rowspan="4" class="al vmi">Endognaths</td> + <td rowspan="4" class="ac vmi">2</td> + <td rowspan="4" class="al br0 vmi">Premandibular</td> + </tr> + <tr class="br"> + <td class="ac">3</td> + <td class="al">3rd <span class="hid">loco</span>"</td> + </tr> + <tr class="br"> + <td class="ac">4</td> + <td class="al">4th <span class="hid">loco</span>"</td> + </tr> + <tr class="br"> + <td class="ac">5</td> + <td class="al">5th <span class="hid">loco</span>"</td> + </tr> + <tr class="br"> + <td class="ac">6</td> + <td class="al">6th <span class="hid">loco</span>"</td> + <td rowspan="2" class="br0"></td> + <td colspan="3" class="al">Ectognath</td> + <td class="ac">3</td> + <td rowspan="2" class="br0 vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; + width:1em;" alt="brace" /></td> + <td rowspan="2" class="al br0 vmi">Mandibular</td> + </tr> + <tr class="br"> + <td class="ac">7</td> + <td class="al">Chilaria</td> + <td colspan="3" class="al">Metastoma</td> + <td class="ac">4</td> + </tr> + <tr class="br bt"> + <td rowspan="7" class="vmi br0 pr0"><img src="images/mesosomatic.png" style="width:1.25em" + alt="Mesosomatic"/></td> + <td rowspan="7" class="vmi pl0 pr0"><img src="images/lbrace7.png" style="height:19.0ex; + width:1em;" alt="brace" /></td> + <td class="ac">8</td> + <td class="al">Operculum</td> + <td rowspan="2" class="br0 vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; + width:1em;" alt="brace" /></td> + <td rowspan="2" class="al vmi br0">Operculum</td> + <td rowspan="2" class="vmi pl0 pr0 br0"><img src="images/lbrace2.png" style="height:4.5ex; + width:1em;" alt="brace" /></td> + <td class="al">Genital</td> + <td class="ac">5</td> + <td rowspan="2" class="br0 vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; + width:1em;" alt="brace" /></td> + <td rowspan="2" class="al br0 vmi">Hyoid</td> + </tr> + <tr class="br"> + <td class="ac">9</td> + <td class="al">1st branchial</td> + <td class="al">1st branchial</td> + <td class="ac">6</td> + </tr> + <tr class="br"> + <td class="ac">10<span class="hid">0</span></td> + <td class="al">2nd <span class="hid">brat</span>"</td> + <td rowspan="5" class="br0"></td> + <td colspan="3" class="al">2nd branchial</td> + <td class="ac">7</td> + <td rowspan="5" class="br0"></td> + <td class="al br0">1st branchial</td> + </tr> + <tr class="br"> + <td class="ac">11<span class="hid">0</span></td> + <td class="al">3rd <span class="hid">bran</span>"</td> + <td colspan="3" class="al">3rd <span class="hid">bran</span>"</td> + <td class="ac">8</td> + <td class="al br0">2nd <span class="hid">brat</span>"</td> + </tr> + <tr class="br"> + <td class="ac">12<span class="hid">0</span></td> + <td class="al">4th <span class="hid">bran</span>"</td> + <td colspan="3" class="al">4th <span class="hid">bran</span>"</td> + <td class="ac">9</td> + <td class="al br0">3rd <span class="hid">bran</span>"</td> + </tr> + <tr class="br"> + <td class="ac">13<span class="hid">0</span></td> + <td class="al">5th <span class="hid">bran</span>"</td> + <td colspan="3" class="al">5th <span class="hid">bran</span>"</td> + <td class="ac">10<span class="hid">0</span></td> + <td class="al br0">4th <span class="hid">bran</span>"</td> + </tr> + <tr class="br"> + <td class="ac">14<span class="hid">0</span></td> + <td class="al">6th <span class="hid">bran</span>"</td> + <td colspan="3"></td> + <td class="ac">11<span class="hid">0</span></td> + <td class="br0"></td> + </tr> + </table> + +<!-- Trimmed down version for handhelds --> + + <table class="sp2 w100 bt bb handonly" title="Cœlomic Cavities" + summary="Cœlomic Cavities"> + <tr class="bb"> + <th colspan="5" class="vmi smaller br">LIMULUS.</th> + <th class="vmi smaller">VERTEBRATE.</th> + </tr> + <tr class="bb"> + <th colspan="2" class="vmi smaller plhs prhs">Segments.</th> + <th class="vmi smaller ba">Appendages.</th> + <th class="vmi smaller ba">Eurypterid<br/> + appendages.</th> + <th class="vmi smaller ba">C.c.</th> + <th class="vmi smaller">Cœlomic<br/> + cavities.</th> + </tr> + <tr class="br"> + <td rowspan="7" class="vmi br0 pr0 pl0"><img src="images/prosomatic.png" style="width:1.25em" + alt="Prosomatic"/></td> + <td class="ar">1</td> + <td class="al">Cheliceræ or 1st locomotor.</td> + <td class="al">Cheliceræ</td> + <td class="ar">1</td> + <td class="al br0">Anterior</td> + </tr> + <tr class="br"> + <td class="ar">2</td> + <td class="al">2nd locomotor</td> + <td rowspan="4" class="al vmi br0" style="padding:0"> + <table class="w100" title="Cœlomic Cavities" summary="Cœlomic Cavities"> + <tr> + <td class="pl0 pr0"><img src="images/rbrace5.png" style="height:12.0ex; width:1em;" + alt="brace" /></td> + <td class="pl0 vmi br">Endognaths</td> + </tr> + </table> + </td> + <td rowspan="4" class="ar vmi">2</td> + <td rowspan="4" class="al br0 vmi">Premandibular</td> + </tr> + <tr class="br"> + <td class="ar">3</td> + <td class="al">3rd <span class="hid">loco</span>"</td> + </tr> + <tr class="br"> + <td class="ar">4</td> + <td class="al">4th <span class="hid">loco</span>"</td> + </tr> + <tr class="br"> + <td class="ar">5</td> + <td class="al">5th <span class="hid">loco</span>"</td> + </tr> + <tr class="br"> + <td class="ar">6</td> + <td class="al">6th <span class="hid">loco</span>"</td> + <td class="al">Ectognath</td> + <td class="ar">3</td> + <td rowspan="2" class="al br0 vmi">Mandibular</td> + </tr> + <tr class="br"> + <td class="ar">7</td> + <td class="al">Chilaria</td> + <td class="al">Metastoma</td> + <td class="ar">4</td> + </tr> + <tr class="br bt"> + <td rowspan="7" class="vmi br0 pr0 pl0"><img src="images/mesosomatic.png" style="width:1.25em" + alt="Mesosomatic"/></td> + <td class="ar">8</td> + <td class="al">Operculum</td> + <td rowspan="2" class="al vmi br0" style="padding:0"> + <table class="w100" title="Cœlomic Cavities" summary="Cœlomic Cavities"> + <tr> + <td class="vmi prhs" rowspan="2">Opc.</td> + <td class="br plhs">Genital</td> + </tr> + <tr> + <td class="br plhs">1st<br/> + branchial</td> + </tr> + </table> + </td> + <td class="ar">5</td> + <td rowspan="2" class="al br0 vmi">Hyoid</td> + </tr> + <tr class="br"> + <td class="ar vmi">9</td> + <td class="al">1st branchial</td> + <td class="ar vmi">6</td> + </tr> + <tr class="br"> + <td class="ar">10</td> + <td class="al">2nd <span class="hid">brat</span>"</td> + <td class="al">2nd branchial</td> + <td class="ar">7</td> + <td class="al br0">1st branchial</td> + </tr> + <tr class="br"> + <td class="ar">11</td> + <td class="al">3rd <span class="hid">bran</span>"</td> + <td class="al">3rd <span class="hid">bran</span>"</td> + <td class="ar">8</td> + <td class="al br0">2nd <span class="hid">brat</span>"</td> + </tr> + <tr class="br"> + <td class="ar">12</td> + <td class="al">4th <span class="hid">bran</span>"</td> + <td class="al">4th <span class="hid">bran</span>"</td> + <td class="ar">9</td> + <td class="al br0">3rd <span class="hid">bran</span>"</td> + </tr> + <tr class="br"> + <td class="ar">13</td> + <td class="al">5th <span class="hid">bran</span>"</td> + <td class="al">5th <span class="hid">bran</span>"</td> + <td class="ar">10</td> + <td class="al br0">4th <span class="hid">bran</span>"</td> + </tr> + <tr class="br"> + <td class="ar">14</td> + <td class="al">6th <span class="hid">bran</span>"</td> + <td></td> + <td class="ar">11</td> + <td class="br0"></td> + </tr> + </table> + +<!-- End of trimmed down version for handhelds --> + + <p>The second cavity would in reality represent four segments belonging to the 2nd, 3rd, 4th, 5th + locomotor appendages, <i>i.e.</i> the very four segments which in the Eurypteridæ are concentrated + together to form the endognaths, and we should be justified in putting this interpretation on it, + because, according to Kishinouye, its ventral portion forms the coxal gland, and, according to + Lankester, the coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th, 5th locomotor + appendages. Similarly in the vertebrate, we find three head-cavities in the region which + corresponds, on my theory, to the prosomatic region of Limulus, (1) the anterior cavity discovered + by Miss Platt, (2) the premandibular cavity, and (3) the mandibular cavity, which, if they + corresponded with the prosomatic cœlomic cavities of Limulus, would represent not three + segments but seven segments, as follows:—the anterior cavity would correspond to the first + cœlomic cavity, <i>i.e.</i> the cavity of the cheliceral segments in both Limulus and the + Eurypteridæ; the premandibular, to the second cœlomic cavity, representing, therefore, the + 2nd, 3rd, 4th, 5th prosomatic segments in Limulus and the endognathal segments in the Eurypteridæ; + and the mandibular to the 3rd and 4th cœlomic cavities, representing the last locomotor and + chilarial segments in Limulus, <i>i.e.</i> the ectognathal and metastomal segments in the + Eurypteridæ.</p> + + <div><span class="pagenum" id="page254">{254}</span></div> + + <p>It is worthy of note that, in respect to their cœlomic cavities, as in the position and + origin of their nerves in the central nervous system, the first pair of appendages, the cheliceræ, + retain a unique position, differing from the rest of the prosomatic appendages.</p> + + <p class="sp3">In the table I have shown how the vertebrate cœlomic cavities may be + compared with those of Limulus. The next question to consider is the evidence obtained by + morphologists and anatomists as to the number of segments supplied by the trigeminal nerve-group; + this question will be considered in the next chapter.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>In Chapters IV. and V. I have dealt with the opisthotic segments of the vertebrate, including + therein the segments supplied by the facial nerve, and shown that they correspond to the + mesosomatic segments of the palæostracan; consequently the facial (VII.), glossopharyngeal + (IX.), and vagus (X.) nerves originally supplied the branchial and opercular appendages.</p> + <p>In this chapter the consideration of the pro-otic segments is commenced, that is, the + segments supplied by the trigeminal (V.) and the eye-muscle nerves (III., IV., VI.). I have + considered the VIth nerve with the rest of the eye-muscle nerves for convenience' sake, though + in reality it belongs to the same segment as the facial. Of these, that part of the trigeminal + which innervates the muscles of mastication corresponds to the splanchnic segments, while the + eye-muscle nerves belong to the corresponding somatic segments; but the pro-otic segments of the + vertebrate ought to correspond to the prosomatic segments of the invertebrate, just as the + opisthotic correspond to the mesosomatic. Therefore the motor part of the trigeminal ought to + supply muscles which originally moved the prosomatic appendages, while the eye-muscles ought to + have belonged to the somatic part of the same segments.</p> + <p>The first question considered is the number of segments which ought to be found in this + region. In Limulus, the Eurypteridæ, and the scorpions there are seven prosomatic segments which + carry (1) the cheliceræ, (2, 3, 4, 5) the four first locomotor appendages—the endognaths, + (6) the large special appendage—the ectognath—and (7) the appendages, which in + Limulus are known as the chilaria, and are small and insignificant, but in Eurypterus and other + forms grow forwards, fuse together, and form a single median lip to an accessory oral chamber, + which lip is known as the metastoma. Of these appendages the cheliceræ and endognaths tend to + dwindle away and become mere tentacles, while the large swimming ectognath and metastoma remain + strong and vigorous.</p> + <p>In this, the prosomatic region, the somatic segmentation is not characterized by the presence + of the longitudinal muscle segments, for they do not extend into this head-region, but only by + the presence of the segmental somatic <span class="pagenum" + id="page255">{255}</span>ventro-dorsal muscles. Among the muscles of the appendages the system + of large tergo-coxal muscles is especially apparent.</p> + <p>From these considerations it follows that the number of segments in this region in the + vertebrate ought to be seven; that the musculature supplied by the trigeminal nerve ought to + represent seven ventral or splanchnic segments, of which only the last two are likely to be + conspicuous; and that the musculature supplied by the eye-muscle nerves ought to be + dorso-ventral in direction, which it is, and represent seven dorsal or somatic segments.</p> + <p>A further peculiarity of this region, both in Limulus and the scorpions, is found in the + excretory organs which are known by the name of coxal glands, because they extend into the basal + joint, or coxa, of certain of the prosomatic limbs. The appendages so characterized are always + the four endognaths, and it follows that if these four endognaths lose their locomotor power, + become reduced in size, and concentrated together to form mere tentacles, then of necessity the + coxal glands will be concentrated together, and tend to form a glandular mass in the region of + the mouth; in fact, take up a position corresponding to that of the pituitary body in + vertebrates.</p> + <p>Taking all these facts into consideration, it is possible to construct a drawing of a + sagittal section through the head-region of Eurypterus, which will represent, with considerable + probability, the arrangement of parts in that animal. This can be compared with the + corresponding section through the head of Ammocœtes.</p> + <p>Now, as pointed out in the last chapter, the early stage of Ammocœtes is remarkably + different from the more advanced stage; at that time the septum between the oral and respiratory + chambers has not yet broken through, and the olfactory or nasal tube, known at this stage as the + tube of the hypophysis, is directed ventrally, not dorsally.</p> + <p>The comparison of the diagram of Eurypterus with that of the early stage of Ammocœtes + is remarkably close, and immediately suggests not only that the single nose of the former is + derived from the corresponding organ in the palæostracan, but that the pituitary body is derived + from the concentrated coxal glands, and the lower lip from the metastoma. The further working + out of these homologies will be discussed in the next chapter.</p> + <p>In addition to the evidence of segmentation afforded by the appendages, there are in this + region, in Limulus and the scorpion group, three other criteria of segmentation available to us, + if from any cause the evidence of appendages fails us. These are—</p> + <p>1. The number of neuromeres are marked out in this region of the brain more or less plainly, + especially in the young animal, just as they are also in the embryo of the vertebrate.</p> + <p>2. The segmentation is represented here, just as in the mesosomatic region, by two sets of + muscle-segments; the one <i>somatic</i>, consisting of the segmentally arranged dorso-ventral + muscles, the continuation of the group already discussed in connection with the mesosomatic + segmentation, and the other <i>appendicular</i> characterized by the tergo-coxal muscles. These + latter segmental muscles are especially valuable, for in such forms as Mygale, Phrynus, etc., + their presence is indicated externally by markings on the prosomatic carapace, and thus + corresponding markings found on fossil carapaces or on dorsal head-shields can be <span + class="pagenum" id="page256">{256}</span>interpreted. These two sets of muscle-segments + correspond in the vertebrate to the somatic and splanchnic segmentations.</p> + <p class="sp0">3. In the vertebrate the segmentation in this region is indicated by the + cœlomic or head-cavities, which are cavities formed in the mesoderm of the embryo, the + walls of which give origin to the striated muscles of the head. In Limulus corresponding + cœlomic cavities are found, which are directly comparable with those found in the + vertebrate.</p> + </div> + + <div><span class="pagenum" id="page257">{257}</span></div> + + <p class="ac">CHAPTER VIII</p> + + <p class="ac"><i>THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">The prosomatic segments of the vertebrate.—Number of segments belonging to + the trigeminal nerve-group.—History of cranial segments.—Eye-muscles and their + nerves.—Comparison with the dorso-ventral somatic muscles of the + scorpion.—Explanation of the oculomotor nerve and its group of muscles.—Explanation + of the trochlearis nerve and its dorsal crossing.—Explanation of the abducens + nerve.—Number of segments supplied by the trigeminal nerves.—Evidence of their motor + nuclei.—Evidence of their sensory ganglia.—Summary.</p> + </div> + + <p>From the evidence given in the last chapter, combined with that given in Chapter IV., the + probability of the theory that the trigeminal group of nerves of the vertebrate have been derived + from the prosomatic group of nerves of the invertebrate can be put to the test by the answers to + the following morphological and anatomical questions<span class="wnw">:—</span></p> + + <p>1. Do we find in the vertebrate two segmentations in this region corresponding to the two + segmentations in the branchial region, <i>i.e.</i> a somatic or dorsal series of segments, and a + splanchnic or ventral series of segments? The latter would not be branchial, but rather of the + nature of free tactile appendages; so that it is useless to look for or talk about gill-slits, + although such appendages, being serially homologous with the branchial mesosomatic appendages, + would readily give rise to the conception of branchial segments.</p> + + <p>2. Is there morphological evidence that the trigeminal nerve is not the nerve belonging to a + single segment, or even to two segments, but is really a concentration of at least six, probably + seven, segmental nerves?</p> + + <p>3. Is there morphological evidence that the oculomotor and trochlear nerves, which on all sides + are regarded as belonging to the trigeminal segments, are not single nerves corresponding each + <span class="pagenum" id="page258">{258}</span>to a single segment, but are the somatic motor + roots belonging to the same segments as those to which the trigeminal supplies the splanchnic + roots?</p> + + <p>4. Do the mesoderm segments, which give origin to the eye-muscles, and therefore do the + head-cavities of this region, correspond with the trigeminal segments? Considering the + concentration of parts in this region and the difficulty already presented by the want of + numerical agreement between the prosomatic appendages and the prosomatic cœlomic cavities + in Limulus, it may very probably be difficult to determine the actual number of the mesoderm + segments.</p> + + <p>5. Is there anatomical evidence that the ganglion of origin of the motor part of the trigeminal + nerve is not a single ganglion, but a representative of many, probably seven?</p> + + <p>6. Is there anatomical evidence that the ganglia of origin of the oculomotor and trochlear + nerves represent many ganglia?</p> + + <p>7. Is there any evidence that the organs originally supplied by the motor part of the + trigeminal nerve are directly comparable with prosomatic appendages?</p> + + <p class="sp3">It is agreed on all sides that in this region of the head there is distinct + evidence of double segmentation, the dorsal mesoderm segments giving origin to the eye-muscles, + and the ventral segments to the musculature innervated by the trigeminal nerve. Originally, + according to the scheme of van Wijhe, two segments only were recognized, the dorsal parts of which + were innervated by the IIIrd and IVth nerves respectively. Since his paper, the tendency has been + to increase the number of segments in this region, as is seen in the following sketch, taken from + Rabl, of the history of cranial segmentation.</p> + + <p class="ac"><span class="sc">History of Cranial Segmentation.</span></p> + + <p>The first attempt to deal with this question was made by Goethe and Oken. They considered that + the cranial skeleton was composed of a series of vertebræ, but as early as 1842 Vogt pointed out + that only the occipital segments could be reduced to vertebræ. In 1869, Huxley showed that + vertebræ were insufficient to explain the cranial segmentation, and that the nerves must be + specially considered. The olfactory and optic nerves he regarded as parts of the brain, not true + segmental nerves; the rest of the cranial nerves <span class="pagenum" + id="page259">{259}</span>were segmental, with special reference to branchial arches and clefts, + the facial, glossopharyngeal, and separate vagus branches supplying the walls of the various + branchial pouches. In a similar manner, the supra- and infra-maxillary branches of the trigeminal + were arranged on each side of the mouth, and the inner and outer twigs of the first (ophthalmic) + branch of the trigeminal on each side of the orbito-nasal cleft, the trabecular and the + supra-maxillary arches being those on each side of this cleft. Thus Huxley considered that there + was evidence of a series of pairs of ventral arches belonging to the skull, viz. the trabecular + and maxillary in front of the mouth, the mandibular, hyoid, and branchial arches behind, and that + the Vth, VIIth, IXth, and Xth nerves were segmental in relation to these arches and clefts. + Gegenbaur, in 1871 and 1872, considered that the branchial arches represented the lower arches of + cranial vertebræ, and therefore corresponded to lower arches in the spinal region, <i>i.e.</i> the + skull was composed of as many vertebræ as there are branchial arches. These vertebræ were confined + to the notochordal part of the skull, the prechordal part having arisen secondarily from the + vertebral part, while the number of vertebræ are at least nine, possibly more. The nerves which + could be homologized with spinal nerves were, he thought, divisible into two great + groups—(1) the trigeminal group, which included the eye-muscle nerves, the facial, and its + dorsal branch, the auditory; (2) the vagus group, which included the glossopharyngeal and + vagus.</p> + + <p>Such was the outcome of the purely comparative anatomical work of Huxley and + Gegenbaur—work that has profoundly influenced all the views of segmentation up to the + present day.</p> + + <p>Now came the investigations of the embryologists, of whom I will take, in the first instance, + Balfour, whose observations on the embryology of the Selachians led him to the conclusion that + besides the evidence of segmentation to be found in the cranial nerves and in the branchial + clefts, further evidence was afforded by the existence of head-cavities, the walls of which formed + muscles just as they do in the spinal region. He came to the conclusion that the first head-cavity + belonged to one or more pre-oral segments, of which the nerves were the oculomotor, trochlearis, + and possibly abducens; while there were seven post-oral segments, each with its head-cavity and + its visceral arch, of which the trigeminal, facial, glossopharyngeal, and the four parts of the + vagus were the respective nerves.</p> + + <div><span class="pagenum" id="page260">{260}</span></div> + + <p>Marshall, in 1882, considered that the cranial segments were all originally respiratory, and + that all the segmental nerves are arranged uniformly with respect to a series of gill-clefts which + have become modified anteriorly and have been lost, to a certain extent, posteriorly. He included + the olfactory nerves among the segmental nerves, and looked upon the olfactory pit, the + orbito-nasal lacrymal duct, the mouth, and the spiracle as all modified gill-slits, so that he + reckoned three pre-oral and oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and + eight post-oral segments belonging respectively to the VIIth and VIth nerves, and to the IXth + nerve, and six segments belonging to the Xth nerve. He pointed out that muscles supplied by the + oculomotor nerve develop from the outer wall of the first head-cavity; not, however, the + <i>obliquus superior</i> and <i>rectus externus</i>, the latter originating probably from the + walls of the third cavity.</p> + + <p>In the same year, 1882, came van Wijhe's well-known paper, in which he showed that the mesoderm + of the head in the selachian divided into two sets of segments, dorsal and ventral; that the + dorsal segments were continuous with the body-somites, and that the ventral segments formed the + lateral plates of mesoblast between each of the visceral and branchial pouches. He concluded that + the dorsal somites were originally nine in number, that each was supplied with a ventral + nerve-root, in the same way as the somites in the trunk, and that to each one a visceral pouch + corresponded, whose walls were supplied by the corresponding dorsal nerve-root; of these nine + segments, the ventral nerve-roots of the first three segments were respectively the oculomotor, + trochlearis, and abducens nerves. The next three segments possessed no definable ventral root or + muscles, and the seventh, eighth, and ninth segments possessed as ventral roots the hypoglossal + nerve, with its muscular supply. The corresponding dorsal nerve-roots were the trigeminal, facial, + auditory, glossopharyngeal and vagus nerves, the difference between cranial and spinal dorsal + roots being that the former contain motor fibres.</p> + + <p>Ahlborn, in 1884, drew a sharp distinction between the segments of the mesoderm and those of + the endoderm. The former segmentation he called mesomeric, the latter branchiomeric. He considered + the two segmentations to be independent, and concluded that the branchiomeric was secondary to the + mesomeric, and therefore not of <span class="pagenum" id="page261">{261}</span>segmental value. As + to the segments of the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ + remain permanently, and correspond to the three last segments in the selachian head. Of the + anterior mesoderm segments, he considered that there were originally six, and that there are six + typical eye-muscles in all Craniota, which have been compressed into three segments, as in + Selachia.</p> + + <p>Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal nerve belongs to + three proto-vertebræ posterior to the vagus region, which were true spinal segments. He therefore + modified Gegenbaur's conceptions to this extent: that portion of the skull designated by Gegenbaur + as vertebral must be divided into two parts—a hind or occipital region, which is clearly + composed of modified vertebræ and is the region of the hypoglossal nerves, and a front region, + extending from the oculomotor to the accessorius nerves, which is characterized segmentally by the + formation of branchial arches, but in which there is no evidence that proto-vertebræ were ever + formed. He therefore divides the head-skeleton into three parts—</p> + + <p>1. Gegenbaur's evertebral part—the region of the olfactory and optic nerves—which + cannot be referred to any metameric segmentation.</p> + + <p>2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be segmented from the + consideration of the nerves and branchial arches, but not referable to proto-vertebræ—the + region of the trigeminal and vagus nerves.</p> + + <p>3. The vertebral spinal part—the region of the hypoglossal nerves.</p> + + <p>He further showed that the ganglia of the specially branchial nerves, the facial, + glossopharyngeal, and vagus, are at one stage in connection with the epidermis, so that these + parts of the epidermis represent sense-organs which do not develop; these organs probably belonged + to the lateral line system. As the connection takes place at the dorsal edge of the gill-slits, + they may also be called rudimentary branchial sense-organs.</p> + + <p>Since this paper of Froriep's, it has been generally recognized, and Gegenbaur has accepted + Froriep's view, that the three hindmost metameres, which distinctly show the characteristics of + vertebræ, belong to the spinal and not to the cranial region, so that the metameric segmentation + of the cranial region proper has become <span class="pagenum" id="page262">{262}</span>more and + more associated with the branchial segmentation. Froriep's discovery of the rudimentary branchial + sense-organs as a factor in the segmentation question has led Beard to the conclusion that the + olfactory and auditory organs represent in a permanent form two of these rudimentary branchial + sense-organs. He therefore includes both the olfactory and auditory nerves in his list of cranial + segmental nerves, and makes eleven cranial branchial segments in front of the spinal segments + represented by the hypoglossal.</p> + + <p>A still larger number of cranial segments is supposed to exist, according to the researches of + Dohrn and Killian, in the embryos of <i>Torpedo ocellata</i>. The former, holding to the view that + vertebrates arose from annelids, considered that the head was formed of a series of metameres, to + each one of which a mesoderm-segment, a gill-arch, a gill-cleft, a segmental nerve and vessel + belonged. He found in the front head-region of a Torpedo embryo, corresponding to van Wijhe's + first four somites, no less than twelve to fifteen mesoderm segments, and concluded, therefore, + that the eye-muscle nerves, especially the oculomotor, represented many segmental nerves, and were + not the nerves of single segments; so, also, that the inferior maxillary part of the trigeminal + and the hyoid nerve of the facial are probably not single nerves, but a fusion of several. Killian + comes to much the same conclusion as Dohrn, for he finds seventeen to eighteen separate mesoderm + segments in the head, of which twelve belong to the trigeminal and facial region.</p> + + <p>Since Rabl's paper, a number of papers have appeared, especially from America, dealing with yet + another criterion of the original segmentation of the head, viz. a series of divisions of the + central nervous system itself, which are seen at a very early stage of development, and are called + neuromeres; the divisions in the cranial region being known as encephalomeres, and those of the + spinal region as myomeres. Locy's paper has especially brought these divisions into prominence as + a factor in the question of segmentation. They are essentially segments of the epiblast and not of + the mesoblast; they are conspicuous in very early stages, and appear to be in relation with the + cranial nerves, according to Locy. He recognizes in <i>Squalus acanthias</i>, in front of the + spino-occipital region, fourteen pairs of such encephalomeres and a median unsegmented + termination, which may represent one more pair fused in the middle line, making at least fifteen. + He distributes these fifteen segments as follows: <span class="pagenum" + id="page263">{263}</span>fore-brain three and unsegmented termination, mid-brain two, and + hind-brain nine.</p> + + <p>Again, Kupffer, in his recent papers on the embryology of Ammocœtes, asserts that + especial information as to the number of primitive segments is afforded by the appearance in the + early stages of a series of epibranchial ganglia in connection with the cranial nerves, which + remain permanently in the case of the vagus nerves, but disappear in the case of pro-otic nerves. + He considers that the evidence points to the number of segments in the mid- and hind-brain region + as being primitively fifteen, viz. six segments belonging to the trigeminal and abducens group, + three segments belonging respectively to the facial, auditory, and glossopharyngeal, and six to + the vagus.</p> + + <p class="sp3">From this sketch we see that the modern tendency is to make six segments at least + out of the region of the trigeminal nerves rather than two. In this region, as already mentioned, + the evidence of segmentation is based more clearly on the somatic than on the splanchnic segments. + We ought, therefore, in the first place, to consider the teaching of the eye-muscles and their + nerves and the cœlomic cavities in connection with them, and see whether the hypothesis + that such muscles represent the original dorso-ventral somatic muscles of the palæostracan + ancestor is in harmony with and explains the facts of modern research.</p> + + <p class="ac"><span class="sc">Eye-Muscles and their Nerves.</span></p> + + <p>The only universally recognized somatic nerves belonging to these segments which exist in the + adult are the nerves to the eye-muscles, of which, according to van Wijhe, the oculomotor is the + nerve of the 1st segment, the trochlearis of the 2nd, and the abducens of the 3rd; while the + nerves and muscles belonging to the 4th and 5th segments, <i>i.e.</i> the 2nd facial and + glossopharyngeal segments respectively, show only the merest rudiments, and do not exist in the + adult. One significant fact appears in this statement of van Wijhe, and is accepted by all those + who follow him, viz. that the oculomotor nerve has equal segmental value with the trochlearis and + the abducens, although it supplies a number of muscles, each of which, on the face of it, has the + same anatomical value as the superior oblique or external rectus. Dohrn alone, as far as I know, + as already pointed out, insists upon the multiple character of the oculomotor nerve.</p> + + <div><span class="pagenum" id="page264">{264}</span></div> + + <p>As far as the anatomist is concerned, the evidence is becoming clearer and clearer that the + nucleus of the IIIrd nerve is a composite ganglion composed of a number of nuclei, each similar to + that of the trochlearis, so that if the trochlearis nucleus is a segmental motor nucleus, then the + oculomotor nucleus is a combined nucleus belonging to at least four segmental nerves, each of + which has the same value as that of the trochlearis.</p> + + <p>The investigations of a number of anatomists, among whom may be mentioned Gudden, Obersteiner, + Edinger, Kölliker, Gehuchten, all lead directly to the conclusion that this oculomotor nucleus is + composed of a number of separate nuclei, of which the most anterior as also the Edinger-Westphal + nucleus contains small cells, while the others contain large cells. Thus Edinger divides the + origin of the oculomotor nerve into a small-celled anterior part and a larger posterior part, of + which the cells are larger and distinctly arranged in three groups—(1) dorsal, (2) ventral, + and (3) median. Between the anterior and posterior groups lies the Edinger-Westphal nucleus, which + is small-celled; naturally, the large-celled group is that which gives origin to the motor nerves + of the eye-muscles, the small-celled being possibly concerned with the motor nerves of the + pupillary and ciliary muscles. I may mention that Kölliker considers that the anterior lateral + nucleus has nothing to do with the oculomotor nerve, but is a group of cells in which the fibres + of the posterior longitudinal bundle and of the deep part of the posterior commissure + terminate.</p> + + <p>These conclusions of Edinger are the outcome of work done in his laboratory by Perlia, who says + that in new-born animals the nucleus of origin of the oculomotor nerve is made up of a number of + groups quite distinct from each other, each group being of the same character as that of the + trochlearis. He finds the same arrangement in various mammals and birds. Further, he finds that + some of the fibres arise from the nucleus of the opposite side, thus crossing, as in the + trochlearis; these crossing fibres belong to the most posterior of the dorsal group of nuclei, + <i>i.e.</i> to the nerve to the inferior oblique muscle.</p> + + <p>The evidence, therefore, points to the conclusion that the oculomotor nucleus is a multiple + nucleus, each part of which gives origin to one of the nerves of one of the eye-muscles.</p> + + <p>Edinger says that such an array of clinical observations exists, <span class="pagenum" + id="page265">{265}</span>and of facts derived from post-mortem dissections, that one may venture + to designate the portion of the nucleus from which the innervation of each individual ocular + muscle comes. He gives Starr's table, the latest of these numerous attempts, begun by Pick. + According to Starr, the nuclei of the nerves to the individual muscles are arranged from before + backward, thus—</p> + + <table class="sp2 mc" title="Order of nuclei according to Starr" + summary="Order of nuclei according to Starr"> + <tr> + <td><i>m. sphincter iridis.</i></td> + <td><i>m. ciliaris.</i></td> + </tr> + <tr> + <td><i>m. levator palpebræ.</i></td> + <td><i>m. rectus internus.</i></td> + </tr> + <tr> + <td><i>m. rectus superior.</i></td> + <td><i>m. rectus inferior.</i></td> + </tr> + <tr> + <td class="pr2"><i>m. obliquus inferior.</i></td> + </tr> + </table> + + <p>Further, the evidence of the well-known physiological experiments of Hensen and Völckers that + the terminal branches of the oculomotor nerve arise from a series of segments of the nucleus, + arranged more or less one behind the other in a longitudinal row, leads them to the conclusion + that the nuclei of origin are arranged as follows, proceeding from head to tail<span + class="wnw">:—</span></p> + + <table class="sp2 mc" title="Order of nuclei according to Hensen and + Völckers" summary="Order of nuclei according to Hensen and + Völckers"> + <tr> + <td rowspan="6">Nearest brain.</td> + <td>1.</td> + <td><i>m. ciliaris.</i></td> + </tr> + <tr> + <td>2.</td> + <td><i>m. sphincter iridis.</i></td> + </tr> + <tr> + <td>3.</td> + <td><i>m. rectus internus.</i></td> + </tr> + <tr> + <td>4.</td> + <td><i>m. rectus superior.</i></td> + </tr> + <tr> + <td>5.</td> + <td><i>m. levator palpebræ.</i></td> + </tr> + <tr> + <td>6.</td> + <td><i>m. rectus inferior.</i></td> + </tr> + <tr> + <td class="pr1">Most posterior.</td> + <td>7.</td> + <td><i>m. obliquus inferior.</i></td> + </tr> + </table> + + <p>It is instructive to compare this arrangement of Hensen and Völckers with the arrangement of + the origin of these muscles from the premandibular cavity as given by Miss Platt.</p> + + <p>Thus she states that the most posterior part of the premandibular cavity is cut off so as to + form a separate cavity, resembling, except in position, the anterior cavity; this separate, most + posterior part gives origin to the inferior oblique muscle. She then goes on to describe how the + dorsal wall of the remainder of the premandibular cavity becomes thickened, to form posteriorly + the rudiment of the inferior rectus and anteriorly the rudiments of the superior and internal + recti, a slight depression in the wall of the cavity separating these rudiments. The internal + rectus is the more median of the two anterior muscles. In other words, her evidence points not + only to a fusion of somites to form the premandibular cavity, but also to the arrangement of these + somites as follows, from head to tail: (1) internal rectus, (2) superior rectus, (3) inferior + rectus, (4) inferior <span class="pagenum" id="page266">{266}</span>oblique—an order + precisely the same as that of Hensen and Völckers, and of Starr.</p> + + <p>I conclude, from the agreement between the anatomical, physiological, and morphological + evidence, that the IIIrd and IVth nerves contain the motor somatic nerves belonging to the same + segments as the motor trigeminal, in other words, to the prosomatic segments, so that the + eye-muscles, innervated by III. and IV., represent segmental muscles belonging to the prosoma. + Further, I conclude that originally there were seven prosomatic segments, the first of which is + represented by the anterior cavity described by Miss Platt, and does not form any permanent + muscles; that the next four belong to the premandibular cavity, and the muscles formed are the + superior rectus, internal rectus, inferior rectus, and inferior oblique; and that the last two + belong to the mandibular cavity, the muscles formed being Miss Platt's mandibular muscle and the + superior oblique. It is, to say the least of it, a striking coincidence that such an arrangement + of the cœlomic cavities as here given should be so closely mimicked by the arrangement in + the prosomatic region of Limulus as already mentioned; it suggests inevitably that the + head-cavities of the vertebrate are nothing more than the prosomatic and mesosomatic segmental + cœlomic cavities, as found in animals such as Limulus. In the table on p. <a + href="#page253">253</a>, I have inserted the segments in the vertebrate for comparison with those + of Limulus.</p> + + <p>Before we can come to any conclusion as to the original position of these eye-muscles, it is + necessary to consider the VIth nerve and the external rectus muscle. This nerve and this muscle + belong to van Wijhe's 4th segment. The muscle is, therefore, the somatic segmental muscle + belonging to the same segment as the facial and is, in fact, a segmental muscle belonging not to + the prosoma, but to the mesosoma. Neal comes to the conclusion that the existing abducens is the + only root which remains permanent among a whole series of corresponding ventral roots belonging to + the opisthotic segments, and further points out that the external rectus was originally an + opisthotic muscle which has taken up a pro-otic position, or, translating this statement into the + language of Limulus, etc., it is a mesosomatic muscle which has taken up a prosomatic + position.</p> + + <p>There is, however, another muscle—the <i>Retractor oculi</i>—belonging to the same + group which is innervated by the VIth nerve. Quite recently Edgeworth has shown that in birds and + reptiles this muscle <span class="pagenum" id="page267">{267}</span>belongs to the hyoid segment; + so that in this respect also the hyoid segment proclaims its double nature.</p> + + <p>With respect to the external rectus muscle, Miss Platt has shown that the mandibular muscle is + formed close alongside the external rectus, so that the two are in close relationship as long as + the former exists.</p> + + <p>Further, as already mentioned, the eye-muscles in Ammocœtes must be considered by + themselves; they do not belong in structure or position to the longitudinal somatic muscles + innervated by the spinal nerves; their structure is not the same as that of the tubular + constrictor or branchial muscles, but resembles that structure somewhat; their position is + dorso-ventral rather than longitudinal; they may be looked upon as a primitive type of somatic + muscles segmentally arranged, the direction of which was dorso-ventral.</p> + + <p>Anderson also has shown that the time of medullation of the nerves supplying these muscles is + much earlier than that of the nerves belonging to the somatic trunk-muscles, their medullation + taking place at the same time as that of the motor nerves supplying the striated visceral muscles; + and Sherrington has observed that these muscles do not possess muscle-spindles, while all somatic + trunk-muscles do. Both these observations are strong confirmation of the view that the eye-muscles + must be classified in a different category to the ordinary somatic trunk muscle group.</p> + + <p>What, then, is the interpretation of these various embryological and anatomical facts?</p> + + <p>Remembering the tripartite division of each segmental nerve-group in Limulus into (1) dorsal or + sensory somatic nerve, (2) appendage-nerve, and (3) ventral somatic nerve, I venture to suggest + that the three nerves—the <i>oculomotorius</i>, the <i>trochlearis</i>, and the + <i>abducens</i>—represent the ventral somatic nerves of the prosoma, and partly also of the + mesosoma; that they are nerves, therefore, which may have originally contained sensory fibres, and + which still contain the sensory fibres of the eye-muscles themselves, as stated by Sherrington. + According to this suggestion, the eye-muscles are the sole survivors of the segmental + dorso-ventral somatic muscles, so characteristic of the group from which I imagine the vertebrates + to have sprung. In the mesosomatic region the dorso-ventral muscles which were retained were those + of the appendages and not of the mesosoma itself, because the presumed ancestor breathed after the + fashion of the water-breathing Limulus, by means of the dorso-ventral muscles of its <span + class="pagenum" id="page268">{268}</span>branchial appendages, and not after the fashion of the + air-breathing scorpion, by means of the dorso-ventral muscles of the mesosoma. The only + mesosomatic dorso-ventral muscles which were retained were those of the foremost mesosomatic + segments, <i>i.e.</i> those supplied by the VIth nerve, which were preserved owing to their having + taken on a prosomatic position and become utilized to assist in the movements of the lateral + eyes.</p> + + <p>Let us turn now to the consideration of the corresponding musculature in Limulus and in the + scorpion group. These muscles constitute the markedly segmental muscles to which I have given the + name 'dorso-ventral somatic muscles.' They are most markedly segmental in the mesosomatic region, + both in Limulus and in Scorpio, each mesosomatic segment possessing a single pair of these + vertical mesosomatic muscles, as Benham calls them (<i>cf.</i> Fig. <a href="#fig58">58</a> + (<i>Dv.</i>)). In the prosomatic region the corresponding muscles are not so clearly defined in + Limulus; they are apparently attached to the plastron forming the group of plastro-tergal muscles. + From Benham's description it is sufficiently evident that they formed originally a single pair to + each prosomatic segment.</p> + + <p>In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles are situated near the + middle line on each side and form the following well-marked series of pairs of muscles, shown in + Fig. <a href="#fig110">110</a>, A, taken from her paper, and thus described by her<span + class="wnw">:—</span></p> + + <p>1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the dorso-ventral muscles. + It is very small, and is attached to the carapace near the median line anteriorly to the central + eyes.</p> + + <p>2. The median dorso-preoral-entosclerite muscle (62) is a large muscle, between which and its + fellow of the opposite side the eyes are situated. It is attached dorsally to the carapace and + ventrally to the pre-oral entosclerite.</p> + + <p>3. The anterior dorso-plastron muscle (63) is attached dorsally to the carapace in the middle + line, being joined to its fellow of the opposite side. They separate, and are attached ventrally + to the plastron. Through the arch thus formed the alimentary canal and the dorsal vessel pass.</p> + + <p>4. The median dorso-plastron muscle (64) is attached dorsally to the posterior part of the + carapace. It runs forward on the anterior surface of the posterior flap of the plastron to the + body of the plastron, to which it is attached.</p> + + <div><span class="pagenum" id="page269">{269}</span></div> + + <div id="fig110"></div> + + <table class="sp2 mc tlf w60" title="Figs. 110 A-C" summary="Figs. 110 A-C"> + <tr> + <td class="w50 ac"><span class="ac w30 fcenter"><a href="images/fig110a.png"><img + style="width:100%" src="images/fig110a.png" alt="" title=""/></a></span></td> + <td class="w50 ac vmi smaller"> + <p>A.</p> + <p class="sp0"><span class="sc">Dorso-ventral Muscles on Carapace of Scorpion.</span> (From + <span class="sc">Miss Beck.</span>)</p> + </td> + </tr> + <tr> + <td class="w50 ac"><span class="ac w30 fcenter"><a href="images/fig110b.png"><img + style="width:100%" src="images/fig110b.png" alt="" title=""/></a></span></td> + <td class="w50 ac vmi smaller"> + <p>B.</p> + <p class="sp0"><span class="sc">Similar Muscles on Carapace of Eurypterus.</span></p> + </td> + </tr> + <tr> + <td class="w50 ac"><span class="ac w30 fcenter"><a href="images/fig110c.png"><img + style="width:100%" src="images/fig110c.png" alt="" title=""/></a></span></td> + <td class="w50 ac vmi smaller"> + <p>C.</p> + <p><span class="sc">Similar Muscles on Head-Shield of a Cephalaspid.</span></p> + <p><i>l.e.</i>, lateral eyes; <i>c.e.</i>, central eyes; <i>Fro.</i>, narial opening.</p> + <p class="sp0">62-65 refer to Miss Beck's catalogue of the scorpion muscles.</p> + </td> + </tr> + <tr> + <td colspan="2" class="ac smaller"><span class="sc">Fig. 110.</span></td> + </tr> + </table> + + <div><span class="pagenum" id="page270">{270}</span></div> + + <p>To these may be added, owing to its attachment to the plastron,</p> + + <p>5. The posterior dorso-plastron muscle (65). This is the first of the dorso-ventral muscles + attached to the mesosomatic tergites, being attached to the tergite of the first segment of the + mesosoma.</p> + + <p>This muscle is of interest, in connection with the prosomatic dorso-ventral muscles, because it + is attached to the plastron, and runs a course in close contact with the muscle (64), the two + muscles being attached dorsally close together, on each side of the middle line, the one at the + very posterior edge of the prosomatic carapace, and the other at the very anterior edge of the + mesosomatic carapace.</p> + + <p>Taking these muscles separately into consideration, it may be remarked with respect to (61) + that the cheliceral segment in its paired dorso-ventral muscles, as in its tergo-coxal muscles, + takes up a separate position isolated from the rest of the prosomatic segments.</p> + + <p>Next comes (62) the median dorso-preoral-entosclerite muscle, which is strikingly different + from all the other dorso-ventral muscles in its large size and the extent of its attachment to the + dorsal carapace, according to Miss Beck's figures. The reason of its large size is clearly seen + upon dissection of the muscles in <i>Buthus</i>, for I find that, strictly speaking, it is not a + single muscle, but is composed of a series of muscle-bundles, separated from each other by + connective tissue. There are certainly three separate muscles included in this large muscle, which + are attached in a distinct series along the pre-oral entosclerite, and present the appearance + given in Fig. <a href="#fig110">110</a>, A, at their attachment to the prosomatic carapace. Of + this muscle-group the most anterior and the most posterior bundle are distinctly separate muscles; + I am not, however, clear whether the middle bundle represents one or two muscles.</p> + + <p>This division of Miss Beck's muscle (62) into three or four muscles brings the prosomatic + region of the scorpion into line with the mesosomatic, and enables us to feel sure that a single + pair of dorso-ventral somatic muscles belongs to each prosomatic segment just as to each + mesosomatic, and, conversely, that each such single pair of muscles possesses segmental value in + this region as much as in the mesosomatic.</p> + + <p>It is very striking to see how in all the Scorpionidæ, in which the two median eyes are the + principal eyes, this muscle group (62) on the two sides closely surrounds these two eyes, so that + with a fixed <span class="pagenum" id="page271">{271}</span>pre-oral entosclerite, a slight + movement of the eyes, laterally or anteriorly, owing to the flexibility of the carapace, might + result as the consequence of their contraction. But this cannot be the main object of these + muscles. The pre-oral entosclerite is firmly fixed to the camerostome, as is seen in Fig. <a + href="#fig94">94</a>, <i>pr. ent.</i>, so that the main object of these muscles is, as Huxley has + pointed out, the movement of this organ.</p> + + <p>In order to avoid repetition of the long name given to this muscle group (62) by Miss Beck, + because of their position, and for other reasons which will appear in the sequel, I will call this + group of muscles the group of recti muscles. These recti muscles belong clearly to the segments + posterior to the first prosomatic or cheliceral segment, and represent certainly three, probably + four, of these segments, <i>i.e.</i> belong to the segments corresponding to the second, third, + fourth, and fifth prosomatic locomotor appendages—the endognaths of the old Eurypterids.</p> + + <p>The next pair of muscles is the pair of anterior dorso-plastron muscles (63). This muscle-pair + evidently belongs to a segment posterior to the segments represented by the group already + discussed, and belongs, therefore, in all probability to the same segment as the sixth pair of + prosomatic appendages—the ectognaths of the old Eurypterids. This can be settled by + considering either the nerve-supply or the embryological development. In the Eurypteridæ it seems + most highly probable that the dorso-ventral muscles of each half of the segments belonging to the + endognaths should be compressed together and separate from the dorso-ventral muscle belonging to + the ectognathal segment, on account of the evident concentration and small size of the endognathal + segments in contradistinction to the separateness and large size of the ectognathal segment.</p> + + <p>The striking peculiarity of this muscle-pair, which distinguishes it from all other muscles in + the scorpion, is the common attachment of the muscles of the two sides in the mid-dorsal line, so + that the pair of muscles forms an arch through which the alimentary canal and dorsal blood-vessel + pass.</p> + + <p>The same dorso-ventral muscles are present in <i>Phrynus</i>, and in this animal the fibres of + this pair of muscles (63) actually interlace before the attachment to the prosomatic carapace, so + that the attachment of the muscle on each side overpasses the mid-dorsal line, and a true crossing + occurs. In Fig. <a href="#fig108">108</a> the position of this pair of <span class="pagenum" + id="page272">{272}</span>muscles is shown just posteriorly to the brain-mass. This muscle I will + call the oblique muscle.</p> + + <p>Finally we come to the muscles (64) and (65), the median and posterior dorso-plastron muscles, + which run close together. Both muscles are attached to the plastron, and, therefore, to that + extent belong to the prosomatic region; they are attached dorsally close to the junction of the + prosoma and mesosoma. This position of the first mesosomatic dorso-ventral muscle belonging to the + opercular segment may be compared with the position of the first mesosomatic dorso-ventral muscle + in Limulus which has become attached to the prosomatic carapace; in both cases we see an + indication that the foremost pair of mesosomatic dorso-ventral somatic muscles tend to take up a + prosomatic position.</p> + + <p>As to the pair of small muscles (64), I believe that they represent the dorso-ventral muscles + of the seventh prosomatic segment (if the pair of muscles (63) belongs to the segment of the sixth + locomotor prosomatic appendages), <i>i.e.</i> they belong to the chilarial segment or + metastoma.</p> + + <p>I desire to draw especial attention to the fact that the dorso-ventral muscle (64), which + represents the seventh segment, always runs close alongside the dorso-ventral muscle (65), which + represents the first mesosomatic or opercular segment.</p> + + <p>The comparison, then, of these two sets of facts leads to the following conclusions<span + class="wnw">:—</span></p> + + <p>The foremost prosomatic or trigeminal segment stood separate and apart, being situated most + anteriorly; the musculature of this segment does not develop, so that the only evidence of its + presence is given by the anterior cœlomic cavity. This corresponds, according to my scheme, + with the first or anterior cœlomic cavity of Limulus, and therefore represents, as far as + the prosomatic appendages are concerned, the first prosomatic appendage-pair, or the cheliceræ; + the appendage-muscles being the muscles of the cheliceræ, and the dorso-ventral somatic muscles + the pair of dorso-cheliceral sternal muscles (61) in the scorpion. Both these sets of muscles, + therefore, dwindle and disappear in the vertebrate.</p> + + <p>Then came four segments fused together to form the premandibular segment, the characteristic of + which is the apparent non-formation of any permanent musculature from the ventral + mesoderm-segments, and the formation of the eye-muscles innervated by the <span class="pagenum" + id="page273">{273}</span>oculomotor nerve from the dorsal mesoderm segments. These four segments + have been so fused together that van Wijhe looked upon them as a single segment, and the + premandibular cavity as the cavity of a single segment. They represent, according to my scheme, + the segments belonging to the endognaths, <i>i.e.</i> the second, third, fourth, fifth pairs of + prosomatic appendages; the premandibular cavity, therefore, represents the second cœlomic + cavity in Limulus, which, according to Kishinouye, is the sole representative of the + cœlomic cavities of the second, third, fourth, fifth prosomatic segments. The muscles + derived from the ventral mesoderm-segments represent the muscles of these appendages, which + therefore dwindle and disappear in the vertebrate, with the possible exception of the muscles + innervated by the descending root of the trigeminal. The muscles derived from the dorsal + mesoderm-segments, <i>i.e.</i> the eye-muscles supplied by the oculomotor nerve, represent the + dorso-ventral somatic muscles of these four segments, muscles which are represented in the + scorpion by the recti group of muscles, <i>i.e.</i> the median dorso-preoral-entosclerite muscles + (62).</p> + + <p>Then came two segments, the mandibular, in which muscles are formed both from the ventral and + from the dorsal mesoderm-segments. From the former arose the main mass of muscles innervated by + the motor root of the trigeminal, from the latter the superior oblique muscle and the mandibular + muscle of Miss Platt, of which the former alone survives in the adult condition. These two + segments are looked upon as a single segment by van Wijhe, of which the mandibular cavity is the + cœlomic cavity. They represent, according to my scheme, the segments belonging to the sixth + pair of prosomatic appendages or ectognaths, and the seventh pair, <i>i.e.</i> the chilaria or + metastoma.</p> + + <p>The first part, then, of the mandibular cavity represents the third cœlomic cavity in + Limulus and the muscles derived from the ventral mesoderm, in all probability the muscles of the + tongue in the lamprey (<i>cf.</i> Chap. IX.), which represents the ectognaths or sixth pair of + prosomatic appendages, while the muscles derived from the dorsal mesoderm, <i>i.e.</i> the + superior oblique muscles, represent the dorso-ventral somatic muscles of this segment, muscles + which are represented in the scorpion group by the pair of anterior dorso-plastron or oblique + muscles (63).</p> + + <p>The second part of the mandibular cavity represents the 4th <span class="pagenum" + id="page274">{274}</span>cœlomic cavity in Limulus and the muscles derived from the ventral + mesoderm, in all probability the muscles of the lower lip in the lamprey (<i>cf.</i> Chap. IX.), + which represents the metastoma; while the muscles derived from the dorsal mesoderm, <i>i.e.</i> + Miss Platt's pair of mandibular muscles, represent the dorso-ventral somatic muscles of this + segment, muscles which are represented in the scorpion group by the pair of median dorso-plastron + muscles (64).</p> + + <p>In connection with this last pair of muscles we find that the external rectus in the vertebrate + represents the first dorso-ventral mesosomatic muscle in the scorpion, <i>i.e.</i> the posterior + dorso-plastron muscle (65), and, as already mentioned (p. <a href="#page267">267</a>), that it + always lies closely alongside the mandibular muscle, just as in the scorpion group muscle (65) + always lies alongside muscle (64).</p> + + <p>In the invertebrate as well as in the vertebrate this muscle is a mesosomatic muscle which has + taken up a prosomatic position.</p> + + <p>The question naturally arises, what explanation can be given of the fact that these + dorso-ventral muscles attached on each side of the mid-dorsal line to the prosomatic carapace + became converted into the muscles moving the eyeballs of the two lateral eyes? An explanation + which must take into account not only the isolated position of the abducens nerve, but also the + extraordinary course of the trochlearis. The natural and straightforward answer to this question + appears to me quite satisfactory, and I therefore venture to commend it to my readers.</p> + + <p>I have argued the case out to myself as follows: The lateral eyes must have been originally + situated externally to the group of muscles innervated by the oculomotor nerve, for a sheet of + muscle representing the superior <i>internal</i> and inferior rectus muscles could only wrap round + the internal surface of each lateral eye; <i>i.e.</i> the arrangement of the muscle-sheet, as in + the scorpion, about two median eyes, is in the wrong position, for if those two eyes, which are + the main eyes in the scorpion, were to move outwards to become two lateral eyes, then such a + muscle-group would form a superior <i>external</i> and inferior rectus group. The evidence, + however, of Eurypterus and similar forms is to the effect that the lateral eyes became big and the + median eyes insignificant and degenerate. If, then, with the degeneration of the one and the + increasing importance of the other, these lateral eyes came near the middle line, then the + muscular group (62), which I have called the recti group, would naturally be pressed into their + <span class="pagenum" id="page275">{275}</span>service, and would form an internal and not an + external group of eye-muscles.</p> + + <p>In Fig. <a href="#fig110">110</a>, A, taken from Miss Beck's paper, I have shown the relative + position of the eyes and the segmental dorso-ventral prosomatic muscles on the carapace of the + scorpion. In Fig. <a href="#fig110">110</a>, B, I have drawn the prosomatic carapace of + <i>Eurypterus Scouleri</i>, taken from Woodward's paper, with the eyes as represented there; in + this I have inserted the segmental dorso-ventral muscles as met with in the scorpion, thereby + demonstrating how, with the degeneration of the median eyes and the large size of the lateral + eyes, the recti muscles of the scorpion would approach the position of an internal recti group to + the lateral eyes, and so give origin to the group of muscles innervated by the oculomotor nerve. + In the Eurypterus these large eyes are large single eyes, not separate ocelli, as in the + scorpion.</p> + + <p>All, then, that is required is that in the first formed fishes, which still possessed the + dorso-ventral muscles of their Eurypterid ancestors, the lateral eyes should be the important + organs of sight, large and near the mid-dorsal line. Such, indeed, is found to be the case. In + amongst the masses of Eurypterids found in the upper Silurian deposits at Oesel, as described by + Rohon, numbers of the most ancient forms of fish are found belonging to the genera Thyestes and + Tremataspis. The nature of the dorsal head-shields of these fishes is shown in Fig. <a + href="#fig14">14</a>, which represents the dorsal head-shield of <i>Thyestes verrucosus</i>, and + Fig. <a href="#fig111">111</a> that of <i>Tremataspis Mickwitzi</i>. They show how the two lateral + eyes were situated close on each side of the mid-dorsal line in these Eurypterus-like fishes, in + the very position where they must have been if the eye-muscles were derived from the dorso-ventral + somatic muscles of a Eurypterid ancestor.</p> + + <div class="ac w20 fcenter sp2"> + <a href="images/fig111.png" id="fig111"><img style="width:100%" src="images/fig111.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 111.—Dorsal Head-Shield of</span> <i>Tremataspis + Mickwitzi</i>. (From <span class="sc">Rohon</span>.)</p> + <p class="sp0"><i>Fro.</i>, narial opening; <i>l.e.</i>, lateral eyes; <i>gl.</i>, glabellum + plate over brain; <i>Occ.</i>, occipital spine.</p> + </div> + </div> + + <p>In Lankester's words, one of the characteristics of the Osteostraci (Cephalaspis, Auchenaspis, + etc.), as distinguished from the Heterostraci (Pteraspis), are the large orbits placed near the + centre of the shield. The apparent exception of Thyestes mentioned by him is no <span + class="pagenum" id="page276">{276}</span>exception, for orbits of the same character have since + been discovered, as is seen in Rohon's figure (Fig. <a href="#fig14">14</a>). In Fig. <a + href="#fig110">110</a>, C, I give an outline of the frontal part of the head-shield of a + Cephalaspid, in which I have drawn the eye-muscles as in the other two figures.</p> + + <p>Although all the members of the Osteostraci possess large lateral eyes towards the centre of + the head-shield, the other group of ancient fishes, the Heterostraci, are characterized by the + presence of lateral eyes far apart, situated on the margin of the head-shield on each side + (<i>cf.</i> Fig. 142, <i>o</i>, p. <a href="#page350">350</a>).</p> + + <p>So, also, on the invertebrate side, the lateral eyes of Pterygotus and Slimonia are situated on + the margin of the prosomatic carapace, while those of Eurypterus and Stylonurus are situated much + nearer the middle line of the prosomatic carapace.</p> + + <p>Next comes the question of the superior oblique muscle and the trochlearis nerve. Why does this + nerve (<i>n.IV.</i> in Fig. <a href="#fig106">106</a>, C and D) alone of all the nerves in the + body take the peculiar position it always does take? The only suggestion that I know of which + sounds reasonable and worth consideration is that put forward by Fürbringer, which is an + elaboration of the original suggestion of Hoffmann. Hoffmann suggested in 1889 that the + trochlearis nerve represented originally a nerve for a protecting organ of the pineal eye, which + became secondarily a motor nerve for the lateral eye as the pineal eye degenerated. Fürbringer + differs from Hoffmann in that he considers that the nerve was originally a motor nerve, and was + not transformed from sensory to motor, yet thinks Hoffmann's suggestion is in the right + direction.</p> + + <p>He points out that the crossing of the trochlearis is not a crossing of fibres between two + centres in the central nervous system, but may be explained by the shifting of the peripheral + organ, <i>i.e.</i> the muscle, from one side to the other, and the nerve following this shift. + Consequently, says Fürbringer, the course of the nerve indicates the original position of the + muscle, and therefore he imagines that the ancestor of the superior oblique muscle was a muscle + the fibres of which were attached in the mid-dorsal line, and interlaced with those of the other + side, the two muscles thus forming an arch through which the nervous system with its central canal + passed. Then, for the sake of getting a more efficient pull, the crossing muscle-fibres became + more definitely attached to the opposite side of the middle line, and finally obtained a new + attachment on the opposite side, with the <span class="pagenum" + id="page277">{277}</span>obliteration of the muscular arch; the nerve on each side, following the + shifts of the muscle, naturally took up the position of the original muscular arch, and so formed + the trochlear nerve, with its dorsal crossing. This explanation of Fürbringer's was associated by + him with movements of the median pineal eyes, the length of their nerve, according to him, even + yet indicating their previous mobility. This assumption is not, it seems to me, necessary. The + length of the nerve is certainly no indication of mobility, for in Limulus and the scorpion group + the nerve to each median eye is remarkably long, yet these eyes are immovably fixed in the + carapace. All that is required is a pair of dorso-ventral muscles belonging to the segment + immediately following the group of segments represented by the oculomotor nerves, the fibres of + which should cross the mid-dorsal line at their attachment; for, seeing that the lateral eyes were + originally so near this position, it follows that such muscles might form part of the muscular + group belonging to the lateral eye without having previously moved the pineal eyes. In fact, + Fürbringer's explanation requires as starting-point that the pair of muscles which ultimately + become the superior oblique should have the exact position of the pair of dorso-ventral muscles in + the scorpion, called by Miss Beck the anterior dorso-plastron muscles (63), which I have named the + oblique muscles. Here, and here only, do we find an interlacement, across the mid-dorsal line, of + the fibres of attachment of the muscles on the two sides, in consequence of which this pair of + muscles is described by her as forming an arch encircling the alimentary canal and dorsal vessel. + If, then, as I have previously argued, the primitive plastron formed a pair of trabeculæ, and the + nervous system grew round the alimentary canal, such an arch would encircle the tubular central + nervous system of the vertebrate.</p> + + <p>Still more striking is this pair of muscles (63) in Phrynus (Fig. 108), where we see how the + arch formed by them almost touches the posterior extremity of the supra-œsophageal + brain-mass, crossing, therefore, over the beginning of the stomach region of the animal. The angle + formed by the arch is much more obtuse than that formed in Scorpio, so that an actual crossing of + the muscle-fibres has taken place at the point of attachment to the carapace. Also, only the part + nearest the carapace is muscular, the rest forming a long tendinous prolongation of the plastron + wall (the primordial cranium), as seen in the figure.</p> + + <div><span class="pagenum" id="page278">{278}</span></div> + + <p>This muscle-pair is, as it should be, the pair of dorso-ventral muscles belonging to the + segment immediately following on the group of segments represented by the recti muscles, + <i>i.e.</i> according to previous argument, the segment belonging to the sixth pair of locomotor + appendages or ectognaths; a muscle, therefore, which would arise in the vertebrate from the + mandibular, and not from the premandibular cavity. A similar muscle probably existed in Eurypterus + (<i>M.obl.</i> in Fig. <a href="#fig106">106</a>, B), and, as in the case of the formation of the + oculomotor group, derived from the recti group of the scorpion, would form the commencement of the + superior oblique muscle in Thyestes and Tremataspis.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig112.png" id="fig112"><img style="width:100%" src="images/fig112.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 112.—A, Diagram of Position of Oblique Muscle in Scorpion; B, + Diagram of Transition Stage; C, Diagram of Superior Oblique Muscle in Vertebrate.</span></p> + <p class="sp0 ac"><i>l.e.</i>, lateral eyes; <i>c.e.</i>, central eyes; <i>C.N.</i>, central + nervous system; <i>Al.</i>, alimentary canal; <i>c.</i>, <i>aqueductus Sylvii</i>.</p> + </div> + </div> + + <p>It is instructive to notice that the original position of attachment of this muscle is + naturally posterior to that of the oculomotor group of muscles, and that Fürbringer, in his + description of the eye-muscles of Petromyzon, asserts that this muscle in this primitive + vertebrate <span class="pagenum" id="page279">{279}</span>form is not attached as in other + vertebrates, but is posterior to the other muscles, so that he calls it the posterior rather than + the superior oblique. The nature of the change by which the muscle known in the scorpion as the + anterior dorso-plastron muscle (63) was probably converted into the superior oblique muscle of the + vertebrate, is represented in the drawings Fig. <a href="#fig112">112</a>, in which also are + indicated the dwindling of the median eyes, and the progressive superiority of the lateral eyes, + as well as the transformation of the recti muscle-group of the scorpion into the muscles supplied + by the oculomotor nerve of the vertebrate.</p> + + <p>With respect to the external rectus muscle, it follows naturally that if the muscles (64) and + (65) are to follow suit with the rest of the group and become attached to the lateral eyes, they + must take up an external position. These two muscles, which always run together, as seen in Fig. + <a href="#fig110">110</a>, A, the one belonging to the prosoma and the other to the mesosoma, are + represented by the mandibular muscle of Miss Platt and the external rectus, the former derived + from the walls of the last pro-otic head-cavity, the latter from the foremost of the opisthotic + head-cavities.</p> + + <p class="sp3">Such, then, is the simple explanation of the origin of the eye-muscles which + follows from my theory, and we see that the successive alterations of the position of the orbit, + and, therefore, of the globe of the eye with its muscles, as we pass from Thyestes to man, is the + natural consequence of the growth of the frontal bone, <i>i.e.</i> of the brain.</p> + + <p class="ac"><span class="sc">The Trigeminal Nerves and the Muscles supplied by them.</span></p> + + <p>Turning now to the evidence as to the number of ventral segments, <i>i.e.</i> the motor and + sensory supply to the prosomatic appendages afforded by the trigeminal nerve, we must, I think, + come to the same conclusion as Dohrn, viz. that if there were originally seven dorsal or somatic + segments in this region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles of + the premandibular cavity, <i>sup. rectus</i>, <i>inf. rectus</i>, <i>int. rectus</i>, <i>inf. + oblique</i>, supplied by IIIrd nerve; 6, 7, muscles of the mandibular cavity, <i>sup. oblique</i>, + supplied by IVth nerve and muscle lost, there must have been also seven corresponding ventral or + splanchnic segments supplied by the trigeminal. At present the evidence for such segments is + nothing like so strong as for the corresponding somatic ones; there are, however, certain + suggestive <span class="pagenum" id="page280">{280}</span>facts which point distinctly in this + direction in connection with both the motor and sensory parts of the trigeminal. The origin of the + trigeminal motor fibres in the central nervous system is most striking. We may take it for granted + that a nucleus of cells giving origin to one or more segmental motor nerves will possess a greater + or less longitudinal extension in the central nervous system, according to the number of fused + separate segmental centres it represents. Thus a nucleus such as that of the IVth nerve or of the + facial is small and compact in comparison to the extensive conjoint nucleus of the vagus and + cranial accessory.</p> + + <p>Upon examination of the motor nucleus of the trigeminal, we find a compact or well-defined + nucleus, the <i>nucl. masticatorius</i>, the nerves of which supply the masseter, temporal, and + other muscles, so that the anatomical evidence at first sight appears to bear out van Wijhe's + conclusion that the motor trigeminal supplies at most two segments. Further examination, however, + shows that this is not all, for the extraordinary so-called descending root of the Vth must be + taken into consideration in any question of the origin of the motor elements, just as the equally + striking ascending root enters into the consideration of the meaning of the sensory elements of + the Vth.</p> + + <p>It is not necessary here to discuss the controversy as to whether this descending root is motor + or sensory. It is universally considered at present to be motor, and is believed to supply, as + Kölliker suggested, among other muscles, the <i>m. tensor tympani</i> and the <i>m. tensor veli + palati</i>. It is thus described by Obersteiner—</p> + + <p>"From the region of the mid-brain the motor root receives an important addition of thick + fibres, which form the cerebral or descending root. The large, round vesicular cells from which + the fibres of the descending root arise form no single compact group, but are partly single, + partly arranged like little bunches of grapes, as far as the region of the anterior corpora + quadrigemina. The further we go brainwards, the smaller is the number of fibres. In the region of + the anterior corpora quadrigemina, the few cells of origin are found more and more median; so that + the uppermost trigeminal fibres descend in curves almost from the mid-line, as is shown by the + exceptional occurrence of one or more of the characteristic cells above the aqueduct. At the + height of the posterior commissure one finds the last of these trigeminal cells."</p> + + <div><span class="pagenum" id="page281">{281}</span></div> + + <p>The anatomy of the Vth nerve reveals, then, three most striking facts<span + class="wnw">:—</span></p> + + <p>1. The motor nucleus of the Vth extends from the very commencement of the infra-infundibular + region to nearly the commencement of the nucleus of the VIIth; in other words, the motor nucleus + of the Vth extends through the whole prosomatic region, just as it must have done originally if + its motor nerves supplied the muscles of the prosomatic appendages. Such an extended range of + origin is indicative of the remains of an equally extended series of segmental centres or + ganglia.</p> + + <p>2. Of these centres the caudalmost have alone remained large and vigorous, constituting the + <i>nucleus masticatorius</i>, which in the fish is divided into an anterior and posterior group, + thus indicating a double rather than a single nucleus; while the foremost ones have dwindled away + until they are represented only by the cells of the descending root, the muscles of these segments + being still represented by possibly the <i>tensor veli palati</i> and the other muscles innervated + from these cells.</p> + + <p>3. The headmost of these cells takes up actually a position dorso-lateral to the central canal, + so that the groups on each side nearly come together in the mid-dorsal line; a very unique and + extraordinary position for a motor cell-group, but not improbable when we recall to mind Brauer's + assertion as to the shifting of the foremost prosomatic ganglion-cells of the scorpion from the + ventral to the dorsal side of the alimentary canal.</p> + + <p>On the sensory side the evidence is also suggestive, the question here being not so much the + distribution of the sensory nerves as the number of ganglia belonging to each of the cranial + nerves.</p> + + <p>With respect to this question, morphologists have come to the conclusion that there is a marked + difference between spinal and cranial nerves, in that whereas the posterior root-ganglia of the + spinal nerves arise from the central nervous system itself, <i>i.e.</i> from the neural crest, the + ganglia of the cranial nerves arise partly from the neural crest, partly from the proliferation of + cells on the surface of the animal; and because of the situation of these proliferating epidermal + patches over the gill-clefts in the case of the vagus and glossopharyngeal nerves, they have been + called by Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia into two + sets, one connected with the neural ridges, called the neural ganglia, <span class="pagenum" + id="page282">{282}</span>and the other connected with the surface-cells, which he calls the + lateral ganglia. This second set corresponds to Kupffer's epibranchial ganglia. Now it is clear + that in the case of the vagus nerve, where, as is well shown in Ammocœtes, the nerve is not + a single segmental nerve, but is in reality made up of a number of nerves going to separate + branchial segments, the indication of such segments is not given by the main vagus ganglion or + neural ganglion, but by the series of lateral ganglia. So also it is argued in the case of the + trigeminal, that if in addition to the ganglion-cells arising from the neural crest separate + ganglion-masses are found in the course of development, in connection with proliferating patches + of the surface (plakodes, Kupffer calls them), then such isolated lateral ganglia are indications + of separate segments, just as in the case of the vagus, even though the separate segments do not + show themselves in the adult. So far the argument appears to me just, but the further conclusion + that the presence of such plakodes shows the previous existence of <i>branchial</i> sense-organs, + and, therefore, that such ganglia are <i>epibranchial</i> ganglia, indicating the position of a + lost gill-slit, is not justified by the premises. If, as I suppose, the trigeminal nerve supplied + a series of non-branchial appendages serially homologous with the branchial appendages supplied by + the vagus, then it is highly probable that the trigeminal should behave with respect to its + sensory ganglia similarly to the vagus nerve, without having anything to do with branchiæ.</p> + + <p>Such plakodal ganglia, then, may give valuable indication of non-branchial segments as well as + of branchial segments. The researches of Kupffer on the formation of the trigeminal ganglia in + Ammocœtes are the chief attempt to find out from the side of the sensory ganglia the number + of segments originally belonging to the trigeminal. The nature and result of these researches is + described in my previous paper (<i>Journal of Anatomy and Physiology</i>, vol. xxxiv.), and it + will suffice here to state that he himself concludes that the trigeminal originally supplied five + at least, probably six, segments. As I have stated there, the evidence as given by him seems to me + to indicate even as many as seven segments.</p> + + <p>In the full-grown Ammocœtes, as is well known, there are two distinct ganglia belonging + to the trigeminal, the one the ganglion of the <i>ramus ophthalmicus</i>, the other the main + ganglion.</p> + + <p>According to Kupffer the larval Ammocœtes possesses three sets of ganglia, not two, for + between the foremost and hindmost ganglion <span class="pagenum" id="page283">{283}</span>he + describes a nerve (<i>x.</i>, Fig. <a href="#fig113">113</a>), with four epibranchial ganglia, + which do not persist as separate ganglia, but either disappear or are absorbed into the two main + ganglia (Fig. <a href="#fig113">113</a>). This discovery of Kupffer's is very suggestive, for, as + already stated, a transformation takes place when the Ammocœtes is 5 mm. long, so that the + arrangement of the parts before that period is distinctly more indicative of the ancestral + arrangement than any later one.</p> + + <p>If we use the name plakodal ganglia to represent that part of these ganglia which was + originally connected with the skin, then Kupffer's researches assert that in the larval + Ammocœtes there were seven such plakodal ganglia, one in front belonging to the foremost + trigeminal ganglion, two behind, parts of the hindmost ganglion, and four in between, which do not + exist later as separate ganglia.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig113.png" id="fig113"><img style="width:100%" src="images/fig113.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 113.—Ganglia of the Cranial Nerves of an + Ammocœtes, 4 mm. in length, projected on to the Median Plane.</span> (After <span + class="sc">Kupffer</span>.)</p> + <p class="sp0"><i>A-B</i>, the line of epibranchial ganglia; <i>au.</i>, auditory capsule; + <i>nc.</i>, notochord; <i>Hy.</i>, tube of hypophysis; <i>Or.</i>, oral cavity; <i>u.l.</i>, + upper lip; <i>l.l.</i> lower lip; <i>vel.</i>, septum between oral and respiratory cavities; + <i>V.</i>, <i>VII.</i>, <i>IX.</i>, <i>X.</i>, cranial nerves; <i>x.</i>, nerve with four + epibranchial ganglia.</p> + </div> + </div> + + <p>In accordance with the views put forward in this book, a possible interpretation of these + plakodal ganglia would be given as follows<span class="wnw">:—</span></p> + + <p class="sp3">Beard, who, after Froriep, drew attention to this relation of the cranial ganglia + to special skin-patches, has compared them with the parapodial ganglia of annelids, <i>i.e</i>. + ganglia in connection with annelidan appendages; whether we are here obtaining a glimpse of the + far-off annelidan ancestry of both arthropods and vertebrates it would be premature at present to + say. It is natural enough to expect, on my view, to find evidence of annelidan ancestry in <span + class="pagenum" id="page284">{284}</span>vertebrate embryology (as has been so often asserted to + be the case), seeing that undoubtedly the Arthropoda are an advanced stage of Annelida; and, + indeed, the way is not a long one when we consider Beecher's evidence that the Trilobita belong to + the Phyllopoda, certainly a primitive crustacean group, which Bernard derives directly from the + annelid group Chætopoda. If, then, these plakodal ganglia indicate the former presence of + appendages, we obtain this result:—The foremost ganglion on each side possesses one plakodal + ganglion, and therefore indicates an anterior pair of appendages, possibly the cheliceræ. Then + comes the peculiar nerve with four plakodal ganglia indicating on each side four appendages close + together, possibly the endognaths. Then, finally, on each side, the second large ganglion with two + plakodal ganglia, indicating two pairs of appendages, possibly the ectognaths and the + metastoma.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The consideration of the history of the cranial segmentation shows that whereas, from the + commencement of that history, the evidence for two ventral segments supplied by the trigeminal + nerve is clear and unmistakable, later observers have tended more and more to increase the + number of these segments, until at the present time the evidence is in favour of at least six, + probably seven, as the number of segments supplied by the motor part of the trigeminal.</p> + <p>So, also, the original evidence for the number of dorsal or somatic segments limits the + number to three, innervated respectively by the oculomotor (III.), trochlear (IV.), and abducens + (VI.) nerves, or rather two, since the last nerve belongs to the facial segment. The muscles + which these three nerves supply are derived respectively from the walls of the premandibular, + mandibular, and hyoid cœlomic cavities.</p> + <p>Later evidence points strongly to the conclusion that the oculomotor nerve and the + premandibular cavity represent not one segment but the fusion of four, while the mandibular + cavity represents two segments. In addition to these, Miss Platt has discovered a still more + anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on + this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the + premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these + dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the + same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental + group, the only representative of which at present known in the mesosomatic region is the + external rectus innervated by the VIth nerve.</p> + <p>These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities + and muscles of the invertebrate to an extraordinary <span class="pagenum" + id="page285">{285}</span>degree, so that it becomes easy to see how the dorso-ventral muscles of + the prosomatic segments of the latter have become converted into the eye-musculature of the + former. The most powerful proof of all that such a conversion has taken place is that a natural + and simple explanation is at once given of the extraordinary course taken by the IVth or + trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention + of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be + directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens, + each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For + generations anatomists have wondered and found no solution, and yet, without any straining of + hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair + of muscles in the scorpion group, the solution is immediately apparent.</p> + <p>This pair of muscles alone, of all the musculature attached to the carapace, crosses the + mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other + side; by a continuation of the same process the relation of the trochlear to the superior + oblique muscle can be explained.</p> + <p class="sp0">The comparison of the eye-muscles of the vertebrate with the dorso-ventral + segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much + clearer.</p> + </div> + + <div><span class="pagenum" id="page286">{286}</span></div> + + <p class="ac">CHAPTER IX</p> + + <p class="ac"><i>THE PROSOMATIC SEGMENTS OF AMMOCŒTES</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">The prosomatic region in Ammocœtes.—The suctorial apparatus of the + adult Petromyzon.—Its origin in Ammocœtes.—Its derivation from + appendages.—The segment of the lower lip or metastomal segment.—The tentacular + segments.—The tubular muscles.—Their segmental arrangement.—Their peculiar + innervation.—Their correspondence with the system of veno-pericardial muscles in + Limulus.—The old mouth or palæostoma.—The pituitary gland.—Its comparison with + the coxal gland of Limulus.—Summary.</p> + </div> + + <p class="sp3">In the last chapter it was seen not to be incompatible with both the anatomical and + morphological evidence to look upon the trigeminal nerves as having originally supplied the seven + prosomatic pairs of appendages of the invertebrate ancestor, the foremost of which, the cheliceræ, + and the four pairs of endognaths dwindled away and became insignificant, leaving as trace of their + former presence the descending root of the Vth nerve; while the two hindmost pairs, the ectognaths + and the chilaria, or metastoma, remained vigorous and developed, leaving as proof of their + presence the <i>nucleus masticatorius</i>. Evidence in favour of this suggestion and of the nature + of the dwindling process is afforded when we examine what the trigeminus does supply in + Ammocœtes. In all vertebrates this nerve supplies the great muscles of mastication which, + in all gnathostomatous fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws; + they take in their food by attaching themselves to their prey and by means of rasping teeth + situated in serried rows within the circular mouth, combined with a powerful suctorial apparatus, + they suck the juices of the fish they feed upon. Not possessing jaws, they feed by suction on the + living animal, a method of feeding which gives them no more claim to be classed as parasitic + animals than the whole group of spiders which feed in a similar manner on living flies.</p> + + <div><span class="pagenum" id="page287">{287}</span></div> + + <p class="ac"><span class="sc">The Origin of the Suctorial Apparatus of Petromyzon.</span></p> + + <p>This powerful suctorial apparatus is innervated entirely by the trigeminal nerve, so that here + in its muscular arrangements any original segmental arrangement of the muscles of mastication + might be expected to be visible. It consists of a large rod or piston, to which are attached + powerful longitudinal muscles; a large muscle, the basilar muscle, which assists the piston in + producing a vacuum, and annular muscles around the circular lip.</p> + + <p>Turn now to the full-grown larval form, Ammocœtes, an animal in the case of + <i>Petromyzon Planeri</i> as large as the full-grown Petromyzon, and seek for this musculature. + There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned, + an oral chamber bounded by the lower and upper lips and the remains of the septum between it and + the respiratory chamber—the velar folds. Attached to its walls a number of tentacles are + situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here + between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but + simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a + massive suctorial apparatus.</p> + + <p>In order, then, to understand the origin of the muscles of mastication, it is necessary to + study the changes which occur at transformation, and thus to find out how the suctorial apparatus + of the adult arises. This most important investigation has been undertaken by Miss Alcock, and + owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series + in the transformation process than has ever been obtained before. Miss Alcock has not yet + published her researches, but has allowed me to make use of some of her facts.</p> + + <p>An enormous proliferation of muscular tissue takes place with great rapidity during this + transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial + apparatus. The starting point of this proliferation can be traced back in all cases to little + groups of embryonic tissue found below the epithelial lining of the oral chamber in + Ammocœtes. Of these groups the most conspicuous one is situated at the base of the large + median ventral tentacles. Others are situated at the base of the tentacular ridge. Further, + although this extraordinary change takes place in the <span class="pagenum" + id="page288">{288}</span>peripheral organ, no marked difference occurs in the arrangement of the + nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new + muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously + in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding + with the proliferation of the muscular elements.</p> + + <p>The clue, then, to the origin of the suctorial apparatus and of the nature of the original + organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by + the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of + government, here is 'headquarters,' from which the answers to all our inquiries must + originate.</p> + + <div class="ac w50 fcenter sp3"> + <a href="images/fig114.jpg" id="fig114"><img style="width:100%" src="images/fig114.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 114.—Distribution of Trigeminal Nerve in + Ammocœtes.</span></p> + <p class="sp0"><i>ps. br.</i>, pseudo-branchial groove; <i>met.</i>, nerve to lower lip, or + metastomal nerve; <i>t.</i>, nerve to tongue; <i>tent.</i>, nerve to tentacles. The mandibular + and internal maxillary nerves are coloured red; the purely sensory nerves to the external + surface are coloured black.</p> + </div> + </div> + + <p class="ac"><span class="sc">The Trigeminal Nerve of Ammocœtes.</span></p> + + <p>Striking is the answer. In Fig. <a href="#fig114">114</a>, Miss Alcock has drawn the + distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as + is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous + nerve, the ophthalmic nerve, and the hindmost to three nerves, the most posterior of which is + purely cutaneous and passes tailwards over the ventral branchial region, as shown in the figure; + the other two nerves, both <span class="pagenum" id="page289">{289}</span>of which contain motor + fibres, are called by Hatschek the mandibular and maxillary nerves. Of these the mandibular or + velar nerve (<i>met.</i>) is a large, conspicuous nerve, which arises so separately from the rest + of the trigeminal as almost to deserve the title of a separate nerve. When it leaves the large + posterior ganglion, it passes into the anterior part of the velum, runs along with the tubular + muscles, which it supplies, to the ventral surface as far as the junction of the lower lip with + the thyroid plate, and has not been followed further by Hatschek. Miss Alcock, however, by means + of serial sections, has traced it further, and shown that at this point it turns abruptly + headwards to terminate in the muscles of the lower lip. If, then, as suggested, the lower lip + represents the metastoma—the last pair of prosomatic appendages—then this mandibular + or velar nerve represents that segmental nerve.</p> + + <p>The other nerve—the maxillary nerve of Hatschek—which constitutes the larger part + of the trigeminal, passes forwards from the ganglion, and at a point somewhere about the anterior + region of the eyeball, divides into two, an external (<i>black</i> in Fig. <a + href="#fig114">114</a>) and an internal (<i>red</i> in Fig. <a href="#fig114">114</a>) nerve. The + external branch is apparently entirely sensory, and supplies the external surfaces of the upper + and lower lips. The internal branch is mainly motor, and supplies the muscles of the upper lip; it + contains also the nerves of the tentacles.</p> + + <p>The nerve to the median ventral tentacle (<i>t.</i>) or tongue leaves the internal division of + the maxillary immediately after its separation from the external; it runs ventralwards, and at the + same time passes internally until it reaches a position between the muco-cartilage and the + epithelium lining the cavity of the throat. It then turns, and passing posteriorly (towards the + tail) to the point where the median ventral tentacle is attached to the lower lip, it supplies + some very rudimentary-looking muscles which run from the tentacle to the adjoining surface, and no + doubt serve to move the tentacle from side to side. A portion of the nerve still continues to run + along the side of the median ventral ridge, as far back as the point where the muscles of the + hyoid segment pass round to the ventral side between the velum and the thyroid; in fact, this + small nerve passes along the whole length of the median ventral ridge.</p> + + <p>This description shows that the trigeminal nerve divides itself into two groups: the one + represented black in the figure, which is purely cutaneous and sensory, corresponding, in the + main, according <span class="pagenum" id="page290">{290}</span>to my theory, to the epimeral + nerves of Limulus; the other coloured red, which supplies muscles belonging to the visceral or + splanchnic muscle-group, and contains also the nerves to the tentacles.</p> + + <p>This latter group, which is formed by two distinct well-defined nerves, viz. the mandibular and + the internal branch of the maxillary, corresponds, according to my theory, to the amalgamated + nerves of the prosomatic appendages, and is clearly divisible into three distinct + nerves—</p> + + <p>1. The lower lip-nerve or the metastomal nerve (<i>met.</i>).</p> + + <p>2. The tongue-nerve (<i>t.</i>).</p> + + <p>3. The nerve (<i>tent.</i>) to the upper lip and tentacles.</p> + + <p>Of these three pairs of nerves it is suggested that the first pair were derived from the nerves + to the metastomal appendage. The second pair of nerves ought, on this theory, originally to have + supplied the pair of appendages immediately in front of the metastoma—that is, the pair of + ectognaths, and therefore the ventral pair of tentacles, known as the tongue, would represent the + last remnant of these ectognaths. Similarly, the other tentacles would represent the endognaths, + and therefore the third pair of nerves would represent the fused nerves to these concentrated + endognaths, which, in the Eurypterids, stand aloof from the ectognaths.</p> + + <p class="sp3">Let us consider these three propositions separately. In the first place, have we + any right to attribute segmental value to the mandibular nerve? What evidence is there of segments + in this region in Ammocœtes?</p> + + <p class="ac"><span class="sc">The Segment of the Lower Lip, or Metastomal Segment.</span></p> + + <p>We have seen that in the branchial or mesosomatic region the segments corresponding to the + mesosomatic appendages were mapped out by means of their supporting or skeletal structures, their + segmental muscles, and their nervous arrangements, as well as by the arrangement of the branchiæ. + Similarly, the segments in front of the branchial region, corresponding to the prosomatic + appendages, ought to be definable by the same means, although, owing to the absence of branchiæ + and the greater concentration in this region, the separate segments would probably not be so + conspicuous.</p> + + <p>The last segment considered was the segment belonging to the VIIth nerve corresponding to the + opercular appendages of the <span class="pagenum" id="page291">{291}</span>Eurypterid. The segment + immediately in front of this is the next for consideration, viz. that corresponding to the + chilarial appendages or metastoma; and as the basal part of this pair of appendages was fused with + the basal part of the operculum, the one cannot be discussed without the other; therefore, the + segment to which the lower lip belongs must be considered in connection with and not apart from + the thyro-hyoid segments already dealt with.</p> + + <p>In Chapter V., p. <a href="#page188">188</a>, I stated that the supporting bars of the foremost + mesosomatic segments, the thyro-hyoid segments, differed from the cartilaginous bars of the + branchial segments, in that they were composed of muco-cartilage. Also in addition to the + muco-cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in Ammocœtes + which covers over the thyroid gland.</p> + + <p>Similarly in the prosomatic segments the skeletal bars are composed of muco-cartilage and the + ventral plate of muco-cartilage continues forward as the plate of the lower lip. It is of special + interest, in connection with the segments indicated by such supporting structures, to find that + this special tissue is entirely confined to the head-region, and disappears absolutely at + transformation, thus indicating the ancestral nature of the segments marked out by its + presence.</p> + + <p>This muco-cartilaginous skeleton is the key to the whole position, and requires, therefore, to + be understood. It is of great importance, not only because it demonstrates the position of the + segments in Ammocœtes which characterized its invertebrate ancestor, but also because it + possesses a structure remarkably similar to that found in the head-plates of the most ancient + fishes. For the present I will confine myself to the consideration of this muco-cartilaginous + skeleton as evidence of the relationship of Ammocœtes to the Eurypterids, and in the next + chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so + that Ammocœtes is really a slightly modified Cephalaspid, the larval form of which was + Eurypterid in character.</p> + + <div><span class="pagenum" id="page292">{292}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig115.jpg" id="fig115"><img style="width:88%" src="images/fig115.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 115.—Dorsal half of Head-region of + Ammocœtes.</span></p> + <p class="sp0"><i>Tr.</i>, trabeculæ; <i>Pit.</i>, pituitary space; <i>Inf.</i>, infundibulum; + <i>Ser.</i>, median serrated flange of velar folds.</p> + </div> + </div> + + + <div><span class="pagenum" id="page293">{293}</span></div> + + <div class="ac w40 fcenter"> + <a href="images/fig116.jpg" id="fig116"><img style="width:100%" src="images/fig116.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 116.—Horizontal Section through the Anterior Part of + Ammocœtes, immediately Ventrally to the Auditory Capsule.</span></p> + <p class="sp0"><i>sk<sub>1</sub></i>-<i>sk<sub>5</sub></i>, skeletal bars; + <i>m<sub>1</sub></i>-<i>m<sub>5</sub></i>, striated visceral muscles; + <i>mt<sub>1</sub></i>-<i>mt<sub>4</sub></i>, tubular muscles; + <i>br<sub>1</sub></i>-<i>br<sub>3</sub></i>, branchiæ; <i>tr.</i>, trabeculæ; <i>inf.</i>, + infundibulum; <i>ped.</i>, pedicle; <i>V.</i>, trigeminal nerve. Muco-cartilage, <i>red</i>; + soft cartilage, blue; hard cartilage, purple.</p> + </div> + </div> + + <div><span class="pagenum" id="page294">{294}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig117.jpg" id="fig117"><img style="width:100%" src="images/fig117.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 117.—Sagittal Lateral Section through the Anterior Part of + Ammocœtes.</span></p> + <p class="sp0">Lettering and colouring same as in Fig. <a href="#fig116">116</a>. <i>aud.</i>, + auditory capsule; <i>j.v.</i>, jugular vein.</p> + </div> + </div> + + <p>In Chapter IV., Figs. 63, 64, I have given a representation of the ventral and dorsal views of + an Ammocœtes cut in half horizontally. Such a section shows with great clearness the series + of branchial appendages with their segmental muscles and cartilaginous bars which form the + branchial segments innervated by the IXth and Xth nerves, according to my view of the branchial + unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid or opercular appendage, which is + clearly serially homologous with the other branchial bars, is composed of muco-cartilage, and not + of cartilage. If we follow this series of horizontal sections nearer to the origin of the + cartilaginous bars from the sub-chordal cartilaginous rod on each side of the notochord, we obtain + a picture, as in Fig. <a href="#fig116">116</a>, in which each branchial segment is defined by the + section of the branchial cartilaginous bar (<i>sk<sub>4</sub></i>, <i>sk<sub>5</sub></i>), by the + section of the separate branchiæ (<i>br<sub>2</sub></i>, <i>br<sub>3</sub></i>), and by the + separate segmental muscles arranged round each bar, these muscles being partly ordinary striated + (<i>m<sub>4</sub></i>, <i>m<sub>5</sub></i>), partly tubular (<i>mt<sub>3</sub></i>, + <i>mt<sub>4</sub></i>). The uppermost of these branchial segments shows the same arrangement; + (<i>sk<sub>3</sub></i>) is the branchial skeletal bar, which is now composed of muco-cartilage, + not cartilage; (<i>br<sub>1</sub></i>) is the branchiæ in the same situation as the others, but + here composed of glandular rather than of respiratory epithelium, while the ordinary striated + branchial muscles of this segment are marked as (<i>m<sub>3</sub></i>), being separated from the + tubular muscles of the segment (<i>mt<sub>2</sub></i>), owing to the large size of the blood-space + in which these latter muscles are lying. In front of this segment so defined we see again another + well-marked skeletal bar (<i>sk<sub>2</sub></i>) of muco-cartilage, evidently indicating a similar + segment anterior to the hyoid segment. In connection with this bar there are no branchiæ, but + again we see two sets of visceral muscles, the one ordinary striated, marked + (<i>m<sub>2</sub></i>), and the other tubular, marked (<i>mt<sub>1</sub></i>). Here, then, the + section indicates the existence of a segment of the same character as the posteriorly situated + branchial segments but belonging to a non-branchial region—a segment which would represent a + non-branchial appendage, the last, therefore, of the prosomatic appendages. Let us, then, follow + <span class="pagenum" id="page295">{295}</span>out these two segmental muco-cartilaginous bars and + their attendant muscles, and see to what sort of segments their investigation leads.</p> + + <p>The bar which comes first for consideration (<i>sk<sub>3</sub></i>) arises immediately behind + the auditory capsule from the first branchial cartilage very soon after it leaves the sub-chordal + cartilaginous ligament; the soft cartilage of the sub-chordal ligament ceases abruptly in its + extension along the notochord at the place where the hard cartilage of the parachordal joins it, + and in a sense it may be said to leave the notochord at this place and pass into the basal part of + the first branchial bar. The most anterior continuation of this branchial system is this + muco-cartilaginous bar (<i>sk<sub>3</sub></i>), which passes forward and ventralwards, being + separated from the axial line by the auditory capsule (<i>cf.</i> Fig. 118, A, B, C). Its position + is well seen in a sagittal section, such as Fig. <a href="#fig117">117</a>. It follows absolutely + the line of the pseudo-branchial groove (<i>ps. br.</i>, Fig. <a href="#fig114">114</a>), and + ventrally joins the plate of muco-cartilage which covers the thyroid gland. It forms a thickened + border to this plate anteriorly, just as the branchial cartilaginous bars border it posteriorly. + In fact, it behaves with respect to the hyoid segment in a manner similar to the rest of the + cartilaginous bars with respect to their respective segments.</p> + + <p>It represents, although composed of muco-cartilage, the cartilaginous bar of the operculum in + Limulus, which also forms the termination of the branchial cartilaginous system, as fully + explained in Chapter III.; it may therefore be called the opercular bar.</p> + + <p>The next bar (<i>sk<sub>2</sub></i>) is extremely interesting, as we are now out of the + branchial or mesosomatic region, and into the region corresponding to the prosoma. It starts from + a cartilaginous projection made of hard cartilage, just in front of the auditory capsule, called + by Parker the 'pedicle of the pterygoid'—a projection (<i>ped.</i>) which defines the + posterior limit of the trabeculæ on each side, where they join on to the parachordals,—and + winding round and below the auditory capsule, joins the opercular bar (<i>cf.</i> Fig. <a + href="#fig118">118</a>), to pass thence into and form part of the muco-cartilaginous plate of the + lower lip. In the section figured (Fig. <a href="#fig116">116</a>), this projection of hard + cartilage is not directly continuous with (<i>sk<sub>2</sub></i>), owing to a slight curvature in + the bar; the next few sections show clearly the connection between (<i>ped.</i>) and + (<i>sk<sub>2</sub></i>), and consequently the complete separation by means of this bar of the + hyoid segment from the segment in front.</p> + + <div><span class="pagenum" id="page296">{296}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig118.jpg" id="fig118"><img style="width:100%" src="images/fig118.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 118.—Skeleton of Head-Region of Ammocœtes. A, + Lateral View; B, Ventral View; C, Dorsal View.</span></p> + <p class="sp0">Muco-cartilage, <i>red</i>; soft cartilage, <i>blue</i>; hard cartilage, + <i>purple</i>. <i>sk<sub>1</sub></i>, <i>sk<sub>2</sub></i>, <i>sk<sub>3</sub></i>, skeletal + bars; <i>c.e.</i>, position of pineal eye; <i>na. cart.</i>, nasal cartilage; <i>ped.</i>, + pedicle; <i>cr.</i>, cranium; <i>nc.</i>, notochord.</p> + </div> + </div> + + <div><span class="pagenum" id="page297">{297}</span></div> + + <p>In the figures, the hard cartilage is coloured purple, the soft cartilage blue, and the + muco-cartilage red, so that the position of this bar is well shown. This bar may be looked upon as + bearing the same relation to the muco-cartilaginous plate of the lower lip as the opercular bar + does to the muco-cartilaginous plate over the thyroid; and seeing that these two plates form one + continuous ventral head-shield of muco-cartilage (Fig. <a href="#fig118">118</a>, B), and also + that this bar fuses with the opercular bar, we may conclude that the segment represented by the + lower lip is closely connected with the hyoid or opercular segments. In other words, if the lower + lip arose from the metastoma, then this pair of skeletal bars might be called the metastomal bars, + which formed the supporting skeleton of the last pair of prosomatic appendages and, as is likely + enough, arose in connection with the posterior lateral horns of the plastron; these posterior + lateral horns, like the rest of the plastron, would give rise to hard cartilage, and so form in + Ammocœtes the two lateral so-called pterygoid projections.</p> + + <p>In the branchial region the muscles which marked out each branchial segment were of two + kinds—ordinary striated visceral muscles and tubular muscles. Of these the former + represented the dorso-ventral muscles of the branchial appendages, while the latter formed a + separate group of dorso-ventral muscles with a separate innervation which may have been originally + the segmental veno-pericardial muscles so characteristic of Limulus and the scorpions. In Figs. + 116, 117, the grouping of these muscles in each branchial segment is well shown, and it is + immediately seen that the hyoid segment possesses its group of striated visceral muscles + (<i>m<sub>3</sub></i>) supplied by the VIIth nerve in the same manner as the posterior groups, as + has already been pointed out by Miss Alcock in her previous paper. Passing to the segment in + front, Fig. <a href="#fig116">116</a> shows that the group of visceral muscles + (<i>m<sub>2</sub></i>) corresponds in relative position with respect to the metastomal bar to the + hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect + to each branchial bar. What, then, is this muscular group? The series of sections show that these + are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig. <a + href="#fig119">119</a> (<i>M.</i>), form a well-marked muscular sheet, whose fibres interlace + across the mid-ventral line of the lower lip. This group of lower lip-muscles is very suggestive, + for these muscles arise, not from the trabeculæ, but from the front dorsal region of the cranium, + just in front of the two lateral <span class="pagenum" id="page298">{298}</span>eyes. In Fig. <a + href="#fig117">117</a> the dorsal part is seen cut across on its way to its dorsal attachment. + Such an origin is reminiscent of the tergo-coxal group of muscles, arising, as they do, from the + primordial cranium and the tergal carapace, and suggests at once that when the chilarial + appendages expanded to form a metastoma, their tergo-coxal muscles formed a sheet of muscles + similar to those of the lower lip of Ammocœtes, by which the movements of the metastoma + were effected. The posterior limit of these muscles ventrally marks out the junction of the + segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma + had fused ventrally with the operculum (Fig. <a href="#fig117">117</a>).</p> + + <div class="ac w20 fcenter sp2"> + <a href="images/fig119.png" id="fig119"><img style="width:100%" src="images/fig119.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 119.—Ventral View Of Head-Region of + Ammocœtes.</span></p> + <p class="sp0"><i>Th.</i>, thyroid gland; <i>M.</i>, lower lip, with its muscles.</p> + </div> + </div> + + <p>Besides the striated visceral muscles, each branchial segment possesses its own tubular + muscles, shown in Fig. <a href="#fig116">116</a> (<i>mt<sub>3</sub></i>) and + (<i>mt<sub>4</sub></i>). As the section shows, there is clearly a group of tubular muscle-fibres + belonging to the hyoid segment (<i>mt<sub>2</sub></i>), and also another group belonging to the + segment in front of the hyoid (<i>mt<sub>1</sub></i>); so that, judging from this section, each of + these segments possesses its own tubular musculature just as do the branchial segments, the + difference being that the tubular muscles are more separated from the striated visceral group than + in the true branchial segments, owing to the size of the blood-spaces surrounding them. What, + then, are these two groups of muscles? Tracing them in the series of sections, both groups are + seen to belong to the system of velar muscles, forming an anterior and a posterior group + respectively; and we see, further, that there is not the slightest trace of any tubular muscles + anterior to these muscles of the velum.</p> + + <p>In the living Ammocœtes the velar folds on each side can be seen <span class="pagenum" + id="page299">{299}</span>to move synchronously with the movements of respiration, contracting at + each expiration, and thus closing the slit by which the oral and respiratory chambers communicate, + and so forcing the waters of respiration through the gill-slits, as described by Schneider. Such a + fact is clear evidence that these tubular muscles of the velar folds belong to the same series as + the tubular muscles of the branchial segments, so that if, as I have already suggested, the latter + muscles were originally the veno-pericardial muscles of segments corresponding to the branchial + appendages, then the former would represent the veno-pericardial muscles of the segments + corresponding to the opercular and metastomal appendages. What, then, are these velar folds, and + how is it that the tubular muscles of these two segments become the velar muscles? I will + consider, in the first instance, the posterior group of muscles (<i>mt<sub>2</sub></i>) in Fig. <a + href="#fig116">116</a>.</p> + + <p>It has already been pointed out that the tubular muscles of the branchial segments are + dorso-ventral, but do not run with the ordinary constrictors, having separate attachments and + running part of their course internally to and part externally to the ordinary constrictors. At + first sight, as is usually stated, the hyoid segment does not appear to possess tubular muscles at + all. If, however, we follow the posterior group of velar muscles (<i>mt<sub>2</sub></i>), we see + (Fig. <a href="#fig117">117</a>) that they pass between the auditory capsule and the opercular bar + (<i>sk<sub>3</sub></i>) of muco-cartilage to reach the region of the jugular vein (<i>j.v.</i>) + posteriorly to the auditory capsule, so that their dorsal origin bears the same relation to the + hyoid segment as the dorsal attachment of the rest of the tubular muscles to their respective + segments. Further, these muscles run along the length of the velar fold, and are attached + ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in + position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular + muscles as regards their respective segments.</p> + + <p>This ventral attachment is shown in Fig. <a href="#fig119">119</a> on each side of the thyroid, + and in Fig. <a href="#fig120">120</a> (<i>mt<sub>2</sub></i>); while in Fig. <a + href="#fig117">117</a> the fibres are seen converging to this ventral position. In other words, + this large posterior muscle of the velar folds is a dorso-ventral muscle, and would actually take + the same position in the hyoid segment as the dorso-ventral tubular muscles in the other branchial + segments, if the velum were put back into its original position as the septum terminating the + branchial chamber. Conversely, the presence of these <span class="pagenum" + id="page300">{300}</span>hyoid tubular muscles in the velum gives evidence that the opercular + segment takes part in the formation of the septum, as already suggested.</p> + + <p>Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid segment, which are + attached to the muco-cartilage. Schaffer also speaks of certain tubular muscles belonging to the + velar group as piercing the muco-cartilage (<i>h. r. s.</i>) in his figures 24 and 25, <i>i.e.</i> + the metastomal bar, near its junction with the opercular bar. In my specimens there is a distinct + group of tubular muscles which pierce the opercular bar of muco-cartilage at its junction with the + metastomal bar, and pass into the posterior group of velar muscles. They clearly belong to the + hyoid segment, as Miss Alcock supposed, but are not attached to the muco-cartilage. It is possible + that they represent a different group to those already considered, and suggest the possibility + that this opercular or thyro-hyoid segment is double with respect to its original veno-pericardial + muscles as well as in other respects.</p> + + <p>The anterior group of tubular muscles (<i>mt<sub>1</sub></i>, Figs. 116, 117) belonging to the + same segment as the metastomal bar must now be taken into consideration. Very different is their + origin to that of the posterior group: they arise close up against the eye, and have given rise to + Kupffer's and Hatschek's misconception that the superior oblique muscle of the eye arises from a + part of the velar musculature. Naturally, as Neal has pointed out, they have nothing to do with + the eye-muscles; the superior oblique muscle is plainly in its true place entirely apart from + these velar muscles, which form the foremost group of the segmental tubular muscles. They pass + into the anterior part of the velar folds and run round to the ventral side just in the same way + as does the posterior group. This anterior group of tubular muscles represents the + veno-pericardial muscle of the segment immediately in front of the opercular, <i>i.e.</i> the + metastomal segment, and is the foremost of these veno-pericardial muscles. Its presence shows that + the velar folds, formed as they were by the breaking down of the septum, are in reality part of + two segments, viz. the opercular and the metastomal, which have fused together in their basal + parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so + apparent in the anatomy of the vertebrate cranial nerves.</p> + + <p>A further piece of evidence that this anterior portion of the velum <span class="pagenum" + id="page301">{301}</span>belongs to the same segment as the lower lip is the fact that in addition + to the tubular muscles a single ordinary striated muscle is found in the velum which, like the + muscles of the lower lip, is innervated by this same mandibular nerve.</p> + + <p>This muscle is attached laterally to the muco-cartilage of the metastomal bar + (<i>sk<sub>2</sub></i>) at its junction with the muco-cartilage of the lower lip, and spreads out + into a number of strands which are attached at intervals along the whole length of the free + anterior edge of the velum. It is the only non-tubular muscle belonging to the velum, and by its + contraction it draws the anterior portions of the velar folds apart from each other, and so opens + the slit between them, through which the food and mud must pass. Clearly from its position it does + not belong to the original tergo-coxal group of muscles as do those of the lower lip; it must have + been one of the intrinsic muscles of the metastoma itself.</p> + + <p>This anterior portion of the velar folds affords yet another striking hint of the correctness + of my comparison of the lower lip segment of Ammocœtes with the chilaria of Limulus or the + metastoma of Eurypterus; for the most dorsal anterior portion, which at its attachment possesses a + wedge of muco-cartilage, forms a separate, well-defined, rounded basal projection marked + <i>Ser.</i> in Fig. <a href="#fig115">115</a>, and <i>B</i> in the accompanying Fig. <a + href="#fig120">120</a>. This is that part of the velar folds which comes together in the middle + line and closes the entrance into the respiratory chamber. The epithelial surface here is most + striking and suggestive, for it is markedly serrated, being covered with a large number of + closely-set projections or serræ. The serration of the surface here is of so marked a character + that Langerhans considered this part of the velar folds to act as a masticating organ, grinding + and rasping the food and mud which passed through the narrow slit. In fact, Langerhans supposed + that this portion of the velum acted in a manner closely resembling the action of the gnatho-bases + of the prosomatic appendages in Limulus or the Eurypteridæ.</p> + + <p>This suggestion of Langerhans is surely most significant, considering that this somewhat + separate portion of the velum, to which he assigns such a function, is in the very place where the + gnathite portion of the metastomal appendages would have been situated if it were true that the + lower lip and anterior portion of the velum of Ammocœtes were derived from the + metastoma.</p> + + <p>In addition to this marked serrated edge the whole surface of <span class="pagenum" + id="page302">{302}</span>the anterior portion of the velum is covered over with a scale-like or + tubercular pattern remarkably like the surface-ornamentation seen in many of the members of the + ancient group Eurypteridæ. In Fig. <a href="#fig121">121</a> I give a picture of this + surface-marking of the velum. It is striking to see that just as in the case of the invertebrate + this marking and these serræ are formed simply by the cuticular surface of the epithelial cells; a + surface which, according to Wolff, possibly contains chitin. The interpretation which I would give + of the velar folds is therefore as follows<span class="wnw">:—</span></p> + + <p>They represent the fused basal parts of the opercular and metastomal appendages, the + gnatho-bases of the latter still retaining in a reduced degree their rasping surfaces, because, + owing to their position on each side of the opening into the respiratory chamber they were still + able to manipulate the food as it passed by them after the closure of the old mouth.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig120.png" id="fig120"><img style="width:66%" src="images/fig120.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 120.—Ammocœtes cut open in Mid-Ventral Line + to show Position of Velum; Velar Folds removed on one side.</span></p> + <p class="sp0"><i>tr.</i>, trabeculæ; vel., velum; <i>B.</i>, anterior gnathic portion of + velum; <i>ps. br.</i>, pseudo-branchial groove; <i>m<sub>2</sub></i>, muscles of lower lip + segment; <i>m<sub>3</sub></i>, muscles of thyro-hyoid segment; <i>mt<sub>2</sub></i>, + insertion of tubular muscles of velum near thyroid.</p> + </div> + </div> + + <div class="ac w15 fcenter sp2"> + <a href="images/fig121.png" id="fig121"><img style="width:100%" src="images/fig121.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 121.—Surface View of Anterior Surface of + Velum.</span></p> + </div> + </div> + + <p>The whole evidence points irresistibly to the conclusion that the mandibular or velar nerve of + the trigeminal does supply a splanchnic <span class="pagenum" id="page303">{303}</span>segment + which is, in all respects, comparable with the segments supplied by the facial, glossopharyngeal, + and vagus nerves, except that it does not possess branchiæ. This simply means that the appendages + which these nerves originally supplied were prosomatic, not mesosomatic, and corresponded, + therefore, to the chilarial or metastomal appendages.</p> + + <p class="sp3">A comparison of the ventral surface of Slimonia, as given in Fig. <a + href="#fig8">8</a>, p. <a href="#page27">27</a>, with that of Ammocœtes (Fig. <a + href="#fig119">119</a>), when the thyroid gland and lower lip muscles have been exposed to view, + enables the reader to recognize at a glance the correctness of this conclusion.</p> + + <p class="ac"><span class="sc">The Tentacular Segments and the Upper Lip.</span></p> + + <p>Anterior to this metastomal segment, Fig. <a href="#fig116">116</a> shows a group of visceral + muscles, <i>m<sub>1</sub></i>, and yet again a muco-cartilaginous bar, <i>sk<sub>1</sub></i>, but, + as already stated, no tubular muscles. These visceral muscles indicate the presence in front of + the lower lip-segment of one or more segments of the nature of appendages. The muscles in question + (<i>m<sub>1</sub></i>) are the muscles of the upper lip, the skeletal elements form a pair of + large bars of muco-cartilage (<i>sk<sub>1</sub></i>), which start from the termination of the + trabeculæ, and pass ventralwards to fuse with the muco-cartilaginous plate of the lower lip (Figs. + 117 and 118). This large bar forms the tentacular ridge on each side, and gives small projections + of muco-cartilage into each tentacle. In addition to this tentacular bar, a special bar of + muco-cartilage exists for the fused pair of median tentacles, the so-called tongue, which extends + in the middle line along the whole length of the lower lip, being separated from the + muco-cartilaginous plate of the lower lip by the muscles of the lower lip. This tongue bar of + muco-cartilage joins with the muco-cartilage of the lower lip at its junction with the thyroid + plate, and also with the tentacular bar just before the latter joins the muco-cartilaginous plate + of the lower lip. This arrangement of the skeletal tissue suggests that the pair of tentacles + known as the tongue stand in a category apart from the rest of the tentacles; a suggestion which + is strongly confirmed by the separate character of its nerve-supply, as already mentioned.</p> + + <p>For three reasons, viz. the separateness both of their nerve-supply and of their skeletal + tissue, and the importance they assume at transformation, this pair of ventral tentacles must, it + seems to me, be put <span class="pagenum" id="page304">{304}</span>into a separate category from + the rest of the tentacles. On the other hand, the innervation of the rest of the tentacles by a + single nerve which sends off a branch as it passes each one, together with the concentration of + their skeletal elements into a single bar, with projections into each tentacle, points directly to + the conclusion that these tentacles must be considered as a group, and not singly.</p> + + <p>I suggest that these tentacles are the remains of the ectognaths and endognaths; the tongue + representing the two ectognaths, and the four tentacles on each side the four pairs of + endognaths.</p> + + <p>As we see, this method of interpretation attributes segmental value to the tentacles, a + conclusion which is opposed to the general opinion of morphologists, who regard them as having no + special morphological importance, and certainly no segmental value. On the other hand, the + importance of the pair of ventral tentacles, the 'tongue' of Rathke, which lie in the mid-line of + the lower lip, has been shown by Kaensche, Bujor, and others, all of whom are unanimous in + asserting that at transformation they are converted into that large and important organ the piston + or tongue of the adult Petromyzon. It is supposed that the rest of the tentacles vanish at + transformation, being absorbed; they appear to me rather to take part in the formation of the + sucking-disc, so that I am strongly inclined to believe that the whole of the remarkable suctorial + apparatus of Petromyzon is derived from the tentacles of Ammocœtes. In other words, on my + view, a conversion of the prosomatic appendages into a suctorial apparatus takes place at + transformation, just as is frequently the case among the Arthropoda.</p> + + <p>It is to the arrangement of the muscles that we look for evidence of segmental value. As long + as it was possible to look upon these tentacles as mere sensory feelers round the mouth entrance, + it was natural to deny segmental value to them. Matters are now, however, totally different since + Miss Alcock's discovery of the rudimentary muscles at the base of the tentacles and their + development at transformation. If these muscles represent some of the appendage muscles belonging + to the foremost prosomatic segments just as the ocular muscles represent the dorso-ventral somatic + muscles of those same segments, then we may expect ultimately to be able to give as good evidence + of segmentation in their case as I have been able to give in the case of these latter muscles; for + the two sets of muscles are curiously alike, seeing that the eye-muscles do not develop until + <span class="pagenum" id="page305">{305}</span>transformation, but throughout the Ammocœtes + stage remain in almost as rudimentary a condition as the tentacular muscles.</p> + + <p>Another difficulty with respect to the tentacles is the determination of the number of them, + owing to the fact that in addition to what may be called well-defined tentacles a large number of + smaller tactile projections are found on the surface of the upper lip, as is seen in Fig. <a + href="#fig115">115</a>. In the very young condition, 7 or 8 mm. in length, it is easier to make + sure on this point. At this stage they may be spoken of as arranged in two groups: an anterior + small group and a posterior larger group. The anterior group consists of a pair of very small + tentacles and a very small median tentacle, all three situated quite dorsally in the front part of + the upper lip. The posterior group, which is separate from the anterior, consists of five pairs of + much larger tentacles, the most ventral pair in the mid-line ventrally on the lower lip being + fused together to form the large ventral median tentacle or tongue already mentioned. This pair, + according to Shipley, is markedly larger than the others. There are, therefore, five conspicuous + tentacles on each side, and in front of them a smaller pair and a small median dorsal one. In the + very young condition the accessory projections above-mentioned are not present, or at all events + are not conspicuous, and the tentacles are also markedly larger in comparison to the size of the + animal than in the older condition, where they have distinctly dwindled.</p> + + <p>This posterior group of five conspicuous tentacles is the one which I suggest represents the + four endognaths and one ectognath. What the significance of the small anterior group is, I know + not. It is possible that the cheliceræ are represented here, for they are situated distinctly + anterior to the other group; I know, however, of no sign of a markedly separate innervation to + these most dorsal tentacles such as I should have expected to find if they represented the + cheliceræ.</p> + + <p>The muscles of the upper lip, which distinctly belong to the visceral and not to the somatic + musculature, form part of the foremost segments, and in these muscles the tentacular nerve reaches + its final destination. From their innervation, then, they must have belonged to the same + appendages as the tentacles supplied by the tentacular nerve, <i>i.e.</i> to the endognaths. What + conclusion can we form as to the probable origin of the upper lip of Ammocœtes? Since the + oral chamber was formed by the forward growth of the metastoma, <i>i.e.</i> the lower lip of + Ammocœtes, it follows that the upper <span class="pagenum" id="page306">{306}</span>lip is + the continuation forwards of the original ventral surface of such an animal as Limulus or a member + of the scorpion group, where there is no metastoma, and corresponds to the endostoma, as Holm + calls it, of Eurypterus. This termination of the ventral surface in all these animals is made up + of two parts: (1) Of sternites composing the true median ventral surface of the body, called by + Lankester the pro- and meso-sternites; and (2) of the sterno-coxal processes of the foremost + prosomatic appendages, called in the case of Limulus gnathites, because they are the main agents + in triturating the food previously to its passage into the mouth. In Limulus, a conjoined + pro-mesosternite forms the median ventral wall to which the sterno-coxal processes are attached on + each side, and in Phrynus and Mygale a well-marked pro-sternite and meso-sternite are present, + forming the posterior limit of the olfactory opening. In Buthus and the true scorpions the + sterno-coxal processes of the 2nd, 3rd, and 4th prosomatic appendages take part in surrounding the + olfactory tubular passage; in Thelyphonus only the processes of the 2nd pair of prosomatic + appendages play such a part, the pro-sternite not being present (<i>cf.</i> Fig. <a + href="#fig97">97</a>).</p> + + <p>Seeing, then, what a large share the sterno-coxal processes of one or more of these prosomatic + appendages plays in the formation of this endostoma, and seeing also that the nerve which supplies + the upper lip-muscles in Ammocœtes is the same as that supplying the tentacles which are + attached to the upper lip, it appears to me more probable than not that the muscles in question + are the vestiges of the sterno-coxal muscles. These muscles differ markedly in their attachments + from the muscles of the lower lip, for whereas the latter resemble the tergo-coxal group in their + extreme dorsal attachment, the former resemble the sterno-coxal group in their attachment to what + corresponds to the endostoma.</p> + + <p>This interpretation of the meaning of the transformation process is in accordance with all the + previous evidence both from the side of the palæostracan as from the side of the vertebrate, for + it signifies that a dwindling process has taken place in the foremost of the original prosomatic + appendages—the cheliceræ and the endognaths; while, on the contrary, the ectognath and the + metastoma have continued to increase in importance right into the vertebrate stage. This process + is simply a continuation of what was already going on in the invertebrate stage, for whereas in + Eurypterus and other cases <span class="pagenum" id="page307">{307}</span>the cheliceræ and + endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage, + and the metastoma was on the upward grade from the two insignificant chilaria of Limulus.</p> + + <p>The transformation of these foremost appendages into a suctorial apparatus is very common among + the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is + in accordance with the evidence that the main mass of that suctorial apparatus should be formed + from appendages corresponding to the ectognath and metastoma rather than from the four endognaths. + In all probability the <i>nucleus masticatorius</i> of the trigeminal nerve with its innervation + of the great muscles of mastication is evidence of the continued development of the musculature of + these two last prosomatic appendages, just as the descending root of the Vth demonstrates the + further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however, + as far as I know, the musculature of the head-region of Petromyzon has not been brought into line + with that of other vertebrates, and until that comparative study has been completed it is + premature to discuss the exact position of the masticating muscles of the higher vertebrates.</p> + + <p>The analysis of these tentacular segments belonging to the trigeminal nerve presents greater + difficulties than that of any of the other cranial segments, owing to the deficiency of our + knowledge of what occurs at transformation. Light is required not only on the origin of the new + muscles but also on the origin of the new and elaborate cartilages which are newly formed at this + time.</p> + + <p>Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we + are not yet in a position to analyze the trigeminal supply in Petromyzon into its component + appendage elements, an analysis which ought ultimately to enable us to determine from which + appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the + muscles are concerned, she gives me the following information<span class="wnw">:—</span></p> + + <p>The tongue-nerve supplies in Ammocœtes the rudimentary muscles which pass laterally from + the base of the large ventral tentacle to the wall of the throat, and even in Ammocœtes + must possess some power of moving that tentacle.</p> + + <p>At transformation these muscles proliferate and develop enormously, and form the bulk of the + large basilar muscle which <span class="pagenum" id="page308">{308}</span>surrounds the throat + ventrally and laterally, and is the most bulky muscle in the suctorial apparatus.</p> + + <p>The velar or mandibular nerve supplies in Ammocœtes the muscles of the lower lip. In + Petromyzon it supplies also the longitudinal muscles of the tongue. The tongue-cartilage first + develops in the region of the median ventral tentacle, and there the longitudinal tongue-muscles + first begin to develop, not from the rudimentary muscles in the tongue but from those in the lower + lip region.</p> + + <p>In Ammocœtes the tentacular nerve supplies the rudimentary muscles in the tentacles and + the muscles of the upper lip. The latter disappear entirely at transformation, and in Petromyzon + the tentacular nerve supplies the circular, pharyngeal, and annular muscles, which are derived + from the rudimentary tentacular muscles.</p> + + <p>For the convenience of my reader I append here a table showing my conception of the manner in + which the endognathal and ectognathal segments of the Palæostracan are represented in + Ammocœtes. It shows well the uniform manner in which all the individual segmental factors + have been fused together to represent the appearance of a single segment (van Wijhe's first + segment) in the case of the four endognathal segments, but have retained their individuality in + the case of the ectognathal segment.</p> + + <table class="sp2 mc ba nothand" title="Endognathal and ectognathal segments" + summary="Endognathal and ectognathal segments"> + <tr> + <th rowspan="2" class="smaller ba">V. Wijhe's<br/> + segments.</th> + <th rowspan="2" colspan="2" class="smaller ba">Eurypterid<br/> + segments.</th> + <th colspan="2" class="smaller">Appendages.</th> + <th rowspan="2" class="smaller ba">Appendage<br/> + nerves.</th> + <th rowspan="2" class="smaller ba">Skeletal<br/> + elements.</th> + <th rowspan="2" class="smaller ba">Somatic<br/> + motor<br/> + nerves.</th> + <th rowspan="2" class="smaller ba">Dorso-<br/> + ventral<br/> + segmental<br/> + muscles.</th> + <th rowspan="2" class="smaller ba">Cœlomic<br/> + cavities.</th> + <th rowspan="2" class="smaller ba">Coxal<br/> + glands.</th> + </tr> + <tr class="ba"> + <th class="smaller">Eurypterid.</th> + <th class="smaller">Ammocœtes.</th> + </tr> + <tr> + <td class="ac vmi ba">1</td> + <td class="ac vmi bb bt pl2 pr0">2<br/> + 3<br/> + 4<br/> + 5</td> + <td class="ac vmi bb bt pl0 pr0"><img src="images/rbrace4.png" style="height:9.5ex; + width:1em;" alt="brace" /></td> + <td class="ac vmi ba">4 Endognaths</td> + <td class="ac vmi ba">4 Tentacles</td> + <td class="ac vmi ba">1 Tentacular to 4 tentacles</td> + <td class="ac vmi ba">1 Tentacular bar to 4 tentacles</td> + <td class="ac vmi ba">1 Oculomotor supplying 4 muscles</td> + <td class="ac vmi ba">Sup. inf. int. rectus and inf. oblique</td> + <td class="ac vmi ba">1 Premandibular fusion of 4</td> + <td class="ac vmi ba">1 Pituitary body; fusion of 4 coxal glands</td> + </tr> + <tr> + <td class="ac vmi ba">2</td> + <td class="ac vmi pl2 pr0">6</td> + <td></td> + <td class="ac vmi ba">1 Ectognath</td> + <td class="ac vmi ba">1 Tongue</td> + <td class="ac vmi ba">1 Tongue nerve</td> + <td class="ac vmi ba">1 Tongue bar</td> + <td class="ac vmi ba">1 Trochlearis supplying 1 muscle</td> + <td class="ac vmi ba">Sup. oblique</td> + <td class="ac vmi ba">1 Mandibular</td> + <td class="ba"></td> + </tr> + </table> + +<!-- Transposed table for handhelds --> + + <table class="sp2 mc ba w100 handonly" title="Endognathal and ectognathal segments" + summary="Endognathal and ectognathal segments"> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">V. Wijhe's<br/> + segments.</td> + <td>1</td> + <td>2</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Eurypterid<br/> + segments.</td> + <td>2 3 4 5</td> + <td>6</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Eurypterid<br/> + Appendages.</td> + <td>4 Endognaths</td> + <td>1 Ectognath</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Ammocœtes<br/> + Appendages.</td> + <td>4 Tentacles</td> + <td>1 Tongue</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Appendage<br/> + nerves.</td> + <td>1 Tentacular to 4 tentacles</td> + <td>1 Tongue nerve</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Skeletal<br/> + elements.</td> + <td>1 Tentacular bar to 4 tentacles</td> + <td>1 Tongue bar</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Somatic<br/> + motor<br/> + nerves.</td> + <td>1 Oculomotor supplying 4 muscles</td> + <td>1 Trochlearis supplying 1 muscle</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Dorso-<br/> + ventral<br/> + segmental<br/> + muscles.</td> + <td>Sup. inf. int. rectus and inf. oblique</td> + <td>Sup. oblique</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Cœlomic<br/> + cavities.</td> + <td>1 Premandibular fusion of 4</td> + <td>1 Mandibular</td> + </tr> + <tr class="ba ac vmi"> + <td class="smaller pt05 pb05">Coxal<br/> + glands.</td> + <td>1 Pituitary body; fusion of 4 coxal glands</td> + <td></td> + </tr> + </table> + +<!-- End of transposed table for handhelds --> + + <div><span class="pagenum" id="page309">{309}</span></div> + + <p class="ac"><span class="sc">The Tubular Muscles.</span></p> + + <p>The only musculature innervated by the trigeminal nerve which remains for further discussion, + consists of those peculiar muscles found in the velum, known by the name of striated tubular + muscles. This group of muscles has already been referred to in Chapter IV., dealing with + respiration and the origin of the heart.</p> + + <p>It is a muscular group of extraordinary interest in seeking an answer to the question of + vertebrate ancestry, for, like the thyroid gland, it bears all the characteristics of a survival + from a prevertebrate form, which is especially well marked in Ammocœtes. I have already + suggested in this chapter that the homologues of these muscles are represented in Limulus by the + veno-pericardial group of muscles. I will now proceed to deal with the evidence for this + suggestion.</p> + + <p>The structure of the muscle-fibres is peculiar and very characteristic, so that wherever they + occur they are easily recognized. Each fibre consists of a core of granular protoplasm, in the + centre of which the nuclei are arranged in a single row. This core is surrounded by a margin of + striated fibrillæ, as is seen in Fig. <a href="#fig122">122</a>. Such a structure is + characteristic of various forms of striated muscle found in various invertebrates, such as the + muscle-fibre of mollusca. It is, as far as I know, found nowhere in the vertebrate kingdom, except + in Ammocœtes. At transformation these muscles entirely disappear, becoming fattily + degenerated and then absorbed.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig122.png" id="fig122"><img style="width:42%" src="images/fig122.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 122.—A Tubular Muscle-fibre of + Ammocœtes.</span></p> + <p class="sp0">A, portion of fibre seen longitudinally; B, transverse section of fibre (osmic + preparation); the black dots are fat-globules.</p> + </div> + </div> + + <p>For all these reasons they bear the stamp of a survival from a prevertebrate form. This alone + would not make this tissue of any great importance, but when in addition these muscles are found + to be arranged absolutely segmentally throughout the whole of the branchial region, then this + tissue becomes a clue of the highest importance.</p> + + <p>As mentioned in Chapter IV., the segmental muscles of respiration consist of the adductor + muscle and the two constrictor muscles—the <span class="pagenum" + id="page310">{310}</span>striated constrictor and the tubular constrictor. Of these muscles, both + the muscles possessing ordinary striation are attached to the branchial cartilaginous skeleton, + whereas the tubular constrictors have nothing to do with the cartilaginous basket-work, but are + attached ventrally in the neighbourhood of the ventral aorta.</p> + + <p>These segmental tubular muscles are found also in the velar folds—the remains of the + septum or velum which originally separated the oral from the respiratory chamber. In the branchial + region they act with the other constrictors as expiratory muscles, forcing the water out of the + respiratory chamber. In the living Ammocœtes, the velar folds on each side can be seen to + move synchronously with the movements of respiration, contracting at each expiration; they thus + close the slit by which the oral and respiratory chambers communicate, and therefore, in + conjunction with the respiratory muscles, force the water of respiration to flow out through the + gill-slits, as described by Schneider.</p> + + <p>These tubular muscles thus form a dorso-ventral system of muscles essentially connected with + respiration; they belong to each one of the respiratory segments, and are also found in the velum; + anterior to this limit they are not to be found. What, then, are these tubular muscles in the + velar folds? Miss Alcock has worked out their topography by means of serial sections, and, as + already fully explained, has shown that they form exactly similar dorso-ventral groups, which + belong to the two segments anterior to the purely branchial segments, <i>i.e.</i> to the facial or + hyoid segments and the lower lip-segment of the trigeminal nerve. If the velar folds could be put + back into their original position as a septum, then the hyoid or facial group of tubular muscles + would take up exactly the same position as those belonging to each branchial segment.</p> + + <p>The presence of these two so clearly segmental groups of muscles in the velum—the one + belonging to the region of the trigeminal, the other to the region of the facial—is strong + confirmation of my contention that this septum between the oral and respiratory chambers was + caused by the fusion of the last prosomatic and the first mesosomatic appendages, represented in + Limulus by the chilaria and the operculum.</p> + + <p>Yet another clue to the meaning of these muscles is to be found in their innervation, which is + very extraordinary and unexpected. Throughout the branchial region the striated muscles of each + segment <span class="pagenum" id="page311">{311}</span>are strictly supplied by the nerve of that + segment, and, as already described, each segment is as carefully mapped out in its innervation as + it is in any arthropod appendage. One exception occurs to this orderly, symmetrical arrangement: a + nerve arises in connection with the facial nerve, and passes tailwards throughout the whole of the + branchial region, giving off a branch to each segment as it passes. This nerve (<i>Br. prof.</i>, + Fig. <a href="#fig123">123</a>) is known by the name of the <i>ramus branchialis profundus</i> of + the facial, and its extraordinary course has always aroused great curiosity in the minds of + vertebrate anatomists. Miss Alcock, by the laborious method of following its course throughout a + complete series of sections, finds that each of the segmental branches which is given off, passes + into the tubular muscles of that segment (Fig. <a href="#fig124">124</a>). The tubular muscles + which belong to the velum, <i>i.e.</i> those belonging to the lower lip-segment and to the hyoid + segments, receive their innervation from the velar or mandibular nerve, and belong, therefore, to + the trigeminal, not to the facial, system.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig123.jpg" id="fig123"><img style="width:100%" src="images/fig123.jpg" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 123.—Diagram showing the Distribution of the Facial + Nerve.</span></p> + <p class="sp0">Motor branches, <i>red</i>; sensory branches, blue.</p> + </div> + </div> + + <p>The evidence presented by these muscles is as follows<span class="wnw">:—</span></p> + + <p>In the ancestor of the vertebrate there must have existed a segmentally arranged set of + dorso-ventral muscles of peculiar structure, concerned with respiration, and confined to the + mesosomatic segments and to the last prosomatic segment, yet differing from the other + dorso-ventral muscles of respiration in their innervation and their attachment.</p> + + <p>Interpreting these facts with the aid of my theory of the origin of vertebrates, and + remembering that the homologue of the vertebrate ventral aorta in such a palæostracan as Limulus + is the longitudinal <span class="pagenum" id="page312">{312}</span>venous sinus, while the + opercular and chilarial segments are respectively the foremost mesosomatic and the last prosomatic + segments; they signify that the palæostracan ancestor must have possessed a separate set of + segmental dorso-ventral muscles confined to the branchial, opercular and chilarial or metastomal + segments, which, on the one hand, were respiratory in function, and on the other were attached to + the longitudinal venous sinus. Further, these muscles must all have received a nerve-supply from + the neuromeres belonging to the chilarial and opercular segments, an unsymmetrical arrangement of + nerves, on the face of it, very unlikely to occur in an arthropod.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig124.jpg" id="fig124"><img style="width:72%" src="images/fig124.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 124.—Diagram constructed from a series of Transverse Sections + through a Branchial Segment, showing the arrangement and relative positions of the Cartilage, + Muscles, Nerves, and Blood-Vessels.</span></p> + <p class="sp0">Nerves coloured red are the motor nerves to the branchial muscles. Nerves + coloured blue are the internal sensory nerves to the diaphragms and the external sensory + nerves to the sense-organs of the lateral line system. <i>Br. cart.</i>, branchial cartilage; + <i>M. con. str.</i>, striated constrictor muscles; <i>M. con. tub.</i>, tubular constrictor + muscles; <i>M. add.</i>, adductor muscle; <i>D.A.</i>, dorsal aorta; <i>V.A.</i>, ventral + aorta; <i>S.</i>, sense-organs on diaphragm; <i>n. Lat.</i>, lateral line nerve; <i>X.</i>, + epibranchial ganglia of vagus; <i>R. br. prof. VII.</i>, <i>ramus branchialis profundus</i> of + facial; <i>J.v.</i>, jugular vein; <i>Ep. pit.</i>, epithelial pit.</p> + </div> + </div> + + <div><span class="pagenum" id="page313">{313}</span></div> + + <p>Is this prophecy borne out by the examination of Limulus? In the first place, these muscles + were dorso-ventral and segmental, and, referring back to Chapter VII., Lankester arranges the + segmental dorso-ventral muscles in three groups: (1) The dorso-ventral somatic muscles; (2) the + dorso-ventral appendage muscles; and (3) the veno-pericardial muscles. Of these the first group is + represented in the vertebrate by the muscles which move the eye, the second group by the striated + constrictor and adductor muscles and the muscles for the lower lip. There is, then, the + possibility of the third group for this system of tubular muscles.</p> + + <p>Looking first at the structure of these muscles as previously described, so different are they + in appearance from the ordinary muscles of Limulus, that Milne-Edwards, as already stated, called + them "brides transparentes," and did not recognize their muscular character, while Blanchard + called them in the scorpion, "ligaments contractils."</p> + + <p>Consider their attachment and their function. They are attached to the longitudinal sinus, + according to Lankester's observation, in such a way that the muscle-fibres form a hollow cone + filled with blood; when they contract they force this blood towards the gills, and thus act as + accessory or branchial hearts. According to Blanchard, in the scorpion they contract synchronously + with the heart; according to Carlson, in Limulus they contract with the respiratory muscles. In + Ammocœtes, where the respiration is effected after the fashion of Limulus, not of Scorpio, + the tubular muscles are respiratory in function.</p> + + <p>Look at their limits. The veno-pericardial muscles in Limulus are limited by the extent of the + heart, they do not extend beyond the anterior limit of the heart. In Fig. <a href="#fig70">70</a> + (p. <a href="#page176">176</a>) two of these muscles are seen in front of the branchial region + also attached to the longitudinal venous sinus, although in front of the gill-region. In + Ammocœtes the upper limit of the tubular muscles is the group found in the velum; this most + anterior group belongs to a region in front of the branchial region—that of the + trigeminal.</p> + + <p>Moreover, the supposition that the segmental tubular muscles belong throughout to the + veno-pericardial group gives an adequate reason why they do not occur in front of the velum; for, + as their existence is dependent upon the longitudinal collecting sinus in Limulus and Scorpio, + which is represented by the ventral aorta in <span class="pagenum" + id="page314">{314}</span>Ammocœtes, they cannot extend beyond its limits. Now, Dohrn + asserts that the ventral aorta terminates in the spiracular artery, which exists only for a short + time; and, in another place, speaking of this same termination of the ventral aorta, he states: + "Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus arteriosus hervorgeht, + die erste Anlage der Thyroidea umfasst, in der Mesodermfalte des späteren Velums in die Höhe + steigt um in die Aorta der betreffenden Seite einzumunden." These observations show that the + vessel which in Ammocœtes represents the longitudinal collecting sinus in the Merostomata + does not extend further forwards than the velum, and in consequence the representatives of the + veno-pericardial muscles cannot extend into the segments anterior to the velum. One of the + extraordinary characteristics of these tubular muscles which distinguishes them from other + muscles, but brings them into close relationship with the veno-pericardial group, is the manner in + which the bundles of muscle-fibres are always found lying freely in a blood-space; this is clearly + seen in the branchial region, but most strikingly in the velum, the interior of which, apart from + its muco-cartilage, is simply a large lacunar blood-space traversed by these tubular muscles.</p> + + <p>All these reasons point to the same conclusion: the tubular muscles in Ammocœtes are the + successors of the veno-pericardial system of muscles.</p> + + <p>If this is so, then this homology ought to throw light on the extraordinary innervation of + these tubular muscles by the <i>branchialis profundus</i> branch of the facial nerve and the velar + branch of the trigeminal. We ought, in fact, to find in Limulus a nerve arising exclusively from + the ganglia belonging to the chilarial and opercular segments, which, instead of being confined to + those segments, traverses the whole branchial region on each side, and gives off a branch to each + branchial segment; this branch should supply the veno-pericardial muscle of that side.</p> + + <p>Patten and Redenbaugh have traced out the distribution of the peripheral nerves in Limulus, and + have found that from each mesosomatic ganglion a segmental cardiac nerve arises which passes to + the heart and there joins the cardiac median nerve, or rather the median heart-ganglion, for this + so-called nerve is really a mass of ganglion-cells. In all the branchial segments the same plan + exists, each cardiac nerve belonging to that neuromere is strictly segmental. <span + class="pagenum" id="page315">{315}</span>Upon reaching the opercular and chilarial neuromeres an + extraordinary exception is found; the cardiac nerves of these two neuromeres are fused together, + run dorsally, and then form a single nerve called the pericardial nerve, which runs outside the + pericardium along the whole length of the mesosomatic region, and gives off a branch to each of + the cardiac nerves of the branchial neuromeres as it passes them.</p> + + <p>This observation of Patten and Redenbaugh shows that the pericardial nerve of Limulus agrees + with the very nerve postulated by the theory, as far as concerns its origin from the chilarial and + opercular neuromeres, its remarkable course along the whole branchial region, and its segmental + branches to each branchial segment.</p> + + <p>At present the comparison goes no further; there is no evidence available to show what is the + destination of these segmental branches of the pericardial nerve, and so far all evidence of their + having any connection with the veno-pericardial muscles is wanting. Carlson, at my request, + endeavoured in the living Limulus to see whether stimulation of the pericardial nerve caused + contraction of the veno-pericardial muscles, but was unable to find any such effect. On the + contrary, his experimental work indicated that each veno-pericardial muscle received its motor + supply from the corresponding mesosomatic ganglion. This is not absolutely conclusive, for if, as + Blanchard asserts in the case of the scorpion, a close connection exists between the action of + these muscles and of the heart, it is highly probable that their innervation conforms to that of + the heart. Now Carlson has shown that this cardiac nerve from the opercular and chilarial + neuromeres is an inhibitory nerve to the heart, while the segmental cardiac nerves belonging to + the branchial ganglia are the augmentor nerves of the heart.</p> + + <p>His experiments, then, show that the motor nerves of the heart and of the veno-pericardial + muscles run together in the same nerves, but he says nothing of the inhibitory nerves to the + latter muscles. If they exist and if they are in accordance with those to the heart, then they + ought to run in the pericardial nerve, and would naturally reach the veno-pericardial muscles by + the segmental branches of the pericardial nerve.</p> + + <p>Moreover, inhibitory nerves are, in certain cases, curiously associated with sensory fibres; so + that the nerve which corresponds <span class="pagenum" id="page316">{316}</span>to the pericardial + nerve, viz. the <i>branchialis profundus</i> of the facial, may be an inhibitory and sensory + nerve, and not motor at all. Miss Alcock's observations are purely histological; no physiological + experiments have been made.</p> + + <p>At present, then, it does not seem to me possible to say that Carlson's experiments have + disproved <i>any</i> connection of the pericardial nerve with the veno-pericardial muscles. We do + not know what is the destination of its segmental branches; they may still supply the + veno-pericardial muscles even if they do not cause them to contract; they certainly do not appear + to pass directly into them, for they pass into the segmental cardiac nerves, and can only reach + the muscles in conjunction with their motor nerves. Such a course would not be improbable when it + is borne in mind how, in the frog, the augmentor nerves run with the inhibitory along the whole + length of the vagus nerve.</p> + + <p>Until further evidence is given both as to the function of the segmental branches of the + pericardial nerve in the Limulus, and of the <i>branchialis profundus</i> in Ammocœtes, it + is impossible, I think, to consider that the phylogenetic origin of these tubular muscles is as + firmly established as is that of most of the other organs already considered. I must say, my own + bias is strongly in favour of looking upon them as the last trace of the veno-pericardial system + of muscles, a view which is distinctly strengthened by Carlson's statement that the latter system + contracts synchronously with the respiratory movements, for undoubtedly in Ammocœtes their + function is entirely respiratory. Then again, although at present there is no evidence to connect + the pericardial nerve in Limulus with this veno-pericardial system of muscles, yet it is + extraordinarily significant that in such animals as Limulus and Ammocœtes, in both of which + the mesosomatic or respiratory region is so markedly segmental, an intrusive nerve should, in each + case, extend through the whole region, giving off branches to each segment. Still more striking is + it that this nerve should arise from the foremost mesosomatic and the last prosomatic neuromeres + in Limulus—the opercular and chilarial segments—precisely the same neuromeres which + give origin to the corresponding nerve in Ammocœtes, for according to my theory of the + origin of vertebrates, the nerves which supplied the opercular and metastomal appendages have + become the facial nerve and the lower lip-branch of the trigeminal nerve.</p> + + <div><span class="pagenum" id="page317">{317}</span></div> + + <p class="sp3">With the formation of the vertebrate heart from the two longitudinal venous sinuses + and the abolition of the dorsal invertebrate heart, the function of these tubular muscles as + branchial hearts was no longer needed, and their respiratory function alone remained. The last + remnant of this is seen in Ammocœtes, for the ordinary striated muscles were always more + efficient for the respiratory act, and so at transformation the inferior tubular musculature was + got rid of, there being no longer any need for its continued existence.</p> + + <p class="ac"><span class="sc">The Palæostoma, or Old Mouth.</span></p> + + <p>The arrangement of the oral chamber in Ammocœtes is peculiar among vertebrates, and, + upon my theory, is explicable by its comparison with the accessory oral chamber which apparently + existed in Eurypterus. According to this explanation, the lower lip of the original vertebrate + mouth was formed by the coalescence of the most posterior pair of the prosomatic + appendages—the chilaria; from which it follows that the vertebrate mouth was not the + original mouth, but a new structure due to such a formation of the lower lip.</p> + + <p>It is very suggestive that the direct following out of the original working hypothesis should + lead to this conclusion, for it is universally agreed by all morphologists that the present mouth + is a new formation, and Dohrn has argued strongly in favour of the mouth being formed by the + coalescence of a pair of gill-slits. Interpret this in the language of my theory, and immediately + we see, as already explained, gill-slits must mean in this region the spaces between appendages + which did not carry gills; the mouth, therefore, was formed by the coalescence of a pair of + appendages to form a lower lip just as I have pointed out.</p> + + <p>Where, then, must we look for the palæostoma, or original mouth? Clearly, as already suggested, + it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of + Ammocœtes was originally the tube of the hypophysis, so that the following out of the + theory points directly to the tube of the hypophysis as the place where the palæostoma must be + looked for.</p> + + <p>This conclusion is not only not at variance with the opinions of morphologists, but gives a + straightforward, simple explanation why the palæostoma was situated in the very place where they + are most inclined to locate it. Thus, if we trace the history of the question, <span + class="pagenum" id="page318">{318}</span>we see that Dohrn's original view of the comparison of + the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by + the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the + dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis. + This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in + 1888, holding the view that the vertebrates were derived from annelids which had lost their + supra-œsophageal ganglia, and that, therefore, there was no question of an + œsophageal tube piercing the central nervous system of the vertebrate, explained the close + connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis + with the annelidan mouth, so that the infundibular or so-called nervous portion was a special + nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many + annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or + palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening + is the palæostoma. basing this view largely upon his observations on Ammocœtes and + Acipenser.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig125.png" id="fig125"><img style="width:100%" src="images/fig125.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 125.—Diagram to show the Meeting of the Four Tubes + in such a Vertebrate as the Lamprey.</span></p> + <p class="sp0"><i>Nc.</i>, neural canal with its infundibular termination; <i>Nch.</i>, + notochord; <i>Al.</i>, alimentary canal with its anterior diverticulum; <i>Hy.</i>, + hypophysial or nasal tube; <i>Or.</i>, oral chamber closed by septum.</p> + </div> + </div> + + <p>As is seen in Fig. <a href="#fig125">125</a>, the position of this palæostoma is a very + suggestive one. At this single point in Ammocœtes, four separate tubes terminate; here is + the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal + tube or tube <span class="pagenum" id="page319">{319}</span>of the hypophysis, and the pre-oral + elongation of the alimentary canal.</p> + + <p>It is perfectly simple and easy for the olfactory tube to open into any one of the other three. + By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by + opening into the gut it produces the actual condition of things seen in Myxine and other + vertebrates; by opening into the notochordal tube it would produce a transitional condition + between the other two.</p> + + <p>The view held by Kupffer is that this nasal tube (tube of the hypophysis) opened into the + anterior diverticulum of the vertebrate gut, and was for this reason the original mouth-tube; then + a new mouth was formed, and this connection was closed, being subsequently reopened as in Myxine. + My view is that this tube originally opened into the infundibulum, in other words, into the + original gut of the palæostracan ancestor, and was for this reason the original mouth-tube, in the + same sense as the olfactory passage of the scorpion may be, and often is, called the mouth-tube. + When, with the breaking through of the septum between the oral and respiratory chambers, the + external opening of the oral chamber became a new mouth, the old mouth was closed but the + olfactory tube still remained, owing to the importance of the sense of smell. Subsequently, as in + Myxine and the higher vertebrates, it opened into the pharynx, and so formed the nose of the + higher vertebrates.</p> + + <p class="sp3">It is not, to my mind, at all improbable that during the transition stage, between + its connection with the old alimentary canal, as in Eurypterus or the scorpions, and its blind + ending, as in Ammocœtes, the nasal tube opened into the tube of the notochord. This + question will be discussed later on when the probable significance of the notochord is + considered.</p> + + <p class="ac"><span class="sc">The Pituitary Gland.</span></p> + + <p>Turning back to the comparison of Fig. <a href="#fig106">106</a>, B, and Fig. <a + href="#fig106">106</a>, C, which represent respectively an imaginary sagittal section through an + Eurypterus-like animal and through Ammocœtes at a larval stage, all the points for + comparison mentioned on p. <a href="#page244">244</a> have now been discussed with the exception + of the suggested homology between the coxal glands of the one animal and the pituitary body of the + other.</p> + + <div><span class="pagenum" id="page320">{320}</span></div> + + <p>This latter gland undoubtedly arises posteriorly to the hypophysial tube, or Rathke's pouch (as + it is sometimes called), and, as already mentioned, is supposed by Kupffer to be formed from the + posterior wall of this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who has + investigated this matter, finds that the glandular hypophysis is not formed from the walls of + Rathke's pouch, but from the tissue of the rudimentary connection or stalk between the two + premandibular cavities, which becomes closely connected with the posterior wall of Rathke's pouch, + and becoming cut off from the rest of the premandibular cavity on each side, becomes permanently a + part of the 'Hypophysis Anlage.'</p> + + <p>The importance of Nusbaum's investigation consists in this, that he derives the glandular + hypophysis from the connecting stalk between the two premandibular cavities, and therefore from + the walls of the ventral continuation of this cavity on each side.</p> + + <p>This may be expressed as follows:—</p> + + <p>The cœlomic cavity, known as the premandibular cavity, divides into a dorsal and a + ventral part; the walls of the dorsal part give origin to the somatic muscles belonging to the + oculomotor nerve, while the walls of the ventral part on each side form the connecting stalk + between the two cavities, and give origin to the glandular hypophysis.</p> + + <p>Now, as already pointed out, the premandibular cavity is homologous with the 2nd prosomatic + cœlomic cavity of Limulus, and this 2nd prosomatic cœlomic cavity divides, according + to Kishinouye, into a dorsal and a ventral part; and, further, the walls of this ventral part form + the coxal gland. Both in the vertebrate, then, and in Limulus, we find a marked glandular tissue + in a corresponding position, and the conclusion is forced upon us that the glandular hypophysis + was originally the coxal gland of the invertebrate ancestor. As in all other cases already + considered, when the facts of topographical anatomy, of morphology and of embryology, all combine + to the same conclusion as to the derivation of the vertebrate organ from that of the invertebrate, + then there must be also a structural similarity between the two. What, then, is the nature of the + coxal gland in the scorpions and Limulus? Lankester's paper gives us full information on this + point as far as the scorpion and Limulus are concerned, and he shows that the coxal gland of + Limulus differs markedly from that of Scorpio in the size of the cells and in the <span + class="pagenum" id="page321">{321}</span>arrangement of the tubes. In Fig. <a + href="#fig126">126</a>, A, I give a picture of a piece of the coxal gland of Limulus taken from + Lankester's paper.</p> + + <p>Turning now to the vertebrate, Bela Haller's paper gives us a number of pictures of the + glandular hypophysis from various vertebrates, and he especially points out the tubular nature of + the gland and its solidification in the course of development in some cases. In Fig. <a + href="#fig126">126</a>, B, I give his picture of the gland in Ammocœtes.</p> + + <p>The striking likeness between Haller's picture and Lankester's picture is apparent on the face + of it, and shows clearly that the histological structure of the glands in the two cases confirms + the deductions drawn from their anatomical and morphological positions.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig126.png" id="fig126"><img style="width:100%" src="images/fig126.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 126.—A, Section of Coxal Gland of Limulus</span> (from <span + class="sc">Lankester</span>); <span class="sc">B, Section of Pituitary Body of + Ammocœtes</span> (from <span class="sc">Bela Haller</span>).</p> + <p class="sp0"><i>n.a.</i>, termination of nasal passage.</p> + </div> + </div> + + <p>The sequence of events which gave rise to the pituitary body of the vertebrate was in all + probability somewhat as follows<span class="wnw">:—</span></p> + + <p>Starting with the excretory glands of the Phyllopoda, known as shell-glands, which existed + almost certainly in the phyllopod Trilobite, we pass to the coxal gland of the Merostomata. + Judging from Limulus, these were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor + appendages. When these appendages became reduced in size and purely tactile they were compressed + and concentrated round the mouth region, forming the endognaths of the Merostomata; as a necessary + consequence of the concentration of the coxæ of the endognaths, the coxal gland also became + concentrated, <span class="pagenum" id="page322">{322}</span>and took up a situation close against + the pharynx, as represented in Fig. <a href="#fig106">106</a>, B. When, then, the old mouth + closed, and the pharynx became the <i>saccus vasculosus</i>, the coxal gland remained in close + contact with the <i>saccus vasculosus</i>, and became the pituitary body, thus giving the reason + why there is always so close a connection between the pituitary body and the infundibular + region.</p> + + <p>Whatever was the condition of the digestive tracts at the transition stage between the + arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was + ultimately closed. The method of its closure was exceedingly simple and evident. The membranous + cranium was in process of formation by the extension of the plastron laterally and dorsally; a + slight growth of the same tissue in the region of the mouth would suffice to close it and thus + separate the infundibulum from the olfactory tube. As evidence that such was the method of + closure, it is instructive to see how in Ammocœtes the glandular tissue of the pituitary + body is embedded in and mixed up with the tissue of this cranial wall; how the termination of the + nasal tube is embedded in this same thickened mass of the cranial wall—how, in fact, both + coxal gland and olfactory tube have become involved in the growth of the tissue of the plastron, + by means of which the mouth was closed.</p> + + <p>I have now passed in review the nature of the evidence which justifies a comparison between the + segments supplied by the cranial nerves of the vertebrate and the prosomatic and mesosomatic + segments of the palæostracan. For the convenience of my readers I have put these conclusions into + tabular form (see p. 323), for all the segments as far as that supplied by the glossopharyngeal + nerves. In both vertebrate and invertebrate this is a fixed position, for in the former, however + variable may be the number of branchial segments which the vagus supplies, the second branchial + segment is always supplied by a separate nerve, the glossopharyngeal, and in the latter, though + the number of segments bearing branchiæ varies, the minimum number of such segments (as seen in + the Pedipalpi) is never less than two.</p> + + <div><span class="pagenum" id="page323">{323}</span></div> + + <p class="ac"><span class="sc">Table of Comparison of Corresponding Segments in the Eurypterids + and in Ammocœtes</span><br/> + (<i>i.e.</i> <span class="sc">in Cephalaspids</span>).</p> + + <table class="sp2 w100 ba nothand" title="Corresponding Segments in Eurypterids + and Cephalaspids" summary="Corresponding Segments in Eurypterids + and Cephalaspids"> + <tr> + <th rowspan="3" class="smaller ba vmi ac"><img src="images/supraoes.png" style="width:1.25em" + alt="supraoes"/></th> + <th colspan="5" class="smaller ba">Median Eyes.</th> + <th colspan="4" class="smaller ba">Pineal Eyes.</th> + <th colspan="2" class="smaller ba">Pineal Nerve.</th> + <th rowspan="3" class="smaller ba"><img src="images/suprainf.png" style="width:1.25em" + alt="suprainf"/></th> + </tr> + <tr> + <th colspan="5" class="smaller ba">Lateral Eyes.</th> + <th colspan="4" class="smaller ba">Lateral Eyes.</th> + <th colspan="2" class="smaller ba">II</th> + </tr> + <tr> + <th colspan="5" class="smaller ba">Camerostome.</th> + <th colspan="4" class="smaller ba">Olfactory Organ.</th> + <th colspan="2" class="smaller ba">I</th> + </tr> + <tr> + <td></td> + <td colspan="5" class="smaller ac ba">Invertebrate (Limulus or Eurypterid).</td> + <td colspan="7" class="smaller ac ba">Vertebrate (Ammocœtes or Cephalaspid).</td> + </tr> + <tr> + <th rowspan="12" class="smaller ba"><img src="images/infraoes.png" style="width:1.25em" + alt="infraoes"/></th> + <th rowspan="9" class="smaller ba"><img src="images/prosomatic.png" style="width:1.25em" + alt="prosomatic"/></th> + <th rowspan="2" class="smaller ba">Segments.</th> + <td colspan="2" class="smaller ac vmi ba">Appendages.</td> + <th rowspan="2" class="smaller ba">Cœlomic<br/> + cavities</th> + <th rowspan="2" class="smaller ba">Appendages.</th> + <th rowspan="2" class="smaller ba">Splanchnic<br/> + Nerves.</th> + <td rowspan="2" class="smaller ac vmi ba">Somatic<br/> + Segmental<br/> + Muscles.</td> + <th rowspan="2" class="smaller ba">Somatic<br/> + Nerves.</th> + <th rowspan="2" class="smaller ba">Cœlomic<br/> + Cavities.</th> + <th rowspan="2" class="smaller ba">V. Wijhe's<br/> + Segments.</th> + <th rowspan="12" class="smaller ba"><img src="images/infrainf.png" style="width:1.25em" + alt="infrainf"/></th> + </tr> + <tr> + <td class="smaller ac vmi ba">Limulus.</td> + <td class="smaller ac vmi ba">Eurypterid.</td> + </tr> + <tr> + <td class="smaller ac ba">1</td> + <td class="smaller ac ba">Cheliceræ</td> + <td class="smaller ac ba">Cheliceræ</td> + <td class="smaller ac ba">1</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">Anterior.</td> + <td class="smaller ac ba">—</td> + </tr> + <tr> + <td class="smaller ac ba">2</td> + <td class="smaller ba">1st Locomotor</td> + <td rowspan="4" class="ac vmi ba" style="padding:0"> + <table title="Corresponding Segments in Eurypterids + and Cephalaspids" summary="Corresponding Segments in Eurypterids + and Cephalaspids"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace4.png" style="height:10.3ex; width:1em;" + alt="brace" /></td> + <td class="smaller vmi pl0">4 Endognaths</td> + </tr> + </table> + </td> + <td rowspan="4" class="smaller ac vmi ba">2<br/> + Ventral part forms coxal gland.</td> + <td rowspan="4" class="smaller ac vmi ba">4 tentacles and upper lip.</td> + <td rowspan="4" class="smaller ac vmi ba">V<br/> + Tentacular and upper lip nerve.</td> + <td rowspan="4" class="smaller ac vmi ba">Muscles supplied by oculomotor nerve.</td> + <td rowspan="4" class="smaller ac vmi ba">III</td> + <td rowspan="4" class="smaller ac vmi ba">Premandibular.<br/> + Ventral part forms pituitary body.</td> + <td rowspan="4" class="smaller ac vmi ba">1</td> + </tr> + <tr> + <td class="smaller ac ba">3</td> + <td class="smaller ba">2nd <span class="hid">Loct</span>"</td> + </tr> + <tr> + <td class="smaller ac ba">4</td> + <td class="smaller ba">3rd <span class="hid">Loco</span>"</td> + </tr> + <tr> + <td class="smaller ac ba">5</td> + <td class="smaller ba">4th <span class="hid">Loco</span>"</td> + </tr> + <tr> + <td class="smaller ac vmi ba">6</td> + <td class="smaller vmi ba">5th <span class="hid">Loco</span>"</td> + <td class="smaller ac vmi ba">Ectognath</td> + <td class="smaller ac vmi ba">3</td> + <td class="smaller ac vmi ba">Tongue</td> + <td class="smaller ac vmi ba">V<br/> + Tongue nerve</td> + <td class="smaller ac vmi ba">Sup. oblique</td> + <td class="smaller ac vmi ba">IV</td> + <td class="smaller ac vmi ba">Mandibular</td> + <td rowspan="2" class="vmi ba" style="padding:0"> + <table class="w100" title="Corresponding Segments in Eurypterids + and Cephalaspids" summary="Corresponding Segments in Eurypterids + and Cephalaspids"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace4.png" style="height:9.5ex; width:1em;" + alt="brace" /></td> + <td class="smaller vmi pl0 ac w80">2</td> + </tr> + </table> + </td> + </tr> + <tr> + <td class="smaller ac vmi ba">7</td> + <td class="smaller ac vmi ba">Chilaria</td> + <td class="smaller ac vmi ba">Metastoma</td> + <td class="smaller ac vmi ba">4</td> + <td class="smaller ac vmi ba">Lower lip</td> + <td class="smaller ac vmi ba">V<br/> + Lower lip nerve</td> + <td class="ba"></td> + <td class="ba"></td> + <td class="smaller ac vmi ba">Mandibular</td> + </tr> + <tr> + <td rowspan="3" class="smaller ac vmi ba"><img src="images/mesosomatic.png" + style="width:1.25em" alt="mesosomatic"/></td> + <td class="smaller ac vmi ba">8</td> + <td class="smaller ac vmi ba">Operculum</td> + <td rowspan="2" class="vmi ba" style="padding:0"> + <table class="w100" title="Corresponding Segments in Eurypterids + and Cephalaspids" summary="Corresponding Segments in Eurypterids + and Cephalaspids"> + <tr> + <td class="smaller vmi pr0 ac w80">Genital</td> + <td rowspan="2" class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; + width:1em;" alt="brace" /></td> + <td rowspan="2" class="smaller vmi pl0 ac w80">Operculum</td> + </tr> + <tr> + <td class="smaller vmi pr0 pt1 pb1 ac w80">Branchial</td> + </tr> + </table> + </td> + <td class="smaller ac vmi ba">5</td> + <td class="smaller ac vmi ba">Thyroid</td> + <td rowspan="2" class="vmi ba" style="padding:0"> + <table class="w100" title="Corresponding Segments in Eurypterids + and Cephalaspids" summary="Corresponding Segments in Eurypterids + and Cephalaspids"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; width:1em;" + alt="brace" /></td> + <td class="smaller vmi pl0 ac w80">VII</td> + </tr> + </table> + </td> + <td rowspan="2" class="smaller ac vmi ba">Ext. rectus<br/> + Retract oculi</td> + <td rowspan="2" class="smaller ac vmi ba">VI</td> + <td class="smaller ac vmi ba">Hyoid<sub>1</sub></td> + <td class="smaller ac vmi ba">3</td> + </tr> + <tr> + <td class="smaller ac vmi ba">9</td> + <td class="smaller vmi ba">1st Branchial</td> + <td class="smaller ac vmi ba">6</td> + <td class="smaller ac vmi ba">Hyoid or 1st Branchial</td> + <td class="smaller ac vmi ba">Hyoid<sub>2</sub></td> + <td class="smaller ac vmi ba">4</td> + </tr> + <tr> + <td class="smaller ac vmi ba">10<span class="hid">0</span></td> + <td class="smaller vmi ba">2nd <span class="hid">Bra</span>"</td> + <td class="smaller ac vmi ba">2nd Branchial</td> + <td class="smaller ac vmi ba">7</td> + <td class="smaller ac vmi ba">Branchial</td> + <td class="smaller ac vmi ba">IX</td> + <td class="smaller ac vmi ba">—</td> + <td class="smaller ac vmi ba">—</td> + <td class="smaller ac vmi ba">2nd Branchial</td> + <td class="smaller ac vmi ba">5</td> + </tr> + </table> + +<!-- Left half for handheld --> + + <table class="sp2 w100 handonly" title="Corresponding Segments in Eurypterids + and Cephalaspids (1 of 2)" summary="Corresponding Segments in Eurypterids + and Cephalaspids (1 of 2)"> + <tr> + <th rowspan="3" class="smaller ba vmi ac"><img src="images/supraoes.png" style="width:1.25em" + alt="supraoes"/></th> + <th colspan="5" class="smaller ba">Median Eyes.</th> + </tr> + <tr> + <th colspan="5" class="smaller ba">Lateral Eyes.</th> + </tr> + <tr> + <th colspan="5" class="smaller ba">Camerostome.</th> + </tr> + <tr> + <td class="ba"></td> + <td colspan="5" class="smaller ac ba">Invertebrate (Limulus or Eurypterid).</td> + </tr> + <tr> + <th rowspan="12" class="smaller ba"><img src="images/infraoes.png" style="width:1.25em" + alt="infraoes"/></th> + <th rowspan="9" class="smaller ba"><img src="images/prosomatic.png" style="width:1.25em" + alt="prosomatic"/></th> + <th rowspan="2" class="smaller ba">Seg.</th> + <td colspan="2" class="smaller ac vmi ba">Appendages.</td> + <th rowspan="2" class="smaller ba">Cœlomic<br/> + cavities</th> + </tr> + <tr> + <td class="smaller ac vmi ba">Limulus.</td> + <td class="smaller ac vmi ba">Eurypterid.</td> + </tr> + <tr> + <td class="smaller ac ba">1</td> + <td class="smaller ac ba">Cheliceræ</td> + <td class="smaller ac ba">Cheliceræ</td> + <td class="smaller ac ba">1</td> + </tr> + <tr> + <td class="smaller ac ba">2</td> + <td class="smaller ba">1st Locomotor</td> + <td rowspan="4" class="ac vmi ba" style="padding:0"> + <table title="Corresponding Segments in Eurypterids + and Cephalaspids (1 of 2)" summary="Corresponding Segments in Eurypterids + and Cephalaspids (1 of 2)"> + <tr> + <td class="vmi pl0 pr0"><img src="images/rbrace4.png" style="height:10.3ex; width:1em;" + alt="brace" /></td> + <td class="smaller vmi pl0">4 Endognaths</td> + </tr> + </table> + </td> + <td rowspan="4" class="smaller ac vmi ba">2<br/> + Ventral part forms coxal gland.</td> + </tr> + <tr> + <td class="smaller ac ba">3</td> + <td class="smaller ba">2nd <span class="hid">Loct</span>"</td> + </tr> + <tr> + <td class="smaller ac ba">4</td> + <td class="smaller ba">3rd <span class="hid">Loco</span>"</td> + </tr> + <tr> + <td class="smaller ac ba">5</td> + <td class="smaller ba">4th <span class="hid">Loco</span>"</td> + </tr> + <tr> + <td class="smaller ac vmi ba">6</td> + <td class="smaller vmi ba">5th <span class="hid">Loco</span>"</td> + <td class="smaller ac vmi ba">Ectognath</td> + <td class="smaller ac vmi ba">3</td> + </tr> + <tr> + <td class="smaller ac vmi ba">7</td> + <td class="smaller ac vmi ba">Chilaria</td> + <td class="smaller ac vmi ba">Metastoma</td> + <td class="smaller ac vmi ba">4</td> + </tr> + <tr> + <td rowspan="3" class="smaller ac vmi ba"><img src="images/mesosomatic.png" + style="width:1.25em" alt="mesosomatic"/></td> + <td class="smaller ac vmi ba">8</td> + <td class="smaller ac vmi ba">Operculum</td> + <td rowspan="2" class="vmi ba" style="padding:0"> + <table class="w100" title="Corresponding Segments in Eurypterids + and Cephalaspids (1 of 2)" summary="Corresponding Segments in Eurypterids + and Cephalaspids (1 of 2)"> + <tr> + <td class="smaller vmi pr0 ac w80">Genital</td> + <td rowspan="2" class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; + width:1em;" alt="brace" /></td> + <td rowspan="2" class="smaller vmi pl0 ac w80">Opc.</td> + </tr> + <tr> + <td class="smaller vmi pr0 pt1 pb1 ac w80">Branchial</td> + </tr> + </table> + </td> + <td class="smaller ac vmi ba">5</td> + </tr> + <tr> + <td class="smaller ac vmi ba">9</td> + <td class="smaller vmi ba">1st Branchial</td> + <td class="smaller ac vmi ba">6</td> + </tr> + <tr> + <td class="smaller ac vmi ba">10<span class="hid">0</span></td> + <td class="smaller vmi ba">2nd <span class="hid">Bra</span>"</td> + <td class="smaller ac vmi ba">2nd Branchial</td> + <td class="smaller ac vmi ba">7</td> + </tr> + </table> + +<!-- Right half for handheld --> + + <table class="sp2 w100 handonly" title="Corresponding Segments in Eurypterids + and Cephalaspids (2 of 2)" summary="Corresponding Segments in Eurypterids + and Cephalaspids (2 of 2)"> + <tr> + <td rowspan="5" class="ba"></td> + <th colspan="4" class="smaller ba">Pineal Eyes.</th> + <th colspan="2" class="smaller ba">Pineal Nerve.</th> + <th rowspan="3" class="smaller ba"><img src="images/suprainf.png" style="width:1.25em" + alt="suprainf"/></th> + </tr> + <tr> + <th colspan="4" class="smaller ba">Lateral Eyes.</th> + <th colspan="2" class="smaller ba">II</th> + </tr> + <tr> + <th colspan="4" class="smaller ba">Olfactory Organ.</th> + <th colspan="2" class="smaller ba">I</th> + </tr> + <tr> + <td colspan="7" class="smaller ac ba">Vertebrate (Ammocœtes or Cephalaspid).</td> + </tr> + <tr> + <th class="smaller ba">Apps.</th> + <th class="smaller ba">Splanchnic<br/> + Nerves.</th> + <td class="smaller ac vmi ba">S.S.M.</td> + <th class="smaller ba">Somatic<br/> + Nerves.</th> + <th class="smaller ba">Cœlomic<br/> + Cavities.</th> + <th class="smaller ba">VW.<br/> + S.</th> + <th rowspan="11" class="smaller ba"><img src="images/infrainf.png" style="width:1.25em" + alt="infrainf"/></th> + </tr> + <tr> + <td class="smaller ar ba">1</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">—</td> + <td class="smaller ac ba">Anterior.</td> + <td class="smaller ac ba">—</td> + </tr> + <tr> + <td class="smaller ar ba">2</td> + <td rowspan="4" class="smaller ac vmi ba">4 tentacles and upper lip.</td> + <td rowspan="4" class="smaller ac vmi ba">V<br/> + Tentacular and upper lip nerve.</td> + <td rowspan="4" class="smaller ac vmi ba">Muscles supplied by oculo-<br/> + motor nerve.</td> + <td rowspan="4" class="smaller ac vmi ba">III</td> + <td rowspan="4" class="smaller ac vmi ba">Pre-<br/> + mandibular.<br/> + Ventral part forms pituitary body.</td> + <td rowspan="4" class="smaller ac vmi ba">1</td> + </tr> + <tr> + <td class="smaller ar ba">3</td> + </tr> + <tr> + <td class="smaller ar ba">4</td> + </tr> + <tr> + <td class="smaller ar ba">5</td> + </tr> + <tr> + <td class="smaller ar vmi ba">6</td> + <td class="smaller ac vmi ba">Tongue</td> + <td class="smaller ac vmi ba">V<br/> + Tongue nerve</td> + <td class="smaller ac vmi ba">Sup. oblique</td> + <td class="smaller ac vmi ba">IV</td> + <td class="smaller ac vmi ba">Mandibular</td> + <td rowspan="2" class="smaller ac vmi ba">2</td> + </tr> + <tr> + <td class="smaller ar vmi ba">7</td> + <td class="smaller ac vmi ba">Lower lip</td> + <td class="smaller ac vmi ba">V<br/> + Lower lip nerve</td> + <td class="ba"></td> + <td class="ba"></td> + <td class="smaller ac vmi ba">Mandibular</td> + </tr> + <tr> + <td class="smaller ar vmi ba">8</td> + <td class="smaller ac vmi ba">Thyroid</td> + <td rowspan="2" class="smaller ac vmi ba">VII</td> + <td rowspan="2" class="smaller ac vmi ba">Ext. rectus<br/> + Retract oculi</td> + <td rowspan="2" class="smaller ac vmi ba">VI</td> + <td class="smaller ac vmi ba">Hyoid<sub>1</sub></td> + <td class="smaller ac vmi ba">3</td> + </tr> + <tr> + <td class="smaller ar vmi ba">9</td> + <td class="smaller ac vmi ba">Hyoid or 1st Branchial</td> + <td class="smaller ac vmi ba">Hyoid<sub>2</sub></td> + <td class="smaller ac vmi ba">4</td> + </tr> + <tr> + <td class="smaller ar vmi ba">10</td> + <td class="smaller ac vmi ba">Branchial</td> + <td class="smaller ac vmi ba">IX</td> + <td class="smaller ac vmi ba">—</td> + <td class="smaller ac vmi ba">—</td> + <td class="smaller ac vmi ba">2nd Branchial</td> + <td class="smaller ac vmi ba">5</td> + </tr> + <tr> + <td class="smaller" colspan="8">S.S.M. = Somatic Segmental Muscles. VW.S. = V. Wijhe's + Segments.</td> + </tr> + </table> + +<!-- End of split table for handheld --> + + <div><span class="pagenum" id="page324">{324}</span></div> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The general consideration of the evidence of the number of segments, and their nature in the + pro-otic region of the vertebrate, as given in the last chapter, is not incompatible with the + view that the trigeminal nerve originally supplied seven appendages, which appendages did not + carry branchiæ, but were originally used for purposes of locomotion as well as of + mastication.</p> + <p>Such appendages clearly no longer exist in the higher vertebrates, the muscles of mastication + only remaining; but in the earliest fish-forms they must have existed, as, indeed, is seen in + <span class="correction" title="Original reads 'Ptericthys'.">Pterichthys</span> and + Bothriolepis. Judging from all the previous evidence some signs of their existence may + reasonably be expected still to remain in Ammocœtes. Such is indeed the case.</p> + <p>In the adult Petromyzon the trigeminal nerve innervates specially a massive suctorial + apparatus, by means of which it holds on to other fishes, or to stones in the bottom of the + stream. There is here no apparent sign of appendages. Very great, however, is the difference in + the oral chamber of Ammocœtes; here there is no sign of any suctorial apparatus, but + instead, a system of tentacles, together with the remains of the septum or velum, which + originally closed off the oral from the respiratory chamber. These tentacles are the last + remnants of the original foremost prosomatic appendages of the palæostracan ancestor. Like the + lateral eyes they do not develop until the transformation comes, but during the whole larval + condition their musculature remains in an embryonic condition, and then from these embryonic + muscles the whole massive musculature of the suctorial apparatus develops; a sucking apparatus + derived from the modification of appendages, as so frequently occurs in the arthropods.</p> + <p>The study of Ammocœtes indicates that the velum and lower lip correspond to the + metastoma of the Eurypterid, <i>i.e.</i> the chilaria of Limulus, while the large ventral pair + of tentacles, called the tongue, correspond to the ectognaths of the Eurypterids, and probably + to the oar-like appendages of <span class="correction" + title="Original reads 'Ptericthys'.">Pterichthys</span> and Bothriolepis. From these two + splanchnic segments the suctorial apparatus in the main arises; the motor supply of these two + segments forms the mass of the trigeminal nerve-supply, and the nerves supplying them, the velar + nerve and the tongue-nerve, are markedly separate from the rest of the trigeminal nerve.</p> + <p>The rest of the tentacles present much less the sign of independent segments. In their + nerves, their muco-cartilaginous skeleton, and their rudimentary muscles, they indicate a + concentration and amalgamation, such as might be expected from the concentrated endognaths. The + continuation of the dwindling process, already initiated in the Eurypterid, would easily result + in the tentacles of Ammocœtes.</p> + <p>The nasal tube of Ammocœtes, which originates in the hypophysial tube, corresponds + absolutely in position and in its original structure, to the olfactory tube of a scorpion-like + animal. From this homology two conclusions of importance follow: (1) the old mouth, or + palæostoma, of the vertebrate was situated at the end of this tube, therefore, at the + termination of the infundibulum; (2) the upper lip, which by its growth, brings the olfactory + tube from a ventral to a dorsal position, was originally formed by the foremost sternites or + endostoma, or else by the sterno-coxal processes of the second pair of prosomatic appendages of + the palæostracan ancestor.</p> + <p>In strict accordance with the rest of the comparisons made in this region, the pituitary body + shows by similarity of structure, as well as of position, that it arose from the coxal glands, + which were situated at the base of the four endognaths.</p> + <div><span class="pagenum" id="page325">{325}</span></div> + <p>One after another, when once the clue has been found, all these mysterious organs of the + vertebrate, such as the pituitary and thyroid glands, fall harmoniously into their place as the + remnants of corresponding important organs in the palæostraca.</p> + <p>Yet another clue is afforded by the tubular muscles of Ammocœtes, that strange set of + non-vertebrate striated muscles, which are so markedly arranged in a segmental manner, which + disappear at transformation, and are never found in any of the higher vertebrates, for the + limits of their distribution correspond to the veno-pericardial muscles of Limulus.</p> + <p>Their nerve-supply in Ammocœtes is most extraordinary; for, although they are + segmentally arranged throughout the whole respiratory region, which is segmentally supplied by + the VIIth, IXth, and Xth nerves, and are found in front of this region only in one segment, that + of the lower lip, which is supplied by the velar branch of the Vth nerve, yet they are not + supplied segmentally, but only by the velar nerve and a branch of the VIIth, the <i>ramus + branchialis profundus</i>. This latter nerve extends throughout the respiratory region, and + gives off segmental branches to supply these muscles.</p> + <p class="sp0">It is also a curious coincidence that in such a markedly segmented animal as + Limulus, a nerve—the pericardial nerve—which arises from the nerves of the chilarial + and opercular segments, should pass along the whole respiratory region and give off branches to + each mesosomatic segment. It is strange, to say the least of it, that the chilarial or + metastomal and the opercular segments of Limulus should, on the theory advocated in this book, + correspond to the lower lip and hyoid segments of the vertebrate. At present the homology + suggested is not complete, for there is no evidence as yet that the veno-pericardial muscles + have anything to do with the pericardial nerve.</p> + </div> + + <div><span class="pagenum" id="page326">{326}</span></div> + + <p class="ac">CHAPTER X</p> + + <p class="ac"><i>THE RELATIONSHIP OF AMMOCŒTES TO THE MOST ANCIENT FISHES—THE + OSTRACODERMATA</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">The nose of the Osteostraci.—Comparison of head-shield of Ammocœtes + and of Cephalaspis.—Ammocœtes the only living representative of these ancient + fishes.—Formation of cranium.—Closure of old mouth.—Rohon's primordial + cranium.—Primordial cranium of Phrynus and Galeodes.—Summary.</p> + </div> + + <p>The shifting of the orifice of the olfactory passage, which led to the old mouth, from the + ventral to the dorsal side, as seen in the transformation of the ventrally situated hypophysial + tube of the young Ammocœtes, to the dorsally situated nasal tube of the full-grown + Ammocœtes, affords one of the most important clues in the whole of this story of the origin + of vertebrates; for, if Ammocœtes is the nearest living representative of the first-formed + fishes, then we ought to expect to find that the dorsal head-shield of such fishes is + differentiated from that of the contemporary Palæostraca by the presence of a median frontal + opening anterior to the eyes. Conversely, if such median nasal orifice is found to be a marked + characteristic of the group, in combination with lateral and median eyes, as in Ammocœtes, + then we have strong reasons for interpreting these head-shields by reference to the head of + Ammocœtes.</p> + + <p class="sp3">The oldest known fishes belong to a large group of strange forms which inhabited + the Silurian and Devonian seas, classed together by Smith Woodward under the name of Ostracodermi. + These are divided into three orders: (1) the Heterostraci, including one family, the Pteraspidæ, + to which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into two families, the + Cephalaspidæ and Tremataspidæ, which include Cephalaspis, Eukeraspis, Auchenaspis or Thyestes, and + Tremataspis; and (3) the Antiarcha, with one family, the Astrolepidæ, including Astrolepis, + Pterichthys, and Bothriolepis. <span class="pagenum" id="page327">{327}</span>Of these, the first + two orders belong to the Upper Silurian, while the third is Devonian.</p> + + <p class="ac"><span class="sc">The Dorsal Head-Shield of the Osteostraci.</span></p> + + <p>Of the three orders above-named, the Heterostraci and Osteostraci are the oldest, and among + them the Cephalaspidæ have afforded the most numerous and best worked-out specimens. At + Rootziküll, in the island of Œsel, the form known as <i>Thyestes (Auchenaspis) + verrucosus</i> is especially plentiful, being found thickly present in among the masses of + Eurypterid remains, which give the name to the deposit. Of late years this species has been + especially worked at by Rohon, and many beautiful specimens have been figured by him, so that a + considerable advance has been made in our knowledge since Pander, Eichwald, Huxley, Lankester, and + Schmidt studied these most interesting primitive forms.</p> + + <p>All observers agree that the head-region of these fishes was covered by a dorsal and ventral + head-shield, while the body-region was in most cases unknown, or, as in Eichwald's specimens, and + in the specimens figured in Lankester and Smith Woodward's memoirs, was made up of segments which + were not vertebral in character, but formed an aponeurotic skeleton, being the hardened + aponeuroses between the body-muscles. This body-skeleton, which possesses its exact counterpart in + Ammocœtes, will be considered more fully when I discuss the origin of the spinal region of + the vertebrate.</p> + + <p>Of the two head-shields, ventral and dorsal, the latter is best known and characterizes the + group. It consists of a dorsal plate, with characteristic horns, which in <i>Thyestes + verrucosus</i> (Fig. <a href="#fig128">128</a>), as described by Rohon, is composed of two parts, + a frontal part and an occipital part (<i>occ.</i>), the occipital part being composed of segments, + and possessing a median ridge—the <i>crista occipitalis</i>. In Lankester's memoir and in + Smith Woodward's catalogue, a large number of known forms are described and delineated, and we may + perhaps say that in some of the forms, such as <i>Eukeraspis pustuliferus</i> (Fig. <a + href="#fig127">127</a>, B), the frontal part of the shield only is capable of preservation as a + fossil, while in Cephalaspis (Fig. <a href="#fig127">127</a>, A) not only the frontal part but a + portion of the occipital region is preserved, the latter being small in extent when compared with + the occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and Didymaspis, the whole + of both frontal <span class="pagenum" id="page328">{328}</span>and occipital region is capable of + preservation, the line of demarcation between these two regions being well marked in the latter + species.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig127.png" id="fig127"><img style="width:100%" src="images/fig127.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 127.—A, Dorsal Head-Shield of Cephalaspis</span> + (from <span class="sc">Lankester</span>); <span class="sc">B, Dorsal Head-Shield of + Keraspis</span> (from <span class="sc">Lankester</span>).</p> + </div> + </div> + + <p>In the best preserved specimens of all this group of fishes a frontal median orifice is always + present; it appears in some specimens obscurely partially divided into two parts. Perhaps the best + specimen of all was obtained by Rohon at Rootziküll, and is thus described by him<span + class="wnw">:—</span></p> + + <p>The frontal part of the dorsal head-plate carried (Fig. <a href="#fig128">128</a>) the two + orbits for the lateral eyes (<i>l.e.</i>), a marked frontal organ (<i>fro.</i>), and a median + depression (<i>gl.</i>), to which he gives the name parietal organ. The occipital part + (<i>occ.</i>) was clearly segmented, and carried, he thinks, the branchiæ. I reproduce Rohon's + figure of the frontal organ in Thyestes (Fig. 129); he describes it as a deeply sunk pit, divided + in the middle by a slit, which leads deeper in, he supposes, towards the central nervous + system.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig128.jpg" id="fig128"><img style="width:100%" src="images/fig128.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 128.</span>—<span class="sc">Dorsal Head-Shield of</span> + <i>Thyestes (Auchenaspis) verrucosus</i>. (From <span class="sc">Rohon</span>.)</p> + <p class="sp0"><i>Fro.</i>, narial opening; <i>l.e.</i>, lateral eyes; <i>gl.</i>, glabellum + or plate over brain; <i>Occ.</i>, occipital region.</p> + </div> + </div> + + <div><span class="pagenum" id="page329">{329}</span></div> + + <p>A similar organ was described by Schmidt in Tremataspis, and considered by him to be a median + nose. Such also is the view of Jaekel, who points out that a median pineal eye exists between the + two lateral eyes in this animal, as in all other of these ancient fishes, so that this frontal + organ does not, as Patten thinks, represent the pineal eye. The whole of this group of fishes, + then, is characterized by the following striking characteristics<span + class="wnw">:—</span></p> + + <p>1. Two well-marked lateral eyes near the middle line.</p> + + <p>2. Between the lateral eyes, well-marked median eyes, very small.</p> + + <p>3. In front of the eye-region a median orifice, single.</p> + + <p>In addition, behind the eye-region a median plate is always found, frequently different in + structure to the rest of the head-shield, being harder in texture—the so-called post-orbital + plate.</p> + + <div class="ac w20 fcenter sp3"> + <a href="images/fig129.jpg" id="fig129"><img style="width:100%" src="images/fig129.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 129.</span>—<span class="sc">Narial Opening and + Lateral Orbits of</span> <i>Thyestes Verrucosus</i>. (From <span class="sc">Rohon</span>.)</p> + </div> + </div> + + <p class="ac"><span class="sc">Structure of Head-Shield of Cephalaspis compared with that of + Ammocœtes.</span></p> + + <p>What is the structure of this head-shield? It has been spoken of as formed of bone because it + possesses cells, being thus unlike the layers of chitin, which are formed by underlying cells but + are not themselves cellular. At the same time, it is recognized on all sides that it has no + resemblance to bone-structure as seen in fossil remains of higher vertebrates. The latest and best + figure of the structure of this so-called bone is given in Rohon's paper already referred to. It + is, so he describes, clearly composed of fibrillæ and star-shaped cells, arranged more or less in + regular layers, with other sets of similar cells and fibrillæ arranged at right angles to the + first set, or at varying angles. The groundwork of this tissue, in which these cells and fibrils + are embedded, contained calcium salts, and so the whole tissue was preserved. In places, spaces + are found in it, in the deepest layer large medullary spaces; more superficially, ramifying spaces + which he considers to be vascular, and calls Haversian canals; the <span class="pagenum" + id="page330">{330}</span>star-like cells, however, are not arranged concentrically around these + spaces, as in true Haversian canals.</p> + + <p>This structure is therefore a calcareous infiltration of a tissue with cells in it. Where is + there anything like it?</p> + + <p>As soon as I saw Rohon's picture (Fig. <a href="#fig130">130</a>), I was astounded at its + startling resemblance to the structure of muco-cartilage as is seen in Fig. <a + href="#fig131">131</a>, taken from Ammocœtes. If such muco-cartilage were infiltrated with + lime salts, then the muco-cartilaginous skeleton of Ammocœtes would be preserved in the + fossil condition, and be comparable with that of Cephalaspis, etc.</p> + + <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:45%;"><a href="images/fig130.png" id="fig130"><img + style="width:100%" src="images/fig130.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:45%;"><a href="images/fig131.png" id="fig131"><img + style="width:100%" src="images/fig131.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 130.—Section of a Head-Plate of a + Cephalaspid.</span> (From <span class="sc">Rohon</span>.)</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 131.—Section of Muco-Cartilage from Dorsal + Head-Plate of Ammocœtes.</span></p> + </div> + </td> + </tr> + </table> + + <p class="sp3">The whole structure is clearly remarkably like Rohon's picture of a section of the + head-plate of a Cephalaspid (Fig. <a href="#fig130">130</a>). In the latter case the matrix + contains calcium salts, in the former it is composed of the peculiar homogeneous mucoid tissue + which stains so characteristically with thionin. With respect to this calcification, it is + instructive to recall the calcification in the interior of the branchial cartilages of Limulus, as + described in Chapter III., for this example shows how easy it is to obtain a calcification in this + chondro-mucoid material. With respect to the medullary spaces and smaller spaces in this tissue, + as described by Rohon, I would venture to suggest that they need not all necessarily indicate + blood-vessels, for similar spaces would appear in the head-shield of Ammocœtes if its + muco-cartilage alone <span class="pagenum" id="page331">{331}</span>were preserved. Of these, some + would indicate the position of blood-vessels, such, for instance, as of the external carotid which + traverses this structure; but the largest and most internal spaces, resembling Rohon's medullary + spaces, would represent muscles, being filled up with bundles of the upper lip-muscles.</p> + + <p class="ac"><span class="sc">The Muco-Cartilaginous Head-Shield of Ammocœtes.</span></p> + + <p>The resemblance between the structure of the head-shield of Thyestes and the muco-cartilage of + Ammocœtes, is most valuable, for muco-cartilage is unique, occurs in no other vertebrate, + and every trace of it vanishes at transformation; it is essentially a characteristic of the larval + form, and must, therefore, in accordance with all that has gone before, be the remnant of an + ancestral skeletal tissue. The whole story deduced from the study of Ammocœtes would be + incomplete without some idea of the meaning of this tissue. So also, as already mentioned, the + skeleton of Ammocœtes is incomplete without taking this tissue into account. It is confined + entirely to the head-region; no trace of it exists posteriorly to the branchial basket-work. It + consists essentially of dorsal and ventral head-shields, connected together by the tentacular, + metastomal, and thyroid bars, as already described. The ventral shield forms the + muco-cartilaginous plate of the lower lip and the plate over the thyroid gland, so that the + skeleton ventrally is represented by Fig. <a href="#fig118">118</a>, B, which shows how the + cartilaginous bars of the branchial basket-work are separated from each other by this thyroid + plate. At transformation, with the disappearance of this muco-cartilaginous plate, the bars come + together in the middle line, as in the more posterior portion of the branchial basket-work.</p> + + <p>The dorsal head-shield of muco-cartilage covers over the upper lip, sends a median prolongation + over the median pineal eyes and a lateral prolongation on each side as far as the auditory + capsules, giving the shape of the head-shield of muco-cartilage, as in Fig. 118, C.</p> + + <p>Not only then is the structure of the head-shield of a Cephalaspid remarkably like the + muco-cartilage of Ammocœtes, but also its general distribution strangely resembles that of + the Ammocœtes muco-cartilage.</p> + + <p>Now, these head-shields in the Cephalaspidæ and Tremataspidæ <span class="pagenum" + id="page332">{332}</span>vary very much in shape, as is seen by the comparison of Tremataspis and + Auchenaspis with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms belong to a + single group, the Osteostraci.</p> + + <p>The conception that Ammocœtes is the solitary living form allied to this group affords a + clue to the meaning of this variation of shape, which appears to me to be possible, if not indeed + probable. There is a certain amount of evidence given in the development of Ammocœtes which + indicates that the branchial region of its ancestors was covered with plates of muco-cartilage as + well as the prosomatic region.</p> + + <p>The evidence is as follows:—</p> + + <p>The somatic muscles of Ammocœtes form a continuous longitudinal sheet of muscles along + the length of the body, which are divided up by connective tissue bands into a series of imperfect + segments or myotomes. This simple muscular sheet can be dissected off along the whole of the + head-region of the animal, with the exception of the most anterior part, without interfering with + the attachments or arrangements of the splanchnic muscular system in the least. The reason why + this separation can be so easily effected is to be found in the fact that the two sets of muscles + are not attached to the same fascia. The sheet of fascia to which the somatic muscles are attached + is separated from the fascia which encloses the branchial cavity by a space (<i>cf.</i> Figs. 63 + and 64) filled with blood-spaces and cells containing fat, in which space is also situated the + cartilaginous branchial basket-work. These branchial bars are closely connected with the branchial + sheet of fascia, and have no connection with the somatic fascia, their perichondrium forming part + of the former sheet. Upon examination, this space is seen to be mainly vascular, the blood-spaces + being large and frequently marked with pigment; but it also possesses a tissue of its own, + recognized as fat-tissue by all observers. The peculiarity of the cells of this tissue is their + arrangement; they are elongated cells arranged at right angles to the plates of fascia, just as + the fibres of the muco-cartilage are largely arranged at right angles to their limiting plates of + perichondrium. These cells do not necessarily contain fat; and when they do, the fat is found in + the centre of each cell, and does not push the protoplasm of the cell to the periphery, as in + ordinary fat cells.</p> + + <div><span class="pagenum" id="page333">{333}</span></div> + + <p>In Fig. <a href="#fig132">132</a>, B, I give a specimen of this tissue stained by osmic acid; + in Fig. <a href="#fig132">132</a>, A, I give a drawing of ordinary muco-cartilage taken from the + plate of the lower lip; and in Fig. <a href="#fig133">133</a>, A, a modification of the + muco-cartilage taken from the velum, which shows the formation of a tissue intermediate between + ordinary muco-cartilage and this branchial fat-tissue.</p> + + <p>Further, in fully-grown specimens of Ammocœtes, in the region of undoubted + muco-cartilage, a fatty degeneration of the cells frequently appears, together with an increase in + the blood spaces,—the precursor, in fact, of the great change which overtakes this tissue + soon afterwards, at the time of transformation, when it is invaded by blood, and swept away, + except in those places where new cartilage is formed. I conclude, then, that the tissue of this + vascular space was originally muco-cartilage, which has degenerated during the life of the + Ammocœtes. The fact that in most cases undoubted muco-cartilage is to be found here and + there in this space, is strong confirmation of the truth of this conclusion.</p> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig132.png" id="fig132"><img style="width:100%" src="images/fig132.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 132.—A, Muco-cartilage of Lower Lip</span> + (<i>Mc.</i>); <i>m.ph.</i>, muscle of lower lip; <i>m.sm.</i>, somatic muscle; <i>Cor.</i>, + laminated layer of skin. <span class="sc">B, Degenerated Muco-cartilage of Branchial + Region.</span> <i>F.</i>, fat layer; <i>P.</i>, pigment; <i>Bl.</i>, blood-space; <i>N.</i>, + somatic nerve; <i>m.br.</i>, branchial muscle; <i>m.sm.</i>, somatic muscle.</p> + </div> + </div> + + <p>If this conclusion is correct, we may expect that it would be confirmed by the embryological + history of the tissue, and we ought to find that in much younger stages a homogeneous tissue of + the same nature as muco-cartilage fills up the spaces in the branchial <span class="pagenum" + id="page334">{334}</span>region, where in the Ammocœtes only blood and fat-containing cells + are present. For this purpose Shipley kindly allowed me to examine his series of sections through + the embryo at various ages. These specimens are very instructive, especially those stained by + osmic acid, which preserves the natural thickness of this space better than other staining + methods. At an age when the branchial cartilages are seen to be formed, when no fat-cells are + present, a distinctive tissue (Fig. <a href="#fig133">133</a>, B) is plainly visible in the velum + and at the base of the tentacles, in the very position where in the more advanced Ammocœtes + muco-cartilage exists. Taking, then, this tissue as our guide, the specimens show that the space + between the skin and the visceral muscles in which the cartilaginous basket-work lies is filled + with a similar material. At this stage a sheet of embryonic tissue occupies the position where, + later on, blood-spaces and fat-cells are found, and this tissue resembles that seen in the velum + and other places where muco-cartilage is afterwards found.</p> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig133.png" id="fig133"><img style="width:100%" src="images/fig133.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 133.—A, Muco-Cartilage of Velum; B, Embryonic + Muco-Cartilage of Tentacular Bar.</span></p> + </div> + </div> + + <p>I conclude, therefore, that originally the branchial or mesosomatic region was covered with a + dorsal plate of muco-cartilage, which carried on its under surface the dorsal part of the + branchial basket-work, and sprang from the central core of skeletogenous tissue around the + notochord; this plate was separated from the plate which covered this region ventrally by the + lateral grove in which the gill-slits are situated. The ventral plate carried on its under surface + the ventral part of the branchial basket-work, and was originally continuous with the plate over + the thyroid gland.</p> + + <div><span class="pagenum" id="page335">{335}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig134.jpg" id="fig134"><img style="width:100%" src="images/fig134.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 134.—Skeleton of Head-Region of Ammocœtes. A, + Lateral View; B, Ventral View; C, Dorsal View.</span></p> + <p class="sp0">Muco-cartilage, <i>red</i>; soft cartilage, <i>blue</i>; hard cartilage, + <i>purple</i>. <i>sk<sub>1</sub></i>, <i>sk<sub>2</sub></i>, <i>sk<sub>3</sub></i>, skeletal + bars; <i>c.e.</i>, position of pineal eye; <i>na. cart.</i>, nasal cartilage; <i>ped.</i>, + pedicle; <i>cr.</i>, cranium; <i>nc.</i>, notochord.</p> + </div> + </div> + + <div><span class="pagenum" id="page336">{336}</span></div> + + <p>In Fig. <a href="#fig134">134</a>, A, B, C, the cranial skeleton of Ammocœtes is + represented from the dorsal, ventral, and lateral aspects. The muco-cartilage is coloured red, the + branchial or soft cartilage blue, and the hard cartilage purple. The degenerated muco-cartilage of + the branchial region is represented as an uncoloured plate, on which the branchial basket-work + stands in relief. If it were restored to its original condition of muco-cartilage, it would + represent a uniform plate, on the <i>under</i> surface of which the basket-work would be situated; + and if it were calcified and made solid, the branchial basket-work would not show at all in these + figures.</p> + + <p>Is it possible to find the reason why this skeletal covering has degenerated so early before + transformation, and why the thyroid plate remains intact until transformation? We see that all + that part which has degenerated is covered over by the somatic muscles,—by, in fact, muscles + which, being innervated by the foremost spinal nerves, belong naturally to the region immediately + following the branchial. I suggest, therefore, that the original skeletal covering of + muco-cartilage has remained intact only where it has not been invaded and covered over by somatic + muscles, but has been invaded by blood and undergone the same kind of degenerative change as + overtakes the great mass of this tissue at transformation wherever the somatic muscles have + overgrown it.</p> + + <p>The covering somatic muscles in the branchial region form a dorsal and ventral group, of which + the latter is formed in the embryo much later than the former, the line of separation between the + two groups being the lateral groove, with its row of branchial openings. This groove ends at the + first branchial opening, but the ventral and dorsal somatic muscles continue further headwards. It + is instructive to see that, although the lateral groove terminates, the separation between the two + groups of muscles is still marked out by a ridge of muco-cartilage, represented in Fig. <a + href="#fig134">134</a>, A, which terminates anteriorly in the opercular bar.</p> + + <p>Passing now to the prosomatic region, we find that here, too, the muco-cartilaginous external + covering is divisible into a dorsal and a ventral head-plate, the ventral head-plate being the + plate of the lower lip, and the dorsal head-plate the plate of muco-cartilage over the front part + of the head. The staining reaction with thionin maps out this dorsal head-plate in a most + beautiful manner, and shows that the whole of the upper lip-region in front of the nasal orifice + is one large plate of muco-cartilage, obscured largely by the invasion of the crossing muscles of + the upper lip, but left pure and uninvaded all around the nasal orifice, and where the upper and + lower lips come together. In addition to this foremost plate, a median tongue of muco-cartilage + covers over the pineal eye and fills up the <span class="pagenum" id="page337">{337}</span>median + depression between the two median dorsal somatic muscles. Also, two lateral cornua pass + caudalwards from the main frontal mass of muco-cartilage over the lateral eyes, forming the + well-known wedge which separates the dorsal and lateral portions of the dorso-lateral somatic + muscle. In fact, similarly to what we find in the branchial region, the muco-cartilaginous + covering can be traced with greater or less completeness only in those parts which are not covered + by somatic muscles.</p> + + <p>In Fig. <a href="#fig134">134</a>, A, B, C, this striking muco-cartilaginous head-shield, both + dorsal and ventral, is shown. Seeing that the upper lip wraps round the lower one on each side, + and that this most ventral edge of the upper lip contains muco-cartilage, as is seen in Fig. <a + href="#fig117">117</a>, the dorsal head-shield of muco-cartilage ought, strictly speaking, to + extend more ventrally in the drawings. I have curtailed it in order not to interfere with the + representation of the lower lip and tentacular muco-cartilages.</p> + + <p>From what has been said, it follows that the past history of the skeletal covering of the whole + head-region of Ammocœtes, both frontal and occipital, can be conjectured by means of the + ontogenetic history of the foremost myomeres.</p> + + <p>Dohrn and all other observers are agreed that during the development of this animal a striking + forward growth of the foremost somatic myomeres takes place, so that, as Dohrn puts it, the + body-musculature has extended forwards over the gill-region, and at the same time the gill-region + has extended backwards. It is therefore probable that in the ancestral form the myotomes, + innervated by the first spinal nerves, immediately succeeded the branchial region. Judging from + Ammocœtes, the forward growth was at first confined to the dorsal region, and therefore + invaded the dorsal head-plate, the ventral musculature being distinctly a later growth. With + respect to this dorsal part of the myotomes, the first myotome is originally situated some + distance behind the auditory capsule, and then grows forward towards the nasal opening; the + lateral part, according to Hatschek, grows forward more quickly than the dorsal part, and splits + itself above and below the eye into a dorso-lateral part, which extends up to the olfactory + capsule, and a ventro-lateral part (<i>m. lateralis capitis</i> anterior, superior, and inferior), + thus giving rise to the characteristic appearance of the muco-cartilaginous head-shield of + Ammocœtes.</p> + + <p>According, then, to the extent of the growth of these somatic <span class="pagenum" + id="page338">{338}</span>muscles, the shape of the muco-cartilaginous head-shield will vary, and + if it were calcified and then fossilized we should obtain fossil head-shields of widely differing + configuration, although such fossils might be closely allied to each other. This is just what is + found in this group. Let the muco-cartilage extend over the whole of the branchial region of + Ammocœtes, the resulting head-shield would be as in Fig. <a href="#fig135">135</a>, A; the + branchial bars below the muco-cartilaginous shield might or might not be evident, and the line + between the branchial and the trigeminal region might or might not be indicated. Such a + head-shield would closely resemble those of Didymaspis and Tremataspis respectively. Now suppose + the somatic musculature to encroach slightly on the branchial region and also laterally to the end + of the anterior branchial region, then we should obtain a shape resembling that of Thyestes (Fig. + <a href="#fig135">135</a>, B). Continue the same process further, the lateral muscle always + encroaching further than the median masses, until the whole or nearly the whole branchial region + is invested, and we get the head-shield of Cephalaspis (Fig. <a href="#fig135">135</a>, C); + further still, that of Keraspis, and yet still further, that of Ammocœtes (Fig. <a + href="#fig135">135</a>, D).</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig135.png" id="fig135"><img style="width:100%" src="images/fig135.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 135.—Diagrams to show the different shapes of Head-Shields due + to the forward growth of the Somatic Musculature.</span></p> + <p class="sp0">A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocœtes.</p> + </div> + </div> + + <p class="sp3">So close is this similarity, from the comparative point of view, between the dorsal + head-shield of the Osteostraci and the dorsal cephalic region of Ammocœtes that it + justifies us in taking Ammocœtes as the nearest living representative of such types; it is + justifiable, therefore, to interpret by means of Ammocœtes the position of other organs in + these forms. First and foremost is the hard plate <span class="pagenum" + id="page339">{339}</span>known as the post-orbital plate, so invariably found. In Fig. <a + href="#fig134">134</a>, C, I have inserted (<i>cr.</i>) the position of the membranous cranium of + Ammocœtes, and it is immediately evident that the primordial cranium of the Osteostraci + must occupy the exact position indicated by this median hard plate. For this very reason this + median plate would be harder than the rest in order to afford a better protection to the brain + underneath. This plate, because of its position, may well receive the same name as the similar + plate in the trilobite and various palæostracans and be called the glabellum.</p> + + <p class="ac"><span class="sc">Evidence of Segmentation in the Head-Shield—Formation of + Cranium.</span></p> + + <p>We may thus conceive the position of the nose, lateral eyes, median eyes, and cranium in these + old fishes. In addition, other indications of a segmentation in this head-region have been found. + The most striking of all the specimens hitherto discovered are some of <i>Thyestes verrucosus</i>, + discovered by Rohon, in which the dorsal shield has been removed, and so we are able to see what + that dorsal shield covered.</p> + + <p>In Fig. <a href="#fig136">136</a>, I reproduce his drawing of one of his specimens from the + dorsal and lateral aspects. These drawings show that the frontal part of the shield covered a + markedly segmented part of the animal; five distinct segments are visible apart from the median + most anterior region. This segmented region is entirely confined to the prosomatic region, + <i>i.e.</i> to the region innervated by the trigeminal nerve. An indication of similar markings is + given in Lankester's figure of <i>Eukeraspis pustuliferus</i> (see Fig. <a href="#fig127">127</a>, + B), and, indeed, evidence of a segmentation under the antero-lateral border of the head-shield is + recognized at the present time, not only in the Cephalaspidæ, but also in the Pteraspidæ, as was + pointed out to me by Smith Woodward in the specimens at the British Museum. Also, in + <i>Cyathaspis</i>, Jaekel has drawn attention to markings of a similar segmental nature (Fig. <a + href="#fig137">137</a>).</p> + + <p>There seems, then, little doubt but that these primitive fishes possessed something in this + region which was of a segmental character, and indicated at least five segments, probably + more.</p> + + <p>Rohon entitles his discovery 'the segmentation of the primordial cranium.' It would, I think, + be better to call it the segmentation of <span class="pagenum" id="page340">{340}</span>the + anterior region of the head, for that is in reality what his figures show, not the segmentation of + the primordial cranium, which, to judge from Ammocœtes, was confined to the region of the + glabellum.</p> + + <p>What is the interpretation of this appearance?</p> + + <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:43%;"><a href="images/fig136.jpg" id="fig136"><img + style="width:100%" src="images/fig136.jpg" alt="" title=""/></a></td> + <td class="vbm ac" style="width:43%;"><a href="images/fig137.png" id="fig137"><img + style="width:100%" src="images/fig137.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 136.—Lateral and Dorsal Views of the Frontal + and Occipital Regions of the Head-Shield of Thyestes, after Removal of the Outer + Surface.</span> (From <span class="sc">Rohon</span>.)</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 137.—Under Surface of Head-Shield of Cyathaspis.</span> + (From <span class="sc">Jaekel</span>.)</p> + <p class="sp0 ac"><i>A.</i>, lateral eyes; <i>Ep.</i>, median eyes.</p> + </div> + </td> + </tr> + </table> + + <p>Any segmentation in the head-region must be indicative of segments belonging to the trigeminal + or prosomatic region, or of segments belonging to the vagus or mesosomatic region. Many + palæontologists, looking upon segmentation as indicative of gills and gill-slits, have attempted + to interpret such markings as branchial segments, regardless of their position. As the figures + show, they extend in front of the eyes and reach round to the front middle line, a position which + is simply impossible for gills, but points directly to a segmentation connected with the + trigeminal nerve. Comparison with Ammocœtes makes it plain enough that the markings in + question are prosomatic in position, and that the gill-region must be sought for in the place + <span class="pagenum" id="page341">{341}</span>where Schmidt and Rohon located it in Thyestes, + viz. the so-called occipital region.</p> + + <p>This discovery of Rohon's is, in my opinion, of immense importance, for it indicates that, in + these early fishes, the prosomatic segmentation, associated with the trigeminal nerve, was much + more well-marked than in any fishes living in the present day. Why should it be more well-marked? + Turning to the palæostracan, it is very suggestive to compare the markings on their prosomatic + carapace with these markings. Again and again we find indications of segmentation in these fossils + similar to those seen in the ancient fishes. Thus in Fig. <a href="#fig138">138</a> I have put + side by side the palæostracan <i>Bunodes</i> and the fish <i>Thyestes</i>, both life size. In the + latter I have indicated Rohon's segments; in the former the markings usually seen.</p> + + <p>From the evidence of Phrynus, Mygale, etc., as already pointed out, such markings in the + palæostracan fossils would indicate the position of the tergo-coxal muscles of the prosomatic + appendages, even though such appendages have not yet been discovered, and it is significant that + in all these cases there is a distinct indication of a median plate or glabellum in addition to + the segmental markings. Especially instructive is the evidence of Phrynus, as is seen by a + comparison of Figs. 107 and 108, which shows clearly that this median plate (<i>glab.</i>) covered + the brain-region, a brain-region which is isolated and protected from the tergo-coxal muscles by + the growth dorsalwards of the flanges of the plastron. In this way an incipient cranium of a + membranous character is formed, which helps to give attachment to these tergo-coxal muscles. As + such cranium is derived directly from the plastron, it is natural that it should ultimately become + cartilaginous, just as occurs when Ammocœtes becomes Petromyzon and the cartilaginous + cranium of the latter arises from the membranous cranium of the former. In Galeodes also the + growth dorsalwards of the lateral flanges of the plastron to form an incipient cranium in which + the brain lies is very apparent.</p> + + <div class="ac w20 fcenter sp2"> + <a href="images/fig138.png" id="fig138"><img style="width:100%" src="images/fig138.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 138.</span>—<span class="sc">A, Outline of</span> <i>Thyestes + Verrucosus</i> <span class="sc">with Rohon's Segments indicated; B, Outline of</span> + <i>Bunodes Lunula</i> <span class="sc">with Lateral Eyes inserted</span>.</p> + <p class="sp0 ac">Both figures natural size.</p> + </div> + </div> + + <div><span class="pagenum" id="page342">{342}</span></div> + + <p>I venture, then, to suggest that in the Osteostraci the median hard plate or glabellum + protected a brain which was enclosed in a membranous cranium, very probably not yet complete in + the dorsal region—certainly not complete if the median pineal eyes so universally found in + these ancient fishes were functional—a cranium derived from the basal trabeculæ, in + precisely the same manner as we see it already in its commencement in Phrynus and other scorpions. + With the completion of this cranium and its conversion into cartilage, and subsequently into bone, + an efficient protection was afforded to the most vital part of the animal, and thus the hard + head-shield of the Palæostraca and of the earliest fishes was gradually supplanted by the + protecting bony cranium of the higher vertebrates.</p> + + <p>Step by step it is easy to follow in the mind's eye the evolution of the vertebrate cranium, + and because it was evolved direct from the plastron, the impossibility of resolving it into + segments is at once manifest; for although the plastron was probably originally segmented, as + Schimkéwitsch thinks, all sign of such segmentation had in all probability ceased, before ever the + vertebrates first made their appearance on the earth.</p> + + <p>It follows further, from the comparison here made, that those antero-lateral markings + indicative of segments, found so frequently in these primitive fishes, must be interpreted as due + not to gills but to aponeuroses, due to the presence of muscles which moved prosomatic appendages, + muscles which arose from the dorsal region in very much the same position as do the muscles of the + lower lip in Ammocœtes; the latter, as already argued, represent the tergo-coxal muscles of + the last pair of prosomatic appendages—the chilaria or metastoma. Such an interpretation of + these markings signifies that the first-formed fishes must have possessed prosomatic appendages of + a more definite character than the tentacles of Ammocœtes, something intermediate between + those of the palæostracan and Ammocœtes.</p> + + <p class="sp3">For my part I should not be in the least surprised were I to hear that something of + the nature of appendages in this region had been found, especially in view of the well-known + existence of the pair of appendages in the members of the Asterolepidæ—large, oar-like + appendages which may well represent the ectognaths.</p> + + <div><span class="pagenum" id="page343">{343}</span></div> + + <p class="ac"><span class="sc">The Relationship of the Ostracoderms.</span></p> + + <p>Of the three groups of fishes—the Heterostraci, the Osteostraci, and the + Antiarcha—the last is Devonian, and therefore the latest in time of the three, while the + earliest is the first group, as both Pteraspis and Cyathaspis have been found in lower levels of + the Silurian age than any of the Osteostraci, and, indeed, Cyathaspis has been discovered in + Sweden in the lower Silurian. This, the earliest of all groups of fishes, is confined to two forms + only—Pteraspis and Cyathaspis,—for Scaphaspis is now recognized to be the ventral + shield of Pteraspis.</p> + + <p>Hitherto a strong tendency has existed in the minds both of the comparative anatomist and the + palæontologist to look on the elasmobranchs as the earliest fishes, and to force, therefore, these + strange forms of fish into the elasmobranch ranks. For this purpose the same device is often used + as has been utilized in order to account for the existence of the Cyclostomata, viz. that of + degeneration. The evidence I have put forward is very strongly in favour of a connection between + the cyclostomes and the cephalaspids, and agrees therefore with all the rest of the evidence that + the jawless fishes are more ancient than those which bore jaws—the Gnathostomata.</p> + + <p>This is no new view. It was urged by Cope, who classified the Heterostraci, Osteostraci, and + Antiarcha under one big group—the Agnatha—from which subsequently the Gnathostomata + arose. Cope's arguments have not prevailed up to the present time, as is seen in the writings of + Traquair, one of the chief authorities on the subject in Great Britain. He is still an advocate of + the elasmobranch origin of all these earliest fishes, and claims that the latest discoveries of + the Silurian deposits (<i>Thelodus Pagei</i>) and other members of the Cœlolepidæ confirm + this view of the question.</p> + + <p>This view may be summed up somewhat as follows:—</p> + + <p>Cartilaginous jaws would not fossilize, and the Ostracoderms may have possessed them.</p> + + <p>They may have degenerated from elasmobranchs just as the cyclostomes are supposed to have + degenerated.</p> + + <p>Seeing that bone succeeds cartilage, the presence of bony shields in Cephalaspis, etc., + indicates that their precursors were cartilaginous, presumably elasmobranch fishes.</p> + + <p>Of these arguments the strongest is based on the supposed bony <span class="pagenum" + id="page344">{344}</span>covering of the Osteostraci, with the consequent supposition that their + ancestors possessed a cartilaginous covering. This argument is entirely upset, if, as I have + pointed out, the structure of the cephalaspid shield is that of muco-cartilage and not of bone. If + these plates are a calcified muco-cartilage, then the whole argument for their ancestry from + animals with a cartilaginous skeleton falls to the ground, for muco-cartilage is the precursor not + only of bone, but also of cartilage itself.</p> + + <p>The evidence, then, points strongly in favour of Cope's view that the most primitive fishes + were Agnatha, after the fashion of cyclostomes, as is also believed by Smith Woodward, Bashford + Dean, and Jaekel.</p> + + <p>Among living animals, as I have shown, the Limulus is the sole survivor of the palæostracan + type, and Ammocœtes alone gives a clue to the nature of the cephalaspid, <i>i.e.</i> the + osteostracan fish. Older than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is it + possible from their structure to obtain any clue as to the actual passage from the palæostracan to + the vertebrate?</p> + + <p>Here again, as in the case of the Osteostraci, a relationship to the elasmobranch has been + supposed, for the following reasons<span class="wnw">:—</span></p> + + <p>The latest discoveries in the Silurian and Devonian deposits have brought to light strange + forms such as Thelodus and Drepanaspis, of which the latter from the Devonian must, according to + Traquair, be included in the Heterostraci. It possessed, as seen in Fig. <a + href="#fig139">139</a>, large plates, after the fashion of Pteraspis, and also many smaller + ones.</p> + + <p>The former, from the upper Silurian, belongs to the Cœlolepidæ, and was covered over + with shagreen composed of small scutes, after the fashion of an elasmobranch. Traquair suggests + that Thelodus arose from the original elasmobranch stock; that by the fusion of scutes such a form + as Drepanaspis occurred, and, with still further fusion, Pteraspis.</p> + + <p>There are always two ways of looking at a question, and it seems to me possible and more + probable to turn the matter round and to argue that the original condition of the surface-covering + was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates, + Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a + stage on the way to the formation of an elasmobranch, and not a backward stage from the + elasmobranch towards Pteraspis.</p> + + <div><span class="pagenum" id="page345">{345}</span></div> + + <p>This method of looking at the problem seems to me to be more in consonance with the facts than + the reverse; for, as pointed out by Jaekel, the fishes with large plates are the oldest, and in + Cyathaspis, the very oldest of all, the size of the plates is most conspicuous; he considers, + therefore, this preconceived view that large plates are formed by the fusion of small ones must + give way to the opposite belief.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig139.png" id="fig139"><img style="width:100%" src="images/fig139.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 139.—Drepanaspis. Ventral and Dorsal Aspects.</span> (After + <span class="sc">Lankester</span>.)</p> + <p class="sp0"><i>A.</i>, anus; <i>E.</i>, lateral eyes.</p> + </div> + </div> + + <p>So also Rohon, as quoted by Traquair, who, in his first paper accepted Lankester's view that + the ridges of the pteraspidian shield were formed by the fusion of a linear arrangement of numbers + of placoid scales, suggests in his second paper that these ridges may have been the most primitive + condition of the dermal skeleton of the vertebrate, out of which, by differentiation, the dermal + denticles (placoid scales) of the selachian, as well as their modifications in the ganoids, + teleosteans, and amphibians, have arisen.</p> + + <p>One thing is agreed upon on all sides; no sign of bone-corpuscles is to be found in this dermal + covering of Pteraspis. In the deeper layers are large spaces, the so-called pulp-cavities leading + into narrow canaliculi, the so-called dentine canals. The structure is <span class="pagenum" + id="page346">{346}</span>looked upon as similar to that of the pulp and dentine canals of many + fish-scales.</p> + + <p>On the other hand, this dermal covering of Pteraspis has been compared by Patten with the + arrangement of the chitinous structure of certain parts of the external covering of Limulus, a + comparison which to my mind presents a great difficulty. The chitin-layers in Limulus are + <i>external</i> to the epidermal cells, being formed by them; the layers in Pteraspis which look + like chitin must have been <i>internal</i> to the epidermal layer; for each vascular canal which + passes from a pulp-cavity on its way to be distributed into the dentine canals of the ridge gives + off short side branches, which open directly into the groove between the ridges. If these canals + were filled with blood they could not possibly open directly into the open grooves between the + ridges; these openings must, therefore, have been covered over with an epithelial layer which + covered over the surface of the animal, and consequently the chitin-like structure must have been + internal to the epidermis, and not external, as on Patten's view. The comparison of this structure + with the dentine of fish-scales signifies the same thing, for in the latter the epidermis is + external to the dentine-plates, the hard skeletal structure is in the position of the cutis, not + of the cuticle.</p> + + <p>The position appears to me to be this: the dermal cranial skeleton of vertebrates, whether it + takes the form of a bony skull or of the dorsal plates of a cephalaspid or a pteraspid is, in all + cases, not cuticular, <i>i.e.</i> is not an external formation of the epidermal cells, but is + formed in tissue of the nature of connective tissue underlying the epidermis. On the contrary, the + hard part of the head-carapace of the palæostracan is an external formation of the epidermal + cells.</p> + + <p>If, then, this tissue of Pteraspis is not to be looked upon as chitin, how can we imagine its + formation? It is certainly not bone, for there are no bone-corpuscles; it is a very regular + laminated structure resembling in appearance chitin rather than anything else.</p> + + <p>As in all cases of difficulty, turn to Ammocœtes and let us see what clue there is to be + found there. The skin of Ammocœtes is peculiar among vertebrates in many respects. It + consists of a number of epidermal cells, as in Fig. <a href="#fig140">140</a>, the varying + function of which need not be considered here, covered over with a cuticular layer which is + extraordinarily thick for the cuticle of a vertebrate skin; this cuticular layer is perforated + with fine canaliculi, through which the <span class="pagenum" id="page347">{347}</span>secretion + of the underlying cells passes, as is seen in Fig. <a href="#fig140">140</a>, A and B. This + cuticle corresponds to the chitinous covering of the arthropod, and like it is perforated with + canaliculi, and, according to Lwoff, possibly contains chitin. The epidermal cells rest on a thick + layer of most striking appearance (Fig. <a href="#fig141">141</a>), for it resembles, in an + extraordinary degree, when examined superficially, a layer of chitin; it is called the laminated + layer, and is characterized by the extreme regularity of the laminæ. This appearance is due, as + the observations of Miss Alcock show, to alternate layers of connective tissue fibres arranged at + right angles to each other, each fibre running a straight course and possessing its own nucleus. + Although the fibres in each layer are packed close together, they are sufficiently apart to form + with the fibres of the alternate layers a meshwork rather than a homogeneous structure, and thus + the surface view of this layer shows a regular network of very fine spaces through which + nerve-fibres and fluid pass. This layer is easily dissolved in a solution of hypochlorite of soda, + a fluid which dissolves chitin. Any one looking at Ammocœtes would say that the only part + of its skin which resembles chitin is this laminated layer, and therefore the only part of its + skin which would afford an indication of the nature of the skeleton of Pteraspis is this laminated + layer, which belongs to the cutis, and not to the cuticle. Yet another significant peculiarity of + this layer is its entire disappearance at transformation. Miss Alcock, in a research not yet + published, has shown that this layer is completely broken up and absorbed at transformation; the + cutis of Petromyzon is formed entirely anew, and no longer presents any regular laminated + character, but resembles rather the sub-epidermal connective tissue layer of the skin of higher + vertebrates. This laminated layer, then, just like the muco-cartilage, shows, by its complete + disappearance at transformation, its ancestral character.</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig140.png" id="fig140"><img style="width:100%" src="images/fig140.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 140.—Epithelial Cells of Ammocœtes to show + the Canaliculi in the Thick Cuticle (B). A, Transverse Section through the Cuticle.</span></p> + </div> + </div> + + <p>Very suggestive is the arrangement of the different skeletal <span class="pagenum" + id="page348">{348}</span>tissues in the head-region of Ammocœtes. Fig. <a + href="#fig141">141</a> represents a section through the head near the pineal eye. Most internally + is <i>a</i>, a section of the membranous cranium, then comes <i>b</i>, the muco-cartilaginous + skeleton, then <i>c</i>, the laminated layer, and finally <i>d</i>, the external cuticle. If in + Ammocœtes we possess an epitome of the history of the vertebrate, how would these layers be + represented in the past ages, supposing they could be fossilized?</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig141.png" id="fig141"><img style="width:100%" src="images/fig141.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 141.—Section of Skin and Underlying Tissues in the Head-Region + of Ammocœtes.</span></p> + <p class="sp0"><i>a</i>, cranial wall; <i>b</i>, muco-cartilage; <i>c</i>, laminated layer; + <i>d</i>, external cuticular layer.</p> + </div> + </div> + + <p>The most internal layer <i>a</i>, by the formation of cartilage and then bone, represents the + great mass of vertebrate fossils; the next layer <i>b</i>, by a process of calcification, as + previously argued, represents the head-shield of the Osteostracan fishes; while the cuticular + layer <i>d</i>, no longer thin, is the remnant of the Palæostracan head-carapace. Between these + two layers, <i>b</i> and <i>d</i>, lies the laminated layer <i>c</i>. Intermediate to the + Palæostracan and the Osteostracan comes the Heterostracan, with its peculiar head-shield—a + head-shield whose origin is more easily conceivable as arising from something of the nature of the + laminated layer than from any other structure represented in Ammocœtes.</p> + + <p>My present suggestion, then, is this: the transition from the skeletal covering of the + Palæostracan to that of the highest vertebrates was brought about by the calcification of + successive layers from without inwards, all of which still remain in Ammocœtes and show how + the external chitinous covering of the arthropod was gradually replaced by the deep-lying internal + bony cranium of the higher vertebrates.</p> + + <p>In Ammocœtes the layer which represents the covering of the <span class="pagenum" + id="page349">{349}</span>Palæostracan has already almost disappeared. At transformation the layers + representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the + stage representing the higher vertebrates, far from disappearing, by the formation of cartilage + reaches a higher stage and prepares the way for the ultimate stage of all—the formation of + the bony cranium.</p> + + <p>So much for the evidence as to the nature of the structure of the head-shield of the + Pteraspidæ.</p> + + <p>It suggests that these fishes were covered anteriorly with armoured plates derived from the + cutis layer of the skin, a layer which was specially thickened and very vascular, apparently, to + enable respiration to be very largely, if not entirely, effected by the surface of the body. It is + difficult to understand how the sea-scorpions breathed, and it is easy to see how the formation of + ventral and dorsal plates enclosing the mesosomatic appendages may at the outset have hindered the + action of the branchiæ. The respiratory chamber, according to my view, had at first the double + function of respiration and digestion. A new digestive apparatus was the pressing need at the + time; it would, therefore, be of distinct advantage to remove, as much as possible, the burden of + respiration from this incipient alimentary canal.</p> + + <p>What can be said as to the shape of these ancient forms of fishes? Certain parts of them are + absolutely known, other parts are guesswork. They are known to have possessed a dorsal shield, a + ventral shield formerly looked upon as belonging to a separate species, called Scaphaspis, and a + spine attached to the dorsal shield. The rest of their configuration, as given in Smith Woodward's + restoration (Fig. <a href="#fig142">142</a>) is guesswork; the fish-like body with its scales, the + heterocercal tail, is based on the most insufficient evidence of something of the nature of scales + having being found near the head-plates.</p> + + <p>The dorsal shield is characterized by a pair of lateral eyes situated on the edge of the + shield, not as in Cephalaspis near the middle line. In the middle line, where the rostrum meets + the large dorsal plate, median eyes were situated. But the slightest sign of any median single + nasal opening, such as is so characteristic of the head-shield of the Osteostraci and of + Ammocœtes has never been discovered. The olfactory organ must have been situated on the + ventral side as in the larval stage of Ammocœtes, or in the Palæostraca. Many of these + head-shields are remarkably well preserved, <span class="pagenum" id="page350">{350}</span>and it + is difficult to believe that an olfactory opening would not be seen if any such had existed, as it + does in Thyestes.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig142.png" id="fig142"><img style="width:100%" src="images/fig142.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 142.—Restoration of Pteraspis.</span> (After <span + class="sc">Smith Woodward</span>.)</p> + </div> + </div> + + <p>The difficulty of interpreting these types is the difficulty of understanding their method of + locomotion; that is largely the reason why the spine has been placed as if projecting from the + back, and a fish-like body with a heterocercal tail-fin added. If, on the contrary, the spine is a + terminal tail-spine, then, as far as the fossilized remains indicate, the animal consisted of a + dorsal shield, a ventral shield, and a tail-spine, to which must be added two apparently lateral + pieces and a few scales. If the animal did not possess a flexible body with a tail-fin, but + terminated in a rigid spike after the fashion of a Limulus-like animal, then it must have moved by + means of <span class="pagenum" id="page351">{351}</span>appendages. At present we have not + sufficient evidence to decide this question.</p> + + <p>That the animal crawled about in the mud by means of free appendages is by no means an + impossible view, seeing how difficult it is to find the remains of appendages in the fossils of + this far-back time, even when we are sure that they existed. Thus, for many generations, the + appendages of trilobites, which occur in such countless numbers, and in such great variety of + form, were absolutely unknown, until at last, in consequence of a fortunate infiltration by + pyrites, they were found by Beecher preserved down to the minutest detail. Even to this day no + trace of appendages has been found in such forms as Hemiaspis, Bunodes, Belinurus, + Prestwichia.</p> + + <p>The whole question of the evidence of any prosomatic appendages in these ancient fishes is one + of very great interest, and of late years has been investigated by Patten. It has long been known + that forms such as Pterichthys and Bothriolepis possessed two large, jointed locomotor appendages, + and Patten has lately obtained better specimens of Bothriolepis than have ever been found before, + which show not only the general configuration of the fish, but also the presence of mandibles or + gnathites in the mouth-region resembling those of an arthropod. These mandibles had been seen + before (Smith Woodward), but Patten's specimens are more perfect than any previously described, + and cause him to conclude that these ancient fish were of the nature of arthropods rather than of + vertebrates.</p> + + <p>Patten has also been able to obtain some excellent specimens of the under surface of the head + of Tremataspis, which, as evident in Fig. <a href="#fig143">143</a>, show the presence of a series + of holes, ranging on each side from the mouth-opening, in a semicircular fashion towards the + middle line. He considers that these openings indicate the attachments of appendages, in + opposition to other observers, such as Jaekel, who look upon them as gill-slits. To my mind, they + are not in the right position for gill-slits; they are certainly in a prosomatic rather than in a + mesosomatic position, and I should not be at all surprised if further research justified Patten's + position. So convinced is he of the presence of appendages in all these old forms, that he + considers them to be arthropods rather than vertebrates, although, at the same time, he looks upon + them as indicating the origin of vertebrates from arthropods. Here, perhaps, it is advisable to + say a few words on Patten's attitude towards this question.</p> + + <div><span class="pagenum" id="page352">{352}</span></div> + + <p>Two years after I had put forward my theory of the derivation of vertebrates from arthropods, + Patten published, in the <i>Quarterly Journal of Microscopical Science</i>, simultaneously with my + paper in that journal, a paper entitled "The Origin of Vertebrates from Arachnids." In this paper + he made no reference to my former publications, but he made it clear that there was an absolutely + fundamental difference between our treatment of the problem; for he took the old view that of + necessity there must be a reversal of surfaces in order that the internal organs should be in the + same relative positions in the vertebrate and in the invertebrate. He simply, therefore, + substituted Arachnid for Annelid in the old theory. Because of this necessity for the reversal of + surfaces he discarded the terms dorsal and ventral as indicative of the surfaces of an animal, and + substituted hæmal and neural, thereby hopelessly confusing the issue and making it often very + difficult to understand his meaning.</p> + + <div class="ac w30 fcenter sp2"> + <a href="images/fig143.png" id="fig143"><img style="width:100%" src="images/fig143.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 143.—Under-Surface of Head-Region in + Tremataspis.</span> (After <span class="sc">Patten</span>.)</p> + </div> + </div> + + <p>He still holds to his original opinion, and I am still waiting to find out when the reversal of + surfaces took place, for his investigations lead him, as must naturally be the case, to compare + the dorsal (or, as he would call it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and + of the Pteraspidæ with the dorsal surface of the Palæostraca.</p> + + <p>All these ancient fishes are, according to him, still in the arthropod stage, have not yet + turned over, though in a peculiarly unscientific manner he argues elaborately that they must have + swum on their back rather than on their front, and so indicated the coming reversal. Because they + were arthropods they cannot have had a <span class="pagenum" id="page353">{353}</span>frontal + nose-organ; therefore, Patten looks upon the nose and the two lateral eyes of the Osteostraci as a + complex median eye, regardless of the fact that the median eyes already existed.</p> + + <p class="sp3">Every atom of evidence Patten has brought forward, every new fact he has + discovered, confirms my position and makes his still more hopelessly confused. Keep the animal the + right side uppermost, and the evidence of the rocks confirms the transition from the Palæostracan + to the Cyclostome; reverse the surfaces, and the attempt to derive the vertebrate from the + palæostracan becomes so confused and hopelessly muddled as to throw discredit on any theory of the + origin of vertebrates from arthropods. For my own part, I fully expect that appendages will be + found not only in the Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue his + researches with increasing success. I feel sure, however, his task will be much simplified if he + abandons his present position and views the question from my standpoint.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The shifting of the nasal tube from a ventral to a dorsal position, as seen in + Ammocœtes, is, perhaps, the most important of all clues in connection with the comparison + of Ammocœtes to the Palæostracan on the one hand, and to the Cephalaspid on the other; + for, whereas the exact counterpart of the opening of such a tube is always found on the dorsal + head-shield in all members of the latter group, nothing of the kind is ever found on the dorsal + carapace of the former group.</p> + <p>The reason for this difference is made immediately evident in the development of + Ammocœtes itself, for the olfactory tube originates as a ventral tube—the tube of + the hypophysis—in exactly the same position as the olfactory tube of the Palæostracan, and + later on in its development takes up a dorsal position.</p> + <p>In fact, Ammocœtes in its development indicates how the Palæostracan head-shield + became transformed into that of the Cephalaspid.</p> + <p>In another most important character Ammocœtes indicates its relationship to the + Cephalaspidæ, for it possesses an external skeleton or head-shield composed of muco-cartilage, + which is the exact counterpart of the so-called bony head-shield of the latter group; and still + more strikingly the structure of the cephalaspidian head-shield is remarkably like that of + muco-cartilage. In the one case, by the deposition of calcium salts, a hard external skeleton, + capable of being preserved as a fossil, has been formed; in the other, by the absence of the + calcium salts, a soft chondro-mucoid matrix, in which the characteristic cells and fibrils are + embedded, distinguishes the tissue.</p> + <p>The recognition that the head-shields of these most primitive fishes were not composed of + bone, but of muco-cartilage, the precursor of both cartilage and bone, immediately clears up in + the most satisfactory manner the whole <span class="pagenum" id="page354">{354}</span>question + of their derivation from elasmobranch fishes; for the main argument in favour of the latter + derivation is the exceedingly strong one that bone succeeds cartilage—not <i>vice + versâ</i>—therefore, these forms, since their head-shield is bony, must have arisen from + some other fishes with a cartilaginous skeleton, most probably of an elasmobranch nature. + Seeing, however, that the structure of their shields resembles muco-cartilage much more closely + than bone, and that Ammocœtes forms a head-shield of muco-cartilage closely resembling + theirs, there is no longer any necessity to derive the jawless fishes from the gnathostomatous; + but, on the contrary, we may look with certainty upon the Agnatha as the most primitive group + from which the others have been derived.</p> + <p>The history of the rocks shows that the group of fishes, Pteraspis and Cyathaspis, are older + than the Cephalaspidæ—come, therefore, phylogenetically between the Palæostraca and the + latter group. In this group the head-shields are of a very different character, without any sign + of any structure comparable with that of bone, and although they possessed both lateral and + median eyes, there is never in any case any trace of a dorsal nasal orifice. Their olfactory + passage, like that of the Palæostraca, must have been ventral.</p> + <p>The remarkable comparison which exists between the head-shields of Ammocœtes and + Cephalaspis, enables us to locate the position of the brain and cranium of the latter with + considerable accuracy, and so to compare the segmental markings found in many of these fossils + with the corresponding markings, found either in fossil Palæostraca or on the head-carapaces of + living scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial region was + covered with a median plate, often especially hard, which corresponded to the glabellum of the + trilobite; the growth of the cranium can be traced from its beginnings as the upturned lateral + flanges of the plastron to the membranous cranium of Ammocœtes.</p> + <p>From such a comparison it follows that the segments, found in the antero-lateral region of + the head-shield, were not segments of the cranium, but of parts beyond the region of the + cranium, and from their position must have been segments supplied by the trigeminal nerve, and + not by the vagus group; segments, therefore, which did not indicate gills and gill-slits, but + muscles, innervated by the trigeminal nerve; muscles which, as indicated by the corresponding + markings on the carapace of Phrynus, Mygale, etc., were the tergo-coxal muscles of the + prosomatic appendages.</p> + <p class="sp0">The discovery of the nature of these appendages in the Pteraspidæ and + Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and Bothriolepis), is a problem of the + future, though in the latter, not only have the well-known oar-like appendages been long since + discovered, but Patten has recently found specimens of Bothriolepis which throw light on the + anterior masticating gnathite-like appendages which these ancient forms possessed.</p> + </div> + + <div><span class="pagenum" id="page355">{355}</span></div> + + <p class="ac">CHAPTER XI</p> + + <p class="ac"><i>THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Lateral line organs.—Function of this group of organs.—Poriferous + sense-organs on the appendages in Limulus.—Branchial sense-organs.—Prosomatic sense + organs.—Flabellum.—Its structure and position.—Sense-organs of + mandibles.—Auditory organs of insects and arachnids.—Poriferous chordotonal + organs.—Balancers of Diptera.—Resemblance to organs of + flabellum.—Racquet-organs of Galeodes.—Pectens of scorpions.—Large size of + nerve to all these special sense-organs.—Origin of parachordals and auditory + capsule.—Reason why VIIth nerve passes in and out of capsule.—Evidence of + Ammocœtes.—Intrusion of glandular mass round brain into auditory + capsule.—Intrusion of generative and hepatic mass round brain into base of + flabellum.—Summary.</p> + </div> + + <p>When speaking of the tripartite arrangement of the cranial nerves, an arrangement which gave + the clue to the meaning of the cranial segments, I spoke of the trigeminal as supplying the + sensory nerves to the skin in the head-region, and I compared this dorsal system of afferent + nerves to the system of epimeral nerves in Limulus which supply the prosomatic and mesosomatic + carapaces of Limulus with sensory fibres. I compared the ventral system of eye-muscle nerves with + the system of nerves supplying the segmental dorso-ventral somatic muscles of the prosomatic + region, and I compared the lateral system of mixed nerves with the nerves supplying the prosomatic + and mesosomatic appendages of Limulus. I compared, also, the optic nerves and the olfactory nerves + with the corresponding nerves in the same invertebrate group. My readers will see at once that one + well-marked group of nerves—the auditory and lateral line system—has been entirely + omitted up to the present, it has not even been mentioned in the scheme of the cranial segments; I + have purposely reserved its consideration until now, because the organs these nerves supply, + though situated in the skin, are of such a special character <span class="pagenum" + id="page356">{356}</span>as to form a category by themselves. These nerves cannot be classed among + the afferent nerves of the skin any more than the nerves of the optic and olfactory apparatus; + they require separate consideration. A very extensive literature has grown up on the subject of + this system of lateral line sense-organs and their innervation, the outcome of which is decisively + in favour of this system being classed with the sense-organs supplied by the auditory nerve, so + that in endeavouring to understand the position of the auditory nerve, we must always bear in mind + that any theory as to its origin must apply to the system of lateral line nerves as well.</p> + + <p>Now, although the auditory apparatus is common to all vertebrates, the lateral line system is + not found in any land-dwelling animals; it belongs essentially to the fishes, and is, therefore, + an old system so far as concerns the vertebrate group. Its sense-organs are arranged along the + lateral line of the fish, and, in addition, on the head-region in three well-marked lines known as + the supra-orbital, infra-orbital, and mandibular line systems. These sense-organs lie in the skin + in a system of canals, and are innervated by a special nervous system different to that + innervating adjacent skin-areas. The great peculiarity of their innervation consists in the fact + that their nerves all belong to the branchial system of nerves; no fibres arise in connection with + the trigeminal, but all of them in connection with the facial, glossopharyngeal and vagus nerves. + In other words, although organs in the skin, their nerve-supply belongs to the lateral nervous + system which supplies splanchnic and not somatic segments, a system which, according to the theory + advanced in this book, originated in the nerves supplying appendages. The conclusion, therefore, + is that in order to obtain some clue as to the origin of the sense-organs of this system in the + assumed palæostracan ancestor, we must examine the mesosomatic appendages and see whether they + possess any special sense-organs of similar function.</p> + + <p>Further, considering that the auditory organ is to be regarded as a specially developed member + of this system, we must especially look for an exceptionally developed organ in the region + supplied by the auditory nerve.</p> + + <p>The question of the origin of this system of lateral line sense-organs possesses a special + interest for all those who attempt to obtain a solution of the origin of vertebrates, for the + upholders of the view that the vertebrates have descended from annelids have always <span + class="pagenum" id="page357">{357}</span>found its strongest support in the similarity of two sets + of segmental organs found in annelids and vertebrates. On the one hand, great stress was laid upon + the similarity of the segmental excretory organs in the two groups of animals, as will be + discussed later; on the other, of the similarity of the segmentally arranged lateral + sense-organs.</p> + + <p>These lateral sense-organs of the annelids have been specially described by Eisig in the + Capitellidæ, and, according to Lang, "there are many reasons for considering these lateral organs + to be homologous with the dorsal cirri of the ventral parapodia of other Polychæta, and in the + family of the Glyceridæ we can follow, almost step by step, the transformation of the cirri into + lateral organs." Eisig describes them in the thoracic prebranchial region as slightly different + from those in the abdominal branchial region; in the latter region, the ventral parapodia are + gill-bearing, so that these lateral organs are in the branchial region closely connected with the + branchiæ, just as is also the case in the vertebrates. It is but a small step from the + gill-bearing ventral parapodia of the annelid to the gill-bearing appendages of the phyllopod-like + protostracan; so that if we assume that this is the correct line along which to search for the + origin of the vertebrate auditory apparatus, then, on my theory of the origin of the vertebrates + from a group resembling the Protostraca, it follows that special sense-organs must have existed + either on or in close connection with the branchial and prebranchial appendages of the + protostracan ancestor of the vertebrates, which would form an intermediate link between the + lateral organs of the annelids and the lateral and auditory organs of the vertebrates.</p> + + <p>Further, these special sense-organs could not have been mere tactile hairs, but must have + possessed some special function, and their structure must have been compatible with that function. + Can we obtain any clear conception of the original function of this whole system of + sense-organs?</p> + + <p>A large amount of experimental work has been done to determine the function of the lateral line + organs in fishes, and they have been thought at one time or another to be supplementary organs for + equilibration, organs for estimating pressure, etc. The latest experimental work done by Parker + points directly to their being organs for estimating slow vibrations in water in contradistinction + to the quicker vibrations constituting sound. He concludes that surface wave-movements, whether + produced by air moving on the water or <span class="pagenum" id="page358">{358}</span>solid bodies + falling into the water, are accompanied by disturbances which are stimuli for the lateral line + organs.</p> + + <p>One of these segmental organs has become especially important and exists throughout the whole + vertebrate group, whether the animal lives on land or in water—this is the auditory organ. + Throughout, the auditory organ has a double function—the function of hearing and the + function of equilibration. If, then, this is, as is generally supposed, a specialized member of + the group, it follows that the less specialized members must possess the commencement of both + these functions, just as the experimental evidence suggests.</p> + + <p>In our search, then, for the origin of the auditory organ of vertebrates, we must look for + special organs for the estimation of vibrations and for the maintenance of the equilibrium of the + animal, situated on the appendages, especially the branchial or mesosomatic appendages; and, + further, we must specially look for an exceptional development of such segmental organs at the + junction of the prosomatic and mesosomatic regions.</p> + + <p>Throughout this book the evidence which I have put forward has in all cases pointed to the same + conclusion, viz. that the vertebrate arose by way of the Cephalaspidæ from some arthropod, either + belonging to, or closely allied to, the group called Palæostraca, of which the only living + representative is Limulus. If, then, my argument so far is sound, the appendages of Limulus, both + prosomatic and mesosomatic, ought to possess special sense-organs which are concerned in + equilibration or the appreciation of the depth of the water, or in some modification of such + function, and among these we might expect to find that somewhere at the junction of the prosoma + and mesosoma such sense-organs were specially developed to form the beginning of the auditory + organ.</p> + + <p class="sp3">Now, it is a striking fact that the appendages of Limulus do possess special + sense-organs of a remarkable character, which are clearly not simply tactile. Thus Gegenbaur, as + already stated, has drawn attention to the remarkable branchial sense-organs of Limulus; and + Patten has pointed out that special organs, which he considers to be gustatory in function, are + present on the mandibles of the prosomatic appendages. I myself, as mentioned in my address to the + British Association at Liverpool in 1896, searched for some special sense-organ at the junction of + the prosoma and mesosoma, and was rewarded by finding that that extraordinary adjunct to the <span + class="pagenum" id="page359">{359}</span>last locomotor appendage, known as the flabellum, was an + elaborate sense-organ. I now propose to show that all these special sense-organs are constructed + on a somewhat similar plan; that the structure of the branchial sense-organs suggests that they + are organs for the estimation of water pressures; that among air-breathing arthropods + sense-organs, built up on a somewhat similar plan, are universally found, and are considered to be + of the nature of auditory and equilibration organs; and, what is especially of importance, in view + of the fact that the most prominent members of the Palæostraca were the sea-scorpions, that the + remarkable sense-organs of the scorpions known as the pectens belong apparently to the same + group.</p> + + <p class="ac"><span class="sc">The Poriferous Sense-Organs of the Appendages in + Limulus.</span></p> + + <p>On all the branchial appendages in Limulus, special sense-organs are found of a most + conspicuous character. They form in the living animal bluish convex circular patches, the + situation of which on the appendages is shown in Fig. <a href="#fig58">58</a>. These organs are + not found on the non-branchial operculum. Gegenbaur, who was the first to describe them, has + pointed out how the surface of the organ is closely set with chitinous goblets shaped as seen in + Fig. <a href="#fig144">144</a>, A, which do not necessarily project free on the surface, but are + extruded on the slightest pressure. Each goblet fits into a socket in the chitinous covering, and + is apparently easily protruded by variations of pressure from within. The whole surface of the + organ on the appendage is slightly bulged in the living condition, and the chitin is markedly + softer here than in the surrounding part of the limb. Each of these organs is surrounded by a + thick protection of strongly branching spines. On the surface of the organ itself no spines are + found, only these goblets, so that the surface-view presents an appearance as in Fig. <a + href="#fig144">144</a>, B. Each goblet possesses a central pore, which is the termination of a + very fine, very tortuous, very brittle chitinous tubule (<i>ch.t.</i>), which passes from the + goblet through the layers of the chitin into the subjacent tissue. The goblets vary considerably + in size, a few very large ones being scattered here and there. The fine chitinous tubule is + especially conspicuous in connection with these largest goblets. In the smaller ones there is the + same appearance of a pore and a commencing tube, but I have not been able to trace the tube + through the chitinous layers, as in the case of the larger goblets.</p> + + <div><span class="pagenum" id="page360">{360}</span></div> + + <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:44%;"><a href="images/fig144.png" id="fig144"><img + style="width:100%" src="images/fig144.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:44%;"><a href="images/fig145.png" id="fig145"><img + style="width:100%" src="images/fig145.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 144.—A, A Goblet from one of the Branchial + Sense-organs of Limulus</span> (<i>ch.t.</i>, chitinous tubule); <span class="sc">B, + Surface View of a Portion of a Branchial Sense-Organ</span>.</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 145.—The Endognaths of Limulus pushed out of + the way on one side in order to show the position of the Flabellum</span> (<i>fl.</i>) + <span class="sc">projecting towards the crack between the Prosomatic and Mesosomatic + Carapaces</span>.</p> + </div> + </td> + </tr> + </table> + + <p>Gegenbaur, in his picture, draws a straight tubule passing from every goblet among the fine + canaliculi of the chitin. He says they are difficult to see, except in the case of the larger + goblets. The tubule from the larger goblets is most conspicuous, and is in my sections always + tortuous, never straight, as represented by Gegenbaur. A special branch of the appendage-nerve + passes to these organs, and upon the fine branches of this nerve groups of ganglion-cells are + seen, very similar in appearance to the groups described by Patten on the terminal branches of the + nerves which supply the mandibular organs. At present I can see no mechanism by which the goblets + are extruded or returned into place. In the case of the Capitellidæ, Eisig describes retractor + muscles by means of which the lateral sense-organs are <span class="pagenum" + id="page361">{361}</span>brought below the level of the surface, and he imagines that the + protrusion is effected by hydraulic means, by the aid of the vascular system. In the branchial + sense-organs of Limulus there are no retractor muscles, and it seems to me that both retraction + and protrusion must be brought about by alterations of pressure in the vascular fluids. Certainly + the cavity of the organ is very vascular. If this be so, it seems likely enough that such an organ + should be a very delicate organ for estimating changes in the pressure of the external medium, for + the position of the goblets would depend on the relation between the pressure of the fluid inside + the organ and that on the surface of the appendage. Whether the chitinous tubule contains a + nerve-terminal or not I am unable to decide from my specimens, but, judging from Patten's + description of the similar chitinous tubules belonging to the mandibular organs, it is most highly + probable that these tubules also contain a fine terminal nerve-fibre.</p> + + <p>These organs, then, represent segmental branchial sense-organs, of which it can be said their + structure suggests that they may be pressure-organs; but the experimental evidence is at present + wanting.</p> + + <p>Passing now from the branchial to the prosomatic region, the first thing that struck me was the + presence of that most conspicuous projection at the base of the last locomotor appendage, which is + usually called the flabellum, and has been described by Lankester as an exopodite of this + appendage. It is jointed on to the most basal portion of the limb (<i>cf.</i> Fig. <a + href="#fig155">155</a>), and projects dorsally from the limb into the open slit between the + prosomatic and mesosomatic carapace, as is seen in Fig. <a href="#fig145">145</a> (<i>fl.</i>). Of + its two surfaces, the undermost is very convex and the uppermost nearly flat from side to side, + the whole organ being bent, so that when the animal is lying half buried in sand, entirely covered + over by the prosomatic and mesosomatic carapaces except along this slit between the two, the upper + flat or slightly convex surface of the flabellum is exposed to any movement of water through this + slit, and owing to its possessing a joint, the direction of the whole organ can be altered to a + limited extent. The whole of this flat upper surface is one large sense-organ of a striking + character, thus forming a great contrast to the convex under surface, which is remarkably free + from tactile spines or special sense-organs.</p> + + <p>The nerve going to the flabellum is very large, almost as large as the nerve to the rest of the + appendage, and the very large majority <span class="pagenum" id="page362">{362}</span>of the + nerve-fibres turn towards the flat, uppermost side, where the sense-organ is situated. Between the + nerve-fibres (<i>n.</i>) and the chitinous surface containing the special sense-tubes masses of + cells (<i>gl.</i>) are seen, as in Fig. <a href="#fig146">146</a>, apparently nerve-cells, which + form a broad border between the nerve-fibres and the pigmented chitinogenous layer (<i>p.</i>). On + the opposite side, nothing of the sort intervenes between the pigmented layer and the blood-spaces + and nerve-fibres which constitute the central mass of the flabellum.</p> + + <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations"> + <tr class="vmi"> + <td class="vbm ac" style="width:46%;"><a href="images/fig146.png" id="fig146"><img + style="width:100%" src="images/fig146.png" alt="" title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 146.—Section through Flabellum.</span></p> + <p class="sp0"><i>ch.</i>, chitinous layers; <i>s.o.</i>, sense-organs; <i>sp.</i>, + spike-organ; <i>p.</i>, pigment layer; <i>gl.</i>, ganglion cell layer; <i>bl.</i> and + <i>n.</i>, blood-spaces and nerves.</p> + </div> + </td> + <td class="vbm ac" style="width:46%;"><a href="images/fig147.png" id="fig147"><img + style="width:100%" src="images/fig147.png" alt="" title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 147.—Section parallel to the Surface of + Flabellum, showing the Porous Terminations of the Sense-Organs and the Arrangement of the + Canaliculi round them.</span></p> + </div> + </td> + </tr> + </table> + + <p>At present I am inclined to look upon this mass of cells as constituting a large ganglion, + which extends over the whole length and breadth of the upper surface of the flabellum. At the same + <span class="pagenum" id="page363">{363}</span>time, my preparations are not sufficiently clear to + enable me to trace out the connections of these cells, especially their connections with the + special sense-organs.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig148.png" id="fig148"><img style="width:67%" src="images/fig148.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 148.—Section through the three Sense-Organs of + Flabellum.</span></p> + <p class="sp0"><i>bl.</i>, blood-spaces; <i>n.</i>, nerve; <i>gl.</i>, layer of + ganglion-cells; <i>p.</i>, pigment layer; <i>ch.</i>, 1, 2, 3, the three layers of chitin; + <i>ch.t.</i>, chitinous tubule in large tube of sense-organ; <i>cap.</i>, capitellum or + swollen extremity of large tube; <i>can.</i>, very fine porous canals or canaliculi of + chitin.</p> + </div> + </div> + + <p>In Fig. <a href="#fig148">148</a> I give a magnified representation of a section through three + of these flabellar sense-organs. As is seen, the section divides itself into four zones: (1) the + chitinous layer (<i>ch.</i>); (2) the layer of pigment (<i>p.</i>) and hypodermal cells; (3) the + layer of ganglion-cells (<i>gl.</i>); and (4) the layer of nerve-fibres (<i>n.</i>) and + blood-spaces (<i>bl.</i>). The chitinous layer is composed of the usual three zones of the Limulus + surface—externally (Fig. <a href="#fig148">148</a>), a thin homogeneous layer, followed by a + thick layer of chitin (3), in which the fine vertical tubules or canaliculi are well marked; the + external portion (2) of this layer is differentiated from the rest by the presence of well-marked + horizontal layers in addition to the canaliculi.</p> + + <p>In this chitinous layer the special sense-organs are found. They consist of a large tube which + passes through all the layers of the chitin except the thin homogeneous most external layer. <span + class="pagenum" id="page364">{364}</span>This tube is conical in shape, its base, which rests on + the pigmented layer, being so large and the organs so crowded together that a section of the + chitin across the base of the tubes gives the appearance of a honeycomb, the septa of which is all + that remains of the chitin. This large tube narrows down to a thin elongated neck as it passes + through the chitin, and then, at its termination, bulges out again into an oval swelling + (<i>cap.</i>) situated always beneath the homogeneous most external layer of chitin. Within this + tube a fine chitinous tubule (<i>ch. t.</i>) is situated similar to that seen in the branchial + sense-organs; it lies apparently free in the tube, not straight, but sinuous, and it passes right + through all the chitinous layers to open at the surface as a pore; in the last part of its course, + where it passes through the most external layer (1) of chitin, it lies always at right angles to + the surface.</p> + + <p>If the flabellum be stained with methylene blue and acid fuchsin, then all the canaliculi in + the chitin show up as fine red lines, and present the appearance given in Fig. <a + href="#fig148">148</a>, and it is seen that each of the terminations of the tubules is surrounded + in the homogeneous layer of chitin by a thick-set circular patch of canaliculi which pass to the + very surface of the chitin, while the canaliculi in other parts terminate at the commencement of + the homogeneous layer and do not reach the surface. Further, the contents of the oval swelling, + and, indeed, of the tube as a whole, are stained blue, the chitinous tubule being either unstained + or slightly pink in colour. We see, then, that the chitinous tubule alone reaches the surface, + while the large tube, which contains the tubule, terminates in an oval swelling, which often + presents a folded or wrinkled appearance, as in Fig. <a href="#fig149">149</a> (see also Patten's + Fig. 1, Plate I.). This terminal bulging of the tube is reminiscent of the bulging in the + chitinous tubes of the lyriform organs of the Arachnida, as described by Gaubert, and of the + poriferous chordotonal organs in insects, as described by Graber (see Fig. <a + href="#fig150">150</a>). This terminal swelling is filled with a homogeneous refringent mass + staining blue with methylene blue, in which I have seen no trace of a nucleus; through this the + chitinous tubule makes its way without any sign of bulging on its part. Patten, in his description + of the sense-organs on the mandibles of Limulus, which are evidently the same in structure as + those on the flabellum, refers to this homogeneous mass as a coagulum. I doubt whether this is an + adequate description; it appears to me to stain rather more <span class="pagenum" + id="page365">{365}</span>readily than a blood-coagulum, yet in the sense of being structureless it + resembles a coagulum.</p> + + <p>The enormous number of these organs crowded together over the whole flat surface of the + flabellum produces a very striking appearance when viewed on the surface. Such a view presents an + appearance resembling that of the surface-view of the branchial sense-organs; in both cases the + surface is covered with a great number of closely set circular plaques, in the centre of each of + which is seen a well-marked pore. The circular plaques in the case of the flabellum are much + smaller than those of the branchial sense-organs, and clearly are not protrusible as in the latter + organs, the appearance as of a plaque being due to the ring of thickly-set canaliculi round the + central tubule, as already described. When stained with methylene blue, the surface view of the + flabellum under a low power presents an appearance of innumerable circular blue masses, from each + of which springs a fine bent hair, terminating in a pore at the surface. The blue masses are the + homogeneous substance (<i>cap.</i>) of the bulgings seen through the transparent external layer of + chitin, and the hairs are the terminal part of the chitinous tubules. Patten has represented their + appearance in the mandibles in his Fig. 2, Plate I.</p> + + <p>The large tubes in the chitin alter in shape according to their position. Those in the middle + of the sensory surface of the flabellum, in their course through the chitinous layers, are hardly + bent at all; as they approach the two lateral edges of this surface, their long thin neck becomes + bent more and more, the bending always being directed towards the middle of the surface (see Fig. + <a href="#fig146">146</a>); in this way the chitinous tubules increase more or less regularly in + length from the centre of the organ to the periphery. The large basal part of the conical tube + contains, besides the chitinous tubule, a number of nuclei which are confined to this part of the + tube; some of these nuclei look like those belonging to nerve-fibres, others are apparently the + nuclei of the chitinogenous membrane lining the tube. I have never seen any sign of nerve-cells in + the tube itself.</p> + + <p>The only other kind of sense-organ I have found in connection with these sense-organs are a few + spike-like projections, the appearance of which is given in Fig. <a href="#fig149">149</a>. I have + always seen these in the position given in Fig. <a href="#fig146">146</a> (<i>sp.</i>), + <i>i.e.</i> at the junction of the surface which contains the sense-organs and the surface which + is free from them. They are, so far as I have seen, not very numerous; I have <span + class="pagenum" id="page366">{366}</span>not, however, attempted to examine the whole sense-organ + for the purpose of estimating their number and arrangement.</p> + + <p>As is seen in Fig. <a href="#fig149">149</a>, they possess a fine tubule of the same character + as that of the neighbouring sense-organs, which apparently terminates at the apex of the + projecting spike. They appear to belong to the same group as the other poriferous sense-organs, + and are of special interest, because in their appearance they form a link between the latter and + the poriferous sense-organs which characterize the pecten of the scorpion (<i>cf.</i> Fig. <a + href="#fig152">152</a>, C).</p> + + <div class="ac w25 fcenter sp2"> + <a href="images/fig149.png" id="fig149"><img style="width:100%" src="images/fig149.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 149.—Spike-Organ of Flabellum.</span></p> + <p class="sp0"><i>ch.t.</i>, chitinous tubule.</p> + </div> + </div> + + <p>Such, then, is the structure of this remarkable sense-organ of the flabellum, as far as I have + been able to work it out with the materials at my disposal. It is evident that the flabellar + organs, apart from the spike-organs, are of the same kind as those described by Patten on the + mandibles and chelæ of Limulus, and therefore it is most probable that the nerve-terminals in the + chitinous tubules, and the origin of the latter, are similar in the two sets of organs.</p> + + <p>These organs, as Patten has described them, are situated in lines on the spines of the + mandibles of the prosomatic locomotor appendages, and are grouped closely together to form a + compact sense-organ on the surface of the inner mandible (Lankester's epicoxite) (<i>i.m.</i> in + Fig. <a href="#fig155">155</a>), so that a surface-view of the organ here gives the characteristic + appearance of these poriferous sense-patches. Precisely similar organs are found on the chilaria, + which are, in function at all events, simply isolated mandibles, to use Patten's terminology.</p> + + <p>On the digging appendage (ectognath), as the comparison of Fig. <a href="#fig155">155</a>, A + and C, shows, the mandibular spines are almost non-existent, and the inner mandible or epicoxite + is not present, so that <span class="pagenum" id="page367">{367}</span>the special sense-organ of + this appendage is represented solely by the flabellum.</p> + + <p>This sketch of the special sense-organs of Limulus shows that all the appendages of Limulus + possess special sense-organs, with the exception of the operculum. All these sense-organs are + formed on the same plan, in that they possess a fine chitinous tubule passing through the layers + of chitin into the underlying hypodermal and nervous tissues, which terminates on the surface in a + pore. The surface of the chitin where these pores are situated is perfectly smooth, although, in + the case of the branchial sense-organs, the goblet-shaped masses of chitin, each of which contains + a pore, are able to be pressed out beyond the level of the surface.</p> + + <p>As to their functions, we unfortunately do not know much that is definite. Patten considers + that he has evidence of a gustatory function in the case of the mandibular organs, and suggests + also a temperature-sense in the case of some of these organs. The large organ of the flabellum and + the branchial organs he has not taken into consideration. The situation of these organs puts the + suggestion of any gustatory function, as far as they are concerned, out of the question; and I do + not think it probable that such large specialized organs would exist only for the estimation of + temperature, when one sees how, in the higher animals, the temperature-nerves and the nerves of + common sensation are universally distributed over the body. As already stated, the structure of + the branchial organs seems to me to point to organs for estimating varying pressures more than + anything else, and I am strongly inclined to look upon the whole set of organs as the derivatives + of the lateral sense-organs of annelids, such as are described by Eisig in the Capitellidæ. This + is Patten's opinion with respect to the mandibular organs; and from what I have shown, these + organs cannot be separated in type of structure from those of the flabellum and the branchial + sense-organs.</p> + + <p>In our search, then, for the origin of the vertebrate auditory organ in Limulus and its allies, + we see so far the following indications<span class="wnw">:—</span></p> + + <p>1. The auditory organ of the vertebrate is regarded as a special organ belonging to a + segmentally arranged set of lateral sense-organs, whose original function was co-ordination and + equilibration.</p> + + <p>2. Such a set of segmentally arranged lateral sense-organs is found in annelids in connection + with the dorsal cirri of the ventral parapodia.</p> + + <div><span class="pagenum" id="page368">{368}</span></div> + + <p>3. If, as has been supposed, there is a genetic connection between (1) and (2) and if, as I + suppose, the vertebrates did not arise from the annelids directly, but from a protostracan group, + then it follows that the lateral sense-organs, one of which gave rise to the auditory organ, must + have been situated on the protostracan appendages.</p> + + <p>4. In Limulus, which is the sole surviving representative of the palæostracan group, such + special sense-organs are found on both the prosomatic and mesosomatic appendages, and therefore + may be expected to give a direct clue to the origin of the vertebrate auditory organ.</p> + + <p class="sp3">5. Both from its position, its size, and its specialization, the flabellum, + <i>i.e.</i> an organ corresponding to the flabellum, must be looked upon as more likely to give a + direct clue to the origin of the auditory organ than the sense-organs of the branchial appendages, + or the so-called gustatory organs of the mandibles.</p> + + <p class="ac"><span class="sc">The Auditory Organs of Arachnids and Insects.</span></p> + + <p>The difficulty of the investigating these organs consists in the fact that so little is known + about them in those Arthropoda which live in the water; the only instance of any organ apparently + of the nature of an auditory organ, is the pair of so-called auditory sacs at the base of the + antennæ in various decapods. We are in a slightly better position when we turn to the land-living + arthropods; here the presence of stridulating organs in so many instances carries with it the + necessity of an organ for appreciating sound. It has now been shown that such stridulating organs + are not confined to the Insecta, but are present also in the scorpion group, and I myself have + added to their number by the discovery of a distinct stridulating apparatus in various members of + the Phrynidæ. We may then take it for granted that arachnids as well as insects hear. Where is the + auditory organ?</p> + + <p>Many observers believe that certain surface-organs found universally among the spiders, to + which Gaubert has given the name of lyriform organs, are auditory in function. His investigations + show that they are universally present on the limbs and pro-meso-sternite of all spiders; that + they are present singly, not in groups, on the limbs of Thelyphonus, and that a group of them + exists on the second segment of each limb in the members of the Phrynus tribe. In the latter case + this organ is the most elaborate of all described by him.</p> + + <div><span class="pagenum" id="page369">{369}</span></div> + + <p>It is especially noticeable that they do not exist in Galeodes or in the scorpions, but in the + former special sense-organs are found in the shape of the so-called 'racquet-organs,' on the basal + segments of the most posterior pair of appendages, and also, according to Gaubert, on the + extremity of the palps and the first pair of feet, while in the latter they occur in the shape of + the pectens.</p> + + <p>This observation of Gaubert suggests that the place of the lyriform organs in other arachnids + is taken in Galeodes by the racquet-organs, and in the scorpions by the pectens. Bertkau, + Schimkéwitsch, and Wagner, as quoted by Gaubert, all suggest that the lyriform organs of the + arachnids belong to the same group of sense-organs as the porous chordotonal organs of the + Insecta, sense-organs which have been found in every group of Insecta, and are generally regarded + as auditory organs. Gaubert does not agree with this, and considers the lyriform organs to be + concerned with the temperature-sense rather than with audition.</p> + + <p>The chordotonal organs of insects have been specially studied by Graber. He divides them into + two groups, the poriferous and the non-poriferous, the former being characterized by the presence + of pores on the surface arranged in groups or lines. These poriferous chordotonal organs are + remarkably constant in position, being found only at the base of the wings on the subcostal ridge, + in marked contrast to the other group of chordotonal organs which are found chiefly on the + appendages in various regions. The striking character of this fixity of position of these organs + and the universality of their presence in the whole group, led Graber to the conclusion that in + these poriferous chordotonal organs we are studying a form of auditory apparatus which + characterized the ancestor of the insect-group. These organs are always well developed on the hind + wings, and in the large group of Diptera the auditory apparatus has usurped the whole of the + function of the wing; for the balancers or 'halteres,' as they are called, are the sole + representatives of the hind wings, and they are usually considered to be of the nature of auditory + organs. It is instructive to find that such an auditory organ serves not only for the purpose of + audition, but also as an organ of equilibration; thus Lowne gives the evidence of various + observers, and confirms it himself, that removal of the balancers destroys the power of orderly + flight in the animal.</p> + + <p>A striking peculiarity of these organs in the Insecta, as described <span class="pagenum" + id="page370">{370}</span>by Graber, is the bulging of the porous canal near its termination (Fig. + <a href="#fig150">150</a>, C). This bulging is filled with a homogeneous, highly refractive + material, from which, according to Lowne, a chordotonal thread passes, to be connected with a + ganglion-cell and nerve. This sphere of refractive material he calls the 'capitellum' of the + chordotonal thread. The presence of this material produces in a surface view an appearance as of a + halo around the terminal plaque with its central pore; Graber has attempted to represent this by + the white area round the central area (in Fig. <a href="#fig150">150</a>, B). A very similar + appearance is presented by the surface view of the flabellum in those parts where the tube runs + straight to the surface, so that the refractive material which fills the oval bulging shines + through the overlying chitin and appears to surround the terminal plaque with a translucent + halo.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig150.png" id="fig150"><img style="width:100%" src="images/fig150.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 150</span> (from <span + class="sc">Graber</span>).—<span class="sc">A, Section of Subcostal Nervure of Hind Wing + of Dytiscus to show patch of Poriferous Organs</span> (<i>s.o.</i>). <span class="sc">B, + Surface View of Poriferous Organs; the White Space round each Organ indicates the deeper lying + Refringent Body which fills the bulging of the Canal seen in Transverse Section in + C.</span></p> + </div> + </div> + + <p>Such a peculiarity must have a very definite meaning, and suggests that the canals in the + flabellum of Limulus and in the hind wings of insects belong to the same class of organ, the + chitinous tubule with its nerve-terminal in the former corresponding to the chordotonal thread in + the latter. One wonders whether this sphere of refractive material or 'capitellum' (to use Lowne's + phraseology) is so universally present in order to act as a damper upon the vibrations of the + chordotonal thread in the one case and of the <span class="pagenum" + id="page371">{371}</span>chitinous tubule in the other, just as the <i>membrana tectoria</i> and + the otoliths act in the case of the vertebrate ear.</p> + + <p>Patten says that the only organs which seem to him to be comparable with the gustatory porous + organs of Limulus are the sense-organs in the extremities of the palps and of the first pair of + legs of Galeodes, as described by Gaubert. I imagine that he was thinking only of arachnids, for + the comparison of his drawings with those of Graber show what a strong family resemblance exists + between the poriferous sense-organs of Limulus and those of the insects. On the course of the + terminal nerve-fibres, between the nerve-cell and their entrance into the porous chitinous canal, + Graber describes the existence of rods or scolophores. On the course of the terminal fibres in the + Limulus organ, between the nerve-cells and their entrance into the porous chitinous canal, Patten + describes a spindle-shaped swelling, containing a number of rod-like thickenings among the fibrils + in the spindle, which present an appearance reminiscent of the rods described by Graber.</p> + + <p class="sp3">It appears as though a type of sense-organ, characterized by the presence of pores + on the surface and a fine chitinous canal which opens at these pores, was largely distributed + among the Arthropoda. According to Graber, this kind of organ represents a primitive type of + sense-organ, which was probably concerned with audition and equilibration, and he expresses + surprise that similar organs have not been discovered among the Crustacea. It is, therefore, a + matter of great interest to find that so ancient a type of animal as Limulus, closely allied to + the primitive crustacean stock, <i>does</i> possess poriferous sense-organs upon its appendages + which are directly comparable with these poriferous chordotonal organs of the Insecta.</p> + + <p class="ac"><span class="sc">The Pectens of Scorpions.</span></p> + + <p>Among special sense-organs such as those with which I am now dealing, the pectens of scorpions + and the 'racquet-organs' of Galeodes must, in all probability, be classed. I have given my reasons + for this conclusion in my former paper.<a id="NtA_2" href="#Nt_2"><sup>[2]</sup></a> At present + such reasons are based entirely upon the structure of the organs; experimental <span + class="pagenum" id="page372">{372}</span>evidence as to their function is entirely wanting. With + respect to the pectens of the scorpion (Fig. <a href="#fig151">151</a>), it has been suggested + that they are of the nature of copulatory organs, a suggestion which may be dismissed without + hesitation, for they are not constructed after the fashion of claspers, but are simply elaborate + sense-organs, and, as such, are found equally in male or female. The only observer who has + hitherto specially studied the structure of the sense-organs in the pecten is, as far as I know, + Gaubert, and he describes their structure together with that of the sense-organs of the racquets + of Galeodes, in connection with the lyriform organs of arachnids, as though he recognized a family + resemblance between the three sets of organs.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig151.png" id="fig151"><img style="width:51%" src="images/fig151.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 151.—Under Surface of Scorpion + (Androctonus).</span></p> + <p class="sp0">The operculum is marked out with dots, and on each side of it is seen one of + the pectens.</p> + </div> + </div> + + <p>The pecten of the scorpions is an elaborate sense-organ, or rather group of sense-organs, the + special organ being developed on each tooth of the comb; its surface, which is frequently + flattened, being directed backwards and inwards, when the axis of the pecten is horizontal at + right angles to the length of the body. The surface view of this part of the tooth resembles that + of the branchial organs or of the flabellum in Limulus, in that it is thickly covered with + circular patches, in the centre of which an ill-defined appearance as of a fine pore is seen. In + Fig. <a href="#fig152">152</a>, B, I give a sketch of the surface view of a part of the organ.</p> + + <p>Transverse sections of a tooth of the comb of <i>Scorpio Europæus</i> present the appearance + given in Fig. <a href="#fig152">152</a>, A, and show that each of these circular patches is the + surface-view of a goblet-shaped chitinous organ, Fig. <a href="#fig152">152</a>, C, from the + centre of which a short, somewhat cylindrical chitinous spike projects. Within this spike, and + running through the goblet into the subjacent tissue, is a fine tubule. The series of goblets + gives rise to the appearance of the circular plaques on the surface-view, while the spike with its + tubule <span class="pagenum" id="page373">{373}</span>is the cause of the ill-defined appearance + of the central pore, just as the terminal pore is much less conspicuous on surface-view in the + spike-organs of the flabellum than in the purely poriferous organs, no part of which projects + beyond the level of the chitinous surface.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig152.png" id="fig152"><img style="width:100%" src="images/fig152.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 152.—A, Section through Tooth of Pecten of Scorpion; B, Surface + View of Sense-Organs; C, Goblet of Sense-Organ more highly magnified.</span></p> + <p class="sp0"><i>bl.</i> and <i>n.</i>, region of blood-spaces and nerves; <i>gl.</i>, + ganglion-cell layer; <i>ch.</i>, modified chitinous layer; <i>s.o.</i>, sense-organ.</p> + </div> + </div> + + <p>The fine tubule is soon lost in the thickened but soft modification of the chitinous layer + (<i>ch.</i>) which is characteristic of the sense-organ; at all events, I have not succeeded in + tracing it through this layer with any more success than in the corresponding case of the tubules + belonging to the smaller goblets of the branchial sense-organ of Limulus already described.</p> + + <p>At the base of the modified chitinous layer a series of cells is seen, many, if not all, of + which belong to the chitinogenous layer. Next to these is the marked layer of ganglion-cells + (<i>gl.</i>), similar to those seen in the flabellum of Limulus. The rest of the space in the + section of the tooth is filled up with nerves (<i>n.</i>) and blood-spaces (<i>bl.</i>) just as in + the section, Fig. <a href="#fig146">146</a>, of the flabellum of Limulus.</p> + + <p>Gaubert does not appear to have seen the goblets at all clearly; <span class="pagenum" + id="page374">{374}</span>he describes them simply as conical eminences, and states that they + "recouvrent un pore analogue a celui des poils mais plus petit; il est rempli par le protoplasma + de la couche hypodermique." From the ganglion, according to him, nervous prolongations pass, which + traverse the chitinogenous layer and terminate at the base of the conical eminences. Each of these + prolongations "présente sur son trajet, mais un peu plus près du ganglion que de sa terminaison + périphérique, une cellule nerveuse fusiforme (<i>g.</i>) offrant, comme celles du ganglion, un + gros noyau." He illustrates his description with the following, Fig. <a href="#fig153">153</a>, + taken from his paper.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig153.png" id="fig153"><img style="width:37%" src="images/fig153.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 153</span> (from <span class="sc">Gaubert</span>).—<span + class="sc">Section of a Tooth of Pecten of Scorpion.</span></p> + <p class="sp0"><i>n.</i>, nerve; <i>gl.</i>, ganglion.</p> + </div> + </div> + + <p>I have not been able to obtain any evidence of a fusiform nerve-cell on the course of the + terminal nerve-fibres as depicted by him; fusiform cells there are in plenty, as depicted in my + drawing, but none with a large nucleus resembling those of the main ganglion. In no case, either + in the flabellum or in the branchial organs of Limulus, or in the pecten-organs, have I ever seen + a ganglion-cell within the chitin-layer; all the nuclei seen there resemble those of the cells of + the hypodermis or else the elongated nuclei characteristic of the presence of nerve-fibres. + Gaubert's drawing is a striking one, and I have looked through my specimens to see whether there + was anything similar, but have hitherto failed to obtain any definite evidence of anything of the + kind.</p> + + <p>I feel, myself, that an exhaustive examination of the structure and function of the pecten of + scorpions ought to be undertaken. At present I can only draw the attention of my readers to the + similarity of the arrangement of parts, and of the nature of the end-organs, in the sense-organs + of the flabellum of Limulus and of the pecten of the scorpion. In both cases the special + nerve-fibres terminate in a massive ganglion, situated just below the chitinogenous layer. In both + cases the terminal fibres from these ganglion-cells pass through the modified chitinous layer to + supply end-organs of a striking character; and although the end-organ of the pecten of the + scorpion does <span class="pagenum" id="page375">{375}</span>not closely resemble the majority of + the end-organs of the flabellum, yet it does resemble, on the one hand, the isolated poriferous + spikes found on the flabellum (Fig. <a href="#fig149">149</a>) and, on the other, the poriferous + goblets found on the sense-patches of the branchial appendages of Limulus (Fig. <a + href="#fig144">144</a>, A), so that a combination of these two end-organs would give an appearance + very closely resembling that of the pecten of the scorpion.</p> + + <p>Finally, the special so-called 'racquet-organs' of Galeodes, which are found on the most basal + segments of the last pair of prosomatic appendages, ought also to be considered here. Gaubert has + described their structure, and shown how the nerve-trunk in the handle of the racquet splits up + into a great number of separate bundles, which spread out fan-shaped to the free edge of the + racquet; each of these separate bundles supplies a special sense-organ, which terminates as a + conical eminence on the floor of a deep groove, running round the whole free edge of the racquet. + This groove is almost converted into a canal, owing to the projection of its two sides. Gaubert + imagines that the sense-organs are pushed forward out of the groove to the exterior by the + turgescence of the whole organ; each of the nerve-fibres forming a bundle is, according to + Gaubert, connected with a nerve-cell before it reaches its termination.</p> + + <p>This sketch of the special sense-organs on the appendages of Limulus, of the scorpions, of + Galeodes, and other arachnids, and their comparison with the porous chordotonal organs of insects, + affords reason for the belief that we are dealing here with a common group of organs, which, + although their nature is not definitely known, have largely been accredited with the functions of + equilibration and audition, a group of organs among which the origin of the auditory organ of + vertebrates must be sought for, upon any theory of the origin of vertebrates from arthropods.</p> + + <p>Whenever in any animal these organs are concentrated together to form a special organ, it is + invariably found that the nerve going to this organ is very large, out of all proportion to the + size of the organ, and also that the nerve possesses, close to its termination in the organ, large + masses of nerve-cells. Thus, although the whole hind wing in the blow-fly has been reduced to the + insignificant balancers or 'halteres,' yet, as Lowne states, the nerves to them are the largest in + the body.</p> + + <p>The pectinal nerve in the scorpion is remarkable for its size, and <span class="pagenum" + id="page376">{376}</span>so, also, is the nerve to the flabellum in Limulus, while the large size + of the auditory nerve in the vertebrate, in distinction to the size of the auditory apparatus, has + always aroused the attention of anatomists.</p> + + <p>Throughout this book my attention has been especially directed to both Limulus and the scorpion + group in endeavouring to picture to myself the ancestor of the earliest vertebrates, because the + Eurypteridæ possessed such marked scorpion-like characteristics; so that in considering the origin + of a special sense-organ, such as the vertebrate auditory organ near the junction of the prosoma + and mesosoma, it seems to me that the presence of such marked special sense-organs as the + flabellum on the one hand and the pecten on the other, must both be taken into account, even + although the former is an adjunct to a prosomatic appendage, while the latter represents, + according to present ideas, the whole of a mesosomatic appendage.</p> + + <p>From the point of view that the VIIIth nerve represents a segment immediately posterior to that + of the VIIth, it is evident that an organ in the situation of the pecten, immediately posterior to + the operculum, <i>i.e.</i> according to my view, posterior to the segment originally represented + by the VIIth nerve, is more correctly situated than an organ like the flabellum, which belongs to + a segment anterior to the operculum.</p> + + <p class="sp3">On the other hand, from the point of view of the relationship between the scorpions + and the king-crabs, it is a possibly debatable question whether the pecten really belongs to a + segment posterior to the operculum. The position of any nerve in a series depends upon its + position of origin in the central nervous system, rather than upon the position of its peripheral + organ. Now, Patten gives two figures of the brain of the scorpion built up from serial sections. + In both he shows that the main portion of the pectinal nerve arises from a swelling, to which he + gives the name <i>ganglion nodosum</i>. This swelling arises on each side in close connection with + the origin of the most posterior prosomatic appendage-nerve, according to his drawings, and + posteriorly to such origin he figures a small nerve which he says supplies the distal parts of the + sexual organs. This nerve is the only nerve which can be called the opercular nerve, and + apparently arises posteriorly to the main part of the pectinal nerve. If this is so, it would + indicate that the pectens arose from sense-organs which were originally, like the flabella, + pre-opercular in position, but have shifted to a post-opercular position.</p> + + <div><span class="pagenum" id="page377">{377}</span></div> + + <p class="ac"><span class="sc">The Origin of the Parachordals and Auditory Cartilaginous + Capsule.</span></p> + + <p>In addition to what I have already said, there is another reason why a special sense-organ such + as the pecten is suggestive of the origin of the vertebrate auditory organ, in that such a + suggestion gives a clue to the possible origin of the parachordals and auditory cartilaginous + capsules.</p> + + <p>In the lower vertebrates the auditory organ is characterized by being surrounded with a + cartilaginous capsule which springs from a special part of the axial cartilaginous skeleton on + each side, known as the pair of parachordals. The latter, in Ammocœtes, form a pair of + cartilaginous bars, which unite the trabecular bars with the branchial cartilaginous basket-work. + They are recognized throughout the Vertebrata as distinct from the trabecular bars, thus forming a + separate paired cartilaginous element between the trabeculæ and the branchial cartilaginous + system, which of itself indicates a position for the auditory capsule between the prosomatic + trabeculæ and the mesosomatic branchial cartilaginous system.</p> + + <p>The auditory capsule and parachordals when formed are made of the same kind of cartilage as the + trabeculæ, <i>i.e.</i> of hard cartilage, and are therefore formed from a gelatin-containing + tissue, and not from muco-cartilage. Judging from the origin already ascribed to the trabeculæ, + viz. their formation from the great prosomatic entochondrite or plastron, this would indicate that + a second entochondrite existed in the ancestor of the vertebrate in the region of the junction of + the prosoma and mesosoma, which was especially connected with the sense-organ to which the + auditory organ owes its origin. This pair of entochondrites becoming cartilaginous would give + origin to the parachordals, and subsequently to the auditory capsules, their position being such + that the nerve to the operculum would be surrounded at its origin by the growth of cartilage.</p> + + <p>On this line of argument it is very significant to find that the scorpions do possess a second + pair of entochondrites, viz. the supra-pectinal entochondrites, situated between the nerve-cord + and the pectens, so that if the ancestor of the Cephalaspid was sufficiently scorpion-like to have + possessed a second pair of entochondrites and at the same time a pair of special sense-organs of + the nature either of <span class="pagenum" id="page378">{378}</span>the pectens or flabella, then + the origin of the auditory apparatus would present no difficulty.</p> + + <p class="sp3">It is also easy to see that the formation of the parachordals from entochondrites + homologous with the supra-pectinal entochondrites, would give a reason why the VIIth or opercular + nerve is involved with the VIIIth in the formation of the auditory capsule, especially if the + special sense-organ which gave origin to the auditory organ was originally a pre-opercular + sense-organ such as the flabellum, which subsequently took up a post-opercular position like that + of the pecten.</p> + + <p class="ac"><span class="sc">The Evidence of Ammocœtes.</span></p> + + <p>As to the auditory apparatus itself, we see that the elaborate organ for hearing—the + cochlea—has been evolved in the vertebrate phylum itself. In the lowest vertebrates the + auditory apparatus tends more and more to resolve itself into a simple epithelial sac, the walls + of which in places bear auditory hairs projecting into the sac, and in part form otoliths. Such a + simple sac forms the early stage of the auditory vesicle in Ammocœtes, according to + Shipley; subsequently, by a series of foldings and growings together, the chambers of the ear of + the adult Petromyzon, as figured and described by Retzius, are formed. Further, we see that + throughout the Vertebrata this sac was originally open to the exterior, the auditory vesicle being + first an open pit, which forms a vesicle by the approximating of its sides, the last part to close + being known as the <i>recessus labyrinthicus</i>; in many cases, as in elasmobranchs, this part + remains open, or communicates with the exterior by means of the <i>ductus endolymphaticus</i>.</p> + + <p>Judging, therefore, from the embryological evidence, it would appear that the auditory organ + originated as a special sense-organ, formed by modified epithelial cells of the surface, which + epithelial surface becoming invaginated, came to line a closed auditory vesicle under the surface. + This special sense-organ was innervated from a large ganglionic mass of nerve-cells, situated + close against the peripheral sense-cells, the axis-cylinder processes of which formed the sensory + roots of the nerve.</p> + + <p>Yet another peculiarity of striking significance is seen in connection with the auditory organ + of Ammocœtes. The opening of the cartilaginous capsule towards the brain is a large one + (Fig. <a href="#fig154">154</a>), and <span class="pagenum" id="page379">{379}</span>admits the + passage not only of the auditory and facial nerves, but also of a portion of the peculiar tissue + which surrounds the brain. The large cells of this tissue, with their feebly staining nuclei and + the pigment between them, make them quite unmistakable; and, as I have already stated, nowhere + else in the whole of Ammocœtes is such a tissue found. When I first noticed these cells + within the auditory capsule, it seemed to me almost impossible that my interpretation of them as + the remnant of the generative and hepatic tissue which surrounds the brain of animals such as + Limulus could be true, for it seemed too unlikely that a part of the generative system could ever + have become included in the auditory capsule. Still, they are undoubtedly there; and, as already + argued with respect to the substance round the brain, they must represent some pre-existing tissue + which was functional in the ancestor of Ammocœtes. If my interpretation is right, this + tissue must be generative and hepatic tissue, and its presence in the auditory capsule immediately + becomes a most important piece of evidence, for it proves that the auditory organ must have been + originally so situated that a portion of the generative and hepatic mass surrounding the cephalic + region of the nervous system followed the auditory nerve to the peripheral sense-organ.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig154.png" id="fig154"><img style="width:100%" src="images/fig154.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 154.—Transverse Section through Auditory Capsules + and Brain of Ammocœtes.</span></p> + <p class="sp0"><i>Au.</i>, auditory organ; <i>VIII</i>, auditory nerve; <i>gl.</i>, ganglion + cells of VIIIth nerve; <i>Au. cart.</i>, cartilaginous auditory capsule; <i>gen.</i>, cells of + old generative tissue round brain and in auditory capsule; <i>bl.</i>, blood-vessels</p> + </div> + </div> + + <div><span class="pagenum" id="page380">{380}</span></div> + + <p>Here there was a test of the truth of my theory ranking second only to the test of the median + eyes; the strongest possible evidence of the truth of any theory is given when by its aid new and + unexpected facts are brought to light. The theory said that in the group of animals from which the + vertebrates arose, a special sense-organ of the nature of an auditory organ must have existed on + the base of one of the appendages situated at the junction of the prosoma and mesosoma, and that + into this basal part of the appendage a portion of the cephalic mass of generative and hepatic + material must have made its way in close contiguity to the nerve of the special organ.</p> + + <p>The only living example which nearly approaches the ancient extinct forms from which, according + to the theory, the vertebrates arose, is Limulus, and, as has already been shown, in this animal, + in the very position postulated by the theory, a large special sense-organ—the + flabellum—exists, which, as already stated, may well have given rise to a sense-organ + concerned with equilibration and audition. If, further, it be found that a diverticulum of the + generative and hepatic material does accompany the nerve of the flabellum in the basal part of the + appendage, then the evidence becomes very strong that the auditory organ of Ammocœtes, + <i>i.e.</i> of the ancient Cephalaspids, was derived from an organ homologous with the flabellum; + that, therefore, the material round the brain of Ammocœtes was originally generative and + hepatic material; that, in fact, the whole theory is true, for all the parts of it hang together + so closely that, if one portion is accepted, all the rest must follow. As pointed out in my + address at Liverpool, and at the meeting of the Philosophical Society at Cambridge, it is a most + striking fact that a mass of the generative and hepatic tissue does accompany the flabellar nerve + into the basal part of this appendage. Into no other appendage of Limulus is there the slightest + sign of any intrusion of the generative and hepatic masses; nowhere, except in the auditory + capsule, is there any sign of the peculiar large-celled tissue which surrounds the brain and upper + part of the spinal cord of Ammocœtes. The actual position of the flabellum on the basal + part of the ectognath is shown in Fig. <a href="#fig155">155</a>, A, and in Fig. <a + href="#fig155">155</a>, B, I have removed the chitin, to show the generative and hepatic tissue + (<i>gen.</i>) lying beneath.</p> + + <p>The reason why, to all appearance, the generative and hepatic mass penetrates into the basal + part of this appendage only is apparent <span class="pagenum" id="page381">{381}</span>when we see + (as Patten and Redenbaugh have pointed out) to what part of the appendage the flabellum in reality + belongs.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig155.png" id="fig155"><img style="width:100%" src="images/fig155.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 155.—A, The Digging Appendage or Ectognath of Limulus; B, The + Middle Protuberance (2) of the Entocoxite opened, to show the Generative and Hepatic + Tissue</span> (<i>gen.</i>) <span class="sc">within it; C, One of the Prosomatic Locomotor + Appendages or Endognaths of Limulus, for comparison with A</span>.</p> + <p class="sp0 ac"><i>fl.</i>, flabellum; <i>cox.</i>, coxopodite; <i>ent.</i>, entocoxite; + <i>m.</i>, mandible; <i>i.m.</i>, inner mandible or epicoxite.</p> + </div> + </div> + + <p>Patten and Redenbaugh, in their description of the prosomatic appendages of Limulus, describe + the segments of the limbs as (1) the dactylopodite, (2) the propodite, (3) the mero- and + carpo-podites, (4) the ischiopodite, (5) the basipodite, and (6) the coxopodite (<i>cox.</i> in + Fig. <a href="#fig155">155</a>). Still more basal than the coxopodite is situated the entocoxite + (<i>ent.</i> in Fig. <a href="#fig155">155</a>), which is composed of three sclerites <span + class="pagenum" id="page382">{382}</span>or sensory knobs, to use Patten's description. The middle + one of these three sclerites enlarges greatly in the digging appendage, and grows over the + coxopodite to form the base from which the flabellum springs. Thus, as they have pointed out, the + flabellum does not belong to the coxopodite of the appendage, but to the middle sensory knob of + the entocoxite. Upon opening the prosomatic carapace, it is seen that the cephalic generative and + hepatic masses press closely against the internal surface of the prosomatic carapace and also of + the entocoxite, so that any enlargement of one of the sensory knobs of the entocoxite would + necessarily be filled with a protrusion of the generative and hepatic masses. This is the reason + why the generative and hepatic material apparently passes into the basal segment of the ectognath, + and not into that of the endognaths; it does not really pass into the coxopodite of the appendage, + but into an enlarged portion of the entocoxite, which can hardly be considered as truly belonging + to the appendage. Kishinouye has stated that a knob arises in the embryo at the base of each of + the prosomatic locomotor appendages, but that this knob develops only in the last or digging + appendage (ectognath) forming the flabellum. Doubtless the median sclerites of the entocoxites of + the endognaths represent Kishinouye's undeveloped knobs.</p> + + <p>I conclude, therefore, that the flabellum, together with its basal part, is an adjunct to the + appendage rather than a part of it, and might, therefore, easily remain as a separate and + well-developed entity, even although the appendage itself dwindled down to a mere tentacle.</p> + + <p>The evidence appears to me very strong that the flabellum of Limulus and the pecten of + scorpions are the most likely organs to give a clue to the origin of the auditory apparatus of + vertebrates. At present both the Eurypterids and Cephalaspids have left us in the lurch; in the + former there is no sign of either flabellum or pecten; in the latter, no sign of any auditory + capsule beyond Rohon's discovery of two small apertures situated dorsally on each side of the + middle line in Tremataspis, which he considers to be the termination of the <i>ductus + endolymphaticus</i> on each side. In both cases it is probable, one might almost say certain, that + any such special sense-organ, if present, was not situated externally, but was sunk below the + surface as in Ammocœtes.</p> + + <p class="sp3">The method by which such a sense-organ, situated externally on <span + class="pagenum" id="page383">{383}</span>the surface of the animal, comes phylogenetically to form + the lining wall of an internally situated membranous capsule is given by the ontogeny of this + capsule, which shows step by step how the sense-organ sinks in and forms a capsule, and finally is + entirely removed from the surface except as regards the <i>ductus endolymphaticus</i>.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The special apparatus for hearing is of a very different character from that for vision or + for smell, for its nerve belongs to the infra-infundibular group of nerves, and not to the + supra-infundibular, as do those of the other two special senses. Of the five special senses the + nerves for touch, taste, and hearing, all belong to the infra-infundibular segmental + nerve-groups. The invertebrate origin, then, of the vertebrate auditory nerve must be sought for + in the infra-œsophageal segmental group of nerves, and not in the + supra-œsophageal.</p> + <p>The organs supplied by the auditory nerve are only partly for the purpose of hearing; there + is always present also an apparatus—the semicircular canals—concerned with + equilibration and co-ordination of movements. Such equilibration organs are not confined to the + auditory nerve, but in the water-living vertebrates are arranged segmentally along the body, + forming the organs of the lateral line in fishes; the auditory organ is but one of these lateral + line organs, which has been specially developed.</p> + <p>These lateral line organs have been compared to similar segmental organs found in connection + with the appendages in worms, especially the respiratory appendages. In accordance with this + suggestion we see that they are all innervated from the region of the respiratory + nerves—the vagus, glosso-pharyngeal, and facial—nerves which originally supplied the + respiratory appendages of the palæostracan ancestor.</p> + <p>The logical conclusion is that the appendages of the Palæostraca possessed special + sense-organs concerned with the perception of special vibrations, especially in the mesosomatic + or respiratory region, and that somewhere at the junction of the prosoma and mesosoma, one of + these sense-organs was specially developed to form the origin of the vertebrate auditory + apparatus.</p> + <p>Impressed by this reasoning I made search for some specially striking sense-organ at the base + of one of the appendages of Limulus, at the junction of the prosoma and mesosoma, and was + immediately rewarded by the discovery of the extraordinary nature of the flabellum, which + revealed itself as an elaborate sense-organ supplied with a nerve out of all proportion to its + size. Up to this time no one had the slightest conception that this flabellum was a special + sense-organ; the discovery of its nature was entirely due to the logical following out of the + theory of the origin of vertebrates described in this book.</p> + <p>The structure of this large sense-organ is comparable with that of the sense-organs of the + pectens of the scorpion, and of many other organs found on the appendages of various members of + the scorpion group, of arachnids and <span class="pagenum" id="page384">{384}</span>other + air-breathing arthropods. Many of these organs, such as the lyriform organs of arachnids, and + the 'halteres' or balancers of the Diptera, are usually regarded as auditory and equilibration + organs.</p> + <p>On all the mesosomatic appendages of Limulus very remarkable sense-organs are found, + apparently for estimating pressures, which, when the appendages sank into the body to form with + their basal parts the branchial diaphragms of Ammocœtes, could easily be conceived as + remaining at the surface, and so giving rise to the lateral line organs.</p> + <p>Further confirmation of the view that an organ, such as the flabellum, must be looked upon as + the originator of the vertebrate auditory organ, is afforded by the extraordinary coincidence + that in Limulus a diverticulum of the generative and hepatic mass accompanies the flabellar + nerve into the basal part of the digging appendage, while in Ammocœtes, accompanying the + auditory nerve into the auditory capsule, there is seen a mass of cells belonging to that + peculiar tissue which fills up the space between the brain and the cranial walls, and has + already, on other grounds, been homologized with the generative and hepatic masses which fill up + the encephalic region of Limulus.</p> + <p class="sp0">For all these reasons special sense-organs, such as are found in the flabellum of + Limulus and in the pectens of scorpions, may be looked upon as giving origin to the vertebrate + auditory apparatus. In such case it is highly probable that the parachordals, with the auditory + capsules attached, arose from a second entochondrite of the same nature as the plastron; a + probability which is increased by the fact that the scorpion does possess a second + entochondrite, which, owing to its special relations to the pecten, is known as the + supra-pectinal entochondrite.</p> + </div> + + <div><span class="pagenum" id="page385">{385}</span></div> + + <p class="ac">CHAPTER XII</p> + + <p class="ac"><i>THE REGION OF THE SPINAL CORD</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Difference between cranial and spinal regions.—Absence of lateral + root.—Meristic variation.—Segmentation of cœlom.—Segmental excretory + organs.—Development of nephric organs; pronephric, mesonephric, + metanephric.—Excretory organs of Amphioxus.—Solenocytes.—Excretory organs of + Branchipus and of Peripatus, appendicular and somatic.—Comparison of cœlom of + Peripatus and of vertebrate.—Pronephric organs compared to coxal glands.—Origin of + vertebrate body-cavity (metacœle).—Segmental duct.—Summary of formation of + excretory organs.—Origin of somatic trunk-musculature.—Atrial cavity of + Amphioxus.—Pleural folds.—Ventral growth of pleural folds and somatic + musculature.—Pleural folds of Cephalaspidæ and of Trilobita.—Significance of the + ductless glands.—Alteration in structure of excretory organs which have lost their duct in + vertebrates and in invertebrates.—Formation of lymphatic glands.—Segmental coxal + glands of arthropods and of vertebrates.—Origin of adrenals, pituitary body, thymus, + tonsils, thyroid, and other ductless glands.—Summary.</p> + </div> + + <p>The consideration of the auditory nerve and the auditory apparatus terminates the comparison + between the cranial nerves of the vertebrate and the prosomatic and mesosomatic nerves of the + arthropod, and leaves us now free to pass on to the consideration of the vertebrate spinal nerves + and the organs they supply. Before doing so, it is advisable to pass in review the conclusions + already attained.</p> + + <p>Starting with the working hypothesis that the central nervous system of the vertebrate has + arisen from the central nervous system of the arthropod, but has involved and enclosed the + alimentary canal of the latter in the process, so that there has been no reversal of surfaces in + the derivation of the one form from the other, we have been enabled to compare closely all the + organs of the head-region in the two groups of animals, and in no single case have we been + compelled to make any startling or improbable assumptions. The simple following out of this clue + has led in every case in the most natural <span class="pagenum" id="page386">{386}</span>manner to + the interpretation of all the organs in the head-region of the vertebrate from the corresponding + organs of the arthropod.</p> + + <p>That it is possible to bring together all the striking resemblances between organs in the two + classes of animals, such as I have done in preceding chapters, has been ascribed to a perverted + ingenuity on my part—a suggestion which is flattering to my imaginative powers, but has no + foundation of fact. There has been absolutely no ingenuity on my part; all I have done is to + compare organs and their nerve-supply, as they actually exist in the two groups of animals, on the + supposition that there has been no turning over on to the back, no reversal of dorsal and ventral + surfaces. The comparison is there for all to read; it is all so simple, so self-evident that, + given the one clue, the only ingenuity required is on the part of those who fail to see it.</p> + + <p>The great distinction that has arisen between the two head-regions is the disappearance of + appendages as such, never, however, of important organs on those appendages. If the olfactory + organs of the one group were originally situated on antennules, the olfactory organs still remain, + although the antennules as such have disappeared. The coxal excretory organs at the base of the + endognaths remain and become the pituitary body. A special sense-organ, such as the flabellum of + Limulus or the pecten of scorpion, remains and gives rise to the auditory organ. A special + glandular organ, the uterus in the base of the operculum, remains, and gives rise to the thyroid + gland. The branchiæ and sense-organs on the mesosomatic appendages remain, and even the very + muscles to a large extent. As will be seen later, the excretory organs at the base of the + metasomatic appendages remain. It is merely the appendage as such which vanishes either by + dwindling away, or by so great an alteration as no longer to be recognizable as an appendage.</p> + + <p>This dwindling process was already in full swing before the vertebrate stage; it is only a + continuation of a previous tendency, as is seen in the dwindling of the prosomatic appendages in + the Merostomata and the inclusion of the branchiæ within the body of the scorpion. Already among + the Palæostraca, swimming had largely taken the place of crawling. The whole gradual + transformation from the arthropod to the vertebrate is associated with a transformation from a + crawling to a swimming animal—with the concomitant loss of locomotor appendages as such, and + the alteration of the shape of <span class="pagenum" id="page387">{387}</span>the animal into the + lithe fish-like form. The consideration of the manner in which this latter change was brought + about, takes us out of the cranial into the spinal region.</p> + + <p>If we take Limulus as the only living type of the Palæostraca, we are struck with the fact that + the animal consists to all intents and purposes of prosomatic and mesosomatic regions only; the + metasoma consisting of the segments posterior to the mesosoma is very insignificant, so that the + large mass of the animal consists of what has become the head-region in the vertebrate; the spinal + region, which has become in the higher vertebrates by far the largest region of the body, can + hardly be said to exist in such an animal as Limulus. As to the Eurypterids and others, similar + remarks may be made, though not to the same extent, for in them a distinct metasoma does + exist.</p> + + <p>In this book I have considered up to the present the cranial region as a system of segments, + and shown how such segments are comparable, one by one, with the corresponding segments in the + prosoma and mesosoma of the presumed arthropod ancestor.</p> + + <p>In the spinal region such direct comparison is not possible, as is evident on the face of it; + for even among vertebrates themselves the spinal segments are not comparable one by one, so great + is the variation, so unsettled is the number of segments in this region. This meristic variation, + as Bateson calls it, is the great distinctive character of the spinal region, which distinguishes + it from the cranial region with its fixed number of nerves, and its substantive rather than + meristic variation. At the borderland, between the two regions, we see how the one type merges + into the other; how difficult it is to fix the segmental position of the spino-occipital nerves; + how much more variable in number are the segments supplied by the vagus nerves than those anterior + to them.</p> + + <p>This meristic variation is a sign of instability, of want of fixedness in the type, and is + evidence, as already pointed out, that the spinal region is newer than the cranial. This + instability in the number of spinal segments does not necessarily imply a variability in the + number of segments of the metasoma of the invertebrate ancestor; it may simply be an expression of + adaptability in the vertebrate phylum itself, according to the requirements necessitated by the + conversion of a crawling into a swimming animal, and the subsequent conversion of the swimming + into a terrestrial or flying animal.</p> + + <div><span class="pagenum" id="page388">{388}</span></div> + + <p>However many may have been the original number of segments belonging to the spinal region, one + thing is certain—the segmental character of this region is remarkably clearly shown, not + only by the presence of the segmental spinal nerves, but also by the marked segmentation of the + mesoblastic structures. The question, therefore, that requires elucidation above all others is the + origin of the spinal mesoblastic segments, <i>i.e.</i> of the cœlomic cavities of the + trunk-region, and the structures derived from their walls.</p> + + <p>Proceeding on the same lines as in the case of the cranial segments, it is necessary in the + first instance to inquire of the vertebrate itself as to the scope of the problem in this region. + In addition to the variability in the number of segments so characteristic of the spinal region, + the complete absence in each spinal segment of a lateral root affords another marked difference + between the two regions. Here, except, of course, at the junction of the spinal and cranial + regions, each segmental nerve arises from two roots only, dorsal and ventral, and these roots are + separately sensory and motor, and not mixed in function as was the lateral root of each cranial + segment. Now, these lateral roots were originally the nerves supplying the prosomatic and + mesosomatic appendages with motor as well as sensory fibres. The absence, therefore, of lateral + roots in the spinal region implies that in the vertebrate none of the musculature belonging to the + metasomatic appendages has remained. Consequently, as far as muscles are concerned, the clue to + the origin of the spinal segments must be sought for in the segmentation of the body-muscles.</p> + + <p>Here, in contradistinction to the cranial region, the segmentation is most marked, for the + somatic spinal musculature of all vertebrates can be traced back to a simple sheet of longitudinal + ventral and dorsal muscles, such as are seen in all fishes. This sheet is split into segments or + myotomes by transverse connective tissue septa or myo-commata; each myotome corresponding to one + spinal segment.</p> + + <p>In addition to the evidence of segmentation afforded by the body-musculature in all the higher + vertebrates, similar evidence is given by the segmental arrangement of parts of the supporting + tissue to form vertebræ. Such segments have received the name of sclerotomes, and each sclerotome + corresponds to one spinal segment.</p> + + <p>Yet another marked peculiarity of this region is the segmental arrangement of the excretory + organs. Just as our body-musculature <span class="pagenum" id="page389">{389}</span>has arisen + from the uniformly segmented simple longitudinal musculature of the lowest fish, so, as we pass + down the vertebrate phylum, we find more and more of a uniform segmental arrangement in the + excretory organs.</p> + + <p class="sp3">The origin of all these three separate segmentations may, in accordance with the + phraseology of the day, be included in the one term—the origin of the spinal mesoblastic + segments—<i>i.e.</i> of the cœlomic cavities of the trunk-region and the structures + derived from their walls.</p> + + <p class="ac"><span class="sc">The Origin of the Segmental Excretory Organs.</span></p> + + <p>Of these three clues to the past history of the spinal region, the segmentation manifested by + the presence of vertebræ is the least important, for in Ammocœtes there is no sign of + vertebræ, and their indications only appear at transformation. Especially interesting is the + segmentation due to the excretory organs, for the evidence distinctly shows that such excretory + organs have steadily shifted more and more posteriorly during the evolution of the vertebrate.</p> + + <p>In Limulus the excretory organs are in the prosomatic region—the coxal glands; these + become in the vertebrate the pituitary body.</p> + + <p>In Amphioxus the excretory organs are in the mesosomatic region, segmentally arranged with the + gills.</p> + + <p>In vertebrates the excretory organs are in the metasomatic region posterior to the gills, and + are segmentally arranged in this region. Their investigation has demonstrated the existence of + three distinct stages in these organs: 1. A series of segmental excretory organs in segments + immediately following the branchial segments. This is the oldest of the three sets, and to these + organs the name of the <i>pronephros</i> is given. 2. A second series which extends more + posteriorly than the first, overlaps them to an extent which is not yet settled, and takes their + place; to them is given the name of the <i>mesonephros</i>. 3. A third series continuous with the + mesonephric is situated in segments still more posterior, supplants the mesonephros and forms the + kidneys of all the higher vertebrates. This forms the <i>metanephros</i>.</p> + + <p>These three sets of excretory organs are not exactly alike in their origin, in that the + pronephric tubules are formed from a different portion of the cœlomic walls to that from + which the meso- and <span class="pagenum" id="page390">{390}</span>metanephric tubules are formed, + and the former alone gives origin to a duct, which forms the basis for the generative and urinary + ducts, and is called the <i>segmental duct</i>. The mesonephric tubules, called also the Wolffian + body, open into this duct.</p> + + <p>In order to make the embryology of these excretory organs quite clear, I will make use of van + Wijhe's phraseology and also of his illustrations. He terms the whole cœlomic cavity the + <i>procœlom</i>, which is divisible into a ventral unsegmented part, the body-cavity or + <i>metacœlom</i>, and a dorsal segmented part, the <i>somite</i>. This latter part again is + divided into a dorsal part—the <i>epimere</i>—and a part connecting the dorsal part + with the body-cavity, to which therefore he gives the name of <i>mesomere</i>.</p> + + <p>The cavity of the epimere disappears, and its walls form the muscle and cutis plates of the + body. The part which forms the muscles is known as the <i>myotome</i>, which separates off from + the mesomere, leaving the latter as a blind sac—the + <i>mesocœlom</i>—communicating by a narrow passage with the body cavity or + <i>metacœlom</i>. At the same time, from the mesomere is formed the <i>sclerotome</i>, + which gives rise to the skeletal tissues of the vertebræ, etc., so that van Wijhe's epimere and + mesomere together correspond to the original term, protovertebra, or somite of Balfour; and when + the myotome and sclerotome have separated off, there is still left the intermediate cell-mass of + Balfour and Sedgwick, <i>i.e.</i> the sac-like mesocœle of van Wijhe, the walls of which + give origin to the mesonephrotome or <i>mesonephros</i>. Further, according to van Wijhe, the + dorsal part of the unsegmented metacœlom is itself segmented, but not, as in the case of + the mesocœle, with respect to both splanchnopleuric and somatopleuric walls. The + segmentation is manifest only on the somatopleuric side, and consists of a distinct series of + hollow somatopleuric outgrowths, called by him <i>hypomeres</i>, which give rise to the + <i>pronephros</i> and the segmental duct.</p> + + <p>Van Wijhe considers that the whole metacœlom was originally segmented, because in the + lower vertebrates the segmentation reaches further ventral-wards, so that in Selachia the + body-cavity is almost truly segmental. Also in the gill-region of Amphioxus the cavities which are + homologous with the body-cavity arise segmentally.</p> + + <div><span class="pagenum" id="page391">{391}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig156.png" id="fig156"><img style="width:100%" src="images/fig156.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 156.—Diagrams to illustrate the Development of the + Vertebrate Cœlom.</span> (After <span class="sc">van Wijhe</span>.)</p> + <p class="sp0"><i>N.</i>, central nervous system; <i>Nc.</i>, notochord; <i>Ao.</i>, aorta; + <i>Mg.</i>, midgut. A, <i>My.</i>, myocœle; <i>Mes.</i>, mesocœle; <i>Met.</i>, + metacœle; <i>Hyp.</i>, hypomere (pronephric). B and C, <i>My.</i>, myotome; + <i>Mes.</i>, mesonephros; <i>S.d.</i>, segmental duct (pronephric); <i>Met.</i>, body + cavity.</p> + </div> + </div> + + <div><span class="pagenum" id="page392">{392}</span></div> + + <p>As is well known, Balfour and Semper were led, from their embryological researches, to compare + the nephric organs of vertebrates with those of annelids, and, indeed, the nature of the + vertebrate segmental excretory organs has always been the fact which has kept alive the belief in + the origin of vertebrates from a segmented annelid. These segmental organs thus compared were the + mesonephric tubules, and doubts arose, especially in the mind of Gegenbaur, as to the validity of + such a comparison, because the mesonephric tubules did not open to the exterior, but into a + duct—the segmental duct—which was an unsegmented structure opening into the cloaca; + also because the segmental duct, which was the excretory duct of the pronephros, was formed first, + and the mesonephric tubules only opened into it after it was fully formed. Further, the pronephros + was said to arise from an outbulging of the somatopleuric mesoblast, which extended over a limited + number of metameres, and was not segmental, but continuous. Gegenbaur and others therefore argued + that the original prevertebrate excretory organ was the pronephros and its duct, not the + mesonephros, from which they concluded that the vertebrate must have been derived from an + unsegmented type of animal, and not from the segmented annelid type.</p> + + <p>Such a view, however, has no further reason for acceptance, as it was based on wrong premises, + for Rückert has shown that the pronephros does arise as a series of segmental nephric tubules, and + is not unsegmented. He also has pointed out that in Torpedo the anterior part of the pronephric + duct shows indications of being segmented, a statement fully borne out by the researches of Maas + on Myxine, who gives the clearest evidence that in this animal the anterior part of the pronephric + duct is formed by the fusion of a series of separate ducts, each of which in all probability once + opened out separately to the exterior.</p> + + <p>Rückert therefore concludes that Balfour and Semper were right in deriving the segmental organs + of vertebrates from those of annelids, but that the annelid organs are represented in the + vertebrate, not by the mesonephric tubules, but by the pronephric tubules and their ducts, which + originally opened separately to the exterior. By the fusion of such tubules the anterior part of + the segmental duct was formed, while its posterior part either arose by a later cœnogenetic + lengthening, or is the only remnant of a series of pronephric tubules which originally extended + the whole length of the body, as suggested also by Maas and Boveri. Rückert therefore supposed + that the mesonephric tubules were a secondary set of nephric organs, which were not necessarily + directly derived from the annelid nephric organs.</p> + + <div><span class="pagenum" id="page393">{393}</span></div> + + <p>At present, then, Rückert's view is the one most generally accepted—the original annelid + nephric organs are represented by the pronephric tubules and the pronephric duct, not by the + mesonephric tubules, which are a later formation. This latter statement would hold good if the + mesonephric tubules were found entirely in segments posterior to those containing the pronephric + tubules; such, however, is said not to be the case, for the two sets of organs are said to overlap + in some cases; even when they exist in the same segments, the former are said always to be formed + from a more dorsal part of the cœlom than the pronephros, always to be a later formation, + and never to give any indication of communicating with the exterior except by way of the + pronephric duct.</p> + + <p>The recent observations of Brauer on the excretory organs of the Gymnophiona throw great doubt + on the existence of mesonephric and pronephric tubules in the same segment. He criticizes the + observations on which such statements are based, and concludes that, as in Hypogeophis, the + nephrotome which is cut off after the separation of the sclero-myotome gives origin to the + pronephros in the more anterior regions, just as it gives origin to the mesonephros in the more + posterior regions. In fact, the observations of van Wijhe and others do not in reality show that + two excretory organs may be formed in one segment, the one mesonephric from the remains of the + mesomere and the other pronephric from the hypomere, but rather that in such cases there is only + one organ—the pronephros—part of which is formed from the mesomere and part from the + hypomere. Brauer goes further than this, and doubts the validity of any distinction between + pronephros and mesonephros, on the ground of the former arising from a more ventral part of the + procœlom than the latter; for, as he says, it is only possible to speak of one part of the + somite as being more ventral than another part when both parts are in the same segment; so that if + pronephric and mesonephric organs are never in the same segment, we cannot say with certainty that + the former arises more ventrally than the latter.</p> + + <p>These observations of Brauer strongly confirm Sedgwick's original statement that the pronephric + and mesonephric organs are homodynamous organs, in that they are both derived from the original + serially situated nephric organs, the differences between them being of a subordinate nature and + not sufficient to force us to believe that the mesonephros is an organ of quite different origin + to the <span class="pagenum" id="page394">{394}</span>pronephros. So, also, Price, from his + investigations of the excretory organs of Bdellostoma, considers that in this animal both + pronephros and mesonephros are derived from a common embryonic kidney, to which he gives the name + <i>holonephros</i>.</p> + + <p>Brauer also is among those who conclude that the vertebrate excretory organs were derived from + those of annelids; he thinks that the original ancestor possessed a series of similar organs over + the whole pronephric and mesonephric regions, and that the anterior pronephric organs, which alone + form the segmental duct, became modified for a larval existence—that their peculiarities + were adaptive rather than ancestral. This last view seems to me very far-fetched, without any + sufficient basis for its acceptance. According to the much more probable and reasonable view, the + pronephros represents the oldest and original excretory organs, while the mesonephros is a later + formation. Brauer's evidence seems to me to signify that the pronephros, mesonephros, and + metanephros are all serially homologous, and that the pronephros bears much the same relation to + the mesonephros that the mesonephros does to the metanephros. The great distinction of the + pronephros is that it, and it alone, forms the segmental duct.</p> + + <p>We may sum up the conclusions at which we have now arrived as follows<span + class="wnw">:—</span></p> + + <p>1. The pronephric tubules and the pronephric duct are the oldest part of the excretory system, + and are distinctly in evidence for a few segments only in the most anterior part of the + trunk-region immediately following the branchial region. They differ also from the mesonephric + tubules by not being so clearly segmental with the myotomes.</p> + + <p>2. The mesonephric tubules belong to segments posterior to those of the pronephros, are + strictly segmental with the myotomes, and open into the pronephric duct.</p> + + <p>3. All observers are agreed that the two sets of excretory organs resemble each other in very + many respects, as though they arose from the same series of primitive organs, and, according to + Sedgwick and Brauer, no distinction of any importance does exist between the two sets of organs. + Other observers, however, consider that the pronephric organs, in part at all events, arise from a + part of the nephrocœle more ventral than that which gives origin to the mesonephric organs, + and that this difference in position of origin, combined <span class="pagenum" + id="page395">{395}</span>with the formation of the segmental duct, does constitute a true + morphological distinction between the two sets of organs.</p> + + <p>4. All the recent observers are in agreement that the vertebrate excretory organs strongly + indicate a derivation from the segmental organs of annelids.</p> + + <p>The very strongest support has been given to this last conclusion by the recent discoveries of + Boveri and Goodrich upon the excretory organs of Amphioxus. According to Boveri, the nephric + tubules of Amphioxus open into the dorsal cœlom by one or more funnels. Around each funnel + are situated groups of peculiar cells, called by him 'Fadenzellen,' each of which sends a long + process across the opening of the funnel. Goodrich has examined these 'Fadenzellen,' and found + that they are typical pipe-cells, or solenocytes, such as he has described in the nephridial + organs of various members of the annelid group Polychæta. Also, just as in the Polychæta, the + ciliated nephric tubule has no internal funnel-shaped opening into the cœlom, but + terminates in these groups of solenocytes. "Each solenocyte consists of a cell-body and nucleus + situated at the distal free extremity of a delicate tube; the proximal end of the tube pierces the + wall of the nephridial canal and opens into its lumen. A single long flagellum arising from the + cells works in the tube and projects into the canal."</p> + + <p>The exceedingly close resemblance between the organs of Amphioxus and those of Phyllodoce, as + given in his paper, is most striking, and, as he says, leads to the conclusion that the excretory + organs of Amphioxus are essentially identical with the nephridia of certain polychæte worms.</p> + + <p>It is to me most interesting to find that the very group of annelids, the Polychæta, which + possess solenocytes so remarkably resembling those of the excretory organs of Amphioxus, are the + highest and most developed of all the Annelida. I have argued throughout that the law of evolution + consists in the origination of successive forms from the dominant group then alive, dominance + signifying the highest type of brain-power achieved up to that time. The highest type among + Annelida is found in the Chætopoda; from them, therefore, the original arthropod type must have + sprung. This original group of Arthropoda gave rise to the two groups of Crustacea and Arachnida, + in my opinion also to the Vertebrata, and, as already mentioned, it is convenient to give it a + generalized <span class="pagenum" id="page396">{396}</span>name, the Protostraca, from which + subsequently the Palæostraca arose.</p> + + <p>The similarity between the excretory organs of Amphioxus and those of Phyllodoce suggests that + the protostracan ancestor of the vertebrates arose from the highest group of the + Chætopoda—the Polychæta. The evidence which I have already given points, however, strongly + to the conclusion that the vertebrate did not arise from members of the Protostraca near to the + polychæte stock, but rather from members in which the arthropod characters had already become well + developed—members, therefore, which were nearer the Trilobita than the Polychæta. Such early + arthropods would very probably have retained in part excretory organs of the same character as + those found in the original polychæte stock, and thus account for the presence of solenocytes in + the excretory organs of Amphioxus.</p> + + <p>In connection with such a possibility, I should like to draw attention to the observations of + Claus and Spangenberg on the excretory organs of Branchipus—that primitive phyllopod, which + is recognized as the nearest approach to the trilobites at present living. According to Claus, an + excretory apparatus exists in the neighbourhood of each nerve-ganglion, and Spangenberg finds a + perfectly similar organ in the basal segment of each appendage—a system, therefore, of + excretory organs as segmentally arranged as those of Peripatus. Claus considers that although + these organs formed an excretory system, it is not possible to compare them with the annelid + segmental organs, because he thought the cells in question arose from ectoderm. Now, the striking + point in the description of the excretory cells in these organs, as described both by Claus and + Spangenberg, is that they closely resemble the pipe-cells or solenocytes of Goodrich; each cell + possesses a long tube-like projection, which opens on the surface. They appear distinctly to + belong to the category of flame-cells, and resemble solenocytes more than anything else. According + to Goodrich, the solenocyte is probably an ectodermal cell, so that even if it prove to be the + case, as Claus thought, that these pipe-cells of Branchipus are ectodermal, they would still claim + to be derived from the segmental organs of annelids, especially of the Polychæta, being, to use + Goodrich's nomenclature, true nephridial organs, as opposed to cœlomostomes.</p> + + <p>These observations of Claus and Spangenberg suggest not only that the primitive arthropod of + the trilobite type possessed segmental <span class="pagenum" id="page397">{397}</span>organs in + every segment directly derived from those of a polychæte ancestor, but also that such organs were + partly somatic and partly appendicular in position. Such a suggestion is in strict accord with the + observations of Sedgwick on the excretory organs of the most primitive arthropod known, viz. + Peripatus, where also the excretory organs, which are true segmental organs, are partly somatic + and partly appendicular. Further, the excretory organs of the Scorpion and Limulus group are again + partly somatic and partly appendicular, receiving the name of coxal glands, because there is a + ventral projection of the gland into the coxa of the corresponding appendage.</p> + + <p>Judging from all the evidence available, it is probable that when the arthropod stock arose + from the annelids, simultaneously with the formation of appendages, the segmental somatic nephric + organs of the latter extended ventrally into the appendage, and thus formed a segmental set of + excretory organs, which were partly somatic, partly appendicular in position, and might therefore + be called coxal glands.</p> + + <p>As already stated, all investigators of the origin of the vertebrate excretory organs are + unanimous in considering them to be derived from segmental organs of the annelid type. I naturally + agree with them, but, in accordance with my theory, would substitute the words "primitive + arthropod" for the word "annelid," for all the evidence I have accumulated in the preceding + chapters points directly to that conclusion. Further, the most primitive of the three sets of + vertebrate segmental organs—the pronephros, mesonephros, and metanephros—is + undoubtedly the pronephros; consequently the pronephric tubules are those which I consider to be + more directly derived from the coxal glands of the primitive arthropod ancestor. Such a derivation + appears to me to afford an explanation of the difficulties connected with the origin of the + pronephros and mesonephros respectively, which is more satisfactory than that given by the direct + derivation from the annelid.</p> + + <p>The only living animal which we know of as at all approaching the most primitive arthropod type + is, as pointed out by Korschelt and Heider, Peripatus; and Peripatus, as is well known, possesses + a true cœlom and true cœlomic excretory organs in all the segments of the body. + Sedgwick shows that at first a true cœlom, as typical as that of the annelids, is formed in + each segment of the body, and that then this cœlom (which represents in the vertebrate van + Wijhe's pro-cœlom) <span class="pagenum" id="page398">{398}</span>splits into a dorsal and + a ventral part. In the anterior segments of the body the dorsal part disappears (presumably its + walls give origin to the mesoblast from which the dorsal body-muscles arise), while the ventral + part remains and forms a nephrocœle, giving origin to the excretory organs of the adult. + According to von Kennel, the cavity becomes divided into three spaces, which for a time are in + communication—a lateral (I.), a median (II.), and a dorso-median (III.). The dorso-median + portion becomes partitioned off, and this, as well as the greater part of the lateral portion, + which lies principally in the foot, is used up in providing elements for the formation of the + body- and appendage-muscles respectively and the connective tissue.</p> + + <p>In Fig. <a href="#fig157">157</a> I reproduce von Kennel's diagram of a section across a + Peripatus embryo, in which I. represents the lateral appendicular part of the cœlom, II. + the ventral somatic part, and III. the dorsal part which separates off from the ventral and + lateral parts, and, as its walls give origin largely to the body-muscles, may be called the + myocœle. The muscles of the appendages are formed from the ventral part of the original + procœlom, just as I have argued is the case with the muscles of the splanchnic segmentation + in vertebrates.</p> + + <p>Sedgwick states that the ventral part of the cœlom extends into the base of each + appendage, and there forms the end-sac of each nephric tubule, into which the nephric funnel + opens, thus forming a coxal gland; this end-sac or vesicle in the appendage is called by him the + internal vesicle (<i>i.v.</i>), because later another vesicle is formed from the ventral + cœlom in the body itself, close against the nerve-cord on each side, which he calls the + external vesicle (<i>e.v.</i>). (<i>Cf.</i> Fig. <a href="#fig158">158</a>, taken from Sedgwick.) + This second vesicle is, according to him, formed later in the development from the nephric tubule + of the internal vesicle, so that it discharges its contents to the exterior by the same opening as + the original tubule. Of course, as he points out, the whole system of internal and external + vesicles and nephric tubules are all simply derivatives of the original ventral part of the + cœlom or nephrocœle.</p> + + <div><span class="pagenum" id="page399">{399}</span></div> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig157.png" id="fig157"><img style="width:100%" src="images/fig157.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 157.—Transverse Section of Peripatus Embryo.</span> (After + <span class="sc">von Kennel</span>.)</p> + <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nerve-cord; <i>App.</i>, appendage; + <i>I</i>, <i>II</i>, <i>III</i>, the three divisions (lateral, median, and dorso-median) of + the cœlom.</p> + </div> + </div> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig158.png" id="fig158"><img style="width:100%" src="images/fig158.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 158.—Section of Peripatus.</span> (After <span + class="sc">Sedgwick</span>.)</p> + <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nerve-cord; <i>App.</i>, appendage; + <i>i.v.</i>, internal, and <i>e.v.</i>, external vesicles of the segmented excretory tubule + (coxal gland).</p> + </div> + </div> + + <p>Here, then, in Peripatus, and presumably, therefore, in members of the Protostraca, we see that + the original segmental organs of the annelid have become a series of nephric organs, which + extended into the base of the appendages, and may therefore be called coxal glands; also it is + clear, from Sedgwick's description, that if the appendages disappeared, the nephric organs would + still remain, not as coxal glands, but as purely somatic excretory glands. They would still be + homologous with the annelid segmental organs, or with the coxal glands, but would arise <i>in + toto</i> from a part of the ventral cœlom or nephrocœle, more dorsal than the former + appendicular part, because the appendages and their enclosed cœlom are always situated + ventrally to the body. Again, according to Sedgwick, the nephric tubules are connected with two + cœlomic vesicles, the one in the appendage the internal vesicle, and the other, the + so-called bladder, or the external vesicle, in the body itself, close against the nerve-cord. + Sedgwick appears to consider that either of these vesicles may form the end-sac of a nephric + tubule, for he discusses the question whether the single vesicle, which in each case gives origin + to the nephridia of the first three legs, corresponds to the internal or external vesicle. He + <span class="pagenum" id="page400">{400}</span>decides, it is true, in favour of the internal + vesicle, and therefore considers the excretory organ to be appendicular, <i>i.e.</i> a coxal + gland, in these segments as well as in those more posterior. Still, the very discussion shows that + in his opinion, at all events, the external vesicle might represent the end-sac of the tubule, in + the absence of the internal or appendicular vesicle.</p> + + <p>Such an arrangement as Sedgwick describes in Peripatus is the very condition required to give + rise to the pronephric and mesonephric tubules, as deduced by me from the consideration of the + vertebrate, and harmonizes and clears up the controversy about the mesonephros and pronephros in + the most satisfactory manner. Both pronephros and mesonephros are seen to be derivatives of the + original annelid segmental organs, not directly from an annelid, but by way of an arthropodan + ancestor; the difference between the two is simply that the pronephric organs were coxal glands, + and indicate, therefore, the presence of the original metasomatic appendages, while the + mesonephric organs were homologous organs, formed in segments of later origin which had lost their + appendages. For this reason the pronephros is said to be formed, in part at least, from a portion + of the cœlom situated more ventrally than the purely somatic part which gives rise to the + mesonephros. For this reason Sedgwick, Brauer, etc., can say that the mesonephros is strictly + homodynamous with the pronephros; while equally Rückert, Semon, and van Wijhe can say it is not + homodynamous, in so far that the two organs are not derived strictly from absolutely homologous + parts of the cœlom. For this reason Semon can speak of the mesonephros as a dorsal + derivative of the pronephros, just as Sedgwick says that the external or somatic vesicle of + Peripatus is a derivative of the appendicular nephric organ. For this reason the pronephros, or + rather a part of it, is always derived from the somatopleuric layer, for, as is clear from Miss + Sheldon's drawing, the part of the cœlom in Peripatus which dips into the appendage is + derived from the somatopleuric layer alone.</p> + + <p>Such a cœlom as that of Peripatus, Fig. <a href="#fig157">157</a>, would represent the + origin of the vertebrate cœlom, and would therefore represent the procœlom of van + Wijhe. In strict accordance with this, we see that it separates into a dorsal part, the walls of + which give origin to the somatic muscles, or at all events to the great longitudinal dorsal + muscles of the animal, and a ventral part, which forms a nephrocœle, <span class="pagenum" + id="page401">{401}</span>dips into the appendage, and gives origin to the muscles of the + appendage. In the vertebrate, after the somatic dorsal part or myocœle has separated off, a + ventral part is left, which forms a nephrocœle in the trunk-region, and gives origin to the + splanchnic striated muscles in the cranial region, <i>i.e.</i> to the muscles which, according to + my theory, were once appendicular muscles. This ventral nephrocœlic part is divisible in + the trunk into a segmented part, which forms the excretory organs proper, and an unsegmented part, + the metacœle or true body-cavity of the vertebrate.</p> + + <p>This comparison of the procœlom of the vertebrate and arthropod signifies that the + vertebrate metacœle was directly derived by ventral downgrowth from the arthropod + nephrocœle, so that if, as I suppose, the vertebrate nervous system represents the + conjoined nervous system and alimentary canal of the arthropod, then the vertebrate + metacœle, or body-cavity, must have been originally confined to the region on each side of + the central nervous system, and from this position have spread ventrally, to enclose ultimately + the new-formed vertebrate gut. This means that the body-cavity (metacœle) of the vertebrate + is not the same as the body-cavity of the annelid, but corresponds to a ventral extension of the + nephrocœle, or ventral part of such body-cavity.</p> + + <p>Such a phylogenetic history is most probable, because it explains most naturally and simply the + facts of the development of the vertebrate body-cavity; for the mesoblast always originates in the + neighbourhood of the notochord and central nervous system, and the lumen of the body-cavity always + appears first in that region, and then extends laterally and ventrally on each side until it + reaches the most ventral surface of the embryo, thus forming a ventral mesentery, which ultimately + disappears, and the body-cavity surrounds the gut, except for the dorsal mesentery. Thus Shipley, + in his description of the formation of the mesoblastic plates which line the body-cavity in + Ammocœtes, describes them as commencing in two bands of mesoblast situated on each side, + close against the commencing nervous system<span class="wnw">:—</span></p> + + <p>"These two bands are separated dorsally by the juxtaposition of the dorsal wall of the + mesenteron and the epiblast, and ventrally by the hypoblastic yolk-cells which are in contact with + the epiblast over two-thirds of the embryo. Subsequently, but at a much later date, the mesoblast + is completed ventrally by the downgrowth on <span class="pagenum" id="page402">{402}</span>each + side of these mesoblastic plates. The subsequent downward growth is brought about by the cells + proliferating along the free ventral edge of the mesoblast, these cells then growing ventralwards, + pushing their way between the yoke-cells and epiblast."</p> + + <p>The derivation of the vertebrate pronephric segmental organs from the metasomatic coxal glands + of a primitive arthropod would mean, if the segmental organs of Peripatus be taken as the type, + that such glands opened to the exterior on every segment, either at the base of the appendage or + on the appendage itself. It is taken for granted by most observers that the pronephric segmental + organs once opened to the exterior on each segment, and then, from some cause or other, ceased to + do so, and the separate ducts, by a process of fusion, came to form a single segmental duct, which + opened into the cloaca. Many observers have been led to the conclusion that the pronephric duct is + epiblastic in origin, although from its position in the adult, it appears far removed from all + epiblastic formations. However, at no time in the developmental history is there any clear + evidence of actual fusion of any part of the pronephric organ with the epidermis, and the latest + observer, Brauer, is strongly of opinion that there is never sufficiently close contact with the + epidermis to warrant the statement that the epiblastic cells take part in the formation of the + duct. All that can be said is, that the formation of the duct takes place at a time when the + pronephric diverticulum is in close propinquity to the epidermis, before the ventral downgrowth of + the myotome has taken place.</p> + + <p>The formation of the anterior portion of the pronephric duct is, according to Maas in Myxine, + and Wheeler in Petromyzon, undoubtedly brought about by the fusion of a number of pronephric + tubules, which, according to Maas, are clearly seen in the youngest specimens as separate + segmental tubes; each of these tubules is supplied by a capillary network from a segmental branch + of the aorta, as in the tubules of Amphioxus according to Boveri, and does not possess a + glomerulus.</p> + + <p>The posterior part of the duct into which the mesonephric tubules enter possesses also a + capillary network, which Maas considers to represent the original capillary network of a series of + pronephric tubules, the only remnant of which is the duct into which the mesonephric tubules open. + He therefore argues that the pronephric duct indicates a series of pronephric tubules, which + originally extended <span class="pagenum" id="page403">{403}</span>along the whole length of the + body, and were supplanted by the mesonephric tubules, which also belonged to the same + segments.</p> + + <p>I also think that the paired appendages which have left the pronephric tubules as signs of + their past existence, existed originally, in the invertebrate stage, on every segment of the body. + But I do not consider that such a statement is at all equivalent to saying that such pairs of + tubules must have existed upon every one of the segments existing at the present day; for it seems + to me that Rückert is much more likely to be right when he says that in Selachians the duct + clearly does grow back, and is not formed throughout <i>in situ</i>; so that he gives a double + explanation of the formation of the duct—a palingenetic anterior part formed by the fusion + of the extremities of the original excretory tubules, to which a posterior cœnogenetic + lengthening has been added.</p> + + <p>It does not seem to me at all necessary that the immediate invertebrate ancestor of the + vertebrate should have possessed excretory organs which opened out separately to the exterior on + each segment; a fusion may already have taken place in the invertebrate stage, and so a single + duct have been acquired for a number of organs. Such a suggestion has been made by Rückert, + because of the fact discovered by Cunningham and E. Meyer, that the segmental organs of <i>Lanice + conchilega</i> are on each side connected together by a single strong longitudinal canal. I would, + however, go further than this and say, that even although the nephric organs of the polychæte + ancestor opened out on every segment, and although the primitive arthropodan ancestor derived from + such polychæte possessed coxal glands which opened out either on to or at the base of each + appendage, similarly to those of Peripatus, yet the immediate arthropodan ancestor, with its + palæostracan affinities, may already have possessed metasomatic coxal glands, all of which opened + into a single duct, with a single opening to the exterior.</p> + + <p>Judging from Limulus, such was very probably the case, for Patten and Hazen have shown (1) that + the coxal glands of Limulus are segmental organs belonging to the prosomatic segments; (2) that + the organs belonging to the cheliceral and ectognathal segments are not developed; (3) that the + four glands belonging to the endognaths become connected together by a <i>stolon</i>, which + communicates with a single nephric duct, opening to the exterior on the basal segment of the 5th + prosomatic appendage (the last endognath). At <span class="pagenum" id="page404">{404}</span>no + time is there any evidence of any separate openings or any fusion with the ectoderm, such as might + indicate separate openings of these prosomatic coxal segmental organs. Thus we see that in + Limulus, which is presumably much nearer the annelid condition than the vertebrate, all evidence + of separate nephric ducts opening to the exterior on each prosomatic segment has entirely + disappeared, just as is the case in the metasomatic coxal glands (<i>i.e.</i> the pronephros) of + the vertebrate. What is seen in the prosomatic region of Limulus, and doubtless also of the + Eurypterids, may very probably have occurred in the metasomatic region of the immediate + invertebrate ancestors of the vertebrate, and so account for the single pronephric duct belonging + to a number of pronephric organs.</p> + + <p>The interpretation of these various embryological investigations may be summed up as + follows<span class="wnw">:—</span></p> + + <p>1. The ancestor of the vertebrates possessed a pair of appendages on each segment; into the + base of each of these appendages the segmental excretory organ sent a diverticulum, thus forming a + coxal gland.</p> + + <p>2. Such coxal glands, even in the invertebrate stage, may have discharged into a common duct + which opened to the exterior most posteriorly.</p> + + <p>3. Then, from some cause, the appendages were rendered useless, and dwindled away, leaving + only the pronephric organs to indicate their former presence. At the end of this stage the animal + possessed vertebrate characteristics.</p> + + <p>4. For the purpose of increasing mobility, of forming an efficient swimming instead of a + crawling animal, the body-segments increased in number, always, as is invariably the case, by the + formation of new ones between those already formed and the cloacal region, and so of necessity + caused an elongation of the pronephric duct. Into this there now opened the ducts of the segmental + organs formed by recapitulation, those, therefore, belonging to the + body-segments—mesonephric—having nothing to do with appendages, for the latter had + already ceased to exist functionally, and would not, therefore, be repeated with each meristic + repetition.</p> + + <p>This, so to speak, passive lengthening of the pronephric duct in consequence of the lengthening + of the early vertebrate body by the addition of metameres, each of which contained only + mesonephric and no pronephric tubules, is, to my mind, an example of a principle <span + class="pagenum" id="page405">{405}</span>which has played an important part in the formation of + the vertebrate, viz. that the meristic variation by which the spinal region of even the lowest of + existing vertebrates has been formed, has largely taken place in the vertebrate phylum itself, and + that such changes must be eliminated before we can picture to ourselves the pre-vertebrate + condition. As an example, I may mention the remarkable repetition of similar segments pictured by + Bashford Dean in Bdellostoma. Such repetition leads to passive lengthening of such parts as are + already formed but are not meristically repeated: such are the notochord, the vertebrate + intestine, the canal of the spinal cord, and possibly the lateral line nerve. The fuller + discussion of this point means the discussion of the formation of the vertebrate alimentary canal; + I will therefore leave it until I come to that part of my subject, and only say here that the + evidence seems to me to point to the conclusion that at the time when the vertebrate was formed, + the respiratory and cloacal regions were very close together, the whole of the metasoma being + represented by the region of the pronephros alone.</p> + + <p>Here, as always, the evidence of Ammocœtes tends to give definiteness to our + conceptions, for Wheeler points out that up to a length of 7 mm. the pronephros only is formed; + there is no sign of the more posteriorly formed mesonephros. Now we know, as pointed out in + Chapter VI., p. <a href="#page228">228</a>, this is the time of Kupffer's larval stage of + Ammocœtes. This is the period during which the invertebrate stage is indicated in the + ontogeny, so that, in accordance with all that has gone before, this means that the metasoma of + the invertebrate ancestor was confined to the region of the pronephros.</p> + + <p>Again, take Shipley's account of the development of Petromyzon. He says—</p> + + <p>"The alimentary canal behind the branchial region may be divided into three sections. + Langerhans has termed these the stomach, midgut, and hindgut, but as the most anterior of these is + the narrowest part of the whole intestine, it would, perhaps, be better to call it + œsophagus. This part of the alimentary canal lies entirely in front of the yolk, and is, + with the anterior region, which subsequently bears the gills, raised from the rest of the egg when + the head is folded off. It is supported by a dorsal mesentery, on each side of which lies the + head-kidney (pronephros)."</p> + + <p>Further on he says—</p> + + <div><span class="pagenum" id="page406">{406}</span></div> + + <p>"The hindgut is smaller than the midgut; its anterior limit is marked by the termination of the + spiral valve, which does not extend into this region. The two segmental ducts open into it just + where it turns ventrally to open to the exterior by a median ventral anus. Its lumen is from an + early stage lined with cells which have lost their yolk, and it is in wide communication with the + exterior from the first. This condition seems to be, as Scott suggests, connected with the + openings of the ducts of the pronephros, for this gland is completed and seems capable of + functioning long before any food could find its way through the midgut, or, indeed, before the + stomodæum has opened."</p> + + <p class="sp3">Is there no significance in this statement of Shipley? Even if it be possible to + find some special reason why the branchial and cloacal parts of the gut are freed from yolk and + lined with serviceable epithelium a long time before the midgut, why should a bit of the midgut, + which Shipley calls the œsophagus, which is connected with the region of the pronephros and + not of the branchiæ, differ so markedly from the rest of the midgut? Surely the reason is that the + branchial region of the gut, the pronephric region of the gut, and the cloacal region of the gut, + belong to a different and earlier phase in the phylogenetic history of the Ammocœtes than + does the midgut between the pronephric and cloacal regions. This observation of Shipley fits in + with and emphasizes the view that the original animal from which the vertebrate arose consisted of + a cephalic and branchial region, followed by a pronephric and cloacal region; the whole + intermediate part of the gut, which forms the midgut, with its large lumen and spiral valve, and + belongs to the mesonephric region, being a later formation brought about by the necessity of + increasing the length of the body.</p> + + <p class="ac"><span class="sc">The Origin of the Somatic Trunk-Musculature and the Formation of an + Atrial Cavity.</span></p> + + <p>Next comes the question, why was the pronephros not repeated in the meristic repetition that + took place during the early vertebrate stage? What, in fact, caused the disappearance of the + metasomatic appendages, and the formation of the smooth body-surface of the fish?</p> + + <p>The embryological evidence given by van Wijhe and others of the manner in which the original + superficially situated pronephros is <span class="pagenum" id="page407">{407}</span>removed from + the surface and caused to assume the deeper position, as seen in the later embryo, is perfectly + clear and uniform in all the vertebrate groups. The diagrams at the end of van Wijhe's paper, + which I reproduce here, illustrate the process which takes place. At first the myotome (Fig. <a + href="#fig159">159</a>, A) is confined to the dorsal region on each side of the spinal cord and + notochord. Then (Fig. <a href="#fig159">159</a>, B) it separates from the rest of the somite and + commences to extend ventrally, thus covering over the pronephros and its duct, until finally (Fig. + <a href="#fig159">159</a>, C) it reaches the mid-ventral line on each side, and the foundations of + the great somatic body-muscles are finally laid.</p> + + <p>In order, therefore, to understand how the obliteration of the appendages took place, we must + first find out what is the past history of the myotomes. Why are they confined at first to the + dorsal region of the body, and extend afterwards to the ventral region, forcing by their growth an + organ that was originally external in situation to become internal?</p> + + <p>In the original discussion at Cambridge, I was accused of violating the important principle + that in phylogeny we must look at the most elementary of the animals whose ancestors we seek, and + was told that the lowest vertebrate was Amphioxus, not Ammocœtes; that therefore any + argument as to the origin of vertebrates must proceed from the consideration of the former and not + the latter animal. My reply was then, and is still, that I was considering the cranial region in + the first place, and that therefore it was necessary to take the lowest vertebrate which possessed + cranial nerves and sense-organs of a distinctly vertebrate character, a criterion evidently not + possessed by Amphioxus. Such argument does not apply to the spinal region, so that, now that I + have left the cranial region and am considering the spinal, I entirely agree with my critics that + Amphioxus is likely to afford valuable help, and ought to be taken into consideration as well as + Ammocœtes. The distinction between the value of the spinal (including respiratory) and + cranial regions of Amphioxus for drawing phylogenetic conclusions is recognized by Boveri, who + says that, in his opinion, "Amphioxus shows simplicity and undifferentiation rather than + degeneration. If truly Amphioxus is somewhat degenerated, then it is so in its prehensile and + masticatory apparatus, its sense organs, and perhaps its locomotor organs, owing to its method of + living."</p> + + <div><span class="pagenum" id="page408">{408}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig159.png" id="fig159"><img style="width:100%" src="images/fig159.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 159.—Diagrams to illustrate the Development of the + Vertebrate Cœlom.</span> (After <span class="sc">van Wijhe</span>.)</p> + <p class="sp0"><i>N.</i>, central nervous system; <i>Nc.</i>, notochord; <i>Ao.</i>, aorta; + <i>Mg.</i>, midgut. A, <i>My.</i>, myocœle; <i>Mes.</i>, mesocœle; <i>Met.</i>, + metacœle; <i>Hyp.</i>, hypomere (pronephric). B and C, <i>My.</i>, myotome; + <i>Mes.</i>, mesonephros; <i>S.d.</i>, segmental duct (pronephric); <i>Met.</i>, + body-cavity.</p> + </div> + </div> + + <div><span class="pagenum" id="page409">{409}</span></div> + + <p>Hatschek describes in Amphioxus how the cœlom splits into a dorsal segmented portion, + the protovertebra, and a ventral unsegmented portion, the lateral plates. He describes in the + dorsal part the formation of myotome and sclerotome, as in the Craniota. Also, he describes how + the myotome is at first confined to the dorsal region in the neighbourhood of the spinal cord and + notochord, and subsequently extends ventrally, until, just as in Ammocœtes, the body is + enveloped in a sheet of somatic segmented muscles, the well-known myomeres.</p> + + <p>The conclusion to be drawn from this is inevitable. Any explanation of the origin of the + somatic muscles in Ammocœtes must also be an explanation of the somatic muscles in + Amphioxus, and conversely; so that if in this respect Amphioxus is the more primitive and simpler, + then the condition in Ammocœtes must be looked upon as derived from a more primitive + condition, similar to that found in Amphioxus. Now, it is well <span class="correction" + title="Original reads 'know'">known</span> that a most important distinction exists between + Amphioxus and Ammocœtes in the topographical relation of the ventral portion of this + muscle-sheet, for in the former it is separated from the gut and the body-cavity by the atrial + space, while in the latter there is no such space. Fürbringer therefore concludes, as I have + already mentioned, that this space has become obliterated in the Craniota, but that it must be + taken into consideration in any attempt at formulating the nature of the ancestors of the + vertebrate.</p> + + <p>Kowalewsky described this atrial space as formed by the ventral downgrowth of pleural folds on + each side of the body, which met in the mid-ventral line and enclosed the branchial portion of the + gut. According to this explanation, the whole ventral portion of the somatic musculature of the + adult Amphioxus belongs to the extension of the pleural folds, the original body-musculature being + confined to the dorsal region. This is expressed roughly on the external surface of Amphioxus by + the direction of the connective tissue septa between the myotomes (<i>cf.</i> Fig. <a + href="#fig162">162</a>, B). These septa, as is well known, bend at an angle, the apex of which + points towards the head. The part dorsal to the bend represents the part of the muscle belonging + to the original body; the part ventral to the bend is the pleural part, and represents the + extension into the pleural folds.</p> + + <p>Lankester and Willey have attempted to give another explanation of the formation of the atrial + cavity; they look upon it as originating from a ventral groove, which becomes a canal by the + meeting of two <span class="pagenum" id="page410">{410}</span>outgrowths from the metapleure on + each side. This canal then extends dorsalwards on each side, and so forms the atrial cavity; the + metapleure still remains in the adult; the somatic muscles in the epipleure of the adult are the + original body-muscles, and not extensions into an epipleuric fold, for there is no such fold.</p> + + <p>This explanation is a possible conception for the post-branchial portion of the atrium, but is + impossible for the branchial region; for, as Macbride points out, as must necessarily be the case, + the point of origin of the atrial wall is, in all stages of development, situated at the end of + the gill-slit. It shifts in position with the position of the gill-slit, but there can be no + backwards extension of the cavity. Macbride therefore agrees with Kowalewsky that the atrial + cavity is formed by the simultaneous ventral extension of pleural folds, and of the branchial part + of the original pharynx. Thus, in his summing up, he states: "In the larva practically the whole + sides and dorsal portion of the pharynx represent merely the hyper-pharyngeal groove and the + adjacent epithelium of the pharynx of the adult, the whole of the branchial epithelium of the + adult being represented by a very narrow strip of the ventral wall of the pharynx of the larva. + The subsequent disproportionate growth of this part of the pharynx of the larva, and of the + adjacent portion of the atrial cavity, has given the impression that the atrial cavity grew + upwards and displaced other structures, which is not the case."</p> + + <p>Further, van Wijhe states that the atrium extends beyond the atriopore right up to the anus, + just as must have been the case if the pleural folds originally existed along the whole length of + the body. His words are: "Allerdings hat sich das Atrium beim <i>Amphioxus lanceolatus</i> + eigenthümlich ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus, d.h. bis an + die Wurzel des Schwanzes ausdehnt."</p> + + <p>We get, therefore, this conception of the origin of the somatic musculature of the vertebrate. + The invertebrate ancestor possessed on each side, along the whole length of its body, a lateral + fold or pleuron which was segmented with the body, and capable of movement with the body, because + the dorsal longitudinal somatic muscles extended segmentally into each segment of the pleuron. By + the ventral extension of these pleural folds, not only was the smooth body-surface of the + vertebrate attained, but also the original appendages obliterated as such, leaving only as signs + of their existence the <span class="pagenum" id="page411">{411}</span>branchiæ, the pronephric + tubules, and the sense-organs of the lateral line system.</p> + + <p>Such an explanation signifies that the somatic trunk-musculature of the vertebrate was derived + from the dorsal longitudinal musculature of the body of the arthropod, and not from the ventral + longitudinal musculature, and that therefore in the primitive arthropod stage the equivalent of + the myotome of the vertebrate did not give origin to the ventral longitudinal muscles of the + invertebrate ancestor. Now, as I have said, von Kennel states that in the procœlom of + Peripatus a dorsal part (III. in Fig. <a href="#fig157">157</a>) is cut off which gives origin to + the dorsal body-musculature, while the ventral part which remains (I. and II. in Fig. <a + href="#fig157">157</a>) gives origin in its appendicular portion (I.) to the muscles of the + appendage, and presumably in its ventral somatic portion (II.) to the ventral longitudinal muscles + of the body. This dorsal cut-off part might be called the myotome, in the same sense as the + corresponding part of the procœlom in the vertebrate is called the myotome. In both cases + the muscles derived from it form only a part of the voluntary musculature of the animal, and in + both cases the muscles in question are the dorsal longitudinal muscles of the body, to which must + be added the dorso-ventral body-muscles. Now, the whole of my theory of the origin of vertebrates + arose from the investigation of the structure of the cranial nerves, which led to the conception + that their grouping is not, like the spinal, a dual grouping of motor and sensory elements, but a + dual grouping to supply two sets of segments, characterized especially by the different + embryological origin of their musculature. The one set I called the somatic segmentation, because + the muscles belonging to it were the great longitudinal body-muscles; the other I called the + splanchnic segmentation, because its muscles were those connected with the branchial and visceral + arches. According to my theory, this latter segmentation was due to the segmentation of the + appendages in the invertebrate ancestor; and in previous chapters, dealing as they do with the + cranial region, attention was especially directed to the way in which the position of the striated + splanchnic musculature could be explained by a transformation of the prosomatic and mesosomatic + appendages. Now, I am dealing with the metasomatic region, in which it is true the appendages take + a very subordinate place, but still something corresponding to the splanchnic segments of the + cranial region might fairly be expected to exist, and I therefore <span class="pagenum" + id="page412">{412}</span>desire to emphasize what appears to me to be the fact, that the + musculature, which in the region of the trunk would correspond to that derived from the ventral + segmentation of the mesoblast in the region of the head, may have arisen not only from the + musculature of the appendages, but also from the ventral longitudinal musculature of the body of + the invertebrate ancestor, for it seems probable that this latter musculature had nothing to do + with the origin of the great longitudinal muscles of the vertebrate body, either dorsal or + ventral.</p> + + <p>The way in which I imagine the obliteration of the atrial cavity to have taken place is + indicated in Fig. <a href="#fig160">160</a>, B, which is a modification of a section across a + trilobite-like animal as represented in Fig. <a href="#fig160">160</a>, A. As is seen, the pleural + folds on each side have nearly met the bulged-out ventral body-surface. A continuation of the same + process would give Fig. <a href="#fig160">160</a>, C, which is, to all intents and purposes, the + same as Fig. <a href="#fig159">159</a>, C, taken from van Wijhe, and shows how the segmental duct + is left in the remains of the atrial cavity. The lining walls of the atrial cavity are represented + very black, in order to indicate the presence of pigment, as indeed is seen in the corresponding + position in Ammocœtes. In these diagrams I have represented the median ventral surface as a + large bulged-out bag, without indicating any structures in it except the ventral extension of the + procœlom to form the metacœlom. At present I will leave the space between the + central nervous system and the ventral mesentery blank, as in the diagrams; in my next chapter I + will discuss the possible method of formation within this blank space of the notochord and midgut. + Boveri considers that the obliteration of the atrial cavity in the higher vertebrates is not + complete, but that its presence is still visible in the shape of the pronephric duct. The evidence + of Maas and others that the duct is formed by the fusion of the pronephric tubules is, it seems to + me, conclusive against Boveri's view; but yet, as may be seen from my diagrammatic figures, the + very place where one would expect to find the last remnant of the atrial cavity is exactly where + the pronephric duct is situated. For my own part I should expect to find evidence of a former + existence of an atrial cavity rather in the pigment round the pronephros and its duct than in the + duct itself.</p> + + <div><span class="pagenum" id="page413">{413}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig160.png" id="fig160"><img style="width:100%" src="images/fig160.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 160.—A, Diagram of Section through a Trilobite-like Animal; B, + Diagram to illustrate Suggested Obliteration of Appendages and the Formation of an Atrial + Cavity by the Ventral Extension of the Pleural Folds; C, Diagram to illustrate the Completion + of the Vertebrate Type by the Meeting of the Pleural Folds in the Mid-ventral Line and the + Obliteration of the Atrial Cavity.</span></p> + <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nervous system; <i>My.</i>, myotome; + <i>Pl.</i>, pleuron; <i>App.</i>, appendage; <i>Neph.</i>, nephrocœle; <i>Met.</i>, + metacœle; <i>S.d.</i>, segmental duct; <i>At.</i>, atrial chamber; <i>V.Mes.</i>, + ventral mesentery; <i>Mes.</i>, mesonephros. The dotted line represents the splanchnopleuric + mesoblast in all figures.</p> + </div> + </div> + + <div><span class="pagenum" id="page414">{414}</span></div> + + <p>The conception that Amphioxus shows us how to account for the great envelope of somatic muscles + which wraps round the vertebrate body, in that the ancestor of the vertebrate possessed on each + side the body a segmented pleuron, is exactly in accordance with the theory of the origin of + vertebrates deduced from the study of Ammocœtes, as already set forth in previous chapters. + For we see that one of the striking characteristics of such forms as Bunodes, Hemiaspis, etc., is + the presence of segmented pleural flaps on each side of the main part of the body; and if we pass + further back to the great group of trilobites, we find in the most manifold form, and in various + degrees of extent, the most markedly segmented pleural folds. In fact, the hypothetical figure + (Fig. <a href="#fig160">160</a>, A) which I have deduced from the embryological evidence, might + very well represent a cross-section of a trilobite, provided only that each appendage of the + trilobite possessed an excretory coxal gland.</p> + + <p>The earliest fishes, then, ought to have possessed segmented pleural folds, which were moved by + somatic muscles, and enveloped the body after the fashion of Ammocœtes and Amphioxus, and I + cannot help thinking that Cephalaspis shows, in this respect also, its relation to + Ammocœtes. It is well known that some of the fossil representatives of the Cephalaspids + show exceedingly clearly that these animals possessed a very well-segmented body, and it is + equally recognized that this skeleton is a calcareous, not a bony skeleton, and does not represent + vertebræ, etc. It is generally called an aponeurotic skeleton, meaning thereby that what is + preserved represents not dermal plates alone, or a vertebrate skeleton, but the calcified septa or + aponeuroses between a number of muscle-segments or myomeres, precisely of the same kind as the + septa between the myomeres in Ammocœtes. The termination of such septa on the surface would + give rise to the appearance of dermal plates or scutes, or the septa may even have been attached + to something of the nature of dermal plates. The same kind of picture would be represented if + these connective tissue dissepiments of Ammocœtes were calcified, and the animal then + fossilized. In agreement with this interpretation of the spinal skeleton of Cephalaspis, it may be + noted that again and again, in parts of these dissepiments, I have found in old specimens of + Ammocœtes nodules of cartilage formed, and at transformation it is in this very tissue that + the spinal cartilages are formed.</p> + + <div><span class="pagenum" id="page415">{415}</span></div> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig161.jpg" id="fig161"><img style="width:100%" src="images/fig161.jpg" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 161.</span>—<span class="sc">A, Facsimile of + Woodward's Drawing of a Specimen of</span> <i>Cephalaspis Murchisoni</i>, <span class="sc">as + seen from the side. The Cephalic Shield is on the Right and Caudal to it the Pleural Fringes + are well shown; B, Another Specimen of</span> <i>Cephalaspis Murchisoni</i> <span + class="sc">taken from the same block of Stone, showing the Dermoseptal Skeleton and in one + place the Pleural Fringes</span>, <i>bc.</i></p> + </div> + </div> + + <p>Now, the specimens of Cephalaspis all show, as seen in Fig. <a href="#fig161">161</a>, that the + skeletal septa cover the body regularly, and then along one line are bent away from the body to + form, as it were, a fringe, or rather a free pleuron, which has been easily pushed at an angle to + the body-skeleton in the process of fossilization. Patten thinks that this fringed appearance is + evidence of a number of segmental appendages which were jointed to the corresponding + body-segments, and in the best specimen at the South Kensington Natural History Museum he thinks + such joints are clearly visible. He concludes, therefore, that the cephalaspids were arthropods, + and not vertebrates. I have also carefully examined this specimen, and do not consider that what + is seen resembles the joint of an arthropod appendage; the appearance is rather such as would be + produced if the line of attachment of Patten's appendages to the body were the place where the + pleural body folds became free from the body, and so with any pressure a <span class="pagenum" + id="page416">{416}</span>bending or fracture of the calcified plates would take place along this + line. There is, undoubtedly, an appearance of finish at the termination of these skeletal fringes, + as though they terminated in a definitely shaped spear-like point, just as is seen in the + trilobite pleuræ. This, again, to my mind, is rather evidence of pleural fringes than of true + appendages.</p> + + <div class="ac w40 fcenter sp2"> + <a href="images/fig162.png" id="fig162"><img style="width:100%" src="images/fig162.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 162.—A, Arrangement of Septa in + Ammocœtes</span> (<i>NC.</i>, position of notochord); <span class="sc">B, Arrangement + of Septa in Amphioxus</span>.</p> + </div> + </div> + + <p>As already argued, I look upon Ammocœtes as the only living fish at all resembling the + cephalaspids; it is therefore instructive to compare the arrangement of this spinal dermo-septal + skeleton of Cephalaspis with that of the septa between the myomeres in the trunk-region of + Ammocœtes and Amphioxus. Such a skeleton in Ammocœtes would be represented by a + series of plates overlapping each other, arranged as in Fig. <a href="#fig162">162</a>, A, and in + Amphioxus as in Fig. <a href="#fig162">162</a>, B. I have lettered the corresponding parts of the + two structures by similar letters, <i>a</i>, <i>b</i>, <i>c</i>. Ammocœtes differs in + configuration from Amphioxus in that it possesses an extra dorsal (<i>a</i>, <i>d</i>) and an + extra ventral bend. Ammocœtes is a much rounder animal than Amphioxus, and both the dorsal + and ventral bends are on the extreme ventral and dorsal surfaces—surfaces which can hardly + be said to exist in Amphioxus. The part, then, of such an aponeurotic skeleton <span + class="pagenum" id="page417">{417}</span>in Ammocœtes which I imagine corresponds to + <i>b</i>, <i>c</i> in Amphioxus, and therefore would represent the pleural fold, is the part + ventral to the bend at <i>b</i>. In both the animals this bend corresponds to the position of the + notochord NC.</p> + + <p>The skeleton of Cephalaspis compares more directly with that of Ammocœtes than that of + Amphioxus, for there is the same extra dorsal bend (Fig. <a href="#fig161">161</a>, <i>a</i>, + <i>d</i>) as in Ammocœtes; the lateral part of the skeleton again gives an angle <i>a</i>, + <i>b</i>, <i>c</i>; the part from <i>b</i> to <i>c</i> would therefore represent the pleural fold. + I picture to myself the sequence of events somewhat as follows<span + class="wnw">:—</span></p> + + <p class="sp3">First, a protostracan ancestor, which, like Peripatus, possessed appendages on + every segment into which cœlomic diverticula passed, forming a system of coxal glands; such + glands, being derived from the segmental organs of the Chætopoda, discharged originally to the + exterior by separate openings on each segment. It is, however, possible, and I think probable, + that a fusion of these separate ducts had already taken place in the protostracan stage, so that + there was only one external opening for the whole of these metasomatic coxal glands, just as there + is only one external opening for the corresponding prosomatic coxal glands of Limulus. Then, by + the ventral growth of pleural body-folds, such appendages became enclosed and useless, and the + coxal glands of the post-branchial segments, with their segmental or pronephric duct, were all + that remained as evidence of such appendages. This dwindling of the metasomatic appendages was + accompanied by the getting-rid of free appendages generally, in the manner already set forth, with + the result that a smooth fish-like body-surface was formed; then the necessity of increasing + mobility brought about elongation by the addition of segments between those last formed and the + cloacal region. Each of such new-formed segments was appendageless, so that its segmental organ + was not a coxal gland, but entirely somatic in position, and formed, therefore, a mesonephric + tubule, not a pronephric one. Such glands could no longer excrete to the exterior, owing to the + enclosing shell of the pleural folds; but the pronephric duct was there, already formed, and so + these nephric tubules opened into that, instead of, as in the case of the branchial slits, forcing + their way through the pleural walls when the atrium became closed.</p> + + <div><span class="pagenum" id="page418">{418}</span></div> + + <p class="ac"><span class="sc">The Meaning of the Ductless Glands.</span></p> + + <p>If it is a right conception that the excretory organs of the protostracan group, which gave + origin to the vertebrates as well as to the crustaceans and arachnids, were of the nature of coxal + glands, then it follows that such coxal glands must have existed originally on every segment, + because they themselves were derived from the segmental organs of the annelids; it is therefore + worth while making an attempt to trace the fate of such segmental organs in the vertebrate as well + as in the crustacean and arachnid.</p> + + <p>Such an attempt is possible, it seems to me, because there exists throughout the animal kingdom + striking evidence that excretory organs which no longer excrete to the exterior do not disappear, + but still perform excretory functions of a different character. Their cells still take up effete + or injurious substances, and instead of excreting to the exterior, excrete into the blood, forming + either ductless glands of special character, or glands of the nature of lymphatic glands.</p> + + <p>The problem presented to us is as follows:—</p> + + <p>The excretory organs of both arthropods and vertebrates arose from those of annelids, and were + therefore originally present in every segment of the body. In most arthropods and vertebrates they + are present only in certain regions; in the former case, as the coxal glands of the prosomatic or + head-region; in the latter, as the nephric glands of the metasomatic or trunk-region, and, in the + case of Amphioxus, of the mesosomatic or branchial region.</p> + + <p>In the original arthropod, judging from Peripatus, they were present, as in the annelid, in all + the segments of the body, and formed coxal glands. Therefore, in the ancestors of the living + Crustacea and Arachnida, coxal glands must have existed in all the segments of the body, and we + ought to be able to find the vestiges of them in the mesosomatic or branchial and metasomatic or + abdominal regions of the body.</p> + + <p>Similarly, in the vertebrates, derived, as has been shown, not from the annelids, but from an + arthropod stock, evidence of the previous existence of coxal glands ought to be manifested in the + prosomatic or trigeminal region, in the mesosomatic or branchial region, as well as in the + metasomatic or post-branchial region.</p> + + <p>How does an excretory organ change its character when it ceases <span class="pagenum" + id="page419">{419}</span>to excrete to the exterior? What should we look for in our search after + the lost coxal glands?</p> + + <p>The answer to these questions is most plainly given in the case of the pronephros, especially + in Myxine, where Maas has been able to follow out the whole process of the conversion of nephric + tubules into a tissue resembling that of a lymph-gland.</p> + + <p>He states, in the first place, that the pronephros possesses a capillary network, which extends + over the pronephric duct, while the tubules of the mesonephros possess not only this capillary + network, equivalent to the capillaries over the convoluted tubules in the higher vertebrates, but + also a true glomerulus, in that the nephric segmental arteriole forms a coil (Knauel), and pushes + in the wall of the mesonephric tubule. He describes the pronephros of large adult individuals as + consisting of—</p> + + <p>1. Tubules with funnels which open into the pericardial cœlom.</p> + + <p>2. A large capillary network (the glomus) at the distal end.</p> + + <p>3. A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel controversy), which Spengel + considers to be composed of the altered epithelium of pronephric tubules, while Semon looks on it + as an amalgamation of glomeruli.</p> + + <p>Maas is entirely on the side of Spengel, and shows that this peculiar tissue is actually formed + by modified pronephric tubules, which become more and more lymphatic in character.</p> + + <p>He says: "The pronephros consists of a number of nephric tubules, placed separately one behind + the other, which were originally segmental in character, each one of which is supplied by a + capillary network from a segmental branch of the aorta. The tubules begin with many mouths + (dorso-lateral and medial-ventral) in the pericardial cavity; on their other blind end they have + lost their original external opening, and there, in the cranial portion of the head-kidney, before + they have joined together to form a collecting duct, they, together with the vascular network, are + transformed into a peculiar adrenal-like tissue. The most posterior of the segmental capillary + nets retain their original character, and are concentrated into the separate capillary mass known + as the glomus."</p> + + <p>Later on he says: "Further, the separate head-kidney is more and more removed in structure from + an excretory organ in the ordinary sense. One cannot, however, speak of it as an organ becoming + rudimentary; this is proved not only by the progressive transformation <span class="pagenum" + id="page420">{420}</span>of its internal tissue into a tissue of a very definite character, but + also by the cilia in its canals, and the steady increase in the number of its funnels. It appears, + therefore, to be the conversion of an excretory organ into an organ for the transference of fluid + out of the cœlom into a special tissue, <i>i.e.</i> into its blood-sinus; in other words, + into an organ which must be classed as belonging to the lymph-system."</p> + + <p>In exact correspondence with this transformation of a nephric tubule into a ductless gland of + the nature of a lymphatic gland, is the formation of the head-kidney in the Teleostea. Thus, + Weldon points out that, though the observations of Balfour left it highly probable that the + "lymphatic" tissue described by him was really a result of the transformation of part of the + embryonic kidney, he did not investigate the details of its development. This was afterwards done + by Emery, with the following results: "In those Teleostea which he has studied, Professor Emery + finds that at an early stage the kidney consists entirely of a single pronephric funnel, opening + into the pericardium, and connected with the segmental duct, which already opens to the exterior. + Behind this funnel, the segmental duct is surrounded by a blastema, derived from the intermediate + cell-mass, which afterwards arranges itself more or less completely into a series of solid cords, + attaching themselves to the duct. These develop a lumen, and become normal segmental tubules, but + it is, if I may be allowed the expression, a matter of chance how much of the blastema becomes so + transformed into kidney tubules, and how much is left as the 'lymphatic' tissue of Balfour, this + 'lymphatic' tissue remaining either in the pronephros only, or in both pro- and <span + class="correction" title="Original reads 'neso-'.">meso</span>-nephros."</p> + + <p>If we turn now to the invertebrates, we see also how close a connection exists between + lymphatic and phagocytic organs and excretory organs. The chief merit for this discovery is due to + Kowalewsky, who, taking a hint from Heidenhain's work on the kidney, in which he showed how easy + it was to find out the nature of different parts of the mammalian excretory organ by the injection + of different substances, such as a solution of ammoniated carmine, or of indigo-carmine, has + injected into a large number of different invertebrates various colouring matters, or litmus, or + bacilli, and thus shown the existence, not only of known excretory organs, but also of others, + lymphatic or lymphoid in nature, not hitherto suspected.</p> + + <p>In all cases he finds that a phagocytic action with respect to solid <span class="pagenum" + id="page421">{421}</span>bodies is a property of the leucocytes, and that these leucocytes which + are found in the cœlomic spaces of the Annelida, etc., are apparently derived from the + epithelium of such spaces. Also by the proliferation of such epithelium in places, <i>e.g.</i> the + septal glands of the terrestrial Oligochæta, segmental glandular masses of such tissue are formed + which take up the colouring matter, or the bacilli. In the limicolous Oligochæta such septal + glands are not found, but at the commencement of the nephridial organ, immediately following upon + the funnel, a remarkable modification of the nephridial wall takes place to form a large cellular + cavernous mass, the so-called filter, which in Euaxes is full of leucocytes; the cells are only + definable by their nuclei, and look like and act in the same way as the free leucocytes outside + this nephridial appendage. As G. Schneider points out, the whole arrangement is very like that + described by Kowalewsky in the leeches Clepsine and Nephelis, where, also immediately succeeding + the funnel of the nephridial organ, a large accessory organ is found, which is part of the + nephridium, and is called the nephridial capsule. This is the organ <i>par excellence</i> which + takes up the solid carmine-grains and bacilli, and apparently, from Kowalewsky's description, + contains leucocytes in large numbers. We see, then, that in such invertebrates, just as in the + vertebrate, modifications of the true excretory organ may give rise to phagocytic glands of the + nature of lymphatic glands. Further, these researches of Kowalewsky suggest in the very strongest + manner that whenever by such means new, hitherto unsuspected glands are discovered, such glands + must belong to the excretory system, <i>i.e.</i> must be derived from cœlomic epithelium, + even when all evidence of any cœlom has disappeared. Kowalewsky himself was evidently so + impressed with the same feeling that he heads one of his papers "The Excretory Organs of the + Pantopoda," although the organs in question had been discovered by him by this method, and + appeared as ductless glands with no external opening.</p> + + <p>To my mind these observations of Kowalewsky are of exceeding interest, for it is immediately + clear that if the segmental organs of the annelids, which must have existed on all the segments of + the forefathers of the Crustacea and Arachnida (the Protostraca), have left any sign of their + existence in living crustaceans and arachnids, then such indication would most likely take the + form of lymphatic glands in the places where the excretory organs ought to have been.</p> + + <p>Now, as already pointed out in Peripatus, such segmental organs <span class="pagenum" + id="page422">{422}</span>were formed by the ventral part of the cœlom, and dipped + originally into each appendage. We know also that each segment of an arachnid embryo possesses a + cœlomic cavity in its ventral part which extends into the appendage on each side; this + cavity afterwards disappears, and is said to leave no trace in the adult of any excretory coxal + gland derived from its walls. If, however, it is found that in the very position where such organ + ought to have been formed a segmentally arranged ductless gland is situated, the existence of + which is shown by its taking up carmine, etc., then it seems to me that in all probability such + gland is the modification of the original coxal gland.</p> + + <p>This is what Kowalewsky has done. Thus he states that Metschnikoff had fed Mysis with + carmine-grains, and found tubules at the base of the thoracic feet coloured red with carmine. He + himself used an allied species, <i>Parapodopsis cornutum</i>, and found here also that the carmine + was taken up by tubules situated in the basal segments of the feet. In Nebalia, feeding + experiments with alizarin blue and carmine stained the antennal glands, and showed the existence + of glands at the base of the eight thoracic feet. These glands resemble the foot-glands of Mysis, + Parapodopsis, and Palæmon, and lie in the space through which the blood passes from the thoracic + feet, <i>i.e.</i> from the gills, to the heart. In Squilla also, in addition to the shell-glands, + special glands were discovered on the branchial feet on the path of the blood to the heart. These + glands form continuous masses of cells which constitute large compact glands at the base of the + branchial feet. Single cells of the same sort are found along the whole course of the branchial + venous canal, right up to the pericardium.</p> + + <p>These observations show that the Crustacea possess not only true excretory organs in the shape + of coxal glands, <i>i.e.</i> antennary glands, shell-glands, etc., in the cephalic region, but + also a series of segmental glands situated at the base of the appendages, especially of the + respiratory appendages: a system, that is to say, of coxal glands which have lost their excretory + function, through having lost their external opening, but have not in consequence disappeared, but + still remain <i>in situ</i>, and still retain an important excretory function, having become + lymphatic glands containing leucocytes. Such glands are especially found in the branchial + appendages, and are called branchial glands by Cuénot, who describes them for all Decapoda.</p> + + <p>Further, it is significant that the same method reveals the <span class="pagenum" + id="page423">{423}</span>existence in Pantopoda of a double set of glands of similar character, + one set in the basal segments of the appendage, and the other in the adjacent part of the + body.</p> + + <p>In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ situated along the + whole length of the nerve-cord in the abdominal region takes up carmine grains and bacilli; an + organ which in Androctonus does not form one continuous gland, but a number of separate, + apparently irregularly grouped, glandular bodies.</p> + + <p>In addition to this median lymphatic gland, Kowalewsky has discovered in the scorpion a pair of + lateral glands, to which he gives the name of lymphoid glands, which communicate with the thoracic + body-cavity (<i>i.e.</i> the pseudocœle), are phagocytic, and, according to him, give + origin to leucocytes by the proliferation of their lining cells, thus, as he remarks, reminding us + of the nephridial capsules of Clepsine. These glands are so closely related in position to the + coxal glands on each side that he has often thought that the lumen of the gland communicated with + that of the coxal gland; he, however, has persuaded himself that there is no true communication + between the two glands. Neither of these organs appears to be segmental, and until we know how + they are developed it is not possible to say whether they represent fused segmental organs or + not.</p> + + <p>The evidence, then, is very strong that in the Crustacea and Arachnida the original segmental + excretory organs do not disappear, but remain as ductless glands, of the nature of lymphatic + glands, which supply leucocytes to the system.</p> + + <p>Further, the evidence shows that the nephric organs, or parts of the cœlom in close + connection with these organs, may be transformed into ductless glands, which do not necessarily + contain free leucocytes as do lymph-glands, but yet are of such great importance as excretory + organs that their removal profoundly modifies the condition of the animal. Such a gland is the + so-called adrenal or suprarenal body, disease of which is a feature of Addison's disease; a gland + which forms and presumably passes into the blood a substance of remarkable power in causing + contraction of blood-vessels, a substance which has lately been prepared in crystalline form by + Jokichi Takamine, and called by him "adrenalin"; a gland, therefore, of very distinctly peculiar + properties, which cannot be regarded as rudimentary, but is of vital importance for the due + maintenance of the healthy state.</p> + + <p>In the Elasmobranchs two separate glandular organs have been <span class="pagenum" + id="page424">{424}</span>called suprarenal; a segmental series of paired organs, each of which + possesses a branch from the aorta and a sympathetic ganglion, and an unpaired series in close + connection with the kidneys, to which Balfour gave the name of interrenal glands. Of these two + sets of glands, Swale Vincent has shown that the extract of the interrenals has no marked + physiological effect, in this respect resembling the extract of the cortical part of the mammalian + gland, while the extract of the paired segmental organs of the Elasmobranch produces the same + remarkable rise of blood-pressure as the extract of the medullary portion of the mammalian + gland.</p> + + <p>The development also of these two sets of glands is asserted to be different. Balfour + considered that the suprarenals were derived from sympathetic ganglion-cells, but left the origin + of the interrenals doubtful. Weldon showed that the cortical part of the suprarenals in the lizard + was derived from the wall of the glomerulus of a number of mesonephric tubules. In Pristiurus, he + stated that the mesoblastic rudiment described by Balfour as giving origin to the interrenals is + derived from a diverticulum of each segmental tubule, close to the narrowing of its funnel-shaped + opening into the body-cavity. With respect to the paired suprarenals he was unable to speak + positively, but doubted whether they were derived entirely from sympathetic ganglia.</p> + + <p>Weldon sums up the results of his observations by saying: "That all vertebrates except + Amphioxus have a portion of the kidney modified for some unknown purpose not connected with + excretion; that in Cyclostomes the pronephros alone is so modified, in Teleostei the pro- and part + of the meso-nephros; while in the Elasmobranchs and the higher vertebrates the mesonephros alone + gives rise to this organ, which has also in these forms acquired a secondary connection with + certain of the sympathetic ganglia."</p> + + <p>Since Weldon's paper, a large amount of literature on the origin of the adrenals has appeared, + a summary of which, up to 1891, is given by Hans Rabl in his paper, and a further summary by + Aichel in his paper published in 1900. The result of the investigations up to this latter paper + may be summed up by saying that the adrenals, using this term to include all these organs of + whatever kind, are in all cases, partly at all events, derived from some part of the walls of + either the mesonephric or pronephric excretory organs, but that in addition a separate origin from + the sympathetic nervous system must <span class="pagenum" id="page425">{425}</span>be ascribed to + the medullary part of the organ and to the separate paired organs in the Elasmobranchs, which are + equivalent to the medullary part in other cases.</p> + + <p>The evidence, then, of the transformation of the known vertebrate excretory organs—the + pronephros and the mesonephros—leads to the conclusion that in our search for the missing + coxal glands of the meso- and pro-somatic regions, we must look for either lymphatic glands, or + ductless glands of distinct importance to the body. I have already considered the question in the + prosomatic region, and have given my reasons why the pituitary gland must be looked upon as the + descendant of the arthropod coxal gland. In this case also the resulting ductless gland is still + of functional importance, for disease of it is associated with acromegaly. If, as is possible, it + is homologous with the Ascidian hypophysial gland, then it is confirmatory evidence that this + latter is said by Julin to be an altered nephridial organ.</p> + + <p>Finally, I come to the mesosomatic or branchial region; and here, strikingly enough, we find a + perfectly segmental glandular organ of mysterious origin—the thymus gland—segmental + with the branchiæ, not necessarily with the myotomes, belonging, therefore, to the appendicular + system; and since the branchiæ represent, according to my theory, the basal part of the appendage, + such segmental glands would be in the position of coxal glands. Here, then, in the thymus may be + the missing mesosomatic coxal glands.</p> + + <p>What, then, is the thymus?</p> + + <p>The answer to this question has been given recently by Beard, who strongly confirms Kölliker's + original view that the thymus is a gland for the manufacture of leucocytes, and that such + leucocytes are directly derived from the epithelial cells of the thymus. Kölliker also further + pointed out that the blood of the embryo is for a certain period destitute of leucocytes. Beard + confirms this last statement, and says that up to a certain stage (varying from 10 to 16 mm. in + length of the embryo) the embryos of <i>Raja batis</i> have no leucocytes in the blood or + elsewhere. Up to this period the thymus-placode is well formed, and the first leucocytes can be + seen to be formed in it from its epithelial cells; then such formation takes place with great + rapidity, and soon an enormous discharge of leucocytes occurs from the thymus into the + tissue-spaces and blood. He therefore concludes that all lymphoid tissues in the body arise + originally from the thymus gland, <i>i.e.</i> from leucocytes discharged from the thymus.</p> + + <div><span class="pagenum" id="page426">{426}</span></div> + + <p>The segmental branchial glands, known by the name of thymus, are, according to this view, the + original lymphatic glands of the vertebrate; and it is to be noted that, in fishes and in + Amphibia, lymphatic glands, such as we know them in the higher mammals, do not exist; they are + characteristic of the higher stages of vertebrate evolution. In the lower vertebrates, the only + glandular masses apart from the cell-lining of the body-cavity itself, which give rise to + leucocyte-forming tissue, are these segmental branchial glands, or possibly also the modified + post-branchial segmental glands, known as the head-kidney in Teleostea, etc.</p> + + <p>The importance ascribed by Beard to the thymus in the formation of leucocytes in the lowest + vertebrates would be considerably reduced in value if the branchial region of Ammocœtes + possessed neither thymus glands nor anything equivalent to them. Such, however, is not the case. + Schaffer has shown that in the young Ammocœtes masses of lymphatic glandular tissue are + found segmentally arranged in the neighbourhood of each gill-slit—tissue which soon becomes + converted into a swarming mass of leucocytes, and shows by its staining, etc., how different it is + from a blood-space. The presence of this thymus leucocyte-forming tissue, as described by + Schaffer, is confirmed by Beard, and I myself have seen the same thing in my youngest specimen of + Ammocœtes.</p> + + <p>Further, the very methods by which Kowalewsky has brought to light the segmental lymph-glands + of the branchial region of the Crustacea, etc., are the same as those by which Weiss discovered + the branchial nephric glands in Amphioxus—excretory organs which Boveri considers to + represent the pronephros of the Craniota. In this supposition Boveri is right, in so far that both + pronephros and the tubules in Amphioxus belong to the same system of excretory organs; but I + entirely agree with van Wijhe that the region in Amphioxus is wrong. The tubules in Amphioxus + ought to be represented in the branchial region of the Craniota, not in the post-branchial region; + van Wijhe therefore suggests that further researches may homologize them with the thymus gland in + the Craniota, not with the pronephros. This suggestion of van Wijhe appears to me a remarkably + good one, especially in view of the position of the thymus glands in Ammocœtes and the + nephric branchial glands in Amphioxus. If, as I have pointed out, the atrial cavity of Amphioxus + has been closed in Ammocœtes by the apposition of <span class="pagenum" + id="page427">{427}</span>the pleural fold with the branchial body-surface, then the remains of the + position of the atrial chamber must exist in Ammocœtes as that extraordinary space between + the somatic muscles and the branchial basket-work filled with blood-spaces and modified + muco-cartilage. It is in this very space, close against the gill-slits, that the thymus glands of + Ammocœtes are found, in the very place where the nephric tubules of Amphioxus would be + found if its atrial cavity were closed completely. Instead, therefore, of considering with Boveri + that the branchial nephric tubules of Amphioxus still exist in the Craniota as the pronephros, and + that the atrial chamber has narrowed down to the pronephric duct, I would agree with van Wijhe + that the pronephros is post-branchial, and suggest that by the complete closure of the atrial + space in the branchial region the branchial nephric tubules have lost all external opening, and + consequently, as in all other cases, have changed into lymphatic tissue and become the segmental + thymus glands.</p> + + <p>As van Wijhe himself remarks, the time is hardly ripe for making any positive statement about + the relationship between the thymus gland and branchial excretory organs. There is at present not + sufficient consensus of opinion to enable us to speak with any certainty on the subject, yet there + is so much suggestiveness in the various statements of different authors as to make it worth while + to consider the question briefly.</p> + + <p>On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and parathymus, are all + stated to be derivatives of the epithelium lining the gill-slits, and Maurer would draw a + distinction between the organs derived from the dorsal side of the gill-cleft and those derived + from the ventral side—the former being thymus, the latter forming the epithelial cell-nests, + <i>i.e.</i> parathyroids. The thymus in Ammocœtes, according to Schaffer, lies both ventral + and dorsal to the gill-cleft; Maurer thinks that only the dorsal part corresponds to the thymus, + the ventral part corresponding to the parathyroids, etc. Structurally, the thymus, parathyroids, + and the epithelial cell-nests are remarkably similar, so that the evidence appears to point to the + conclusion that, in the neighbourhood of the gill-slits, segmentally arranged organs of a + lymphatic character are situated, which give origin to the thymus, parathyroids, tonsils, etc. + Now, among these organs, <i>i.e.</i> among those ventrally situated, Maurer places the carotid + gland, so that, if he is right, the origin of the carotid gland <span class="pagenum" + id="page428">{428}</span>might be expected to help in the elucidation of the origin of the + thymus.</p> + + <p>The origin of the carotid gland has been investigated recently by Kohn, who finds that it is + associated with the sympathetic nervous system in the same way as the suprarenals. He desires, in + fact, to make a separate category for such nerve-glands, or paraganglia, as he calls them, and + considers them all to be derivatives of the sympathetic nervous system, and to have nothing to do + with excretory organs. The carotid gland is, according to him, the foremost of the suprarenal + masses in the Elasmobranchs, viz. the so-called axillary heart.</p> + + <p>In my opinion, nests of sympathetic ganglion-cells necessarily mean the supply of efferent + fibres to some organ, for all such ganglia are efferent, and also, if they are found in the organ, + would have been brought into it by way of the blood-vessels supplying the organ, so that Aichel's + statement of the origin of the suprarenals in the Elasmobranchs seems to me much more probable + than a derivation from nerve-cells. If, then, it prove that Aichel is right as to the origin of + the suprarenals, and Kohn is right in classifying the carotid gland with the suprarenals, then + Maurer's statements would bring the parathyroids, thymus, etc., into line with the adrenals, and + suggest that they represent the segmented glandular excretory organs of the branchial region, into + which, just as in the interrenals of Elasmobranchs, or the cortical part of the adrenals of the + higher vertebrates, there has been no invasion of sympathetic ganglion-cells.</p> + + <p>Wheeler makes a most suggestive remark in his paper on Petromyzon: he thinks he has obtained + evidence of serial homologues of the pronephric tubules in the branchial region of + Ammocœtes, but has not been able up to the present to follow them out. If what he thinks to + be serial homologues of the pronephric tubules in the branchial region should prove to be the + origin of the thymus glands of Schaffer, then van Wijhe's suggestion that the thymus represents + the excretory organs of the branchial region would gain enormously in probability. Until some such + further investigation has been undertaken, I can only say that it seems to me most likely that the + thymus, etc., represent the lymphatic branchial glands of the Crustacea, and therefore represent + the missing coxal glands of the branchial region.</p> + + <p>This, however, is not all, for the appendages of the mesosomatic region, as I have shown, do + not all bear branchiæ; the foremost or <span class="pagenum" id="page429">{429}</span>opercular + appendage carries the thyroid gland. Again, the basal part of the appendage is all that is left; + the thyroid gland is in position a coxal gland. It ought, therefore, to represent the coxal gland + of this appendage, just as the thymus, tonsils, etc., represent the coxal glands of the rest of + the mesosomatic appendages. In the thyroid gland we again see a ductless gland of immense + importance to the economy, not a useless organ, but one, like the other modified coxal glands, + whose removal involves far-reaching vital consequences. Such a gland, on my theory, was in the + arthropod a part of the external genital ducts which opened on the basal joint of the operculum. + What, then, is the opinion of morphologists as to the meaning of these external genital ducts?</p> + + <p>In a note to Gulland's paper on the coxal glands of Limulus, Lankester states that the + conversion of an externally-opening tubular gland (coxal gland) into a ductless gland is the same + kind of thing as the history of the development of the suprarenal from a modified portion of + mesonephros, as given by Weldon. Further, that in other arthropods with glands of a tubular + character opening to the exterior at the base of the appendages, we also have coxal nephridia, + such as the shell-glands of the Entomostraca, green glands of Crustacea (antennary coxal gland); + and further on he writes: "When once the notion is admitted that ducts opening at the base of + limbs in the Arthropoda are possibly and even probably modified nephridia, we immediately conceive + the hypothesis that the genital ducts of the Arthropoda are modified nephridia."</p> + + <p>So, also, Korschelt and Heider, in their general summing up on the Arthropoda, say: "In + Peripatus, where the nephridia appear, as in the Annelida, in all the trunk-segments, a + considerable portion of the primitive segments is directly utilized for the formation of the + nephridia. In the other groups, the whole question of the rise of the organs known as nephridia is + still undecided, but it may be mentioned as very probable that the salivary and anal glands of + Peripatus, the antennal and shell-glands of the Crustacea, the coxal glands of Limulus and the + Arachnida, as well as the efferent genital ducts, are derived from nephridia, and in any case are + mesodermal in origin."</p> + + <p>The necessary corollary to this exceedingly probable argument is that glandular structures such + as the uterine glands of the scorpion already described, which are found in connection with these + terminal <span class="pagenum" id="page430">{430}</span>genital ducts, may be classed as modified + nephridial glands, and that therefore the thyroid gland of Ammocœtes, which, on the theory + of this book, arose in connection with the opercular genital ducts of the palæostracan ancestor, + represents the coxal glands of this fused pair of appendages. Such a gland, although its function + in connection with the genital organs had long disappeared, still, in virtue of its original + excretory function, persisted, and even in the higher vertebrates, after it had lost all semblance + of its former structure and become a ductless gland of an apparently rudimentary nature, still, by + its excretory function, demonstrates its vital importance even to the highest vertebrate.</p> + + <p>By this simple explanation we see how these hitherto mysterious ductless glands, pituitary, + thymus, tonsils, thyroid, are all accounted for, are all members of a common stock—coxal + glands—which originally, as in Peripatus, excreted at the base of the prosomatic and + mesosomatic appendages, and are still retained because of the importance of their excretory + function, although ductless owing to the modification of their original appendages.</p> + + <p>Finally, there is yet another organ in the vertebrate which follows the same law of the + conversion of an excretory organ into a lymphatic organ when its connection with the exterior is + obliterated, and that is the vertebrate body-cavity itself. According to the scheme here put + forth, the body-cavity of the vertebrate arose by the fusion of a ventral prolongation of the + original nephrocœle on each side; prolongations which accompanied the formation of the new + ventral midgut, and by their fusion formed originally a pair of cavities along the whole length of + the abdomen, being separated from each other by the ventral mesentery of the gut. Subsequently, by + the ventral fusion of these two cavities, the body-cavity of the adult vertebrate was formed.</p> + + <p>This is simply a statement of the known method of formation of the body-cavity in the embryo, + and its phylogenetic explanation is that the body-cavity of the vertebrate must be looked upon as + a ventral prolongation of the original ancestral body-cavity. Embryology clearly teaches that the + original body-cavity or somite was confined to the region of the notochord and central nervous + system, and there, just as in Peripatus, was divisible into a dorsal part, giving origin to the + myocœle, and a ventral part, forming the nephrocœle. From this original + nephrocœle are formed the pronephric excretory organs, the mesonephric excretory organs, + and the body-cavity.</p> + + <div><span class="pagenum" id="page431">{431}</span></div> + + <p class="sp3">That the vertebrate body-cavity was originally a nephrocœle is generally + accepted, and its excretory function is shown by the fact that it communicates with the exterior + in all the lower vertebrates, either through abdominal pores or by way of nephridial funnels. Bles + has shown how largely these two methods of communicating with the exterior mutually exclude each + other. In the higher vertebrates both channels become closed, except in the case of the Fallopian + tubes, and thus, so to speak, the body-cavity becomes a ductless gland, still, however, with an + excretory function, but now, as in all other cases, forming a part of the lymphatic rather than of + the true excretory system.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>The consideration of the formation of the vertebrate cranial region, as set forth in previous + chapters, indicates that the ancestor of the vertebrates was not an arachnid purely or a + crustacean purely, but possessed partly crustacean and partly arachnid characters. In order to + express this conclusion, I have used the term Protostraca, invented by Korschelt and Heider, to + indicate a primitive arthropod group, from which both arachnids and crustaceans may be supposed + to have arisen, and have therefore stated that the vertebrate did not arise directly from the + annelids, but from the Protostraca. Such an origin signifies that the origin of the excretory + organs of the vertebrate must not be looked for in the segmental organs of the annelid, but + rather in such modified annelid organs as would naturally exist in a primitive arthropod group. + The nature of such organs may be inferred, owing to the fortunate circumstance that so primitive + an arthropod as Peripatus still exists, and we may conclude that the protostracan ancestor + possessed in every segment a pair of appendages and a pair of cœlomic cavities, which + extended into the base of these appendages. The ventral portion of each of these cœlomic + cavities separated off from the dorsal and formed a nephrocœle, giving origin to a + segmental excretory organ, which, seeing that its end-vesicle was in the base of the appendage, + and seeing also the nature of the known arachnid and crustacean excretory organs, may fitly be + termed a coxal gland. This, then, is the working hypothesis to explain the difficulties + connected with the origin of the pronephros and mesonephros—that the original segmental + organs were coxal glands, and therefore indicated the presence of appendages. This hypothesis + leads to the following conclusions<span class="wnw">:—</span></p> + <p>1. The coxal glands belonging to the post-branchial appendages of the invertebrate ancestor + are represented by the pronephric tubules, and existed over the whole metasomatic region.</p> + <p>2. Such glands discharged into a common duct—the pronephric duct—which opened + into the cloacal region, either in the protostracan stage, when the metasomatic appendages were + still in existence, just as the coxal glands of the prosomatic region in Limulus discharge into + a common duct, or else the pronephric duct was formed when the appendages were obliterated.</p> + <div><span class="pagenum" id="page432">{432}</span></div> + <p>3. The metasomatic appendages disappeared owing to their enclosure by pleural folds, which, + meeting in the mid-ventral line, not only caused the obliteration of the appendages, and gave a + smooth fish-like body-surface to the animal, but also caused the formation of an atrial + cavity.</p> + <p>4. Into these pleural folds the dorsal longitudinal muscles of the body extended, and + ultimately reached to the ventral surface, thus forming the somatic muscles of the vertebrate + body.</p> + <p>5. When the pleural folds had met in the mid-ventral line the animal had <span + class="correction" title="Original reads 'became'">become</span> a vertebrate, and was dependent + for its locomotion on the movements of these somatic muscles, and not on the movements of + appendages. Consequently, elongation of the trunk-region took place, for the purpose of + increasing mobility, by the formation of new metameres.</p> + <p>6. Each of such metameres possessed its own segmental excretory organ, formed in the same way + as the previous pronephric organs, but, as there were no appendages in these new-formed + segments, the excretory organs took on the characters of a mesonephros, not a pronephros, and + opened into the pronephric duct, because the direct way to the exterior was blocked by the + enveloping pleural folds.</p> + <p>7. The group of annelids from which the protostracan ancestor of the vertebrates arose was + the highest annelidan group, viz. the Polychæta, as shown by the nature of the excretory organs + in Amphioxus.</p> + <p>8. The coxal glands of the protostracan ancestor existed on all the segments, and were, + therefore, divisible into three groups, prosomatic, mesosomatic, and metasomatic; these three + groups of coxal glands still exist in the vertebrate as ductless glands.</p> + <p>9. The prosomatic coxal glands form the pituitary body.</p> + <p>10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids, tonsils, etc.</p> + <p>11. The metasomatic coxal glands form the adrenals.</p> + <p>12. The procœlom of the vertebrate is the procœlom of the protostracan + ancestor, which splits into a dorsal part, the myocœle, and a ventral part, the + nephrocœle. This latter part not only forms the pronephros and mesonephros, but also by a + ventral extension gives origin to the walls of the vertebrate body-cavity or + metacœle.</p> + <p class="sp0">13. This ventral extension of the original nephrocœle at first excreted to + the exterior, through abdominal pores, or through peritoneal funnels. When such paths to the + exterior became closed, it also became a ductless gland, belonging to the lymphatic system.</p> + </div> + + <div><span class="pagenum" id="page433">{433}</span></div> + + <p class="ac">CHAPTER XIII</p> + + <p class="ac"><i>THE NOTOCHORD AND ALIMENTARY CANAL</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">Relationship between notochord and gut.—Position of unsegmented tube of + notochord.—Origin of notochord from a median groove.—Its function as an accessory + digestive tube.—Formation of notochordal tissue in invertebrates from closed portions of + the digestive tube.—Digestive power of the skin of Ammocœtes.—Formation of + new gut in Ammocœtes at transformation.—Innervation of the vertebrate + gut.—The three outflows of efferent nerves belonging to the organic system.—The + original close contiguity of the respiratory chamber to the cloaca.—The elongation of the + gut.—Conclusion.</p> + </div> + + <p>In the previous chapters all the important organs of the arthropod have been found in the + vertebrate in their appropriate place, of similar structure, and innervated from corresponding + parts of the central nervous system. Such comparison is possible only as long as the ventral and + dorsal surfaces of the vertebrate correspond with the respective surfaces of the arthropod, and no + reversal is assumed. This method of comparative anatomy is the surest and most certain guide to + the relationship between two animals, and when the facts obtained by the anatomical method are so + strikingly confirmatory of the palæontological evidence, the combined evidence becomes so strong + as to amount almost to a certainty that vertebrates did arise from arthropods in the manner mapped + out in previous chapters, and not from a hypothetical group of animals, such as is postulated in + the theory of their origin from forms like Balanoglossus.</p> + + <p class="sp3">The latter theory derives the alimentary canal of the vertebrate from that of the + invertebrate, and finds in the latter the commencement of the notochord. In the comparison which I + have made the alimentary canal of the invertebrate ancestor has become the tube of the central + nervous system of the vertebrate, and there is no sign of a notochord whatever. All the organs of + the arthropod have already been allocated; where the notochord is situated in the <span + class="pagenum" id="page434">{434}</span>vertebrate there is nothing but a gap in the + invertebrate, but the position of that gap can be settled with great accuracy from the previous + comparison of organs in the two groups. So, also, the alimentary canal of the vertebrate is from + the very nature of the case a new organ, yet, as has been shown in Chapter V., the comparison of + the respiratory organs in the two groups gives a strong suggestion of the manner in which such a + canal was formed.</p> + + <p class="ac"><span class="sc">The Origin of the Notochord.</span></p> + + <p>The time has now come to endeavour to frame a plausible theory of the method of formation of + the notochord and the new alimentary canal, and thus to complete the diagram on p. <a + href="#page413">413</a>. The comparative method is no longer available, for these structures are + both unrepresented as such in the arthropod; any suggested explanation, therefore, must be more + tentative, and cannot give the same feeling of certainty as is the case with all the organs + already considered. Our only chance of finding out the past history of the notochord lies in the + embryological method, in the hope that, according to the 'law of recapitulation,' the ancestral + history may be repeated in the ontogeny with sufficient clearness to enable some conclusion to be + drawn.</p> + + <p>At the outset, one point comes out clearly—the close relationship between the notochord + and the vertebrate gut; they are both derived from the same layer, both parts of the same + structure. On this point all embryologists are agreed; it is expressed in such statements as, "the + notochord, as well as the alimentary canal, is formed from hypoblast"; "the notochord arises as a + thickening in the dorsal wall of the alimentary canal." The two structures are so closely + connected together that they must be considered together. If we can conjecture the origin of the + one, we may be sure that we have the clue to the origin of the other. The two together form the + one new organ which distinguishes the vertebrate from the arthropod, the only thing left which + requires explanation for the completion of this strange history.</p> + + <p>What, then, is the notochord? What are its characteristics? In the highest vertebrates it is + conspicuous only in the embryo; with the development of the axial skeleton it is more and more + squeezed out of existence, until in the adult it is no longer visible. By the 'law of + recapitulation' this developmental history implies that, as we descend the vertebrate phylum, the + notochord ought to be more and <span class="pagenum" id="page435">{435}</span>more conspicuous, + more and more permanent during the life of the animal. Such is, indeed, found to be the case, + until at last, in the lowest vertebrates, such as the lamprey, and in forms like Amphioxus, the + notochord persists throughout the life of the animal as a large important axial supporting + rod.</p> + + <p>This rod has a number of striking characteristics which distinguish it from all other + structures, and are the only means of guessing its probable origin. Its position in the body is + always the same in all vertebrates and is very significant, for it lies just ventrally to the + central nervous system, along nearly the whole length of the animal, not quite the whole length, + for it invariably terminates close to the place where the infundibulum comes to the surface of the + brain; it is, in fact, always confined to the infra-infundibular and spinal cord part of the + central nervous system. Interpreting this into the language of the arthropod, it means that a rod + was formed just ventrally to the nervous system, which extended the whole length of the + infraœsophageal and ventral chain of ganglia, and terminated at the orifice of the mouth. + Moreover, this rod was unsegmented, for the notochord is devoid of segmentation.</p> + + <p>At the anterior end the rod tapers to a point, as in Fig. <a href="#fig166">166</a>. In its + middle part it is very large and conspicuous, cylindrical in shape; its interior is filled with a + peculiar vacuolated tissue, different to any other known vertebrate tissue, which has therefore + received the name of notochordal tissue. Outside this is a thick sheath formed of many layers, of + which the external one gives the staining reactions of elastin, and is called the external elastic + layer. Between this sheath and the notochordal tissue a thin layer of lining cells, of normal + appearance, is conspicuous in Ammocœtes. These cells secrete the layers of the sheath, and + have originally, by proliferation, given rise to the notochordal tissue. In the notochord of + Ammocœtes there is no sign of either nerves, blood-vessels, or muscles.</p> + + <p>The centre of the notochord presents the appearance of a slight slit, as though it had + originated from a tube, and that is the opinion now generally held, for its mode of formation in + the embryo is as that of a tube formed from an open groove, as will be explained immediately.</p> + + <p>We may, then, conceive of the notochord as originally a tube lying in the mid-line just + ventrally to the central nervous system, and extending from the original mouth to the end of the + body. Translate this into the language of the arthropod and it denotes a tube on the <span + class="pagenum" id="page436">{436}</span>mid-ventral surface of the body, which extended from + mouth to anus. Such a tube might be formed from the mid-ventral surface as follows<span + class="wnw">:—</span></p> + + <p>In Fig. <a href="#fig163">163</a>, A, the lining of the ventral surface between two appendages + is represented flat, in B is shown how the formation of a solid rod may arise from the bulging of + that ventral surface, and in C how a groove on that surface may lead to the formation of a tube + between the two appendages. The difference between a notochordal rod formed as in B from that in C + would be shown in the sheath, for in B the sheath would be formed from the cuticle of the lining + cells, and in C from the basement membrane. The structure of the sheath is in accordance with the + embryological evidence that the notochord is formed as a tube from a groove, as in C, and not as a + solid rod as in B, for it possesses a well-marked elastin layer, and elastin has never yet been + found as a constituent of any cuticular secretion, but invariably in connection with + basement-membranes.</p> + + <div class="ac w35 fcenter sp2"> + <a href="images/fig163.png" id="fig163"><img style="width:100%" src="images/fig163.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 163.—Diagram of two possible methods of the + Formation of a Notochord.</span></p> + </div> + </div> + + <p>The position, then, of the notochord and its method of formation suggests that the mid-ventral + surface of the arthropod ancestor of the vertebrate formed a deep groove between the bases of all + the prosomatic, mesosomatic, and metasomatic appendages, which was subsequently converted into a + tube extending along the whole of the body between mouth and anus, and finally, by the + proliferation of its lining cells and their conversion into notochordal tissue, became the + notochordal rod of the vertebrate.</p> + + <p>As already frequently stated, Apus and Branchipus are the two living arthropods which most + nearly resemble the extinct trilobites. The beautiful specimens of Triarthrus (Fig. <a + href="#fig165">165</a>) found by Beecher give an idea of the under surface of the trilobite such + as has never been obtained before, and demonstrate how closely the condition of things found in + Apus (Fig. <a href="#fig164">164</a>) was similar to that occurring in the trilobites. In both + cases the mid-ventral surface of the animal formed a deep groove which extended the whole length + of the <span class="pagenum" id="page437">{437}</span>animal; on each side of this groove in Apus + are closely set the gnatho-bases of the appendages, in such a manner that the groove can be easily + converted into a canal by the movements of these bases—a canal which, owing to the great + number of the appendages and their closeness to each other, can be completely and efficiently + closed.</p> + + <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations"> + <tr> + <td class="vbm ac" style="width:44%;"><a href="images/fig164.png" id="fig164"><img + style="width:100%" src="images/fig164.png" alt="" title=""/></a></td> + <td class="vbm ac" style="width:44%;"><a href="images/fig165.png" id="fig165"><img + style="width:100%" src="images/fig165.png" alt="" title=""/></a></td> + </tr> + <tr> + <td> + <div class="smaller ac"> + <p class="sp0"><span class="sc">Fig. 164.—Under-Surface of Apus.</span> (After <span + class="sc">Bronn</span>.)</p> + </div> + </td> + <td> + <div class="smaller aj it1"> + <p class="sp0"><span class="sc">Fig. 165.</span>—<span class="sc">Under-Surface of a + Trilobite</span> (<i>Triarthrus</i>). (From <span class="sc">Beecher</span>.)</p> + </div> + </td> + </tr> + </table> + + <p>All those who have seen Apus in the living state assert that this canal so formed is actually + used by the animal for feeding purposes. By the movements of the gnatho-bases food is passed up + from the hind end of the animal along the whole length of this ventral canal to the mouth, where + it is taken in and swallowed. In this way Apus has been seen to swallow its own eggs.</p> + + <p>In the trilobites there is a similar deep channel formed by the mid-ventral surface, similar + gnatho-bases, and closely set appendages, and the membrane of this ventral groove was extremely + thin.</p> + + <p>Here, then, in the very group of animals which were the progenitors of the presumed + palæostracan ancestor of the vertebrate—a group which is characterized by its extensive + prevalence and its <span class="pagenum" id="page438">{438}</span>enormous variety of form during + the great trilobite era—the formation of a mid-ventral canal out of this deep ventral groove + is seen to be not only easy to imagine, but most probable, provided that a necessity arose for + such a conversion.</p> + + <p>For what purpose might such a tube have been formed? I would suggest that it might have acted + as an accessory food-channel, which was of sufficient value at the time to give some advantage in + the struggle for existence to those members of the group who were thus able to supplement their + intake of food, but at the same time was so inefficient that it was quickly superseded by the new + alimentary canal, and thus losing its temporary function, became solid, and was utilized to form + an axial supporting rod.</p> + + <p>There is a very considerable amount of evidence in favour of the view that the notochord was + originally a digestive tube; in fact, as far as I know, this conclusion is universally accepted. + The evidence is based essentially upon its development and upon its structure. It is formed in the + vertebrate from the same layer as the alimentary canal, <i>i.e.</i> the hypoblast, and in + Amphioxus it commences as a groove in the dorsal wall of the future alimentary canal; this groove + then closes to form the tube of the notochord, and separates from the alimentary canal. + Embryologically, then, the notochord is looked upon as a tube formed directly from the alimentary + canal.</p> + + <p>As regards its structure, its tissue is, as already stated, something <i>sui generis</i>. + Notochordal tissue has no resemblance to bone or cartilage, or any of the usual supporting + tissues. Such a tissue is not, however, entirely confined to the notochord of the vertebrates, but + tissue closely resembling it has been found not only in Amphioxus and the Tunicata, but in certain + other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in Cephalodiscus, and in + Actinotrocha. In all these latter cases, such a tissue is invariably found in disused portions of + the alimentary canal; a diverticulum of the alimentary canal becomes closed, vacuolation of its + lining cells takes place, and a tissue resembling notochordal tissue is formed.</p> + + <p>Owing to the notochord being invariably so striking and mysterious a feature of the lowest + vertebrates, the term vertebrate, which is inappropriate in the members of the group which do not + yet possess vertebræ, has been largely superseded by the term chordate, with the result of + attributing an undue preponderance to this tissue in any system of classification. Hence, wherever + any animal has been found <span class="pagenum" id="page439">{439}</span>with a tissue resembling + that of the notochord, enthusiasts have immediately jumped to the conclusion that a relationship + must exist between it and the chordate animals; and, accordingly, they have classified such + animals as follows: Amphioxus belongs to the group <i>Cephalochorda</i> because the notochord + projects beyond the central nervous system; the Tunicata are called <i>Urochorda</i> because it is + confined to the tail; the Enteropneusta, <i>Hemichorda</i>, because this tissue is confined to a + small diverticulum of the gut, and, finally, <i>Diplochorda</i> has been suggested for + Actinotrocha and Phoronis because two separate portions of the gut are transformed in this + way.</p> + + <p>This exaggerated importance given to any tissue resembling in structure that of the notochord + is believed in by many of those who profess to be our teachers on this subject, the very men who + can deliberately shut their eyes to the plain reading of the story of the pineal eyes, and say, + "In our opinion this pineal organ was not an eye at all."</p> + + <p>The only legitimate inference to be drawn from the similarity of structure between the + notochord and these degenerated gut-diverticula, is that the structure of the notochord may have + arisen in the same way, and that therefore the notochord may once have functioned as a gut. With + cessation of its function its cells became vacuolated, as in these other cases, and its lumen + became filled with notochordal tissue. This evidence strongly confirms the suggestion that the + notochord was once a digestive tube, but by no means signifies that such tissue, wherever found, + indicates the presence of a notochord.</p> + + <p>In order to resemble a notochord, this tissue must possess not only a definite structure but a + definite position, and this position is a remarkably striking and suggestive one. The notochordal + tube is unsegmented, although the vertebrate is markedly segmented. But in all segmented animals + the only unsegmented tube which extends the whole length of the body, from mouth to anus, is + invariably the gut. In the vertebrate there are three such tubes: (1) the gut itself, (2) the + central canal of the nervous system, and (3) the notochordal tube.</p> + + <p>The first is the present gut, the second the gut of the invertebrate ancestor, and the third + the tube in question.</p> + + <p>These three unsegmented tubes, extending along the whole length <span class="pagenum" + id="page440">{440}</span>of the segmented animal, constitute the great peculiarity of the + vertebrate group; it is not the unsegmented notochord alone which requires explanation, but the + presence of three such tubes in the same animal. Any one of them might be the unsegmented gut of + the segmented animal. The most ventral tube is the actual gut of the present vertebrate; the most + dorsal—the neural canal—was, according to my view, the original gut of the + invertebrate ancestor; the middle one—the notochordal tube—was, in all probability, + also once a gut, formed at the time when the exigencies of the situation made it difficult for + food to pass along the original gut.</p> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig166.png" id="fig166"><img style="width:100%" src="images/fig166.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p class="ac"><span class="sc">Fig. 166.—Diagram to show the Meeting of the Four Tubes + in such a Vertebrate as the Lamprey.</span></p> + <p class="sp0"><i>Nc.</i>, neural canal with its infundibular termination; <i>Nch.</i>, + notochord; <i>Al.</i>, alimentary canal with its anterior diverticulum; <i>Hy.</i>, + hypophysial or nasal tube; <i>Or.</i>, oral chamber closed by septum.</p> + </div> + </div> + + <p>Yet another circumstance in favour of this suggestion is the very striking position of the + anterior termination of the notochord. It terminates at the point of convergence of three + structures<span class="wnw">:—</span></p> + + <p>(1) The tube of the hypophysis or nasal tube.</p> + + <p>(2) The infundibulum or old mouth-termination.</p> + + <p>(3) The notochordal tube.</p> + + <p>To these may be added, according to Kupffer, in the embryonic stage, the anterior diverticulum + of the gut (Fig. <a href="#fig166">166</a>).</p> + + <p>This is a very significant point. Here originally, in the invertebrate stage, the olfactory + passage opened into the old mouth and œsophagus. Here, finally, in the completed vertebrate + the same olfactory passage opens into the new pharynx. In the stage between the two it may well + have opened into an intermediate gut, the notochordal tube, its separation from which would leave + the end of the <span class="pagenum" id="page441">{441}</span>notochord blind, just as it had + already left the end of the infundibulum blind.</p> + + <p>The whole evidence points to the derivation of the notochord from a ventral groove on the + surface of the animal, which closed to form a tube capable of acting as an accessory gut at the + critical period before the new gut was fully formed. The essentials of a gut tube are absorption + and digestion of food; is it likely that a tube formed as I have suggested would be efficient for + such purposes?</p> + + <p>As far as absorption is concerned, no difficulty would arise. The gut of the arthropod is lined + with a thin layer of chitin, which is traversed, like all other chitinous surfaces, by fine + canaliculi. Through these canaliculi, absorption of fluid material takes place, from the gut to + the body. Similar canaliculi occur in the chitin covering the animal externally, so that, if such + external surface formed a tube, and food in the right condition for absorption passed along it, + absorption could easily take place through the chitinous surface. The evidence of Apus proves that + food does pass along such a tube in the open condition, and in the trilobites the chitinous + surface lining a similar groove was apparently very thin, a condition still more favourable to + such an absorption process.</p> + + <p>At first sight the second essential of a gut-tube—the power of digestion—appears to + present an insuperable difficulty to this method of forming an accessory gut-tube, for it + necessitates the formation of a secretion capable of digesting proteid material by the external + cells of the body, whereas until recently it was supposed that such a function was confined to + cells belonging to the so-called hypoblastic layer. Experiments were made now years ago of turning + a Hydra inside out so that its internal layer should become external, and <i>vice versâ</i>, and + they were said to have been successful. Such an animal could go on living and absorbing and + digesting food, although its epiblastic surface was now its digestive internal surface. More + recent observations have shown that these experiments were fallacious. At night-time, when the + observer was not looking, the hydra reinverted itself, so that again its original digestive + surface was inside and it lived and prospered as before.</p> + + <p>Another piece of evidence of somewhat similar kind, which has not as yet been discredited, is + seen in the Tunicata. In many of these, new individuals are formed from the parent by a process of + budding, and it has been proved that frequently the gut of the new <span class="pagenum" + id="page442">{442}</span>individual thus budded off arises not from the gut or hypoblastic layer + of the parent, but from the surface or epiblastic layer. Such gut so formed possesses as efficient + digestive powers as the gut of the parent.</p> + + <p>The most remarkable evidence of all has been afforded by Miss Alcock's experiments. She + examined the different tissues of Ammocœtes for the express purpose of finding out their + power of digesting fibrin, with the result that the most active cells were those of the liver. + Next in activity came the extract of the lining cells of the respiratory chamber and of the skin. + The intestine itself when freed from the liver-secretion had very little digestive power; extracts + of muscle, nervous system, and thyroid gland had no power whatever, but the extract of the + skin-cells possessed a powerful digesting action.</p> + + <p>Furthermore, it is not necessary to make an extract of the skin in order to obtain this + digestive fluid, for under the influence of chloroform the skin of Ammocœtes secretes + copiously, and this fluid thus secreted was found to possess strong digestive powers. So, also, + Miss Alcock has demonstrated the power of digesting fibrin in a similar secretion of the + epithelial cells lining the carapace of the crayfish. In both cases a very plausible reason for + the presence of a digestive ferment in a skin-secretion is found in the necessity of preventing + the growth of parasites, fungoid, or otherwise, especially in those parts where the animal cannot + keep itself clean by 'preening.' Thus in a crayfish, in which the œsophageal commissures + had been cut, fungus was found to grow on the ventral side, but not on the dorsal carapace. The + animal was accustomed to keep its ventral surface clean by preening; owing to the paralysis it + could not do so, and consequently the fungus grew there. In the lamprey I found that wherever + there was a removal of the surface-epithelium, from whatever cause, that spot was immediately + covered with a fungoid growth, although in the intact lamprey the skin was invariably smooth and + clean.</p> + + <p>I imagine, then, that this digestive power of the skin arose as a protective mechanism against + parasitic attacks; it is self-evident how a tube formed of such material must <i>ab initio</i> act + as a digestive tube.</p> + + <p>In yet another respect this skin secretion of Ammocœtes is most instructive. The surface + of Ammocœtes is absolutely smooth, no scales <span class="pagenum" + id="page443">{443}</span>of any kind exist; this smoothness is due to the presence of a very + well-defined cuticular layer secreted by the underlying epithelial cells. This cuticle is very + much thicker than is usually found in vertebrates, and, strangely enough, has been thought to + contain chitin. Whether it really contains chitin or not I am unable to say, but it certainly + resembles a chitinous layer in one respect; it is perforated by innumerable very fine tubes or + canaliculi, along which, by appropriate staining, it is easy to see the secretion of the + underlying cell pass to the exterior (Fig. <a href="#fig140">140</a>). This marked digestive power + of the skin of Ammocœtes, together with the easy passage of the secretion through the thin + cuticular layer, renders it almost certain that a tube formed from the deep ventral groove of the + trilobite would, from the very first, act as a digestive as well as an absorbent tube; in other + words, the notochord as soon as formed was able to act as an accessory digestive tube.</p> + + <p>This suggested origin of the notochord from a groove along the mid-ventral surface of the body + not only indicates a starting-point from a markedly segmented portion of the body, but also points + to its formation at a stage previous to the formation of the operculum by the fusion of the two + foremost mesosomatic appendages—indicates therefore its formation at a stage more nearly + allied to the trilobite than to the sea-scorpion. The chance of ever finding any direct evidence + of such a chordate trilobite stage appears to me exceedingly improbable, and I greatly fear that + this conception of the mode of formation of the notochord can never be put to direct proof, but + must always remain guesswork.</p> + + <p>On the other hand, evidence of a kind in favour of its origin from a segmented part of the body + does exist, and that evidence has this special value, that it is found only in that most primitive + animal, Amphioxus.</p> + + <p>This evidence is as follows:—</p> + + <p class="sp3">At fairly regular intervals, the sheath of the notochord is interrupted on each + side of the mid-dorsal line by a series of holes, which penetrate the whole thickness of the + sheath. This dorsal part is pressed closely against the spinal cord, and through these holes + fibres appear to pass from the spinal cord to the interior of the notochord. So greatly do these + fibres present the appearance of ventral roots to the notochord, that Miss Platt looks upon them + as paired motor roots to the notochord, or at all events as once having been such motor <span + class="pagenum" id="page444">{444}</span>roots. Lwoff and Rolph both describe a direct + communication between the spinal cord and the notochord by means of fibres passing through these + holes, without however looking upon this connection as a nervous one. Joseph alone asserts that no + absolute connection exists, for the internal elastic layer of the notochord, according to him, is + not interrupted at these holes, and forms, therefore, a barrier between the fibres from the spinal + cord and those from the interior of the notochord. Still, whatever is the ultimate verdict as to + these fibres, the suggestive fact remains of the spaces in the notochordal sheath and of the + corresponding projecting root-like fibres from the spinal cord. The whole appearance gives the + impression of some former connection, or rather series of connections, between the spinal cord and + the notochord, such as would have occurred if nerves had once passed into the notochord. On the + other hand, such nerves were not arranged segmentally with the myotomes, for, according to Joseph, + in the middle of the animal ten to twelve such holes occur in one body-segment. In Apus the + appendages are more numerous than the body-segments, so that it is not necessary for a segmental + arrangement to coincide with that of the body-segments.</p> + + <p class="ac"><span class="sc">The Origin of the Alimentary Canal.</span></p> + + <p>In close connection with the notochord is the alimentary canal. Any explanation of the one must + be of assistance in explaining the other.</p> + + <p>According to the prevalent embryological teaching, the body is formed of three layers, + epiblast, hypoblast, and mesoblast, and the gastræa theory of the origin of all Metazoa implies of + necessity that the formation of every individual commences with the formation of the gut. For this + reason the alimentary canal must in every case be regarded as the earliest formed organ, however + late in the development it may attain its finished appearance. Hence the notochord is spoken of as + developed from the mid-dorsal wall of the alimentary canal. It is possible to look at the question + the other way round, and suppose that the organ whose development is finished first is older than + the one still in process of making. In this case it would be more right to say a ventral extension + of the tissue, which gives rise to the notochord, takes place and forms the alimentary canal. It + is, to my mind, perfectly possible, and indeed probable, that <span class="pagenum" + id="page445">{445}</span>the formation of the vertebrate alimentary canal was a repetition of the + same process which had already led to the formation of the notochordal tube. The formation of the + anterior part of the alimentary canal in Ammocœtes at the time of transformation strongly + suggests the marked similarity of the two processes.</p> + + <p>Of all the startling surprises which occur at transformation, this formation of a new anterior + gut is the most startling. From the oral chamber of Petromyzon two tubes start: the one leads into + the gill-chambers, is known as the bronchus, and is entirely concerned with respiration; the other + leads without a break from the mouth to the anus, has no connection with respiration, and is the + alimentary canal of the animal. Any one looking at Petromyzon would say that its alimentary canal + was absolutely non-respiratory in character. Before transformation, this kind of alimentary canal + commences at the end of the respiratory chamber; from here to the anus it is of the same character + as in Petromyzon, but in Ammocœtes the non-respiratory anterior part simply does not exist: + the whole anterior chamber is both respiratory and affords passage to food. This part of the + alimentary canal of the adult is formed anew. We see, then, here the formation of a part of the + alimentary canal taking place, not in an embryo full of yolk, but in a free-living, independent, + grown-up larval form in which all yolk has long since disappeared: a condition absolutely unique + in the vertebrate kingdom, but one which more than any other may be expected to give a clue to the + method of formation of a vertebrate gut.</p> + + <p>The formation of this new gut can be easily followed at transformation, and was originally + described by Schneider. His statement has been confirmed by Nestler, and its absolute truth has + been demonstrated to me again and again by Miss Alcock, in her specimens illustrative of the + transformation process. First, in the mid-dorsal line of the respiratory chamber a distinct groove + is formed, the edges of which come together and form a solid rod. This solid rod blocks the + opening of the respiratory chamber into the mid-gut, so that during this period of the + transformation no food can pass out of the pharyngeal chamber. A lumen then begins to appear in + this solid rod at the posterior end, which steadily advances mouthwards until it opens into the + oral chamber and thus forms an open tube connecting the mouth with the gut.</p> + + <p>Here, then, is the foundation of a new gut on very similar lines <span class="pagenum" + id="page446">{446}</span>to that of the notochord, by the conversion of a groove into a tube. + Still more suggestive is it to find that the tube so formed has no appearance whatever of + segmentation; it is as unsegmented as the rest of the gut, although, as is seen in Fig. <a + href="#fig62">62</a>, the dorsal wall of the respiratory chamber from which it arose is as + markedly segmented as any part of the animal. Here under our very eyes, in the course of a few + days or weeks, an object-lesson in the process of the manufacture of an alimentary canal is + carried out and completed, and the teaching of that lesson is that a gut-tube may be formed in the + same way as the notochordal tube, by the conversion of a grooved surface into a canal, and that + gut-tube so formed, like the notochord, loses all sign of segmentation, even although the original + grooved surface was markedly segmented.</p> + + <p>The suggestion then is, that the new gut may have been formed by a repetition of the same + process which had already given origin to the notochord.</p> + + <p class="sp3">Such a method of formation is not, in my opinion, opposed to the evidence given by + embryology, but in accordance with it; the discussion of this point will come best in the next + chapter, which treats of the embryological evidence as a whole, and will therefore be left till + then.</p> + + <p class="ac"><span class="sc">The Evidence given by the Innervation of the Vertebrate Alimentary + Canal.</span></p> + + <p>Throughout this investigation the one fixed landmark to which all other comparisons must be + referred, is the central nervous system, and the innervation of every organ has given the clue to + the meaning of that organ. So also it must be with the new alimentary canal; by its innervation we + ought to obtain some insight into the manner of its origination. In any organ the nerves which are + specially of value in determining its innervation, are of necessity the efferent or motor nerves, + for the limits of their distribution in the organ are much more easily determined than those of + the afferent or sensory nerves. The question therefore of primary importance in endeavouring to + determine the nature of the origin of the alimentary canal from its innervation is the + determination of the efferent supply to the musculature of its walls.</p> + + <p>Already in previous chapters a commencement has been made in <span class="pagenum" + id="page447">{447}</span>this direction; thus the musculature of the oral chamber has been derived + directly from the musculature of the prosomatic appendages; the muscles which move the eyes from + the prosomatic and mesosomatic dorso-ventral somatic muscles; the longitudinal body-muscles from + the dorsal longitudinal somatic muscles of the arthropod; the muscles of respiration from the + dorso-ventral muscles of the mesosomatic appendages.</p> + + <p>In all these cases we have been dealing with striated musculature and consequently with only + the motor nerves of the muscle; but the gut posterior to the pharyngeal or respiratory chamber + contains unstriped instead of striped muscle, and is innervated by two sets of nerves, those which + cause contraction and are motor, and those which cause relaxation and are inhibitory. It is by no + means certain that these two sets of nerves possess equal value from a morphological point of + view. The meaning of an inhibitory nerve is at present difficult to understand, and in this + instance, is rendered still more doubtful owing to the presence of Auerbach's plexus along the + whole length of the intestine—an elaborate system of nerve-cells and nerve-fibres situated + between the layers of longitudinal and circular muscles surrounding the gut-walls, which has been + shown by the recent experiments of Magnus, to constitute a special enteric nervous system.</p> + + <p>One of the strangest facts known about the system of inhibitory nerves is their marked tendency + to leave the central nervous system at a different level to the corresponding motor nerves, as is + well known in the case of the heart, where the inhibitory nerve—the vagus—arises from + the medulla oblongata, while the motor nerve—the augmentor or accelerator—leaves the + spinal cord in the upper thoracic region. It is very difficult to obtain any idea of the origin of + such a peculiarity; I know of only one suggestive fact, which concerns the innervation of the + muscles which open and close the chela of the crayfish, lobster, etc. These muscles are + antagonistic to each other, and both possess inhibitory as well as motor nerves. The central + nervous system arrangements are of such a character that the contraction of the one muscle is + accompanied by the inhibition of its opposer, and the nerves which inhibit the contraction of the + one, leave the central nervous system with the nerves which cause the other to contract. Thus the + inhibitory and motor nerves of either the abductor (opener) or adductor (closer) muscles of the + crayfish claw do not leave the central nervous system together, but in separate nerves.</p> + + <div><span class="pagenum" id="page448">{448}</span></div> + + <p>If now for some cause the one set of muscles either disappeared, or were so altered as no + longer to present any appearance of antagonism, then there would be left a single set of muscles, + the inhibitory and motor nerves of which would leave the central nervous system at different + levels, and the older such systems might be, the greater would be the modification in the shape + and arrangements of parts in the animal, so that the two sets of fibres might ultimately arise + from very different levels.</p> + + <p>As mentioned in the introductory chapter, the whole of this investigation into the origin of + vertebrates arose from my work on the system of efferent nerves which innervate the vascular and + visceral systems. One of the main points of that investigation was the proof that such nerves did + not leave the central nervous system uniformly along the whole length of it, but in three great + outflows, cranial, thoracico-lumbar, and sacral; there being two marked gaps separating the three + outflows, caused by the interpolation of the plexuses for the innervation of the anterior and + posterior limbs respectively. All these nerves are characterized by the presence of ganglion-cells + in their course to the periphery, they are, therefore, distinguished from ordinary motor nerves to + striated muscle in that their impulses pass through a ganglion-cell before they reach the + muscle.</p> + + <p>The ganglia of the large middle thoracico-lumbar outflow constitute the ganglia of the + sympathetic system.</p> + + <p>The functions of the nerves constituting these three outflows are very different, as I pointed + out in my original papers. Since then a large amount of further information has been obtained by + various observers, especially Langley and Anderson, which enable the following statements to be + made<span class="wnw">:—</span></p> + + <p>All the nerves which cause contraction of the unstriped muscles of the skin, whether pilomotor + or not, all the nerves which cause secretion of sweat glands wherever situated, all the nerves + which cause contraction or augmentation of the action of muscles belonging to the vascular system, + all the nerves which are motor to the muscles belonging to all organs derived from the Wolffian + and Müllerian ducts, <i>e.g.</i> the uterus, ureters, urethra, arise from the thoracico-lumbar + outflow, never from the cranial or sacral outflows. It is essentially an efferent skin-system.</p> + + <p>On the other hand, the latter two sets of nerves are concerned <span class="pagenum" + id="page449">{449}</span>with the supply of motor nerves to the alimentary canal; they form + essentially an efferent gut-system in contradistinction to the sympathetic or skin-system.</p> + + <p>A marked distinction exists between these cranial and sacral nerves. The vagus never supplies + the large intestine, the sacral nerves never supply the small intestine. Associated with the large + intestine is the bladder, the whole system arising from the original cloacal region; the vagus + never supplies the bladder, its motor nerves belong to the sacral outflow. The motor nerves to the + ureters, to the urethra, and to the trigonal portion of the bladder between the ureters and the + urethra, do not arise from the sacral outflow, but from the thoracico-lumbar. These muscles belong + really to the muscles in connection with the Müllerian and Wolffian ducts and skin, not to the + cloacal region.</p> + + <p>The motor innervation then of the alimentary canal reveals this striking and suggestive state + of affairs. The motor innervation of the whole of the small intestine arises from the cranial + region, and is immediately followed by an innervation from the sacral region for the whole of the + muscles of the cloaca. It thus indicates a head-region and a tail-region in close contiguity, the + whole of the spinal cord region between these two extremes being apparently unrepresented. Not, + however, quite unrepresented, for Elliott has shown recently that the ileo-colic valve at the + junction of the small and large intestine is in reality an ileo-colic sphincter muscle, and that + this muscle receives its motor nerves neither from the vagus nor from the sacral nerves, but from + the thoracico-lumbar outflow or sympathetic system. This may mean one of two things, either that a + band of fibres belonging to the skin-system has been added to the gut-musculature, for the purpose + of forming a sphincter at this spot, or that the region between the vagus territory and the cloaca + is represented by this small band of muscle. The second explanation seems to me the more probable + of the two. Between the mesosomatic region represented by the vagus, and the cloacal region, there + existed a small metasomatic region, represented by the pronephros, with its segmental duct, as + already discussed in Chapter XII. That part of the new alimentary canal which belonged to this + region is the short piece indicated by the ileo-colic sphincter, and innervated, therefore, from + the same region as the organs derived from the segmental duct.</p> + + <p>Such innervation seems to me to suggest that originally the <span class="pagenum" + id="page450">{450}</span>vertebrate consisted, as far as its gut was concerned, of a prosomatic + and mesosomatic (branchial) region, close behind which came the cloaca and anus. Between the two + there was a short metasomatic region (possibly pronephric), so that the respiratory chamber did + not open directly into the cloaca.</p> + + <p>Such an interpretation is, I think, borne out by the study of the most ancient forms of fish. + In Bothriolepis, according to Patten, and in Drepanaspis, according to Traquair, the cloacal + region and anus follow immediately upon the posterior end of the head-shield, <i>i.e.</i> + immediately after that region which presumably contained the branchiæ. Similarly, on the + invertebrate side, all those forms which resembled Limulus must have possessed a very short region + between the branchial and cloacal parts of the body. The original cloacal part of the vertebrate + gut may well have been the original cloaca of the arthropod, into which its intestine emptied + itself, especially when we see the tendency of the scorpion group of animals to form an accessory + cloacal pouch known as the stercoral pouch or pocket.</p> + + <p>Again, it is striking to see how, in certain of the scorpion group, <i>e.g.</i> Thelyphonus and + Phrynus, there is a caudal massing of the central nerve-cells as well as a cephalic massing, so + that their central nervous system is composed of a cephalic and caudal brain. These two brains are + connected together by commissures extending the whole length of the body, in which I have been + unable to find any sign of ganglion-cells. What this caudal brain innervates I do not know; it is, + I think, a matter worth further investigation, especially as there are many indications in the + vertebrate that the lumbo-sacral region of the cord possesses higher functions than the thoracic + region.</p> + + <p>The method of formation of the alimentary canal as indicated by its innervation is as + follows<span class="wnw">:—</span></p> + + <p>In front an oral chamber, formed, as already pointed out, by the modification of the prosomatic + appendages, followed by a respiratory chamber, the muscles and branchiæ of which were the muscles + and branchiæ of the mesosomatic appendages. This mesosomatic, or branchial, part was in close + contiguity to the cloaca and anus, being separated from it only by a short tube formed in the + metasomatic or pronephric region.</p> + + <p>I imagine that this connection was originally in the form of an <span class="pagenum" + id="page451">{451}</span>open groove, as already explained for both notochord and the anterior + part of the gut itself in Ammocœtes; an open groove formed from the mid-ventral surface of + the body, on each side of which were the remnants of the pronephric appendages. By the closure of + this groove ventrally, and the growing round of the pleural folds, as already suggested, the + remains of the pronephric appendages are indicated by the segmental duct and the form of the + vertebrate body is attained.</p> + + <p>Even in the branchial region the same kind of thing must, I think, have occurred. The grooved + ventral surface became a tube, on each side of which were lying in regular order the in-sunk + branchial appendages, the whole being subsequently covered by the pleural folds to form an atrial + chamber. A tube thus formed from the grooved ventral surface would carry with it to the new + ventral surface the longitudinal venous sinuses, and thus form, in the way already suggested, the + heart and ventral aorta. Posterior to the heart in the pronephric region, the same process would + give rise to the sub-intestinal vein.</p> + + <p>The evidence of comparative anatomy bears out most conclusively the suggestion that in the + original vertebrate the gut was mainly a respiratory chamber. In man and all mammals the oral + chamber opens into a small pharynx, followed by the œsophagus, stomach and small intestine. + Of this whole length, a very small part is taken up by the pharynx, in which, in the embryo, the + branchial arches are found, showing that this represents the original respiratory part of the gut. + In the ordinary fish this branchial part is much more conspicuous, occupies a large proportion of + the gut, and in the lowest fishes, such as Ammocœtes and Amphioxus, the branchial region + extends over a large portion of the animal, while the intestine proper is a straight tube, the + length of which is insignificant in comparison with its length in the higher vertebrates.</p> + + <p>Such a tube was able to act as a digestive tube, owing, as already pointed out, to the + digestive powers of the skin-epithelium, and I imagine at first the respiratory chamber, seeing + that it composed very nearly the whole of the gut, was at the same time the main digestive + chamber; even in Ammocœtes its digestive power is superior to that of the intestine + itself.</p> + + <p>Just posterior to the branchial part a diverticulum of the gut was formed at an early stage, as + seen in Amphioxus, and provided the <span class="pagenum" id="page452">{452}</span>commencement of + the liver. This simple liver-diverticulum became the tubular liver of Ammocœtes, and + formed, curiously enough, not a glandular organ of the same character as the liver of the higher + vertebrates, but a hepato-pancreas, like the so-called liver of the arthropods, which also is a + special diverticulum of the gut, or rather the main true gut of the animal. In both cases the + liver is the chief agent in digestion, for in Ammocœtes the liver-extract is very much more + powerful in the digestion of proteids than the extract of any other organ tried by Miss Alcock. + Subsequently in the vertebrate the gastric and pancreatic glands arise and relieve the liver of + the burden of proteid digestion.</p> + + <p class="sp3">It is, to my mind, somewhat significant that the liver on its first formation in + the vertebrate should have arisen as a digestive organ of the same character as the so-called + liver in the arthropods; whether it originally belonged to any separate segment is in our present + state of knowledge difficult to say.</p> + + <p class="ac"><span class="sc">Conclusion.</span></p> + + <p>In conclusion, I will endeavour to illustrate crudely the way in which, on my theory, the + notochord and vertebrate gut may have been formed, the agencies at work being in the main two, + viz. the dwindling of appendages as mere organs of locomotion, and the conversion of a ventral + groove into a tube.</p> + + <p>I imagine that, among the Protostraca, forms were found somewhat resembling trilobites with + markedly polychætan affinities; which, like Apus, possessed a deep ventral groove from one end of + the body to the other, and also pleural fringes, as in many trilobites. This might be called the + Trilobite stage (Fig. <a href="#fig167">167</a>, A).</p> + + <p>This groove became converted into a tube and so gave rise to the notochord, while the + appendages were still free and the pleuræ had not met to form a new ventral surface. This might be + called the Chordate Trilobite stage (Fig. <a href="#fig167">167</a>, B).</p> + + <p>Then, passing from the protostracan to the palæostracan stage, the oral and respiratory + chambers were formed, not communicating with each other, in the manner described in previous + chapters, a ventral groove in the metasomatic region being the only connection between respiratory + chamber and cloaca. This might be called the Chordate Palæostracan stage (Fig. <a + href="#fig167">167</a>, C).</p> + + <div><span class="pagenum" id="page453">{453}</span></div> + + <div class="ac w50 fcenter sp2"> + <a href="images/fig167.png" id="fig167"><img style="width:100%" src="images/fig167.png" alt="" + title=""/></a> + <div class="smaller aj it1"> + <p><span class="sc">Fig. 167.—A, Diagram of Section through a Trilobite-like Animal; B, + Diagram to illustrate the Suggested Formation of the Notochord from a Ventral Groove; C, + Diagram to illustrate the Suggested Formation of the Post-Branchial Gut by the continuation of + the same process of Ventral Groove-Formation, combined with Obliteration of Appendages and + Growth of Pleural Folds; D, Diagram to illustrate the Completion of the Vertebrate Type by the + Meeting of the Pleural Folds in the Mid-Ventral Line with the Obliteration of the Atrial + Cavity and the Conversion of the Ventral Groove into the closed Alimentary Canal.</span></p> + <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nervous system; <i>My.</i>, myotome; + <i>Pl.</i>, pleuron; <i>App.</i>, appendage; <i>Neph.</i>, <span class="correction" + title="Original reads 'nephrocele'">nephrocœle</span>; <i>Met.</i>, metacœle; + <i>Sd.</i>, segmental duct; <i>Mes.</i>, mesonephros; <i>At.</i>, atrial chamber; <i>Nc.</i>, + notochord; <i>H.</i>, heart; <i>F.</i>, fat body; <i>Ng.</i>, notochordal groove. (These + diagrams are intended to complete the diagrams on p. <a href="#page413">413</a>, which, as + stated there, were purposely left incomplete.)</p> + </div> + </div> + + <div><span class="pagenum" id="page454">{454}</span></div> + + <p>Finally, with the conversion of this groove into a tube, the opening of the oral into the + respiratory chamber, and the formation of an atrium by the ventralwards growth of the pleural + folds, the formation of a Vertebrate was completed (Fig. <a href="#fig167">167</a>, D).</p> + + <p>In my own mind I picture to myself an animal which possessed eurypterid and trilobite + characters combined, in which a notochordal tube had been formed in the way suggested, and a + respiratory chamber which communicated with the cloaca by means of a grooved channel along the + mid-ventral line of the metasomatic portion of the body. On each side of this channel were the + remains of the metasomatic appendages (pronephric). The whole was enveloped in the pleural folds, + which probably at this time did not yet meet in the middle line to form a new ventral surface. + This respiratory chamber, owing to the digestive power of the epidermis, assisted in the process + of alimentation to such an extent as to supersede the temporary notochordal tube, with the effect + of bringing about the conversion of the metasomatic groove into a closed canal, and so the + formation of an alimentary tube continuous with the respiratory chamber. The amalgamation of the + pleural folds ventrally completed the process, and so formed an animal resembling the + Cephalaspidæ, Ammocœtes, or Amphioxus.</p> + + <p class="sp5">I have endeavoured in this chapter to make some suggestions upon the origin of the + notochord and of the vertebrate gut in accordance with my theory of the origin of vertebrates. I + feel, however, strongly that these suggestions are much more speculative than those put forward in + the previous chapters, and of necessity cannot give the same feeling of soundness as those based + directly upon comparative anatomy and histology. Still, the fact remains that the origin of the + notochord is at present absolutely unknown, and that my speculation that it may have originated as + an accessory digestive tube is at all events in accordance with the most widely spread opinion + that it arises in close connection with an alimentary canal.</p> + + <div><span class="pagenum" id="page455">{455}</span></div> + + <p class="ac">CHAPTER XIV</p> + + <p class="ac"><i>THE PRINCIPLES OF EMBRYOLOGY</i></p> + + <div class="bq1 it1 smaller sp3"> + <p class="sp0">The law of recapitulation.—Vindication of this law by the theory advanced + in this book.—The germ-layer theory.—Its present position.—A physiological not + a morphological conception.—New fundamental law required.—Composition of adult + body.—Neuro-epithelial syncytium and free-living cells.—Meaning of the + blastula.—Derivation of the Metazoa from the Protozoa. Importance of the central nervous + system for Ontogeny as well as for Phylogeny.—Derivation of free-living cells from + germ-cells.—Meaning of cœlom.—Formation of neural canal.—Gastrula of + Amphioxus and of Lucifer.—Summary.</p> + </div> + + <p>In a discussion upon this theory of mine, which took place at Cambridge on November 25 and + December 2, 1895, it was said that such a theory was absolutely and definitely put out of court, + because it contravened the principles of embryology, was opposed, therefore, to our surest guide + in such matters; and the law was laid down with great assurance that no claim for genetic + relationship between two groups of animals can be allowed which is based upon topographical and + structural coincidences revealed by the study of the anatomy of two adult animals, however + numerous and striking they may be, if there are fundamental differences in the embryology of the + members of these two groups.</p> + + <p>According to my theory the old gut of the arthropod still exists in the vertebrate as the + tubular lining of the central nervous system, and the vertebrate has formed a new gut. According + to the principles of embryology as held up to the present, in all animals above the Protozoa, the + different structures of the body arise from three definite embryonic layers, the epiblast, + mesoblast, and hypoblast, and in all cases the gut arises from the hypoblastic layer. In the + vertebrate the gut also arises from the hypoblast, while the neural canal is epiblastic. My + theory, then, makes the impossible assertion that what was hypoblast in the arthropod has become + epiblast in the vertebrate, and what was epiblast in the arthropod has become hypoblast in the + vertebrate. Such a conception is supposed to be so <span class="pagenum" + id="page456">{456}</span>absolutely impossible that it only requires to be stated to be dismissed + as an absurdity.</p> + + <p>Against this opinion I claim boldly that my theory is not only not contrary to the principles + of embryology, but is mainly based upon the teachings of embryology. I wish here not to be + misunderstood. The great value of the study of embryology for questions of the sequence of the + evolution of animals is to be found in what is known as the Law of Recapitulation, which asserts + that every animal gives some indication in the stages of its individual development of its + ancestral history. Naturally enough it cannot pass through all the stages of its past history with + equal clearness, for what has taken millions of years to be evolved has to be compressed into an + evolution lasting only a few months or weeks, or even less.</p> + + <p>When in the highest vertebrate a vestigial organ, such as the pineal gland, can be traced back + without leaving the vertebrate kingdom to a distinct median eye, such as is found in the lamprey, + that rudimentary organ is evidence of an organ which was functional in the earliest vertebrates or + their immediate ancestors. So it is generally with well defined vestigial organs found in the + adult animal; they always indicate an organ which was functional in the near ancestor.</p> + + <p>Passing from the adult to the embryo we still find the same law. Here, also, vestigial organs + are met with, which may leave no trace in the adult, but indicate organs which were functional in + the near ancestor. Thus, but for embryology, we should have no certainty that the air-breathing + vertebrates had been derived from water-breathing fishes; the indication is not given by any close + resemblance between the formation of the embryos in their earliest stages, but by the formation of + vestigial gill-arches even in the embryos of the highest mammal.</p> + + <p>For all questions of evolution the presence of vestigial organs in the embryo is the important + consideration, for they give an indication of near ancestry; the early formation of the embryo + concerns a much more remote ancestral period, all vestigial organs of which may well have been + lost and obscured by cœnogenetic changes. Let us, then, consider the two things—the + vestigial organs and the early formation of the embryo—separately, and see how far my + opponents are justified in their statement that my theory contravenes the principles of + embryology.</p> + + <div><span class="pagenum" id="page457">{457}</span></div> + + <p>First, I will take the teachings of vestigial organs and the arrangement of organs found in the + vertebrate embryo. Here it is impossible to say that my theory is contrary to the teaching of + embryology, for as the previous chapters have shown again and again, the argument is based very + largely upon the facts of embryology. In the first place, the comparison which I have chiefly made + is a comparison between the larval form of a very low vertebrate and the arthropod group, a + comparison which exists only for the larval form, and not for the adult. The whole theory, then, + is based upon a developmental stage of the vertebrate, and not upon the anatomy of the adult.</p> + + <p>Throughout the whole history it seems to me perfectly marvellous how completely the law of + recapitulation is vindicated by my theory of the origin of the vertebrate. The theory asserts that + the clue to the origin of vertebrates is to be found in the tubular nature of the central nervous + system of the vertebrate; in that the vertebrate central nervous system is in reality formed of + two things: (1) a central nervous system of the arthropod type, and (2) an epithelial tube in the + position of the alimentary canal of the arthropod.</p> + + <p>Is it possible for embryology to recapitulate such a phylogenetic history more clearly than is + here the case? In order to avoid all possibility of our mistaking the clue, the nerve-tube in the + embryo always opens into the anus at its posterior end, while in the larval Amphioxus it is + actually still open to the exterior at the anterior end. The separateness of the tube from the + nervous system at its first origin is shown especially well in the frog, where, as Assheton has + pointed out, owing to the pigment in the cells of the external layer of epithelium, a pigmented + tube is formed, on the outside of which the nervous tissue is lying, and step by step the gradual + intermingling of the nerve-cells and the pigmented lining cells can be followed out.</p> + + <p>Consider the shape of the nerve-tube when first formed in the vertebrate. At the cephalic end a + simple bulged-out tube with two simple anterior diverticula, which passes into a narrow straight + spinal tube; from this large cephalic bulging a narrow diverticulum, the infundibulum, passes to + the ventral surface of the forming brain. This tube is the embryological expression of the simple + dilated cephalic stomach, with its ventral œsophagus and two anterior diverticula, which + opens into the straight intestine of the arthropod. Nay, more, by its very shape, and the + invariable presence of two anterior <span class="pagenum" id="page458">{458}</span>diverticula, it + points not only to an arthropod ancestry, but to a descent from a particular group of primitive + arthropods. Then comes the formation of the cerebral vesicles, and the formation of the optic cup, + telling us as plainly as can be how the invasion of nervous material over this simple cephalic + stomach and its diverticula has altered the shape of the original tube, and more and more enclosed + it with nervous elements.</p> + + <p>So, too, in the spinal cord region. When the tube is first formed, it is a large tube, the + latero-ventral part of which presents two marked bulgings; connecting these two bulgings is the + anterior commissure. These two lateral bulgings, with their transverse commissure, represent, with + marked fidelity, the ventral ganglion-masses of the arthropod with their transverse commissure, + and occupy the same position with respect to the spinal tube, as the ganglion-masses do with + respect to the intestine in the arthropod. Then the further development shows how, by the + subsequent growth of the nervous material, the calibre of the tube is diminished in size, and the + spinal cord is formed.</p> + + <p>Again, I say, is it possible to conceive that embryology should indicate the nature of the + origin of the vertebrate nervous system more clearly than it does?</p> + + <p>It is the same with all the other organs. Take, for instance, the skeletal tissues. The study + of the vertebrate embryo asserts that the cartilaginous skeleton arose as simple branchial bars + and a simple cranio-facial skeleton, and also that the parenchymatous variety of cartilage + represents the embryonic form. Word for word, the early embryonic stage of the vertebrate skeleton + closely resembles the stage reached in the arthropod, as shown by Limulus, and again records, + unmistakably, the past history of the vertebrate.</p> + + <p>So, too, with the whole of the prosomatic region; the situation of the old mouth, the manner in + which the nose of the cephalaspidian fishes arose from the palæostracan, are all shown with vivid + clearness by Kupffer's investigations of the early stage of Ammocœtes, while at the same + time the closure of the oral cavity by the septum shows how the oral chamber was originally + bounded by the operculum. Nay, further, the very formation of this chamber embryologically was + brought about by the forward growth of the lower lip, just as it must have been if the chilaria + grew forward to form the metastoma.</p> + + <p>So, too, the study of the embryo teaches that the branchiæ arise as <span class="pagenum" + id="page459">{459}</span>ingrowths, that the heart arises as two longitudinal veins, just as the + theory supposes from the facts provided by Limulus and the scorpions. No indication of the origin + of the thyroid gland is given by the study of its structure in any adult vertebrate, but in the + larval form of the lamprey there is still preserved for us a most graphic record of its past + history.</p> + + <p>The close comparisons which it is possible to make between the eye-muscles of the vertebrate + and the recti muscles of the scorpion group on the one hand, and between the pituitary and coxal + glands on the other, are based upon, or at all events are strikingly confirmed by, the study of + the cœlomic cavities and the origin of these muscles in the two groups. In fact the + embryological evidence of the double segmentation in the head and the whole nature of the cranial + segments is one of the main foundation-stones on which the whole of my theory rests.</p> + + <p>So it is throughout. Turn to the excretory organs—it is not the kidney of the adult + animal which leads direct to the excretory organs of the primitive arthropod, but the early + embryonic origin of that kidney.</p> + + <p>So far from having put forward a theory which runs counter to the principles of embryology, I + claim to have vindicated the great Law of Recapitulation which is the foundation-stone of + embryological principles. My theory is largely based upon embryological facts, and its strength + consists in the manner in which it links together into one harmonious whole, the facts of + Embryology, Palæontology, Anatomy, and Physiology. Why, then, is it possible to assert that my + theory disregards the principles of embryology, when, as we have seen, embryology is proclaiming + as loudly as possible how the vertebrate arose? In my opinion, it is because the embryologists + have to a large extent gone wrong in their fundamental principles, and have attached more weight + to these faulty fundamental principles than to the obvious facts which, looked at thoughtfully, + could not have failed to suggest a doubt as to the correctness of these 'principles.'</p> + + <p>The current laws of embryology upon which such weight is laid are based on the homology of the + germinal layers in all Metazoa, and state that in all cases after segmentation is finished a + blastula is formed, from which there arises a gastrula, formed of an internal layer, the + hypoblast, and an external layer, the epiblast; subsequently <span class="pagenum" + id="page460">{460}</span>between these arises a third layer, the mesoblast. These layers are + strictly morphological conceptions, and are stated to be homologous in all cases, so that the + hypoblast of one animal must be homologous to the hypoblast of another. In order, therefore, to + compare two adult animals for the purpose of finding kinship between them, it is necessary to find + whether parts such as the gut, which in both cases have the same function, arise from the same + germinal layer in the embryo. We can, in fact, have no certainty of kinship, even although the two + animals are built up as far as the adult state is concerned on a remarkably similar plan, unless + we can study their respective embryos and find out what parts arise from the hypoblast and what + from the epiblast. The homology of the germinal layers constitutes in all cases of disputed + relationship the court of final appeal. A new gut, therefore, in any animal can only be formed + from hypoblast, and any theory, such as that advocated in this book, which deals with the + formation of a new gut, and does not form that gut from pre-existing hypoblast, must of necessity + be wrong and needs no further consideration.</p> + + <p>Such is the result of current conceptions—conceptions which to be valid must be based + upon an absolutely clear morphological definition of the formation of the germinal layers, a + definition not based on their subsequent history and function, but determined solely by the + uniformity of the manner of their origin.</p> + + <p>What, then, is a germinal layer? How can we identify it when it first arises? What is the + morphological criterion by which hypoblast can be distinguished from epiblast, or mesoblast from + either?</p> + + <p>This is the question put by Braem, in an admirable series of articles in the <i>Biologisches + Centralblatt</i>, and is one that must be answered by every worker who bases his views of the + process of evolution upon embryological investigation. As Braem points out, the germinal layers + are definable either from a morphological or physiological standpoint. In the one case they must + arise throughout on the same plan, and whatever be their fate in the adult, they must form at an + early stage structures strictly homologous in all animals. In the other case the criterion is + based on function, and the hypoblast, for instance, is that layer which is found afterwards to + form the definitive alimentary canal. There is no longer any morphological homology; such layers + are analogous; they may be, but are not necessarily, homologous. Braem gives a sketch of the + history of the views held on <span class="pagenum" id="page461">{461}</span>the germinal layers, + and shows how they were originally a purely physiological conception, and how gradually such + conception changed into a morphological one, with the result that what had up to that time been + looked upon as analogous structures became strictly homologous and of fundamental importance in + deciding the position of any animal in the whole animal series.</p> + + <p>This change of opinion was especially due to the lively imagination of Haeckel, who taught that + the germinal layers of all Metazoa must be strictly homologous, because they were all derived from + a common ancestral stock, represented by a hypothetical animal to which he gave the name Gastræa; + an animal which was formed by the simple invagination of a part of the blastula, thus giving rise + to the original hypoblast and epiblast, and he taught that throughout the animal kingdom the + germinal layers were formed by such an invagination of a part of the blastula to form a simple + gastrula. If further investigation had borne out Haeckel's idea, if therefore the hypoblast was in + all cases formed as the invagination of a part of a single-layered blastula, then indeed the dogma + of the homology of the germinal layers would be on so firm a foundation that no speculation which + ran counter to it could be expected to receive acceptance; but that is just what has not taken + place. The formation of the gastrula by simple invagination of the single-layered blastula is the + exception, not the rule, and, as pointed out by Braem, is significantly absent in the earliest + Metazoa; in those very places where, on the Gastræa theory, it ought to be most conspicuous.</p> + + <p>Braem discusses the question most ably, and shows again and again that in every case the true + criterion upon which it is decided whether certain cells are hypoblastic or not is not + morphological but physiological. The decision does not rest upon the answer to the question, Are + these cells in reality the invaginated cells of a single-celled blastula? but to the question, Do + these cells ultimately form the definitive alimentary canal? The decision is always based on the + function of the cells, not on their morphological position. Not only in Braem's paper, but + elsewhere, we see that in recent years the physiological criterion is becoming more and more + accepted by morphologists. Thus Graham Kerr, in his paper on the development of Lepidosiren, says: + "It seems to me quite impossible to define a layer as hypoblastic except by asking one or other of + the two questions: (1) Does it form the lining of an archenteric cavity? and (2) <span + class="pagenum" id="page462">{462}</span>Does it become a certain part of the definitive + epithelial lining of the gut?"</p> + + <p>The appearance of Braem's paper was followed by a criticism from the pen of Samassa, who agrees + largely with Braem, but thinks that he presses the physiological argument too far. He considers + that morphological laws must exist for the individual development as well as for the phylogenetic, + and finishes his article with the following sentence, a sentence in which it appears to me he + expresses what is fast becoming the prevailing view: "Mit dem Satz, den man mitunter lesen kann: + 'es muss doch auch für die Ontogenie allgemeine Gesetze geben' kann leicht Missbrauch getrieben + werden; diese allgemeinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand und bis zu + ihrer Erkenntnis hat es noch gute Wege; das eine kann man aber wohl heute schon sagen, die + Keimblätterlehre gehört zu diesen allgemeinen Gesetzen nicht."</p> + + <p>I conclude, then, that we ought to go back to a time previous to that of Haeckel and ask + ourselves seriously the question, When we lay stress on the germinal layers and speak of this or + that organ arising from this or that germinal layer, are we thereby adding anything to the + knowledge that we already possess from the study of the anatomy and physiology of the adult body? + If by hypoblast we only mean the internal surface or alimentary canal and its glands, etc., and by + epiblast we mean the external surface or skin and its glands, etc., while mesoblast indicates the + middle structures between the other two, then I fail to see what advantages we obtain by using + Greek terms to express in the embryo what we express in English in the adult.</p> + + <p>The evidence given by Braem, and it could be strengthened considerably, is conclusive against + the morphological importance of the theory of the germinal layers, and transfers the fundamental + importance of the early embryonic formation, from that of a three-layered embryo to that of a + single-layered embryo—the blastula—from which, in various ways, the adult animal has + arisen.</p> + + <p>The derivation of both arthropod and vertebrate from such a single-layered animal is perfectly + conceivable, even though the gut of the latter is not homologous with the gut of the former. We + have seen that the teachings of embryology, as far as its later stages are concerned, afford one + of the main supports upon which this theory rests. What, therefore, is required to complete the + story is the way <span class="pagenum" id="page463">{463}</span>in which these later stages arise + from the blastula stage; here, as in all cases, the ontogenetic laws must be in harmony with the + phylogenetic; of the latter the most important is the steady development of the central nervous + system for the upward progress of the animal race. The study of comparative anatomy indicates the + central nervous system, not the gut, as the keystone of the edifice. So, also, it must be with + ontogeny; here also the central factor in the formation of the adult from the blastula ought to be + the formation of the central nervous system, not that of the gut.</p> + + <p>Such, it appears to me, is the case, as may be seen from the following considerations.</p> + + <p>The study of the development of any animal can be treated in two ways: either we can trace back + from the adult to the very beginning in the ovum, or we can trace forward from the fertilized egg + to the adult. Both methods ought to lead to the same result; the difference is, that in the first + case we are passing from the more known to the less known, and are expressing the unknown in terms + of the known. In the second case we are passing from the less known to the more known, and are + expressing the known in speculative terms, invented to explain the unknown. What has just been + said with respect to the germinal layers means that, however much we may study the embryo and try + to express the adult in terms of it, we finally come back to the first way of looking at the + question, and, starting with the adult, trace the continuity of function back to the first + formation of cells having a separate function.</p> + + <p>Let us, then, apply this throughout, and see what are the logical results of tracing back the + various organs and tissues from the adult to the embryo.</p> + + <p>The adult body is built up of different kinds of tissues, which fall naturally, from the + standpoint of physiology, into groups. Such groups are, in the first place—</p> + + <div class="bq1 sp2"> + <p>1. All those tissues which are connected with the central nervous system, including in that + group the nervous system itself.</p> + <p class="sp0">2. All those tissues which have no connection with the nervous system.</p> + </div> + + <p>In the second group the physiologist places all germinal cells, all blood- and + lymph-corpuscles, all plasma-cells and connective tissue and its derivatives—in fact, all + free-living cells, whether in a free state or in a quiescent, so to speak encysted, condition, + such as is <span class="pagenum" id="page464">{464}</span>found in connective tissue. In the first + group the physiologist recognizes that the central nervous system is connected with all muscular + tissues, whether striped or unstriped, somatic or splanchnic, and that such connection is of an + intimate character. Further, all epithelial cells, either of the outer or inner surfaces, whether + forming special sense-organs and glands, such as the digestive and sweat-glands, or not, are + connected with the nervous system. Besides these structures, there is another set of organs as to + which we cannot speak definitely at present, which must be considered separately, viz. all the + cells, together with their derived organs, which line the body-spaces. Whatever may be the + ultimate decision as to this group of cells, it must fall into one or other of the two main + groups.</p> + + <p>The members of these two groups are so interwoven with one another that either, if taken alone, + would still give the form of the body, so that, in a certain sense, we can speak of the body as + formed of two syncytia, separate from each other, but interlaced, of which the one forms a + continuous whole by means of cells connected together by a fluid medium or by solid threads formed + in such fluid medium, while the other does not form a syncytium in the sense that any cell of one + kind may be connected with any cell of another kind, but a syncytium of which all the different + elements are connected together only through the medium of the nervous system.</p> + + <p>If we choose to speak of the body as made up of two syncytia in this way, we must at the same + time recognize the fundamental difference in character between them. In the one case the elements + are connected together only by what may be called non-living material; there is no direct + metabolic activity caused by the action of one cell over a more distant cell in consequence of + such connection, it is not a true syncytium; in the second case there is a living connection, the + metabolism of one part is directly influenced by the activity of another, and the whole utility of + the system depends upon such functional connection.</p> + + <p>The tissues composing this second syncytium may be spoken of as the master-tissues of the body, + and we may express this conception of the building up of the body of the higher Metazoa by saying + that it is composed of a syncytial host formed of the master-tissues, which contains within its + meshes a system of free-living cells, none of which have any connection with the nervous system. + This syncytial <span class="pagenum" id="page465">{465}</span>host is in the adult composed of a + number of double elements, a nerve-cell element, and an epithelial element, such as muscle-cell, + gland-cell, etc., connected together by nerves; and if such connection is always present as we + pass from the adult to the embryo, if there is no period when, for example, the neural element + exists alone free from the muscle-cell, no period when the two can be seen to come together and + join, then it follows that when the single-layered blastula stage is reached, muscle-cell and + nerve-cell must have fused together to form a neuro-muscular cell. Similarly with all the other + neuro-epithelial organs; however far apart their two components may be in the adult, they must + come together and fuse in the embryo to form a neuro-epithelial element.</p> + + <p>The close connection between muscle and nerve which has always been recognized by + physiologists, together with the origin of muscle from a myo-epithelial cell in Hydra and other + Cœlenterata, led the older physiologists to accept thoroughly Hensen's views of the + neuro-epithelial origin of all tissues connected with the central nervous system. Of late years + this conception has been largely given up owing to the statement of His that the nervous system + arises from a number of neuroblasts, which are entirely separate cells, and have at first no + connection with muscle-cells or any peripheral epithelial cells, but subsequently, by the + outgrowing of an axial fibre, find their way to the muscle, etc., and connect with it. I do not + think that His' statement by itself would have induced any physiologist to give up the conception + of the intimate connection of muscle and nerve, if the work of Golgi, Ramón y Cajal, and others + had not brought into prominence the neurone theory, <i>i.e.</i> that each element of the central + nervous system is an independent element, without real connection with any other element and + capable of influencing other cells by contact only. These two statements, emanating as they did + from embryological and anatomical studies respectively, have done much to put into the background + Hensen's conceptions of the syncytial nature of the motor, neural, and sensory elements, which + make up the master-tissues of the body, and have led to the view that all the elements of the body + are alike, in so far as they are formed of separate cells each leading an independent existence, + without any real intimate connection with each other.</p> + + <p>The further progress of investigation is, it seems to me, bringing us back to the older + conception, for not only has the neuroblast theory <span class="pagenum" + id="page466">{466}</span>proved very difficult for physiologists to accept, but also Graham Kerr, + in his latest papers on the development of Lepidosiren, has shown that there is continuity between + the nerve-cell and the muscle-cell from the very first separation of the two sets of elements; in + fact, Hensen is right and His wrong in their respective interpretation of the earliest stages of + the connection between muscle and nerve. So also, it seems to me, the intimate connection between + the metabolism of the gland-cell, as seen in the submaxillary gland, and the integrity of its + nervous connection implies that the connection between nerve-cell and gland-cell is of the same + order as that between nerve-cell and muscle-cell. Graham Kerr also states in his paper that from + the very commencement there is, he believes, continuity between nerve-cell and epithelial cell, + but so far he has not obtained sufficiently clear evidence to enable him to speak positively on + this point.</p> + + <p>Further, according to the researches of Anderson, the cells of the superior cervical ganglion + in a new-born animal will continue to grow healthily as long as they remain connected with the + periphery, even though entirely separated from the central nervous system by section of the + cervical sympathetic nerve, and conversely, when separated from the periphery, will atrophy, even + though still connected with the central nervous system. So, also, on the sensory side, Anderson + has shown that the ganglion-cells of the posterior root-ganglion will grow and remain healthy + after separation of the posterior roots in a new-born animal, but will atrophy if the peripheral + nerve is cut, even though they are still in connection with the central nervous system. Further, + although section of a posterior root in the new-born animal does not affect the development of the + nerve-cells in the spinal ganglion, and of the nerve-fibres connecting the posterior root-ganglion + with the periphery, it does hinder the development of that part of the posterior root connected + with the spinal ganglion.</p> + + <p>These experiments of Anderson are of enormous importance, and force us, it seems to me, to the + same conclusion as that to which he has already arrived. His words are (p. 511): "I suggest, + therefore, that the section of peripheral nerves checked the development of motor and sensory + neurones, not because it blocked the passage of efferent impulses in the first case and the + reception of stimuli from the periphery in the second, but for the same reason in both cases, + <span class="pagenum" id="page467">{467}</span>viz. that the lesion disturbed the chemico-physical + equilibrium of an anatomically continuous (neuro-muscular or neuro-epithelial) chain of cells, by + separating the non-nervous from the nervous, and that the changes occurring in denervated muscle, + which I shall describe later (and possibly those in denervated skin), are in part due to the + reciprocal chemico-physical disturbance effected in these tissues by their separation from the + nervous tissues; also that the section of the posterior roots checked the development of those + portions of them still attached to the spinal ganglia, because the chemico-physical equilibrium in + those processes is maintained not only by the spinal ganglion-cells, but also by the intra-spinal + cells with which these processes are anatomically continuous."</p> + + <p>What is seen so strikingly in the new-born animal can be seen also in the adult, and in + Anderson's paper references are given to the papers of Lugaro and others which lead to the same + conclusion.</p> + + <p>These experiments seem to me distinctly to prove that the connection between the elements of + the peripheral organ and the proximate neurone is more than one of contact.</p> + + <p>We can, however, go further than this, for, apart from the observations of Apathy, there is + direct physiological evidence that the vitality of other neurones besides the terminal neurones is + dependent upon their connection with the peripheral organ, even though their only connection with + the periphery is by way of the terminal neurone. Thus, as is seen from Anderson's experiments, + section of the cervical sympathetic nerve in a very young animal causes atrophy of many of the + cells in the corresponding intermedio-lateral tract, cells which I supposed gave origin to all the + vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking experiment given by + Anderson is the effect of the removal of the periphery upon the medullation of those efferent + fibres which arise from these same spinal cells, for, as he has shown, section of the nerves from + the superior cervical ganglion to the periphery in a very young animal delays the medullation in + the fibres of the cervical sympathetic—that is, in preganglionic fibres which are not + directly connected with the periphery but with the terminal neurones in the superior cervical + ganglion. So also on the afferent side a sufficiently extensive removal of sensory field will + cause atrophy of the cells of Clarke's column, so that, just as in the case of the primary + neurones, <span class="pagenum" id="page468">{468}</span>the secondary neurones show by their + degenerative changes the importance of their connection with the peripheral organs.</p> + + <p>In this way I can conceive the formation of a series of both efferent and afferent relays in + the nervous system by proliferation of the original neural moiety of the neuro-epithelial + elements, every one of which is dependent upon its connection with the peripheral epithelial + elements for its due vitality, the whole system being a scheme for co-ordination of a larger and + larger number of peripheral elements. Thus the cells of the vasomotor centre are in connection + with the whole system of segmental vaso-constrictor centres in the lateral horns of the thoracic + region of the cord, so that to cause atrophy of these cells a very extensive removal of the + vascular system would be required. Each of the segmental centres in the cord supplies a number of + sympathetic segments, the connection with all of which would have to be cut in order to ensure + complete removal of the connection of each of its cells with the periphery, and finally each of + the cells in the sympathetic ganglia supplies a number of peripheral elements, all of which must + be removed to ensure complete severance.</p> + + <p>Thus, if we take any arbitrary number, such as 4, to represent the number of peripheral + organ-elements with which each terminal neurone is connected, and suppose that each neurone has + proliferated into sets of 4, then a cell of the third order, such as a cell of the vasomotor + centre, would require the removal of 64 peripheral elements to cause its complete separation from + the periphery, one of the second order (a cell of the thoracic lateral horn) 16 elements, one of + the first order (a cell of a sympathetic ganglion) 4 elements.</p> + + <p>Such intimate inter-relationship between the neurones, both afferent and efferent, and their + corresponding peripheral organs does not imply that all nerve-cells are necessarily as closely + dependent upon some connection with the periphery, for just as the proliferation of epithelial or + muscle-cells forms an epithelial or muscular sheet, the elements of which are so loosely, if at + all, connected together that their metabolism is in no way dependent upon such connection, so also + a similar proliferation of the neural elements may form connections between nerve-cell and + nerve-cell of a similarly loose nature.</p> + + <p>It is this kind of proliferation which, in my opinion, would bind together the separate relays + of efferent and afferent neurones, and <span class="pagenum" id="page469">{469}</span>so give + origin to reflex actions at different levels. Such neurones would not be in the direct chain of + either the afferent or efferent neurones, and so not directly connected with the periphery, and + could therefore be removed without affecting the vitality of either the efferent or afferent chain + of neurones. In other words, the vitality of the cells on the efferent side ought not to be + dependent on the integrity of the reflex arc. With regard to the development of the anterior + roots, Anderson has shown that this is the case, for section of all the posterior roots conveying + afferent impulses from the lower limb in a new-born animal does not hinder the normal development + of the anterior roots supplying that limb. Also Mott, who originally thought that section of all + the posterior roots to a limb caused atrophy of the corresponding anterior roots, has now come to + the same conclusion as other observers, and can find no degeneration on the efferent side due to + removal of afferent impulses.</p> + + <p>Again, the process of regeneration after section of a nerve is not in favour of the neuroblast + theory. There is no evidence that the cut end of a nerve can grow down and attach itself to a + muscular or epithelial element without the assistance of a nerve tube down which to grow. When the + cut nerves connected with the periphery degenerate, that applies only to the axis-cylinder and the + medullary sheath, not to the neurilemma; the connective tissue elements remain alive and form a + tube into which the growing axon finds its way, and so is conducted to the end-plate or end-organ + of the peripheral structure.</p> + + <p>Possibly, as suggested by Mott and Halliburton, the products of degeneration of the + axis-cylinder and medullary sheath stimulate these connective tissue sheath-cells into active + proliferation, and so bring about the great multiplication of cells arranged as cell-chains, which + are so often erroneously spoken of as forming the young nerves. These sheath-cells are then + supposed to re-form and secrete a pabulum which is important for the process of regeneration of + the down-growing axis-cylinder and medullary sheath. Without such pabulum regeneration does not + take place, as is seen in the central nervous system, where the sheath of Schwann is absent.</p> + + <p>Again, it is becoming more and more doubtful whether the peripheral terminations of nerves are + ever really free. As far as efferent nerves are concerned the nervous element may entirely <span + class="pagenum" id="page470">{470}</span>predominate over the muscular or glandular, as in the + formation of the electric organs of the Torpedo and Malapterurus, but still the final effect is + produced by the alteration of the muscle or gland-cell. On the afferent side especially free + nerve-terminations are largely recognized, or, as in Barker's book, nerves are spoken of as + arising in connective tissue. Thus the numerous kinds of special sense-organs, such as Pacinian + bodies, tendon-organs, genital corpuscles, etc., are all referred to by Barker under the heading + of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these organs has been known + for a long time in the shape of Grandry's corpuscles or the tactile corpuscles in the duck's bill, + where it has been proved that the nerve terminates in special large tactile cells derived from the + surface-epithelium.</p> + + <p>So also with all the others, further investigation tends to put them all in the same category, + all special sensory organs originating from a localized patch of surface-epithelium. Thus Anderson + has shown me in his specimens how the young Pacinian body is composed of rows of epithelial cells, + into each of which a twig from the nerve passes. He has also shown me how, in the case of the + tendon-organ, each nerve-fibre passes towards the attachment of the tendon and then bends back to + supply the tendon-organ, thus indicating, as he suggests, how the nest of epithelial cells has + wandered inwards from the surface to form the tendon-organ. Again, Meissner's corpuscles and + Herbst's corpuscles are evidently referable to the same class as those of Grandry and Pacini.</p> + + <p>Yet another instance of the same kind is to be found in the chromatophores of the frog and + other animals which are under the influence of the central nervous system and yet have been + supposed by various observers to be pigmented connective tissue cells. The most recent work of Leo + Loeb and others has conclusively shown that such cells are invariably derived from the + surface-epithelium.</p> + + <p>Finally, in fishes we find the special sense-organs of the lateral line and other accessory + sensory organs, all of which are indisputably formed from modified surface epithelial cells.</p> + + <p>The whole of this evidence seems to me directly against Barker's classification of sensory + nerve-beginnings in mesoblastic tissues; in none of these cases are we really dealing with free + nervous tissue alone, the starting point is always a neuro-epithelial couple.</p> + + <p>We may then, I would suggest, look upon the adult as formed of <span class="pagenum" + id="page471">{471}</span>a neural syncytium, which we may call the host, which carries with it in + its meshes a number of free cells not connected with the nervous system. If, then, we confine our + attention to the host and trace back this neural syncytium to its beginnings in the embryo, we see + that, from the very nature of the neuro-epithelial couple, each epithelial moiety must approach + nearer and nearer to its neural moiety, until at last it merges with it; the original + neuro-epithelial cell results, and we must obtain, as far as the host is concerned, a + single-layered blastula as the origin of all Metazoa. It follows, further, that there must always + be continuity of growth in the formation of the host, <i>i.e.</i> in the formation of the + neuro-epithelial syncytium; that therefore cells which have been previously free cannot settle + down and take part in its formation, as, for instance, in the case of the formation of any part of + the gut-epithelium or of muscle-cells from free-living cells.</p> + + <p>Further, since the neural moiety is the one element common to all the different factors which + constitute the host, it follows that the convergence of each epithelial moiety to the neural + moiety, as we pass from the adult to the embryo, is a convergence of all outlying parts to the + neural moiety, <i>i.e.</i> to the central nervous system, if there is a concentrated nervous + system. Conversely, in the commencing embryo the place from which the spreading out of cells takes + place, <i>i.e.</i> from which growth proceeds, must be the position of the central nervous system, + if the nervous system is concentrated. If the nervous system is diffuse, and forms a general + sub-epithelial layer, then the growth of the embryo would take place over the whole surface of the + blastula.</p> + + <p>Turning now to the consideration of the second group of tissues, those that are not connected + with the central nervous system, we find that they include among them such special cells as the + germinal cells, free cells of markedly phagocytic nature, and cells which were originally free and + phagocytic, but have settled down to form a supporting framework of connective tissue, and are + known as plasma-cells. In the embryo we find also in many cases free cells in the yolk, forming + more or less of a layer, which function as phagocytes and prepare the pabulum for the fixed cells + of the growing embryo; these cells are known by the name of vitellophags, and in meroblastic + vertebrate eggs form somewhat of a layer known by the name of periblast. Such cells must be + included in the second group, and, <span class="pagenum" id="page472">{472}</span>indeed, have + been said again and again to give origin to the free-living blood-corpuscles of the adult. In + other cases they are said to disintegrate after their work is done.</p> + + <p>In the adult the free-living lymphocytes and hæmocytes reproduce themselves from already + existing free-living cells, but as we pass back to the embryo there comes a time, comparatively + late in the history of the embryo, when such free-living cells are not found in the fluids of the + body, and they are said to arise from the proliferation and setting free of cells which form a + lining epithelium. Such formation of leucocytes has been especially described in connection with + the lining epithelium of the cœlomic cavities, as stated in Chapter XII., so that + anatomists look upon the origin of these free cells as being largely from the cœlomic + epithelium, or mesothelium, as Minot calls it.</p> + + <p>Then, again, the free cells which form the germinal cells can be traced back to a germinal + epithelium, which also is part of the cœlom. Thus the suggestion arises that in the embryo + a cellular lining is formed to a cœlomic cavity (mesothelium) composed of cells which have + no communication with the nervous system, and are capable of a separate existence as free + individuals, either in the form of germinal cells or of lymphocytes, hæmocytes, and plasma-cells, + so that these latter free cells may be considered as living an independent existence in the body, + and ministering to it in the same sense as the germ-cells live an independent existence in the + body. Again, the function of this mesothelium apart from the germ-cell is essentially excretory + and phagocytic. It is the cells of the excretory organs as well as the lymphocytes which pick up + carmine-grains when injected. It is the cells of the modified excretory organs, as mentioned in + Chapter XII., which, according to Kowalewsky and others, give origin to the free leucocytes.</p> + + <p>We see, then, that the conception of a syncytial neuro-epithelial host holding in its meshes a + number of free cells leads directly to the questions: What is the cœlom? To which category + does its lining membrane belong? and further, also, What is the origin of these free cells?</p> + + <p>The Metazoa have been divided into two great groups—those which possess a cœlom + (the Cœlomata; Lankester's Cœlomocœla) and those which do not + (Cœlenterata; Lankester's Enterocœla). As an example of the latter we may take + Hydra, because it is a very <span class="pagenum" id="page473">{473}</span>primitive form, and + because its development has been carefully worked out recently by Brauer.</p> + + <p>In Hydra we find a dermal layer of cells and an inner layer of cells separated by a gelatinous + mass known as mesoglœa; in this mass between the dermal and inner layers scattered cells + are found, the interstitial cells. Now, according to Brauer the position of the germ in Hydra is + the interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the others have their + substance changed into yolk-grains, forming the so-called pseudo-cells, and as such afford pabulum + to the growing egg-cell. Thus we see that in between the dermal and gastral layer of cells a third + layer of cells is found, composed of free living germ-cells, some of which, by the formation of + yolk-granules, become degraded into pabulum for their more favoured kinsfolk. These interstitial + cells are said to arise from the dermal layer, or ectoderm, but clearly, as in other cases, + germ-cells constitute a class by themselves and cannot be spoken of as originating from + ectoderm-cells or from hypoderm-cells.</p> + + <p>So also in Porifera, Minchin states: "In addition to the collared cells of the gastral layer, + and the various cell-elements of the dermal layer, the body-wall contains numerous wandering cells + or amœbocytes, which occur everywhere among the cells and tissues. Though lodged + principally in the dermal layer, they are not to be regarded as belonging to it, but as + constituting a distinct class of cells by themselves. They are concerned probably with the + functions of nutrition and excretion, and from them arise the genital products." Further (p. 31): + "At certain seasons some of these cells become germ-cells; hence the wandering cells and the + reproductive cells may be included together under the general term archæocytes." Also (p. 51): + "The mesoglœa is the first portion to appear as a structureless layer between the dermal + and gastral epithelia, and is probably a secretion of the former."</p> + + <p>He also points out that in these, the very lowest of the Metazoa, the separate origin of these + archæocytes can be traced back to a very early period of embryonic life. Thus in <i>Clathrina + blanca</i> the ovum undergoes a regular and total cleavage, resulting in the formation of a hollow + ciliated blastula of oval form. At one point, the future posterior pole of the larva, are a pair + of very large granular cells with vesicular nuclei, which represent undifferentiated blastomeres + and are destined to give rise to the archæocytes, and, therefore, also to the <span + class="pagenum" id="page474">{474}</span>sexual cells of the adult. Thus, as he says, from the + very earliest period a distinction is made between the "tissue-forming" cells (my syncytial host) + and the archæocytes.</p> + + <p>We see, then, that the origin of all these free-living cells can be traced back to the very + earliest of the Metazoa. Here between the dermal and gastral layers a gelatinous material, the + mesoglœa is secreted by these layers. This material is non-living, non-cellular. In it live + free cells which may either be germ-cells, amœbocytes, or 'collencytes' (connective tissue + cells). If this mesoglœa were a fluid secretion, then we should have a tissue of the nature + of blood or lymph; if it were solid, then we should have the foundation of connective tissue, + cartilage, and bone.</p> + + <p>From this primitive tissue it is easy to see how the special elements of the vascular, + lymphatic, and skeletal tissues gradually arose, the matrix being provided by the cells of the + syncytial host and the cellular elements by the archæocytes. In fact, we have no right to speak of + these lowest members of the Metazoa as not being triploblastic, as possessing nothing + corresponding to mesoblast, for in these free cells in the mesoglœa we have the origin of + the mesenchyme of the higher groups. Thus Lankester, talking of mesenchyme, says: "I think we are + bound to bring into consideration here the existence in many Cœlentera of a tissue + resembling the mesenchyme of Cœlomocœla. In <span class="correction" + title="Original reads 'Scyphomedusœ'">Scyphomedusæ</span>, in Ctenophora, and in Anthozoa, + branched fixed and wandering cells are found in the mesoglœa which seem to be the same + thing as a good deal of what is distinguished as mesenchyme in Cœlomocœla. These + appear to be derived from both the primitive layers; some produce spicules, others fibrous + substance, others again seem to be amœbocytes with various functions. It appears to be + probable that, though it may be necessary to distinguish other elements in it, the mesenchyme of + Cœlomocœla is largely constituted by cells, which are the mother-cells of the + skeletotrophic group of tissues, and are destined to form connective tissues, blood-vessels, and + blood."</p> + + <p>Thus we see that the earliest Metazoa were composed of a dermal and gastral epithelium, with a + sub-epithelial nervous system connecting the parts together, which formed, as it were, a host, + carrying around free living cells of varying function, all of which may be looked on as derived + from archæocytes, <i>i.e.</i> germ-cells. From these the cœlomatous animals arose, and here + also we find, according to <span class="pagenum" id="page475">{475}</span>present-day opinion, + that the cœlom arose in the first place in the very closest connection with the germ-cells + or gonads. Thus Lankester, in his review of the history of the cœlom, states<span + class="wnw">:—</span></p> + + <p>"The numerous embryological and anatomical researches of the past twenty years seem to me to + definitely establish the conclusion that the cœlom is primarily the cavity, from the walls + of which the gonad cells (ova or spermata) develop, or which forms around those cells. We may + suppose the first cœlom to have originated by a closing or shutting off of that portion of + the general archenteron of Enterocœla (Cœlentera), in which the gonads developed as + in Aurelia or as in Ctenophora. Or we may suppose that groups of gonad mother cells, having + proliferated from the endoderm, took up a position between it and the ectoderm, and there acquired + a vesicular arrangement, the cells surrounding the cavity in which liquid accumulated.</p> + + <p>"The cœlom is thus essentially and primarily (as first clearly formulated by Hatschek) + the perigonadial cavity or gonocœl, and the lining cells of gonadial chambers are + cœlomic epithelium. In some few groups of Cœlomocœla the cœloms have + remained small and limited to the character of gonocœls. This seems to be the case in the + Nemertina, the Planarians, and other Platyhelmia. In some Planarians they are limited in number, + and of individually large size; in others they are numerous."</p> + + <p>When Lankester says that "the lining cells of gonadial chambers are cœlomic epithelium," + that is equivalent to saying that the lining cells of the cœlom form an epithelium which + was originally gonadial, provided that, as seems to me most probable, his second suggestion, of + the cœlom being formed from gonadial mother-cells which have taken up an intermediate + position between endoderm and ectoderm and there acquired a vesicular arrangement, is the true + one. It does not seem to me possible to conceive of the gonads arising from cells of the epiblast + or of the hypoblast, in the sense that such cells are differentiated cells belonging to a layer + with a definite meaning. When we consider that the gonad gives origin to the whole of a new + individual, that in the protozoan ancestors of the Metazoa their ultimate aim and object was the + formation of gonads, it seems a wrong conception to speak of the gonads as formed from cells + belonging either to the gut-wall or to the external epithelium. The gonads must stand in a + category by themselves; they represent a whole, <span class="pagenum" + id="page476">{476}</span>while the other cells represent only a part; they cannot therefore be + derived from the latter. They may, and indeed do, give rise to cells of a subordinate character, + but they cannot rightly be spoken of as derived from such cells. The very fact mentioned by + Lankester, that in the lowest cœlomatous Metazoa, the Platyhelminthes, the cœloms + are limited to the character of simple gonocœls, strongly points to the conclusion that all + the cœlomic cells were originally of the nature of gonadial cells, and therefore + free-living and independent of the rest of the cells of the body. Whether the germ-cells appear, + as in Hydra, to be derived from the ectoblast, or, as is usually stated, from the endoblast, in + neither case ought they to be classed with the internal or external epithelium; they are + germ-cells, and the epithelium which they form is neither epiblastic nor hypoblastic, but + germinal, forming originally a simple gonocœle, afterwards, in the higher forms, the + cœlom with its cells of various function. Thus, to quote again from Lankester, "The + cœlomic fluid and the cœlomic epithelium, as well as the floating corpuscles derived + from that epithelium, acquire special properties and importance over and above the original + functions subservient to the maturation of the gonadial cells ... the most important developments + of the cœlom are in connection with the establishment of an exit for the generative + products through the body-wall to the outer world, and further in the specialization of parts of + its lining epithelium for renal excretory functions."</p> + + <p>Such exits led very early to the formation of cœlomoducts, which are true outgrowths of + the cœlom itself (p. 14): "The cœlomoducts and the gonocœls of which they are + a part, frequently acquire a renal excretory function, and may retain both the function of genital + conduits and of renal organs, or may, where several pairs are present (metamerized or segmented + animals), subserve the one function in some segments of the body, and the other function in other + segments."</p> + + <p>The origin of the cœlom and its derivatives from a germinal membrane, as suggested by + Lankester, appears to me most probable, and, if true, it carries with it conclusions of + far-reaching importance, for it necessitates that all the cells which line true cœlomic + cavities, and their derivatives, belong to the category of free-living cells, and are not + connected with the nervous system. The cells in question are essentially those which line serous + cavities and those which form excretory glands such as the kidneys. In the latter organ we ought + especially to be able to obtain a clear answer to this question, for is <span class="pagenum" + id="page477">{477}</span>it not a gland which secretes into a duct and might therefore be expected + to be innervated in the same way as other secretory glands? Although there is a strong <i>primâ + facie</i> presumption in favour of the existence of renal secretory nerves, yet according to the + universal opinion of physiologists no evidence in favour of such nerves has hitherto been found; + all the phenomena of excretion of urine consequent on nerve stimulation are explicable by the + action of nerves on the renal vessels, not on the renal cells. Not only is the physiological + evidence negative up to the present time, but also, I think, the histological. On the one hand, + Retzius has failed to find nerve-connections with kidney-cells; on the other, Berkley has obtained + such evidence with the Golgi method, but failed entirely with methylene blue. I do not myself + think that the evidence of the Golgi method alone is sufficient without corroboration by other + methods, and, in any case, Berkley's evidence does not show the nerve-fibres terminating in the + kidney-cells, in the same way as can be shown by modern methods to exist in the case of epithelial + cells of the surface, etc. Quite recently another paper on this subject has appeared by Smirnow, + who appears to have obtained better results than those given by Berkley.</p> + + <p>Apart from these physiological and histological considerations, this question is also dependent + upon the nature of the development of the excretory organs, for, according to Lankester, all + excretory organs may be divided into the two classes of nephridial organs and cœlomostomes, + of which the former are largely derived from epiblast. We should, therefore, expect to find + secretory nerves to nephridial organs, though possibly not to cœlomostomes. The kidneys of + the Mammalia are supposed to be true cœlomostomes, although, according to Goodrich's + researches, the excretory organs in Amphioxus are solenocytes, <i>i.e.</i> true nephridia.</p> + + <p>As to the lining epithelium of the peritoneal, pleural, and pericardial + cavities—<i>i.e.</i> the mesothelium—there is no definite evidence that these cells + are provided with nerves. Such surfaces are remarkably insensitive in the healthy condition, and + the pain in such cavities is essentially a pressure phenomenon and referable to special + sense-organs, such as Pacinian bodies, etc., rather than to the mesothelium itself.</p> + + <p>These sense-organs are identical in structure with those in the skin, and, as Anderson has + shown, the nerves of these organs <span class="pagenum" id="page478">{478}</span>medullate at the + same time as those in the skin, and both obtain their medullary sheaths earlier than any other + nerves, whether afferent or efferent. However difficult it may be to explain this fact, only one + conclusion seems to me possible—these Pacinian bodies, like the skin Pacinians, originate + from a nest of surface epithelial cells, a conclusion which is extremely probable on my theory of + the origin of vertebrates, but not, as far as I can see, on any other.</p> + + <p>At the present moment the weight of evidence is, to my mind, in favour of the lining + endothelium of the cœlomic cavities being composed of free cells, unconnected with the + nervous system rather than the reverse, but I must confess that the question is undecided. If it + be true that the cœlomic lining is partly enterocœlic and partly gonocœlic, + as Lankester teaches, then it would be natural that its cells should be in connection with the + nervous system, to some extent at all events. This view is, however, based on very slender + foundations. If the mesothelium is composed of cells capable of becoming free, it cannot give rise + to the skeletal muscles, and it cannot therefore be right to speak of the skeletal muscles as + derived from the lining cells of a part of the primary cœlom. The phylogenetic history of + the musculature of the different animals points strongly to its intimate connection with and + derivation from surface epithelial cells rather than from cœlomic mesothelial cells. Thus + in the cœlenterates, as seen in Hydra, the muscular layer arises directly from a + modification of the surface epithelial cells; and right up to the annelids, even to the highest + form in the Polychæta, we still see it stated that the musculature, both circular and + longitudinal, arises from the ectoderm. In the Oligochæta and Hirudinea, according to Bergh, there + are five rows of teloblasts on each side, of which four are ectodermic and give rise to the + nerve-ganglia and the circular muscles, while one is mesoblastic and forms the nephridial organs + and the longitudinal muscles. (The latter statement is, according to Bergh, well known, and is not + particularly shown by him. These longitudinal muscle-bands always lie close against the nervous + system at their first formation, and may well have been derived in connection with it.)</p> + + <p>It is apparently only in the Vertebrata that the lining cells of the cœlomic cavity are + definitely stated to give origin to the body-musculature, and taking into account on the one hand + the evidence of Graham Kerr as to the intimate connection between nerve-cell and <span + class="pagenum" id="page479">{479}</span>muscle-cell from the very beginning, and on the other the + manner in which all the skeletal muscles of the adult are lined with a lymphatic endothelium, I am + strongly inclined to believe that at the closing up of the myocœle, when the myomere + separates from the mesomere, the lining cells remain scattered in among the forming muscle-cells + and form the ultimate lymphatic tissue of the muscles. If this is really so, then the evidence in + favour of the mesothelium being composed of free cells not connected with the nervous system would + be much strengthened, for, on the one hand, an intimate relation exists between the connective + tissue cells and the endothelium of the roots of the lymphatic vessels, a relation which, + according to Virchow, has rendered it impossible to draw any sharp line of distinction between the + two; and, on the other, the lymphatic endothelium merges into the lining cells of the great serous + cavities of the body.</p> + + <p>It is impossible to conceive of an animal possessing a nervous system which is not in + connection with sensory and muscular tissues; an isolated nerve-cell is a meaningless possession; + but it is equally natural to conceive of a germ-cell being isolated, capable of living an + independent existence. Such a difference between the two kinds of tissues must have existed from + the very commencement of the Metazoa, so that we must, it seems to me, imagine that in the + formation of the Metazoa from the Protozoa the whole of the body of the latter did not break up + into a mass of separate gonads, each capable of becoming a free-living protozoan similar to its + parent, but that a portion proliferated into a multinucleated syncytium while the remainder formed + the free-living gonads. This multinucleated syncytium, or host, as it might be called, would still + continue to exist for the purpose of carrying further afield the immortal gonads, which need no + longer be all shed at one time.</p> + + <p>In such an animal as <i>Volvox globator</i> we have an indication of the very kind of animal + postulated as connecting the single-celled Protozoa and the multi-cellular Metazoa, for it + consists of a many-celled case which forms a hollow sphere, each of the cells being provided with + flagella for the purpose of locomotion of the sphere, except a certain number which are not + flagellated; the latter leave the case to swim freely in the fluid contained within the sphere, + and forming spermaries and ovaries, conjugate, maturate, and then are set free by the rupture of + the encircling locomotor host.</p> + + <div><span class="pagenum" id="page480">{480}</span></div> + + <p>This conception of the predecessors of the Metazoa being composed of a mortal host, holding + within itself the immortal sexual products, leads naturally to the idea of the separate + development of the host from that of the germ-cells <i>ab initio</i>, so that the study of the + development of the Metazoa means the study of two separate constituents of the metazoan + individual—on the one hand, the elaboration of the elements forming the syncytial host, on + the other, of those derived from the free-living independent germ-cells. The elaboration of the + host means the differentiation of the protoplasm into epithelial, muscular, and nervous elements, + by means of which the gonads were carried further afield and their nourishment as well as that of + the host ensured.</p> + + <p>The <i>rôle</i> of the nervous system as the middleman between internal and external muscular + and epithelial surfaces was, I imagine, initiated from the very earliest time. The further + evolution of the host consisted in a greater and greater differentiation and elaboration of this + neuro-epithelial syncytium, with the result of a steadily increasing concentration and + departmental centralization of the main factor of the syncytium; in other words, it led to the + origin and elaboration of a central nervous system. In the interstices of this syncytium the + gonads were placed, and at first, doubtless, the life of the host ended when all the germ-cells + had been set free. 'Reproduce and die' was, I imagine, the law of the Metazoa at its earliest + origin, and throughout the ages, during all the changes of evolution, the reminiscence of such law + still manifests itself even up to the highest forms as yet reached. With the differentiation of + the syncytial host there came also differentiation of the free-living gonads, so that only some of + them attained to the perfection of independent existence, capable of continuing the species; while + others became subordinate to the first and provided them with pabulum, manufacturing within + themselves yolk-spherules, and thus in the shape of yolk-cells ministered to the developing + egg-cell. Thus arose a germinal epithelium of which only a few of the elements passed out of the + host as perfect individuals, the remainder being utilized for the nutrition of these few. Such + yolk-cells of the germinal epithelium would still, however, retain their character as free cells + totally independent of the syncytial host, and, situated as they were between the internal and + external epithelium, capable of amœboid movement, would naturally have their phagocytic + action <span class="pagenum" id="page481">{481}</span>utilized either as yolk-cells for the + providing of pabulum to the egg-cell, or as excretory cells for the removal and rendering harmless + of deleterious products of all kinds. Thus the free cells of the body would become differentiated + into the three classes of germ-cells, yolk-cells, and excretory cells.</p> + + <p>Further, the mass of gonads, which originally occupied so large a space within the interior of + the host, necessarily, as the tissues of the host differentiated more and more, took up less and + less space in proportion to the whole bulk of the host and formed a germinal mass of cells between + the outer and inner epithelial layers. This germinal mass formed an epithelium, some of the + members of which acted as scavengers for the inner and outer layers of the host, with the result + that fluid accumulated between the two parts of the germinal epithelium in connection respectively + with the external and internal epithelial surfaces of the host, and thus led to the formation of a + gonocœle, which, by obtaining an external opening, a cœlomostome, gave origin to the + cœlom.</p> + + <p>Again, with the longer life of the host, the setting free of the gonads no longer necessitating + the destruction of the host, and also the gonads themselves requiring a longer and longer time to + be fed up to maturity, the bulk and complexity of the whole organism increased and special + supporting structures became a necessity. The host itself could and did provide these to a certain + extent by secretions from its epithelial elements, but the intermediate supports were provided by + the system of phagocytic cells utilizing the fluids of the body, at first in the shape of + plasma-cells able to move from place to place, then settling down to form a connective tissue + framework, and, later on, cartilage and bone.</p> + + <p>So also were gradually evolved the whole of the endothelial structures; the lymph-cells, + blood-cells, etc., all having their origin from the free cells of the body, which themselves + originated in the extension of a germinal epithelium. Just as in a bee-hive the egg-cells may form + the fully developed sexual animal, whether drone or queen bee, or the asexual host of workers, so + in the body of the Metazoa the free cells may form either male or female germ-cells spermatozoa, + or ova, or a host of workers, scavengers, repairers, food-providers, all useful to the community, + all showing their common origin by their absolute independence of the nervous system.</p> + + <p>Two points of great importance follow from this method of looking <span class="pagenum" + id="page482">{482}</span>at the problem. First, the evolution of the animal kingdom means + essentially the evolution of the host, for that is what forms the individual; secondly, as the + host is composed of a syncytium, the common factor of whose elements is the neural moiety, it + follows that the tissue of central importance for the evolution of the host must be, as indeed it + is, the nervous system. Further, seeing that the growth of the individual means the orderly + spreading out of the epithelial moiety away from the neural moiety, it follows that the germ-band + or germ-area from which growth starts must be in the position of the nervous system. If then, the + nervous system in the animal is a concentrated one, then the growth will emanate from the position + of such nervous system. If, on the other hand, the nervous system is diffused, then the growth + will also be diffused.</p> + + <p>In this book I have throughout argued that the ancestors of vertebrates belonged to a great + group of animals which gave origin also to Limulus and scorpion-like animals; it is therefore + instructive to see what is the nature of the development of such animals. For this purpose I will + take the development of the scorpion, as given by Brauer, for he has worked out its development + with great thoroughness and care. His papers show that the segmentation is discoidal, and results + in an oval blastodermic area lying on a large mass of yolk. Very early there separates out in this + area genital cells and yolk-cells, which latter move freely into the yolk and prepare it into a + fluid pabulum for the nutrition of the cells of the embryonic shield or germ-band. These free + yolk-cells do not take part in the formation of the germinal layers, nor does the endoderm when + formed give origin to free yolk-cells.</p> + + <p>The cells of the germ-band form a small compact area, in which by continual mitosis the cells + become more than one-layered, and soon it is found that those cells which lie close against the + fluid pabulum form a continuous layer and absorb the nutritious material for themselves and the + rest of the embryo. While this area is thus increasing in thickness by continuous development, the + group of genital cells remains always apart, increasing in number, but being always in a state of + isolation from the cells of the rest of the growing area. Thus from the very first Brauer's + observations on the development of the scorpion point to the formation of a syncytial host + containing separate genital cells. The continuous layer of cells against the fluid pabulum, which + is already functioning as a gut, and may <span class="pagenum" id="page483">{483}</span>therefore + be called hypoblast, spreads continuously over the yolk, as also does the surface epithelial + layer, or epiblast. Such spreading is always a continuous one for both surfaces, so that the yolk + is gradually enclosed by a continuous orderly growth from the germ-band, and not by the settling + down of free cells in the yolk here and there to form the gut-lining. This steady orderly + development proceeds owing to the nourishment afforded by the activity of the free cells or + vitellophags and the absorbing power of the hypoblast, a steady growth round the yolk which + results in the formation of the gut-tube, the outer covering and all the muscular and excretory + organs. Where, then, is this starting-point, this germ-band from which the whole embryo grows? It + forms the mid ventral area of the adult animal, it corresponds exactly to the position of the + central nervous system. The whole phenomenon of embryonic growth in the scorpion is exactly what + must take place on the argument deduced from the study of the adult that the animal arises as a + neuro-epithelial syncytium, and we see that that layer of cells which is situated next to the + food-material forms the alimentary tube. It is not a question whether such layer is ventral or + dorsal to the neural cells, but whether it is contiguous to or removed from the food-material.</p> + + <p>Take, again, a meroblastic vertebrate egg as of the bird. Again we find free cells passing into + the yolk to act as vitellophags, the so-called periblast cells; again we see that the embryo + starts from a germ-band or embryonic shield, and spreads from there continuously and steadily; + again we see that the layer of cells which lies against the yolk absorbs the fluid pabulum for the + growing cells; again we see that the area from which the whole process of growth starts is that of + the central nervous system, and again we see that those cells which are contiguous to the food + form the commencing gut, and are therefore called hypoblast, though in this case they are ventral + not dorsal to the neural layer.</p> + + <p>The comparison of these two processes shows that there is one common factor, one thing + comparable in the two, one thing that is homologous and is the essential in the formation of that + part of the animal which I have called the host, and that is the central nervous system. Whether + the epithelial layer which lies ventrally to it or the one that is dorsal forms the gut depends + upon the position of the food-mass. Where the food is, there will be the absorbing layer. <span + class="pagenum" id="page484">{484}</span>Where the food is not, there will be no gut formation, + whatever may have been the previous history of that layer. If, then, we suppose, as I do, that the + vertebrate arose from a scorpion-like animal without any reversal of dorsal and ventral surfaces, + and that the central nervous system remained the same in the two animals, then the comparison of + the development of the two embryos shows that the one would be derived from the other if the + yolk-mass shifted from the dorsal to the ventral side of the nervous system. This would leave the + dorsal epithelial layer of the original syncytium free from pabulum; it would no longer form the + definite gut, <i>but it would still tend to form itself in the same manner as before, would still + grow from a ventrally situated germ-band dorsalwards to form a tube, would recapitulate its past + history, and show how the alimentary canal of the arthropod became the neural canal of the + vertebrate</i>. Although this alimentary canal is formed in the same way as before, it is no + longer recognized as homologous with the scorpion's alimentary canal, but because it no longer + absorbs pabulum, and does not therefore form the definite gut, it is called an epiblastic tube, + and, in the words of Ray Lankester, has no developmental importance.</p> + + <p>All the arthropods are built up on the same type, and in all the development may in its broad + outlines be referred to the type just mentioned. So also with the vertebrate group; in both cases + the position of the central nervous system determines the starting area of embryonic growth. In + both cases the absorbing layer shows the position of the definite gut. A concentrated nervous + system of this type is common to all the segmented animals from the annelids to the vertebrates, + and in all cases the germ-band which indicates the first formation of the embryo is in the + position of this nervous system.</p> + + <p>As far as the embryo is concerned, there is no great difficulty in the conception that the + yolk-mass may have shifted from one side to the other in passing from the arthropod to the + vertebrate, for in the arthropod the embryo at first is surrounded by yolk and then passes to the + periphery of the egg. If it is permissible to speak of a dorsal and ventral surface to an egg, and + we may imagine the egg held with such dorsal surface uppermost, then the yolk would be situated + ventrally to the embryo, as in the vertebrate, if the protoplasmic cells of the embryo rose from + their central position to the surface through the yolk, while if they sank through the yolk, the + yolk would be situated dorsally to the embryo, as in the arthropod.</p> + + <div><span class="pagenum" id="page485">{485}</span></div> + + <p>In cases where there is no yolk, or very little, as in Lucifer and Amphioxus respectively, the + embryo is compelled to feed itself at a very early age; such embryos form a free-swimming pelagic + ciliated blastula, the invagination of which, for the purpose of collecting food material out of + the open sea, is the simplest method of obtaining nutriment. Here, as in other cases, it is the + physiological necessity which determines the method of formation of the gut, and such similarity + of appearance as exists between the gastrula of Lucifer and that of Amphioxus, by no means implies + that the gut of the adult Lucifer is homologous with the gut of Amphioxus.</p> + + <p class="sp3">I have compared two meroblastic eggs of the two classes respectively, because the + scorpion's egg is meroblastic. I imagine that no real difficulty arises with respect to + holoblastic eggs, for the experiments of O. Hertwig and Samassa show that by centrifugalizing, + stimulating, and breaking down of large spheres the holoblastic amphibian egg may be converted + into a meroblastic one, and then development will proceed regularly, <i>i.e.</i> in this case also + the growth proceeds from the animal pole; the large cells of the vegetal pole, like the yolk-cells + of the meroblastic egg, manufacture pabulum for the growing syncytial host.</p> + + <p class="ac"><span class="sc">Summary.</span></p> + + <div class="bq1 smaller sp5"> + <p>Any attempt to discover how vertebrates arose from invertebrates must be based upon the study + of Comparative Anatomy, of Palæontology, and of Embryology. The arguments and evidence put + forward in the preceding chapters show most clearly how the theory of the origin of vertebrates + from palæostracans is supported by the geological evidence, by the anatomical evidence, and by + the embryological evidence. Of the three the latter is the strongest and most conclusive, if it + be taken to include the evidence given by the larval stage of the lamprey.</p> + <p>The stronghold of embryology for questions of this sort is the Law of Recapitulation, which + asserts that the history of the race is recapitulated to a greater or less extent in the + development of the individual. In the previous chapters such recapitulation has been shown for + all the organs of the vertebrate body. In this respect, then, embryology has proved of the + greatest value in confirming the evidence of relationship between the palæostracan and the + vertebrate, given by anatomical and geological study.</p> + <p>There is, however, another side to embryology, which claims that the tissues of all the + Metazoa are built up on the same plan; that in all cases in the very early stage of the embryo + three layers are formed, the epiblast, mesoblast, and hypoblast; that in all animals above the + Protozoa these three layers are <span class="pagenum" id="page486">{486}</span>homologous, the + epiblast in all cases forming the external or skin-layer, the hypoblast the internal or + gut-layer.</p> + <p>Such a theory, therefore, as is advocated in this book, which turns the gut of the arthropod + into the neural canal of the vertebrate, and makes a new gut for the vertebrate from the + external surface must be wrong, as it flatly contradicts the fundamental germ-layer theory.</p> + <p>Of recent years grave doubts have been thrown upon the validity of this theory, doubts which + have increased in force year by year as more and more facts have been discovered which are not + in agreement with the theory. So much is it now discredited that any criticism against my + theory, which is based upon it, weighs nothing in the balance against the positive evidence of + recapitulation already stated. If the germ-layer theory is no longer credited, upon what + fundamental laws is embryology based?</p> + <p>In this chapter I have ventured to suggest a reply to this question, based on the uniformity + of the laws of growth throughout the existence of the individual.</p> + <p>In the adult animal the body is composed of two kinds of tissues, those which are connected + with or at all events are under the control of the nervous system, and those which are capable + of leading a free life independent of the nervous system. These two kinds of tissues can be + traced back from the adult to the embryo, and it is the task of embryology to find out how these + two kinds of tissue originate.</p> + <p>The following out of this line of thought leads to the conception that, throughout the + Metazoa, the body is composed of a host which consists of the master-tissues of the body, and + takes the form of a neuro-epithelial syncytium, within the meshes of which free living + independent organisms or cells live, so to speak, a symbiotic existence.</p> + <p>The evidence points to the origin of all these free cells from germ-cells, and thus leads to + the conception that the blastula stage of every embryo represents two kinds of cells, the one + which will form the mortal host being the locomotor neuro-epithelial cell, the other the + independent immortal symbiotic germ-cell. Such conception leads directly to the conclusion that + the blastula stage of every member of the Metazoa is the embryonic representation of a Protozoan + ancestor of the Metazoa; an ancestor, whose nature may be illustrated by such a living form as + <i>Volvox globator</i>, which, like a blastula, is composed of a layer of cells forming a hollow + sphere. These cells partly bear cilia, and so form a locomotor host, partly are of a different + character, and form male and female germ-cells. The latter leave the surface of the sphere, pass + as free individuals into its fluid contents, form spermaries and ovaries, and then by the + rupture of the mortal locomotor host pass out into the external medium, as free swimming young + Volvox.</p> + <p>It is of interest to note that such members of the Protozoa are among the most highly + developed of the members of this great group.</p> + <p>From such a beginning arose in orderly evolution, on the one hand, all the neuro-muscular and + neuro-epithelial structures of the body—the so-called master-tissues; on the other, the + germ-cells, the blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective tissue + cells, cartilage and bone-cells, etc., all of them independent of the central nervous system, + all traceable to a modification of the original germ-cells.</p> + <div><span class="pagenum" id="page487">{487}</span></div> + <p>Such a view of the processes of embryology brings embryology into harmony with comparative + anatomy and phylogeny, for it makes the central nervous system and not the alimentary canal the + most important factor in the development of the host.</p> + <p>The growth of the individual, whether arthropod or vertebrate, spreads from the position of + the central nervous system, regardless of whether that position is a ventral or dorsal one with + respect to the yolk-mass. Where the pabulum is, there is the definite gut, the lining walls of + which are called in the embryo, hypoblast; but when the pabulum is no longer there, although a + tube is formed in the same manner as the alimentary canal of the arthropod, it is now called an + epiblastic tube, and is known as the neural tube of the vertebrate.</p> + <p>This is the great fallacy of the germ-layer theory, a fallacy which consists of an argument + in a vicious circle: thus the alimentary canal is homologous in all of the Metazoa, because it + is formed of hypoblast, but there is no definition of hypoblast, except that it is always that + layer which forms the definitive alimentary canal.</p> + <p>When, after the process of segmentation has been completed, a free swimming blastula results, + unprovided with any store of pabulum in the shape of yolk, then the same physiological necessity + causes such a form to obtain its nutriment from the surrounding medium. The simplest way to do + this is by a process of invagination, in consequence of which food particles are swept into the + invaginated part and then absorbed. For this reason in such cases true gastrulæ are formed, as + in the case of Amphioxus among the vertebrates, and Lucifer among the crustaceans; such a + formation does not in the least imply that the gut of the arthropod is homologous with that of + the vertebrate. The resemblance between the two is not a morphological one, but due to the same + physiological necessity. They are analogous formations, not homologous.</p> + <p class="sp0">The muscular tissues are found to be formed in close connection with the nervous + tissues, and in very many cases are described as formed from epiblast, so that there are strong + reasons for placing them in a special category of the so-called mesoblastic tissues. If they be + separated out, then it seems to me, the rest of the mesoblast would consist of the free-living + cells of the body, which are not connected with the central nervous system. In watching, then, + the formation of mesoblast, defined in this way, we are watching the separation out from the + master-tissues of the body of the independent skeletal and excretory cells.</p> + </div> + + <div><span class="pagenum" id="page488">{488}</span></div> + + <p class="ac">CHAPTER XV</p> + + <p class="ac"><i>FINAL REMARKS</i></p> + + <div class="bq1 it1 smaller sp3"> + <p>Problems requiring investigation—</p> + <p>Giant nerve-cells and giant-fibres; their comparison in fishes and in arthropods; blood- and + lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; origin of the + sympathetic nervous system; biological test of relationship.</p> + <p class="sp0">Criticism of Balanoglossus theory.—Theory of parallel + development.—Importance of the theory advocated in this book for all problems of + Evolution.</p> + </div> + + <p>The discussion in the last chapter on the "Principles of Embryology" completes the evidence + which I am able to offer up to the present time in favour of my theory of the "Origin of + Vertebrates." There are various questions which I have left untouched, but still are well worth + discussion, and may be mentioned here. The first of these is the significance of the giant + nerve-cells and giant nerve-fibres so characteristic of the brain-region of the lower vertebrates. + In most fishes two very large cells are most conspicuous objects in any transverse section of the + <i>medulla oblongata</i> at the level of entrance of the auditory nerves. Each of these cells + gives off a number of processes, some of which pass in the direction of the auditory nerves and + one very large axis-cylinder process which forms a giant-fibre, known by the name of a Mauthnerian + fibre. Each Mauthnerian fibre crosses the middle line soon after its origin from the giant-cell, + and passes down the spinal cord on the opposite side right to the tail. Here, near the end of the + spinal cord, it breaks up into smaller fibres, which are believed by Fritsch and others to pass + out directly into the ventral roots to supply the muscles of the tail. Thus Bela Haller says: "The + Mauthnerian fibres are known to give origin to certain fibres which supply the ventral roots of + the last three spinal nerves, so that their terminal branches serve, in all probability, for the + innervation of the muscles of the tail-fin." They do not occur in the eel, according to Haller, or + in Silurus, according to Kölliker. <span class="pagenum" id="page489">{489}</span>Their absence in + those fishes, in which a well-developed tail-fin is also absent, increases the probability of the + truth of Fritsch's original conclusion that these giant-fibres are associated axis-cylinders for + certain definite co-ordinated movements of the fish, especially for the lateral movement of the + tail.</p> + + <p>In Ammocœtes, instead of two Mauthnerian fibres, a number of giant-fibres are found. + They are called Müllerian fibres, and arise from giant-cells which are divisible into two groups. + The first group consists of three pairs situated headwards of the level of exit of the trigeminal + nerves. Two of these lie in front of the level of exit of the oculomotor nerves, and one pair is + situated at the same level as the origin of the oculomotor nerves. The second group consists of a + number of cells on each side at the level of the entrance of the fibres of the auditory + nerves.</p> + + <p>The Müllerian fibres largely decussate, as described by Ahlborn, and then become the most + anterior portion of the white matter of the spinal cord, forming a group of about eight fibres on + each side (Fig. <a href="#fig73">73</a>). A few fibres are also found laterally, and slightly + dorsally, to the grey matter. These giant-fibres pass down the spinal cord right to the anal + region; their ultimate destination is unknown. Mayer considers that in the first part of their + course they correspond to those tracts of fibres known as the "posterior longitudinal bundles" in + other vertebrates. I imagine, therefore, that the spinal part of their course represents the two + antero-lateral descending tracts. The second group of giant-cells, which appears to have some + connection with the auditory nerves, may represent "Deiter's nucleus." The whole system is + probably the central nervous part of a co-ordination mechanism, which arises entirely in the + pro-otic or prosomatic region of the brain—the great co-ordinating and equilibrating region + <i>par excellence</i>.</p> + + <p>If we turn now to the arthropod it is a striking coincidence that in the crayfish and in the + lobster the work of Retzius, of Celesia, of Allen, and of many others demonstrates the existence + of an equilibration-mechanism for the swimming movements of the tail-muscles, which is carried out + by means of giant-fibres. These giant-fibres are the axis-cylinder processes of giant-cells, + situated exclusively in the brain-region, and they run through the whole ventral ganglionic chain + in order to supply the muscles of the tail. In the ventral nerve-cord of the crayfish, according + to Retzius, two specially large <span class="pagenum" id="page490">{490}</span>giant-fibres exist, + each of which breaks up, at the last abdominal ganglion, into smaller fibres, which pass directly + out with the nerves to the tail-fin. Allen has shown that, in addition to these two specially + large giant-fibres, there are a number of others, some of which, similarly to the Müllerian fibres + of Ammocœtes, cross the middle line, while some do not. Each of these arises from a large + nerve-cell and passes to one or other of the last pair of abdominal ganglia. The latter fibres, he + says, send off collaterals, while the two specially large giant-fibres do not. The cells which + give origin to all these large, long fibres are situated in or in front of the prosomatic region + of the brain, similarly to the giant-cells, which give rise to the corresponding Müllerian fibres + of Ammocœtes. I do not know how far this system is represented in Limulus or Scorpio.</p> + + <p>It is, to my mind, improbable that the Mauthnerian fibres pass out directly as motor fibres to + the muscles of the tail-fin; it is more likely that they are conducting paths between the + equilibration-mechanism in connection with the VIIIth nerve and the spinal centres for the + movements of the tail. Similarly, with respect to the arthropod, it is difficult to believe that + the motor fibres for the tail-muscles arise in the brain-region. In either case, the striking + coincidence remains that the movements of the tail-end of the body are regulated by means of + giant-fibres which arise from giant-cells in the head-region of the body in both the Arthropoda + and the lowest members of the Vertebrata.</p> + + <p>The meaning of this system of giant-cells and giant-fibres in both classes of animals is well + worthy of further investigation.</p> + + <p>Another important piece of comparative work which ought to help in the elucidation of this + problem is the comparison of the blood- and lymph-corpuscles of the vertebrate with those of the + invertebrate groups. As yet, I have not myself made any observations in this direction, and feel + that it is inadvisable to discuss the results of others until I know more about the facts from + personal observation.</p> + + <p>The large and important question of the manner of formation of the vertebrate skin has only + been considered to a slight extent. A much more thorough investigation requires to be made into + the nature of the skin of the oldest fishes in comparison with the skin of Ammocœtes on the + one side, and of Limulus and the Palæostraca on the other.</p> + + <p>The muscular system requires further investigation, not so much <span class="pagenum" + id="page491">{491}</span>the different systems of the striated voluntary musculature—for + these have been for the most part compared in the two groups of animals in previous + chapters—as the involuntary unstriped musculature, about which no word has been said. The + origin of the different systems of unstriped muscles in the vertebrate is bound up with the origin + of the sympathetic system and its relation to the cranial and sacral visceral systems. The reason + why I have not included in this book the consideration of the sympathetic nervous system is on + account of the difficulty in finding any such system in Ammocœtes. Also, so far as I know, + the distribution of unstriped muscle in Ammocœtes has not been worked out.</p> + + <p>One clue has arisen quite recently which is of great importance, and must be worked out in the + future, viz. the extraordinary connection which exists between the action of the sympathetic + nervous system and the action of adrenalin. This substance, which is obtained from the medullary + part of the adrenal or suprarenal glands, when injected into an animal produces the same effects + as stimulation of the nerves, which belong to the lumbo-thoracic outflow of visceral nerves, + <i>i.e.</i> the system known as the sympathetic nervous system, which is distinct from both the + cranial and sacral outflows of visceral nerves. The similarity of its action to stimulation of + nerves is entirely confined to the nerves of this sympathetic system, and never resembles that of + either the cranial or sacral visceral nerves.</p> + + <p>Another most striking fact which confirms the great importance of this connection between the + adrenals and the sympathetic nervous system from the point of view of the evolution of the latter + system is that the extract of the adrenals always produces the same effect as that of stimulation + of the nerves of the sympathetic system, whatever may be the animal from which the extract is + obtained. Thus adrenalin obtained from the elasmobranch fishes will produce in the highest mammal + all the effects known to occur upon stimulation of the nerves of its sympathetic system.</p> + + <p>Further, the cells, which are always associated with the presence of this peculiar + substance—adrenalin—stain in a characteristic manner in the presence of chromic salts. + In Ammocœtes patches of cells which stain in this manner have been described in connection + with blood-vessels in certain parts, so that, although I know of no definite evidence of the + existence of cell-groups in Ammocœtes corresponding to the ganglia of the sympathetic + system in other vertebrates, it is <span class="pagenum" id="page492">{492}</span>possible that + further investigation into the nature and connection of these "chromaffine" cells may afford a + clue to the origin of the sympathetic nervous system. At present it is premature to discuss the + question further.</p> + + <p>Finally, another test as to the kinship of two animals of different species must be considered + more fully than I have been able to do up to the present time. This test is of a totally different + nature to any put forth in previous pages. It is known as the "biological test" of relationship, + and is the outcome of pathological rather than of physiological or anatomical research. It is + possible that this test may prove the most valuable of all. At present we do not know sufficiently + its limitations and its sources of error, especially in the case of cold-blooded animals, to be + able to look upon it as decisive in a problem of the kind considered in this book.</p> + + <p>The nature of this test is as follows: It has been found that the serum of the blood of another + animal, when injected in sufficient quantity into a rabbit, will cause such a change in the serum + of that rabbit's blood that when it is added to the serum of the other animal a copious + precipitate is formed, although the serum of normal rabbit's blood when mixed with that of another + animal will cause no precipitate whatever. This extraordinary production of a precipitate in the + one case and not in the other indicates the production of some new substance in the rabbit's serum + in consequence of the introduction of the foreign serum into the rabbit, which brings about a + precipitate when the rabbit's serum containing it is mixed with the serum originally injected. The + barbarous name "antibody" has been used to express this supposed substance in accordance with the + meaning of such a word as "antitoxin," which has been a long time in use in connection with + preventive remedies against pathogenic bacteria. Now, it is found that the rabbit's serum + containing a particular "antibody" will cause a precipitate only when added to the serum of the + blood of the animal from which the "antibody" was produced or to the serum of the blood of a + nearly related animal.</p> + + <p>Further, if that animal is closely related a precipitate will be formed nearly as copious as + with the original serum, if more distantly related a cloudiness will occur rather than a + precipitate, and if the relationship is still more distant the mixture of the two sera will remain + absolutely clear. Thus this test demonstrates the close relationship of man to the anthropoid apes + and his more distant <span class="pagenum" id="page493">{493}</span>relationship to monkeys in + general. By this method very evident blood-relationships have been demonstrated, especially + between members of the Mammalia.</p> + + <p>I therefore started upon an investigation into the possibility of proving relationship in this + way between Limulus and Ammocœtes, with the kind assistance of Mr. Graham Smith. I must + confess I was not sanguine of success, as I thought the distance between Limulus and + Ammocœtes was too great. Dr. Lee, of New York, kindly provided me with most excellent serum + of Limulus, and the first experiments showed that the anti-serum of Limulus gave a most powerful + precipitate with its own serum. Graham Smith then tried this anti-serum of Limulus with the serum + of Ammocœtes, and to his surprise, and mine, he obtained a distinct cloudiness, indicative + of a relationship between the two animals. This, however, is not considered sufficient, the + reverse experiment must also succeed. I therefore, with Graham Smith, obtained a considerable + amount of blood from the adult lampreys at Brandon, and produced an anti-serum of Petromyzon, + which gave some precipitate with its own serum, but not a very powerful one. This anti-serum tried + with Limulus gave no result whatever, but at the same time it gave no result with serum from + Ammocœtes, so that the experiment not only showed that Petromyzon was not related to + Limulus, but also was not related to its own larval form, which is absurd.</p> + + <p>Considerable difficulties were encountered in preparing the Petromyzon anti-serum owing to the + extreme toxic character of the lamprey's serum to the rabbit; in this respect it resembled that of + the eel. It is possible that the failure of the lamprey's anti-serum was due to the necessity of + heating the serum sufficiently to do away with its toxicity before injecting it into the rabbit. + At this point the experiments have been at present left. It will require a long and careful + investigation before it is possible to speak decisively one way or the other. At present the + experiment is positive to a certain extent, and also negative; but the latter proves too much, for + it proves that the larva is not related to the adult.</p> + + <p>Some day I hope this "biological test" will be of use for determining the relationships of the + Tunicata, the Enteropneusta, Amphioxus, etc., as well as of Limulus and Ammocœtes.</p> + + <p>The origin of Vertebrates from a Palæostracan stock, as put forward in this book, gives no + indication of the systematic position <span class="pagenum" id="page494">{494}</span>of the + Tunicata or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility be on the + direct line of ascent from the invertebrate to the vertebrate. They must both be looked upon as + persistent failures, relics of the time when the great change to the vertebrate took place. The + Enteropneusta are on a different footing; in their case any evidence of affinity with vertebrates + is very much more doubtful.</p> + + <p>The observer Spengel, who has made the most exhaustive study of these strange forms, rejects + <i>in toto</i> any connection with vertebrates, and considers them rather as aberrant annelids. + The so-called evidence of the tubular central nervous system is worth nothing. There is not the + slightest sign of any tubular nervous system in the least resembling that of the vertebrate. It is + simply that in one place of the collar-region the piece of skin containing the dorsal nerve of the + animal, owing to the formation of the collar, is folded, and thus forms just at this region a + short tube. My theory explains in a natural manner every portion of the elaborate and complicated + tube of the vertebrate central nervous system. In the Balanoglossus theory the evolution of the + vertebrate tube in all its details from this collar-fold is simple guesswork, without any + reasonable standpoint. Similarly, the small closed diverticulum of the gut in Balanoglossus, which + is dignified with the name of "notochord," has no right to the name. As I have already said, it + may help to understand why the notochord has such a peculiar structure, but it gives no help to + understanding the peculiar position of the notochord. The only really striking resemblance is + between the gill-slits of Amphioxus and of the Enteropneusta. In this comparison there is a very + great difficulty, very similar to that of the original attempts to derive vertebrates from + annelids—the gill-slits open ventrally in the one animal and dorsally in the other. In both + animals an atrial cavity exists which is formed by pleural folds, and in these pleural folds the + gonads are situated so that the similarity of the two branchial chambers seems at first sight very + complete. In the Enteropneusta, however, there are certain forms—Ptychodera—in which + these pleural folds have not met in the mid-line in this branchial region, and in these it is + plainly visible that these folds, with their gonads, spring from the ventral mid-line and arch + over the dorsal region of the body. Equally clearly Amphioxus shows that its pleural folds, with + the gonads, spring from the dorsal side of the animal, <span class="pagenum" + id="page495">{495}</span>and grow ventralwards until they fuse in the ventral mid-line (<i>cf.</i> + Fig. <a href="#fig168">168</a>).</p> + + <p>As far, then, as this one single striking similarity between Amphioxus and the Enteropneusta is + concerned it necessitates the reversal of dorsal and ventral surfaces to bring the two branchial + chambers into harmony.</p> + + <div class="ac w45 fcenter sp2"> + <a href="images/fig168.png" id="fig168"><img style="width:100%" src="images/fig168.png" alt="" + title=""/></a> + <div class="smaller ac"> + <p><span class="sc">Fig. 168.—Diagram illustrating the Position of the Pleural Folds and + Gonads in Ptychodera (A) and Amphioxus (B) respectively.</span></p> + <p class="sp0"><i>Al.</i>, alimentary canal; <i>D.A.</i>, dorsal vessel; <i>V.A.</i>, ventral + vessel; <i>g.</i>, gonads; <i>NC.</i>, notochord; <i>C.N.S.</i>, central nervous system.</p> + </div> + </div> + + <p>In a mud-dwelling animal, like Balanoglossus, which possesses no appendages, no special + sense-organs, it seems likely enough that ventral and dorsal may be terms of no particular + meaning, and consequently what is called ventral in Balanoglossus may correspond to what is dorsal + in Amphioxus; in this way the branchial regions of the two animals may be closely compared. Such + comparison, however, immediately upsets the whole argument of the vertebrate nature of + Balanoglossus based on the relative position of the central nervous system and gut, for now that + part of its nervous system which is looked upon as the central nervous system in Balanoglossus is + ventral to the gut, just as in a worm-like animal, and not dorsal to it as in a vertebrate.</p> + + <p>There is absolutely no possibility whatever of making such a detailed comparison between + Balanoglossus and any vertebrate, as I have done between a particular kind of arthropod and + Ammocœtes. In the latter case not only the topographical anatomy of the organs in the two + animals is the same, but the comparison is valid even to microscopical structure. In the former + case the origin of almost all <span class="pagenum" id="page496">{496}</span>the vertebrate organs + is absolutely hypothetical, no clue is given in Balanoglossus, not even to the segmented nature of + the vertebrate. The same holds good with the evidence from Embryology and from Palæontology. I + have pointed out how strongly the evidence in both cases confirms that of Comparative Anatomy. In + neither case is the strength of the evidence for Balanoglossus in the slightest degree comparable. + In Embryology an attempt has been made to compare the origin of the cœlom in Amphioxus and + in Balanoglossus. In Palæontology there is nothing, only an assumption that in the Cambrian and + Lower Silurian times a whole series of animals were evolved between Balanoglossus and the earliest + armoured fishes, which have left no trace, although they were able to hold their own against the + dominant Palæostracan race. The strangeness of this conception is that, when they do appear, they + are fully armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck for the + believers in the Balanoglossus theory that no intermediate less armoured forms have been found, + especially in consideration of the fact that the theory of the origin from the Palæostracan does + not require such intermediate forms, but finds that those already discovered exactly fulfil its + requirements.</p> + + <p>One difficulty in the way of accepting the theory which I have advocated is perhaps the + existence of the Tunicata. I cannot see that they show any affinities to the Arthropoda, and yet + they are looked upon as allied to the Vertebrata. I can only conclude that both they and Amphioxus + arose late, after the vertebrate stock had become well established, so that in their degenerated + condition they give indications of their vertebrate ancestry and not of their more remote + arthropod ancestry.</p> + + <p>In conclusion, the way in which vertebrates arose on the earth as suggested in this book + carries with it many important far-reaching conclusions with respect to the whole problem of + Evolution.</p> + + <p>When the study of Embryology began, great hopes were entertained that by its means it would be + possible to discover the pedigree of every group of animals, and for this end all the stages of + development in all groups of animals were sought for and, as far as possible, studied. It was soon + found, however, that the interpretation of what was seen was so difficult, as to give rise to all + manner of views, depending upon the <span class="correction" + title="Original reads 'idiosyncracy'.">idiosyncrasy</span> of the observer. At his will he decided + whether any appearance was cœnogenetic or palingenetic, <span class="pagenum" + id="page497">{497}</span>with the result that, in the minds of many, embryology has failed to + afford the desired clue.</p> + + <p>At the same time, the geological record was looked upon as too imperfect to afford any real + help; it was said, and is said, that the Cambrian and pre-Cambrian periods were so immense, and + the animals discovered in the lower Silurian so highly organized, as to compel us to ascribe the + origination of all the present-day groups to this immense early period, the animals of which have + left no trace of their existence as fossils.</p> + + <p>In consequence of, or at all events following upon, the supposed failure of embryology and of + geology to solve the problem of the sequence of evolution of animal life, a new theory has arisen, + which goes very near to the denial of evolution altogether. This is the theory of parallel + development. It discards the old picture of a genealogical tree with main branches arising at + different heights, these again branching and branching into smaller and smaller twigs, and + substitutes instead the picture of the ribs of a fan, every rib running independently of every + other, each group represented by a rib reaching its highest development on the circumference of + the fan and coming nearer and nearer to a common point at the handle of the fan. This point of + convergence, where all the groups ultimately meet, is so far back as to reach to the lowest living + organisms.</p> + + <p>This, in my opinion, unscientific and inconceivable suggestion has arisen largely in + consequence of a conception which has become firmly fixed in the minds of very many writers on + this subject—the conception that in the evolution of every group, the higher members of the + group are the most specialized in the peculiarities of that group, and it is impossible to obtain + a new group with different peculiarities from such specialized members. If, then, a higher group + is to arise from a lower, it must arise from the generalized members of that lower group, in other + words, from the lowest members or those nearly akin to the next lower group.</p> + + <p>Similarly, the highest members of this latter group are too specialized, and again we must go + to the more generalized members of the group. In this way each separate specialized group is put + on one side, and so the conception of parallel development comes into being.</p> + + <p>The evidence given in this book dealing with the origin of vertebrates strikes at the + foundations of this belief, for it presents an <span class="pagenum" + id="page498">{498}</span>image of the sequence of evolution of animal forms in orderly upward + progress, caused by the struggle for existence among the members of the race dominant at the time, + which brought about the origin of the next higher group not from the lowest members of the + dominant group, but from some one of the higher members of that group.</p> + + <p>The great factor in evolution has been throughout the growth of the central nervous system; + from that group of animals which possessed the highest nervous system evolved up to that time the + next higher group must have arisen.</p> + + <p>In this way we can trace without a break, always following out the same law, the evolution of + man from the mammal, the mammal from the reptile, the reptile from the amphibian, the amphibian + from the fish, the fish from the arthropod, the arthropod from the annelid, and we may be hopeful + that the same law will enable us to arrange in orderly sequence all the groups in the animal + kingdom.</p> + + <p>This very same law of the paramount importance of the development of the central nervous system + for all upward progress will, I firmly believe, lead to the establishment of a new and more + fruitful embryology, the leading feature of which will be, as suggested in the last chapter, not + the attempt to derive from the blastula three germ-layers common to all animals, but rather two + sets of organs—those which are governed by the nervous system and those which are + not—and thus by means of the development of the central nervous system obtain from + embryology surer indications of relationship than are given at present.</p> + + <p>The great law of recapitulation, which asserts that the past history of the race is indicated + more or less in the development of each individual, a law which of late years has fallen somewhat + into disrepute, owing especially to the difficulty of interpreting the embryological history of + the vertebrate, is triumphantly vindicated by the theory put forward in this book. Each separate + vertebrate organ, one after the other, as shown in the last chapter, indicates in its development + the manner in which it arose from the corresponding organ of the arthropod. There is no failure in + the evidence of embryology, the failure is in the interpretation thereof.</p> + + <p>So, too, my theory vindicates the geological method. There is no failure here; on the contrary, + the record of the rocks proclaims with startling clearness not only the sequence of evolution in + the <span class="pagenum" id="page499">{499}</span>vertebrate kingdom itself, but the origin of + the vertebrate from the most highly-developed invertebrate race.</p> + + <p>The study of the comparative anatomy of organs down to the finest details has always been a + most important aid in finding out relationship between animals or groups of animals. My theory + endorses this view to the uttermost, and especially indicates the study of the central nervous + system and its outgoing nerves as that comparative study which is most likely to afford valuable + results.</p> + + <p>As for the individual, so for the nation; as for the nation, so for the race; the law of + evolution teaches that in all cases brain-power wins. Throughout, from the dawn of animal life up + to the present day, the evidence given in this book suggests that the same law has always held. In + all cases, upward progress is associated with a development of the central nervous system.</p> + + <p class="sp5">The law for the whole animal kingdom is the same as for the individual. "Success in + this world depends upon brains."</p> + + <div><span class="pagenum" id="page501">{501}</span></div> + + <p class="sp3 ac" style="margin-bottom:3.9ex;"><span class="larger">BIBLIOGRAPHY AND INDEX OF + AUTHORS</span></p> + + <table class="sp5 mc w75" title="Bibliography and Index of Authors" + summary="Bibliography and Index of Authors"> + <tr class="bb bt"> + <th class="br smaller">Author's name.</th> + <th class="br smaller">Title of Paper.</th> + <th class="smaller w18">Pages of<br/> + reference.</th> + </tr> + <tr> + <td rowspan="2" class="br it1p05">AHLBORN</td> + <td class="it1p05">"Untersuchungen über das Gehirn der Petromyzonten"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page210">210</a>, <a + href="#page489">489</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 39. 1883</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber die Segmentation des Wirbelthierkörpers"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page260">260</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 40. 1884</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">AICHEL</td> + <td class="it1p05">"Vergleichende Entwicklungsgeschichte und Stammesgeschichte der + Nebennieren"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a>, <a + href="#page428">428</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. f. Mikr. Anat.</i> Vol. 56. 1900</td> + </tr> + <tr> + <td class="br pb05 it1p05">ALCOCK</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page135">135</a>, <a href="#page287">287</a>, <a + href="#page288">288</a>, <a href="#page289">289</a>, <a href="#page304">304</a>, <a + href="#page307">307</a>, <a href="#page347">347</a>, <a href="#page445">445</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Peripheral Distribution of the Cranial Nerves of + Ammocœtes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page164">164</a>, <a href="#page171">171</a>, <a + href="#page177">177</a>, <a href="#page188">188</a>, <a href="#page202">202</a>, <a + href="#page297">297</a>, <a href="#page300">300</a>, <a href="#page310">310</a>, <a + href="#page311">311</a>, <a href="#page316">316</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 33. 1898</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On Proteid Digestion in Ammocœtes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page58">58</a>, <a href="#page213">213</a>, <a + href="#page442">442</a>, <a href="#page452">452</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 33. 1898</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">ALLEN</td> + <td class="it1p05">"Studies on the Nervous System of Crustacea"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page489">489</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 36. 1894</td> + </tr> + <tr> + <td class="br pb05 it1p05">ANDERSON, H. K.</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page448">448</a>, <a href="#page470">470</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Nature of the Lesions which hinder the Development of Nerve-cells and + their Processes</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page466">466</a>, <a href="#page467">467</a>, <a + href="#page469">469</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 28. 1902</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Myelination of Nerve-fibres"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page467">467</a>, <a + href="#page477">477</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Report of the Brit. Assn.</i> 1898</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">APATHY</td> + <td class="it1p05">"Das leitende Element des Nervensystems und seine topographischen Beziehung + zu den Zellen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page467">467</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. zu Neapel.</i> Vol. 12. 1896</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">ASSHETON</td> + <td class="it1p05">"On the Phenomenon of the Fusion of the Epiblastic Layers in the Rabbit and + in the Frog"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page42">42</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"An Experimental Examination into the Growth of the Blastoderm of the + Chick"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Proc. of Roy. Soc.</i> Vol. 60. 1896</td> + </tr> + <tr> + <td rowspan="2" class="br"><span class="pagenum" id="page502">{502}</span></td> + <td class="it1p05">"On the Growth in Length of the Frog Embryo"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"A Re-investigation into the Early Stages of the Development of the + Rabbit"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Primitive Streak of the Rabbit: the Causes which may determine its + Shape, and the part of the Embryo formed by its Activity"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BALFOUR</td> + <td class="it1p05">'Comparative Embryology.' Vol. 2</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page73">73</a>, <a href="#page74">74</a>, <a + href="#page94">94</a>, <a href="#page103">103</a>, <a href="#page104">104</a>, <a + href="#page120">120</a>, <a href="#page181">181</a>, <a href="#page259">259</a>, <a + href="#page424">424</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. 1881. Macmillan & Co.</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Origin and History of the Urino-genital Organs of Vertebrates"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a>, <a + href="#page392">392</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 10. 1876</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Nature of the Organ in Adult Teleosteans and Ganoids, which is + usually regarded as the Head-kidney or Pronephros"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page420">420</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 22. 1882</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BARKER</td> + <td class="it1p05">'The Nervous System'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page470">470</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. 1901</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BATESON</td> + <td class="it1p05">"The Ancestry of the Chordata"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page11">11</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 26. 1886</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">'Materials for the Study of Variation'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page387">387</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BEARD</td> + <td class="it1p05">"The System of Branchial Sense Organs and their Associated Ganglia in + Ichthyopsida"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a>, <a href="#page281">281</a>, <a + href="#page283">283</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 26. 1885</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Development of the Peripheral Nervous System in Vertebrates"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a>, <a href="#page281">281</a>, <a + href="#page283">283</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 29. 1888</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Old Mouth and the New"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page318">318</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> 1888</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Source of Leucocytes and the True Function of the Thymus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page425">425</a>, <a + href="#page426">426</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 18. 1900</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Parietal Eye of the Cyclostome Fishes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page84">84</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 29. 1882</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BECK <span class="smaller">AND</span> LANKESTER</td> + <td class="it1p05">"On the Muscular and Endo-skeletal Tissues of Scorpio"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page171">171</a>, <a href="#page222">222</a>, <a + href="#page224">224</a>, <a href="#page247">247</a>, <a href="#page268">268</a>-<a + href="#page277">277</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 11. 1885</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BEECHER</td> + <td class="it1p05">"Natural Classification of the Trilobites"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page283">283</a>, <a href="#page351">351</a>, <a + href="#page436">436</a>, <a href="#page437">437</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Amer. Journ. of Sci.</i> Ser. 4. Vol. 3. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BELL, C.</td> + <td class="it1p05">'The Nervous System of the Human Body'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page155">155</a>, <a href="#page156">156</a>, <a + href="#page183">183</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. 1830</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BELLONCI</td> + <td class="it1p05">"Système Nerveux et Organes des sens du <i>Sphæroma serratum</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page62">62</a>, <a href="#page90">90</a>, <a + href="#page92">92</a>, <a href="#page101">101</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. Ital. de Biol.</i> Vol. 1. 1882</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Sur la structure et les rapports des lobes olfactives dans les Arthropods + superieurs et les Vertébrés"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page221">221</a>,<a href="#page225">225</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. Ital. de Biol.</i> Vol. 3. 1883</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BENHAM <span class="smaller">AND</span> LANKESTER</td> + <td class="it1p05">"On the Muscular and Endo-skeletal Systems of Limulus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page143">143</a>, <a href="#page171">171</a>, <a + href="#page176">176</a>, <a href="#page177">177</a>, <a href="#page247">247</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 11. 1885</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page503">{503}</span> + <p class="sp0">BERGER</p> + </td> + <td class="it1p05">"Untersuchungen über den Bau des Gehirns und der Retina der + Arthropoden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page88">88</a>-<a href="#page92">92</a>, <a + href="#page97">97</a>, <a href="#page100">100</a>, <a href="#page101">101</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arbeit. a. d. Zool. Instit. Wien.</i> Vol. 1. 1878</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BERGH</td> + <td class="it1p05">"Neue Beiträge zur Embryologie der Anneliden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page478">478</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 50. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BERKLEY</td> + <td class="it1p05">"The Intrinsic Nerves of the Kidney"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page477">477</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bulletin of the Johns Hopkins Hospital.</i> Vol. 4</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BERNARD</td> + <td class="it1p05">'The Apodidæ: a Morphological Study'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page284">284</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Nature Series.</i> 1892</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BERTKAU</td> + <td class="it1p05">"Beiträge zur Kenntniss der Sinnesorgane der Spinnen. 1. Die Augen der + Spinnen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page369">369</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. mikr. Anat.</i> Vol. 27. 1886</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BIEDERMANN</td> + <td class="it1p05">'Electro-physiology'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page20">20</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Translated by F. A. Welby. London. 1896</td> + </tr> + <tr> + <td class="br pb05 it1p05">BLANCHARD</td> + <td class="pb05 it1p05">Quoted by Huxley</td> + <td class="bl vbm pb05"><a href="#page225">225</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">'L'Organisation du Règne Animal. Arachnides'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page109">109</a>, <a href="#page177">177</a>, <a + href="#page190">190</a>, <a href="#page206">206</a>, <a href="#page313">313</a>, <a + href="#page315">315</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Paris. 1852</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BLES</td> + <td class="it1p05">"The Correlated Distribution of Abdominal Pores and Nephrostomes in + Fishes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page431">431</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 32. 1898</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BOBRETSKY</td> + <td class="it1p05">'Development of Astacus and Palæmon'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page74">74</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Kiew. 1873</td> + </tr> + <tr> + <td class="br pb05 it1p05">BOURNE <span class="smaller">AND</span> LANKESTER</td> + <td class="pb05 it1p05"><i>See</i> Lankester and Bourne.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BOVERI</td> + <td class="it1p05">"Die Nieren Canälchen des Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a>, <a href="#page395">395</a>, <a + href="#page402">402</a>, <a href="#page407">407</a>, <a href="#page412">412</a>, <a + href="#page426">426</a>, <a href="#page427">427</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 5. 1892</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BRAEM</td> + <td class="it1p05">"Was ist ein Keimblatt"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page460">460</a>, <a href="#page461">461</a>, <a + href="#page462">462</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Biol. Centralblatt.</i> Vol. 15. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BRAUER</td> + <td class="it1p05">"Beiträge zur Kenntniss der Entwicklungsgeschichte des Skorpions"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page62">62</a>, <a href="#page167">167</a>, <a + href="#page222">222</a>, <a href="#page237">237</a>, <a href="#page281">281</a>, <a + href="#page482">482</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeit. f. wiss. Zool.</i> Part I. Vol. 57. 1894<br/> + Part II. Vol. 59. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Beiträge zur Kenntniss der Entwicklung und Anatomie der Gymnophionen." + III.</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page393">393</a>, <a href="#page394">394</a>, <a + href="#page400">400</a>, <a href="#page402">402</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">"Die Entwicklung der Excretionsorgane" <i>Zool. Jahrbuch.</i> Vol. + 16. 1902</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber die Entwicklung von Hydra"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page473">473</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeit. f. wiss. Zool.</i> Vol. 52. 1891</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BÜTSCHLI</td> + <td class="it1p05">"Notiz zur Morphologie des Auges der Muscheln"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page114">114</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Festschrift des Natur-hist-med. Vereins zu Heidelberg.</i> + 1886</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">BUJOR</td> + <td class="it1p05">"Contribution a l'étude de la métamorphose de <i>l'Ammocœtes + branchialis</i> en <i>Petromyzon Planeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page135">135</a>, <a + href="#page304">304</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Revue Biologique du Nord de la France.</i> Vol. 3. 1891</td> + </tr> + <tr> + <td class="br pb05 it1p05">CARLSON</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page177">177</a>, <a href="#page315">315</a>, <a + href="#page316">316</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">CELESIA</td> + <td class="it1p05">'Differenziamento della proprietà inibitoria e dei funzioni coordinatrici + nella catena gangliare dei crustacei decapodi'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page489">489</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Genoa. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">CLAUS</td> + <td class="it1p05">"Untersuchungen über den Organismus und Entwicklung von Branchipus und + Artemia"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page90">90</a>-<a href="#page92">92</a>, <a + href="#page97">97</a>, <a href="#page100">100</a>, <a href="#page396">396</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arbeit a.d. Zool. Institut. Wien.</i> Vol. 6. 1886</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page504">{504}</span> + <p class="sp0">COPE</p> + </td> + <td class="it1p05">"On the Phylogeny of the Vertebrata"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page343">343</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Proc. Amer. Philos. Soc.</i> Vol. 30. 1892</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">CRONEBERG</td> + <td class="it1p05">"Ueber die Mundtheile der Arachniden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page221">221</a>-<a href="#page224">224</a>, <a + href="#page241">241</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Naturgeschichte.</i> 1880</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">CUÉNOT</td> + <td class="it1p05">"Études sur le sang et les glandes lymphatiques dans la série animale; 2nd + partie; invertébrés"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page422">422</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. d. Zool. exper. gen.</i> 2nd Ser. Vol. 9. 1891</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">CUNNINGHAM, J. T.</td> + <td class="it1p05">"The Significance of Kupffer's Vesicle, with Remarks on other Questions of + Vertebrate Morphology"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page318">318</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 25. 1885</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Nephridia of <i>Lanice conchilega</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page403">403</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Nature.</i> Vol. 36. 1887</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">DANA</td> + <td class="it1p05">"On Cephalization"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page53">53</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mag. of Nat. Hist.</i> 1863</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">DEAN-BASHFORD</td> + <td class="it1p05">'Fishes, Living and Fossil'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page344">344</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">New York. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Embryology of <i>Bdellostoma Stouti</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page405">405</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Festschr. z. siebenzigsten Geburtstag. von C. v. Kupffer.</i> + Jena. 1899</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">DENDY</td> + <td class="it1p05">"On the Parietal Sense-organs and Associated Structures in the New Zealand + Lamprey (<i>Geotria australis</i>)"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page80">80</a>, <a href="#page82">82</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 51. 1907</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">DIETL</td> + <td class="it1p05">"Die Organisation des Arthropoden <span class="correction" + title="Original reads 'Gehirus'.">Gehirns</span>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page101">101</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 27. 1876</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">DOHRN</td> + <td class="it1p05">'Der Ursprung der Wirbelthiere und das Princip des Functionswechsels'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page14">14</a>, <a href="#page60">60</a>, <a + href="#page185">185</a>, <a href="#page186">186</a>, <a href="#page317">317</a>, <a + href="#page318">318</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Leipzig. 1875</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">Studien zur Urgeschichte des Wirbelthiere Körpers. VIII. "Die Thyroidea bei + Petromyzon, Amphioxus, und Tunicaten"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page188">188</a>, <a href="#page195">195</a>-<a + href="#page198">198</a>, <a href="#page199">199</a>, <a href="#page212">212</a>, <a + href="#page213">213</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. Zool. Stat. z. Neapel.</i> Vol. 6. 1886</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Neue Grundlagen zur Beurtheilung der Metamerie des Kopfes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a>, <a href="#page263">263</a>, <a + href="#page279">279</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. Zool. Stat. z. Neapel.</i> Vol. 9. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">Studien zur Urgeschichte des Wirbelthiere Körpers. XIII. "Ueber Nerven und + Gefässe bei Ammocœtes und <i>Petromyzon Planeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page167">167</a>, <a href="#page314">314</a>, <a + href="#page337">337</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. Zool. Stat. z. Neapel.</i> Vol. 8. 1888</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">DREVERMANN</td> + <td class="it1p05">"Ueber <i>Pteraspis dunensis</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page29">29</a>, <a href="#page30">30</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitschr. d. Deutsch. Geol. Gesellschaft.</i> Vol. 56. 1904</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">EDGEWORTH</td> + <td class="it1p05">"The Development of the Head-muscles in <i>Gallus domesticus</i>, and the + Morphology of the Head-muscles in the Sauropsida"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page266">266</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 51. 1907</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">EDINGER</td> + <td class="it1p05">'Anatomy of Central Nervous System in Man and in Vertebrates'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page17">17</a>, <a href="#page264">264</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Translated by Hall. 1899</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. EICHWALD</td> + <td class="it1p05">"Die Thier- und Pflanzenreste des alten rothen Sandsteins und Bergkalks im + Nowgorodschen Gouvernement"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bull. Sci. de l'Acad. Impér. d. St. Petersbourg.</i> 1840</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page505">{505}</span> + <p class="sp0">EISIG</p> + </td> + <td class="it1p05">"Die Seiten-organe und becherförmigen Organe der Capitelliden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page357">357</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 1. 1879</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Capitelliden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page357">357</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Faun. u. Flor. d. Golfes v. Neapel.</i> Vol. 16. 1887</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">ELLIOTT</td> + <td class="it1p05">"On the Innervation of the Ileo-colic Sphincter"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page449">449</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 31. 1904</td> + </tr> + <tr> + <td class="br pb05 it1p05">EMERY</td> + <td class="pb05 it1p05">Quoted by Weldon</td> + <td class="bl vbm pb05"><a href="#page420">420</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">FOSTER, M.</td> + <td class="pb05 it1p05">Text-book of Physiology</td> + <td class="bl vbm pb05"><a href="#page108">108</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">FREUND</td> + <td class="it1p05">"Die Beziehungen der Schilddrüse zu den weiblichen Geschlechtsorganen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page215">215</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Deutsch. Zeitsch. f. Chirugie.</i> Vol. 18. 1883</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">FRITSCH, G.</td> + <td class="it1p05">'Untersuchungen über den feineren Bau des Fischgehirns'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page488">488</a>, <a + href="#page489">489</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Berlin. 1878</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">FRORIEP</td> + <td class="it1p05">"Ueber Anlagen von Sinnesorganen am Facialis, Glossopharyngeus und Vagus, + über die genetische Stellung des Vagus zum Hypoglossus, und über die Herkunft der + Zungenmusculatur"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page261">261</a>, <a href="#page262">262</a>, <a + href="#page281">281</a>, <a href="#page283">283</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. f. Anat. u. Physiol; Anat. Abtheil.</i> 1885</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">FÜRBRINGER, M.</td> + <td class="it1p05">'Ueber die Spino-occipetalen Nerven der Selachier und Holocephalen'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page276">276</a>-<a href="#page278">278</a>, <a + href="#page409">409</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Fest-schrift für Carl Gegenbaur. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">GAUBERT</td> + <td class="it1p05">'Recherches sur les organes des sens et sur les systèmes tegumentaire, + glandulaire et musculaire des appendices des arachnides'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page364">364</a>, <a href="#page368">368</a>-<a + href="#page375">375</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Paris. 1892</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">GEGENBAUR</td> + <td class="it1p05">"Anatomische Untersuchung eines Limulus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page20">20</a>, <a href="#page358">358</a>-<a + href="#page360">360</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Abhandl. d. Naturforsch. Gesellsch. z. Halle.</i> Vol. 4. + 1858</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber die Skeletgewebe der Cyclostomen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page181">181</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Jen. Zeitschrift.</i> Vol. 5. 1870</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">Untersuchungen zur vergleichende Anatomie der Wirbelthiere III. Heft. 'Das + Kopfskeletder Selachiern'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page151">151</a>, <a href="#page259">259</a>, <a + href="#page261">261</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Leipzig. 1872</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">'Grundriss der vergleichenden Anatomie'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Leipzig. 1878</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. GEHUCHTEN</td> + <td class="it1p05">"De l'origine du pathétique et de la racine supérieure du trijumeau"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page264">264</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Acad. d. Sci. Belg. Bulletin.</i> 3rd Ser. Vol. 29. 1895</td> + </tr> + <tr> + <td class="br pb05 it1p05">GOETHE</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page258">258</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">GÖTTE</td> + <td class="it1p05">'Entwicklungsgeschichte der Unke'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page101">101</a>, <a href="#page102">102</a>, <a + href="#page114">114</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Leipzig. 1875</td> + </tr> + <tr> + <td class="br pb05 it1p05">GOLGI</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page72">72</a>, <a href="#page465">465</a>, <a + href="#page477">477</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">GOODRICH</td> + <td class="it1p05">"On the Structure of the Excretory Organs of Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page395">395</a>, <a href="#page396">396</a>, <a + href="#page477">477</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 45. 1902</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Nephridia of the <span class="correction" + title="Original reads 'Polychœta'.">Polychæta</span>." Parts I., II., III.</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page395">395</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vols. 40, 41, 43</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Excretory Organs of Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page477">477</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Proc. Roy. Soc.</i> Vol. 69. 1902</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page506">{506}</span> + <p class="sp0">GRABER</p> + </td> + <td class="it1p05">"Die <span class="correction" title="Original reads 'Chordo-tonalem'.">Chordo-tonalen</span> + Sinnesorgane und das Gehör der Insecten"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page364">364</a>, <a href="#page369">369</a>-<a + href="#page371">371</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vols. 20 and 21. 1882</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">GRENACHER</td> + <td class="it1p05">'Untersuchungen über das Sehorgan der Arthropoden'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page76">76</a>, <a href="#page100">100</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Göttingen. 1879</td> + </tr> + <tr> + <td class="br pb05 it1p05">GUDDEN</td> + <td class="pb05 it1p05">Quoted in Obersteiner</td> + <td class="bl vbm pb05"><a href="#page264">264</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">HAECKEL</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page461">461</a>, <a href="#page462">462</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HALLER, BELA</td> + <td class="it1p05">"Untersuchungen über die Hypophyse und die Infundibulärorgane"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page320">320</a>, <a + href="#page321">321</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Morph. Jahrbuch.</i> Vol. 25. 1898</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Untersuchungen über das Rückenmark der Teleostier"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page488">488</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Morph. Jahrbuch.</i> Vol. 23. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HARDY</td> + <td class="it1p05">"On the Histological Features and Physiological Properties of the + Post-œsophageal Nerve-cord of the Crustacea"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page110">110</a>, <a + href="#page159">159</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc.</i> 1894. B.</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HARDY <span class="smaller">AND</span> MACDOUGALL</td> + <td class="it1p05">"On the Structure and Functions of the Alimentary Canal of Daphnia"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page112">112</a>, <a + href="#page206">206</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Proc. Camb. Phil. Soc.</i> Vol. 8. 1893</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HATSCHEK</td> + <td class="it1p05">"Die Metamerie des Amphioxus und des Ammocœtes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page289">289</a>, <a href="#page300">300</a>, <a + href="#page337">337</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeig.</i>, 7 Jahrgang, 1892. <i>Verhandl. d. Anat. Gesell. + in Wien</i>, p. 136</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Studien über Entwicklung des Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page407">407</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arbeit. d. Zool. Inst. z. Wien.</i> Vol. 4. 1881</td> + </tr> + <tr> + <td class="br pb05"></td> + <td class="pb05 it1p05">Quoted by Lankester</td> + <td class="bl vbm pb05"><a href="#page475">475</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">HAZEN</td> + <td class="pb05 it1p05"><i>See</i> Patten and Hazen.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td class="br pb05 it1p05">HEIDENHAIN</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page258">258</a>, <a href="#page259">259</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">HEIDER</td> + <td class="pb05 it1p05"><i>See</i> Korschelt and Heider.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HENSEN</td> + <td class="it1p05">"Zur Entwicklung des Nervensystem"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page465">465</a>, <a + href="#page466">466</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Virchows Archiv.</i> Vol. 30. 1864</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HENSEN <span class="smaller">AND</span> VÖLCKERS</td> + <td class="it1p05"> </td> + <td rowspan="2" class="bl vbm pb05"><a href="#page265">265</a>, <a + href="#page266">266</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Opthalmol.</i> Vol. 24. 1878</td> + </tr> + <tr> + <td class="br pb05 it1p05">HERTWIG, O., <span class="smaller">AND</span> SAMASSA</td> + <td class="pb05 it1p05">Quoted in Zeigler's 'Lehrbuch der vergleichenden + Entwicklungsgeschichte der niederen Wirbelthiere.' 1902</td> + <td class="bl vbm pb05"><a href="#page485">485</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HIS</td> + <td class="it1p05">"Die Neuroblasten und deren Entstehung im embryonalen Mark"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page465">465</a>, <a + href="#page466">466</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Anat. u. Physiol. Anat. Abth.</i> 1889</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HOFFMANN</td> + <td class="it1p05">"Ueber die Metamerie des Nachhirns und Hinterhirns, und ihre Beziehung zu + den segmentalen Kopfnerven bei Reptilien embryonen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page276">276</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zool. Anzeiger.</i> Vol. 12. 1889</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HOLM</td> + <td class="it1p05">"Ueber die Organisation des <i>Eurypterus Fischeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page192">192</a>, <a href="#page240">240</a>, <a + href="#page241">241</a>, <a href="#page306">306</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mem. d. l'Acad. Imp. d. Sci. d. St. Petersbourg.</i> Vol. 8. + 1898</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">HOYER</td> + <td class="it1p05">"Ueber den Nachweis des Mucins in Geweben Mittelst der Färbe-Methode"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page131">131</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 36. 1890</td> + </tr> + <tr> + <td class="br pb05 it1p05"><span class="pagenum" id="page507">{507}</span> + <p class="sp0">HUXLEY</p> + </td> + <td class="pb05 it1p05">"Hunterian Lectures." 1869</td> + <td class="bl vbm pb05"><a href="#page124">124</a>, <a href="#page258">258</a>, <a + href="#page259">259</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Structure of the Mouth and Pharynx of the Scorpion"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page222">222</a>, <a href="#page225">225</a>, <a + href="#page271">271</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 8. 1860</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Anatomy and Affinities of the Genus Pterygotus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page238">238</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mem. of the Geol. Survey.</i> Monograph I. 1859</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On Cephalaspis and Pteraspis"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. of Geol. Soc.</i> Vol. 14. 1858</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">JAEKEL</td> + <td class="it1p05">"Ueber Tremataspis und Patten's Ableitung der Wirbelthiere von + Arthropoden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page329">329</a>, <a href="#page339">339</a>, <a + href="#page340">340</a>, <a href="#page351">351</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Protocoll der Deutschen Geolog. Gesellschaft</i>, p. 84; in + <i>Zeitsch. d. Deutschen Geologischen Gesellsch.</i> Vol. 55. 1903</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber die Organisation und systematische Stellung der Asterolepiden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page345">345</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ibid.</i>, p. 41</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">JOHNSON</td> + <td class="it1p05">"Contributions to the Comparative Anatomy of the Mammalian Eye, chiefly + based on Opthalmoscopic Examination"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page70">70</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc. B.</i> Vol. 194. 1901</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">JOSEPH</td> + <td class="it1p05">"Ueber das Achsenskelett des Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page444">444</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 59. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">JULIN <span class="smaller">AND</span> VAN BENEDEN</td> + <td class="it1p05">Recherches sur l'Organisation des Ascidies simples. "Sur l'hypophyse," + etc.</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page425">425</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archives de Biologie.</i> Vol. 2. 1881</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KAENSCHE</td> + <td class="it1p05">"Beiträge zur Kenntniss der Metamorphose des <i>Ammocœtes + branchialis</i> in <i>Petromyzon</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page135">135</a>, <a + href="#page304">304</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Schneider's Beiträge.</i> Vol. 2. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. KENNEL</td> + <td class="it1p05">"Entwickelungsgeschichte von <i>Peripatus Edwardsii</i> und <i>Peripatus + torquatus</i>." II. Theil</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page398">398</a>, <a href="#page399">399</a>, <a + href="#page411">411</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arbeit. a. d. Zool. Zoot. Instit. Würzburg.</i> Vol. 8. 1888</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KERR</td> + <td class="it1p05">"On some Points in the Early Development of Motor Nerve-trunks and Myotomes + in <i>Lepidosiren paradoxa</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page461">461</a>, <a href="#page466">466</a>, <a + href="#page478">478</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Roy. Soc. Edin.</i> Vol. 41. 1904</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KILLIAN</td> + <td class="it1p05">"Zur Metamerie des Selachierkopfes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Verhandl. d. Anat. Gesell. Versamml. in München.</i> 1891</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KISHINOUYE</td> + <td class="it1p05">"On the Development of <i>Limulus longispina</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page167">167</a>, <a href="#page238">238</a>, <a + href="#page252">252</a>, <a href="#page253">253</a>, <a href="#page273">273</a>, <a + href="#page320">320</a>, <a href="#page382">382</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Coll. of Sci., Tokio.</i> Vol. 5. 1891</td> + </tr> + <tr> + <td class="br pb05 it1p05">KLEINENBERG</td> + <td class="pb05 it1p05">Quoted by Beard</td> + <td class="bl vbm pb05"><a href="#page318">318</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. KÖLLIKER</td> + <td class="it1p05">"Die obere Trigeminus-Wurzel"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page280">280</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. f. Mikr. Anat.</i> Vol. 53. 1899</td> + </tr> + <tr> + <td class="br pb05 it1p05">v. KÖLLIKER <span class="smaller">AND</span> TERTERJANZ</td> + <td class="pb05 it1p05">Handbuch der Gewebe-Lehre. 6th Auflage. 1893</td> + <td class="bl vbm pb05"><a href="#page264">264</a>, <a href="#page425">425</a>, <a + href="#page488">488</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KOHL</td> + <td class="it1p05">"Rudimentäre Wirbelthieraugen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page94">94</a>, <a href="#page96">96</a>, <a + href="#page99">99</a>, <a href="#page101">101</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bibliotheca Zoologica. Leukart und Chun.</i> Vol. 4 and Vol. + 5</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KOHN</td> + <td class="it1p05">"Ueber den Bau und die Entwicklung der sogenannten Carotis-drüse"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page428">428</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 56. 1900</td> + </tr> + <tr> + <td class="br pb05 it1p05"><span class="pagenum" id="page508">{508}</span> + <p class="sp0">KORSCHELT <span class="smaller">AND</span> HEIDER</p> + </td> + <td class="pb05 it1p05">'Text-book of the Embryology of the Invertebrates.' Translated by M. + Bernard. 1900. Part III. and Part IV.</td> + <td class="bl vbm pb05"><a href="#page27">27</a>, <a href="#page73">73</a>, <a + href="#page88">88</a>, <a href="#page114">114</a>-<a href="#page116">116</a>, <a + href="#page397">397</a>, <a href="#page429">429</a>, <a href="#page431">431</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KOWALEWSKY</td> + <td class="it1p05">"Ein Beitrag zur Kenntniss der Excretionsorgane der Pantopoden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page421">421</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mem. d. l'Acad. d. Imp. d. Sci. d. St. Petersbourg.</i> Ser. VII. + Vol. 38. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Une nouvelle glande lymphatique chez le scorpion d'Europe"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page423">423</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ibid.</i> Ser. VIII. Vol. 5. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Étude Biologique sur les Clepsines"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page421">421</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ibid.</i> Ser. VIII. Vol. 5. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ein Beitrag zur Kenntniss der Excretionsorgane"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page420">420</a>, <a href="#page422">422</a>, <a + href="#page472">472</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Biologisches Centralblatt.</i> 1889</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Weitere Studien über die Entwicklungsgeschichte des <i>Amphioxus + lanceolatus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page409">409</a>, <a + href="#page410">410</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 13. 1877</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">KRIEGER</td> + <td class="it1p05">"Ueber das Centralnervensystem des Flusskrebses</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page101">101</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 33. 1880</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. KUPFFER</td> + <td class="it1p05"> + <p>'Studien zur vergleichenden Entwicklungsgeschichte des Kopfes der Kranioten.'</p> + <p class="sp0"><span class="gap" style="width:2em"> </span>Heft. 1. 'Die Entwicklung + des Kopfes von <i>Acipenser</i>'</p> + </td> + <td rowspan="2" class="bl vbm pb05"><a href="#page318">318</a>, <a href="#page319">319</a>, <a + href="#page320">320</a>, <a href="#page440">440</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">München. 1893</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05"><span class="gap" style="width:2em"> </span>Heft. 2. 'Die Entwicklung + des Kopfes von <i>Ammocœtes Planeri</i>'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page300">300</a>, <a + href="#page440">440</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">München. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05"> + <p><span class="gap" style="width:2em"> </span>Heft. 3. 'Die Entwicklung der Kopfnerven + von <i>Ammocœtes Planeri</i>.'</p> + <p class="sp0"><span class="gap" style="width:2em"> </span>Dritter Abschnitt. 'Die + Metamorphose des larvalen Nervensystems des Kopfes'</p> + </td> + <td rowspan="2" class="bl vbm pb05"><a href="#page228">228</a>, <a href="#page263">263</a>, <a + href="#page282">282</a>, <a href="#page283">283</a>, <a href="#page405">405</a>, <a + href="#page458">458</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">München. 1895</td> + </tr> + <tr> + <td class="br pb05 it1p05">LANG</td> + <td class="pb05 it1p05">'Text-book of Comparative Anatomy.' Translated by H. M. and M. + Bernard</td> + <td class="bl vbm pb05"><a href="#page357">357</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LANGERHANS</td> + <td class="it1p05">"Untersuchungen über <i>Petromyzon Planeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page94">94</a>-<a href="#page101">101</a>, <a + href="#page301">301</a>, <a href="#page405">405</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bericht v. d. Verhandl. d. Naturforsch. Gesellsch. z. + Freiburg.</i> 1873</td> + </tr> + <tr> + <td class="br pb05 it1p05">LANGLEY</td> + <td class="pb05 it1p05">Schäfer's 'Text-book of Physiology.' Vol. 2. 1900</td> + <td class="bl vbm pb05"><a href="#page2">2</a>, <a href="#page3">3</a>, <a + href="#page448">448</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">LANKESTER</td> + <td class="pb05 it1p05">Article "Vertebrata" in the 'Encyclopædia Britannica'</td> + <td class="bl vbm pb05"><a href="#page484">484</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Skeleto-trophic Tissues and Coxal Glands of Limulus, Scorpio, and + Mygale</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page137">137</a>, <a href="#page139">139</a>, <a + href="#page253">253</a>, <a href="#page320">320</a>, <a href="#page321">321</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 24. 1884</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Limulus an Arachnid"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page62">62</a>, <a href="#page238">238</a>, <a + href="#page241">241</a>, <a href="#page306">306</a>, <a href="#page361">361</a>, <a + href="#page366">366</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 21. 1881</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">'Extinct Animals'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page22">22</a>, <a href="#page150">150</a>, <a + href="#page345">345</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. Constable & Co. 1906</td> + </tr> + <tr> + <td class="br pb05"></td> + <td class="pb05 it1p05">A treatise on Zoology. Edited by E. Ray Lankester. Part II. 'The + Entero-cœla and the Cœlomocœla'</td> + <td class="bl vbm pb05"><a href="#page472">472</a>-<a href="#page478">478</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page509">{509}</span> + <p class="sp0">LANKESTER <span class="smaller">AND</span> POWRIE</p> + </td> + <td class="it1p05">"A Monograph of the Fishes of the Old Red Sandstone of Britain." Part I. + "The Cephalaspidæ"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page29">29</a>, <a href="#page275">275</a>, <a + href="#page327">327</a>, <a href="#page339">339</a>, <a href="#page345">345</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Palæontographical Soc.</i> 1868</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LANKESTER, BENHAM, <span class="smaller">AND</span> + BECK</td> + <td class="it1p05">"On the Muscular and Endo-skeletal Systems of Limulus and Scorpio, with + some Notes on the Anatomy and Generic Characters of Scorpions"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page177">177</a>, <a href="#page222">222</a>, <a + href="#page224">224</a>, <a href="#page313">313</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 11. 1885</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LANKESTER <span class="smaller">AND</span> BOURNE</td> + <td class="it1p05">"The Minute Structure of the Lateral and Central Eyes of Scorpio and + Limulus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page74">74</a>, <a href="#page81">81</a>-<a + href="#page83">83</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 23</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LANKESTER <span class="smaller">AND</span> WILLEY</td> + <td class="it1p05">"The Development of the Atrial Chamber of Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page409">409</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 31. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LANKESTER <span class="smaller">AND</span> GULLAND</td> + <td class="it1p05">"Evidence in Favour of the View that the Coxal Gland of Limulus and of + other Arachnids is a Modified Nephridium"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page429">429</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 25. 1885</td> + </tr> + <tr> + <td class="br pb05 it1p05">LATREILLE</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page221">221</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LAURIE</td> + <td class="it1p05">"The Anatomy and Relations of the Eurypteridæ"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page237">237</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Roy. Soc. Edin.</i> Vol. 37. 1893</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On a Silurian Scorpion and some Additional Eurypterid Remains from the + Pentland Hills</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page238">238</a>, <a + href="#page239">239</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ibid.</i> Vol. 34. 1899</td> + </tr> + <tr> + <td class="br pb05 it1p05">LEYDIG</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page91">91</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LOCY</td> + <td class="it1p05">"Contributions to the Structure and Development of the Vertebrate + Head"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a>, <a + href="#page262">262</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. Morph.</i> Vol. 11. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LOEB, LEO, <span class="smaller">AND</span> R. M. + STRONG</td> + <td class="it1p05">"On Regeneration in the Pigmented Skin of the Frog, and on the Character of + the Chromatophores"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page470">470</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Amer. Jour. of Anat.</i> Vol. 3. 1904</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LOWNE</td> + <td class="it1p05">'The Anatomy, Physiology, Morphology, and Development of the Blow-fly'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page369">369</a>, <a href="#page370">370</a>, <a + href="#page375">375</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. 1895</td> + </tr> + <tr> + <td class="br pb05 it1p05">LUGARO</td> + <td class="pb05 it1p05">Quoted by Anderson</td> + <td class="bl vbm pb05"><a href="#page467">467</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">LWOFF</td> + <td class="it1p05">"Ueber den Zusammenhang von Markrohr und Chorda beim Amphioxus und ähnliche + Verbältnisse bei Anneliden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page444">444</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 56. 1893</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MAAS</td> + <td class="it1p05">"Ueber Entwicklungstadien der Vorniere und Urniere bei Myxine"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a>, <a href="#page402">402</a>, <a + href="#page412">412</a>, <a href="#page419">419</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 10. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MACBRIDE</td> + <td class="it1p05">"Further Remarks on the Development of Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page410">410</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 43. 1900</td> + </tr> + <tr> + <td class="br pb05 it1p05">McDOUGALL</td> + <td class="pb05 it1p05"><i>See</i> Hardy and McDougall.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page510">{510}</span> + <p class="sp0">MACLEOD</p> + </td> + <td class="it1p05">"Recherches sur la structure et la signification de l'appareil respiratoire + des Arachnides"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page169">169</a>, <a + href="#page174">174</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. de Biol.</i> Vol. 5. 1881</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MAGNUS</td> + <td class="it1p05">"Versuche am überlebenden Dünndarm von Säugethieren"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page447">447</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. d. Ges. Physiologie.</i> Vols. 102, 103. 1904</td> + </tr> + <tr> + <td class="br pb05 it1p05">MARK</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page115">115</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MARSHALL</td> + <td class="it1p05">"On the Head-cavities and Associated Nerves of Elasmobranchs"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page185">185</a>, <a + href="#page186">186</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 21. 1881</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Segmental Value of the Cranial Nerves"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page260">260</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 16. 1882</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MASTERMAN</td> + <td class="it1p05">"On the Diplochorda"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page16">16</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 43. 1900</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MAURER</td> + <td class="it1p05">"Die Schilddrüse, Thymus und andere Schlundspaltenderivate bei den + Eidechse"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page427">427</a>, <a + href="#page428">428</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Morph. Jahrbuch.</i> Vol. 27. 1899</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MAYER, F.</td> + <td class="it1p05">"Das Centralnervensystem von Ammocœtes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page489">489</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeig.</i> Vol. 13. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MAYER, P.</td> + <td class="it1p05">"Ueber die Entwicklung des Herzens und der grossen Gefässstämme bei den + Selachiern"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 7. 1887</td> + </tr> + <tr> + <td class="br pb05 it1p05">METSCHNIKOW</td> + <td class="pb05 it1p05">Quoted by Kowalewsky</td> + <td class="bl vbm pb05"><a href="#page422">422</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MEYER</td> + <td class="it1p05">"Studien über den Körperbau der Anneliden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page403">403</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 7. 1887</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MILNE-EDWARDS</td> + <td class="it1p05">"Anatomie des Limules"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page157">157</a>, <a href="#page159">159</a>, <a + href="#page176">176</a>, <a href="#page177">177</a>, <a href="#page313">313</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Annales des Sciences Naturelles.</i> Ser. 5. Vol. 17. 1872</td> + </tr> + <tr> + <td class="br pb05 it1p05">MINCHIN</td> + <td class="pb05 it1p05">A treatise on Zoology. Edited by Ray Lankester. Part II. "The Porifera + and Cœlenterata"</td> + <td class="bl vbm pb05"><a href="#page473">473</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MITSUKURI</td> + <td class="it1p05">"On the Fate of the Blastopore, the Relations of the Primitive Streak, and + the Formation of the Posterior End of the Embryo in Chelonia," etc.</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. Coll. Sci.</i> Tokyo. Vol. 10. 1896</td> + </tr> + <tr> + <td class="br pb05 it1p05">MOTT</td> + <td class="pb05 it1p05">"Croonian Lectures of the Roy. Coll. of Physicians," 1900</td> + <td class="bl vbm pb05"><a href="#page469">469</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MOTT <span class="smaller">AND</span> HALLIBURTON</td> + <td class="it1p05">"On the Chemistry of Nerve-degeneration"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page469">469</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc. B.</i> Vol. 194. 1901</td> + </tr> + <tr> + <td class="br pb05 it1p05">MÜLLER, J.</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page1">1</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Vergleichende Anatomie der Myxinoiden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page126">126</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Abhandl. d. Kgl. Akad. d. Wiss.</i> Berlin. 1834</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">MÜLLER, W.</td> + <td class="it1p05">"Ueber die Stammes Entwickelung des Sehorgans der Wirbelthiere"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page96">96</a>-<a href="#page100">100</a>, <a + href="#page105">105</a>, <a href="#page108">108</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Festgabe C. Ludwig. Leipzig. 1874</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">NEAL</td> + <td class="it1p05">"The Segmentation of the Nervous System in <i>Squalus acanthias</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a>, <a href="#page266">266</a>, <a + href="#page300">300</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bull. of Mus. Comp. Zool.</i> Harvard. Vol. 31. 1898</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">NESTLER</td> + <td class="it1p05">"Beiträge zur Anatomie und Entwicklungsgeschichte von <i>Petromyzon + Planeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page168">168</a>, <a href="#page171">171</a>, <a + href="#page175">175</a>, <a href="#page445">445</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Naturgesch. Jahrgang</i>, 56. Vol. I. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page511">{511}</span> + <p class="sp0">NIESKOWSKI</p> + </td> + <td class="it1p05">"Der <i>Eurypterus Remipes</i> aus den obersilurischen Schichten der Insel + Oesel"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page26">26</a>, <a href="#page239">239</a>, <a + href="#page240">240</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. f. d. Naturkunde Liv-Ehst-und Kurlands.</i> 1st Ser. Vol. 3. + 1858</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">NUSBAUM, J.</td> + <td class="it1p05">"Einige neue Thatsachen zur Entwicklungsgeschichte des <i>Hypophysis + Cerebri</i> bei Säugethieren"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page320">320</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 12. 1896</td> + </tr> + <tr> + <td class="br pb05 it1p05">OBERSTEINER</td> + <td class="pb05 it1p05">'Central Nervous System.' Translated by Hill. 1896</td> + <td class="bl vbm pb05"><a href="#page264">264</a>, <a href="#page280">280</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">OKEN</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page258">258</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">OWEN</td> + <td class="pb05 it1p05">"Essays on the Conario-Hypophysial Tract, and the Aspects of the Body + in Vertebrate and Invertebrate Animals"</td> + <td class="bl vbm pb05"><a href="#page14">14</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Anatomy of the American King-crab (<i>Limulus polyphemus</i>)"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page211">211</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Linn. Soc.</i> Vol. 28. 1873</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PANDER</td> + <td class="it1p05">'Monographie der fossilen Fische des Silurischen Systems des + russisch-baltischen Gouvernements'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">St. Petersbourg. 1856</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PARKER, G. H.</td> + <td class="it1p05">"The Retina and Optic Ganglia in Decapods, especially in Astacus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page91">91</a>, <a href="#page93">93</a>, <a + href="#page97">97</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 12. 1895</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Compound Eyes in Crustaceans"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page99">99</a>, <a href="#page100">100</a>, <a + href="#page114">114</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bull. of Harvard Mus. of Comp. Zool.</i> Vol. 20. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"The Function of the Lateral-line Organs in Fishes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page357">357</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bull. of the Fisheries Bureau.</i> Washington. Vol. 24. 1904</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Studies on the Eyes of Arthropods"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page73">73</a>, <a href="#page79">79</a>, <a + href="#page83">83</a>-<a href="#page85">85</a>, <a href="#page114">114</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Morphology.</i> Vols. 1 and 2. 1887 and 1889</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PARKER, W. K.</td> + <td class="it1p05">"On the Skeleton of the Marsipobranch Fishes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page120">120</a>, <a href="#page125">125</a>, <a + href="#page126">126</a>, <a href="#page131">131</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc.</i> 1883</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PATTEN</td> + <td class="it1p05">"On the Origin of Vertebrates from Arachnids"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page352">352</a>, <a + href="#page353">353</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 31. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Morphology and Physiology of the Brain and Sense-organs of + Limulus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page358">358</a>-<a href="#page367">367</a>, <a + href="#page371">371</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 35. 1893</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"New Facts concerning Bothriolepis"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page32">32</a>, <a href="#page351">351</a>, <a + href="#page450">450</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Biological Bulletin.</i> Vol. 7. 1904</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Structure and Classification of the Tremataspidæ"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page329">329</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mem. d. l'Acad. Imp. d. Sci. de St. Petersbourg.</i> Vol. 13. + 1903</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Structure of the Pteraspidæ and Cephalaspidæ"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page415">415</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>The American Naturalist.</i> Vol. 37. 1903</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On the Appendages of Tremataspis"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page351">351</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>The American Naturalist.</i> Vol. 37. 1903</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On Structures Resembling Dermal Bones in Limulus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page346">346</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeig.</i> Vol. 9. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page512">{512}</span> + <p class="sp0">PATTEN <span class="smaller">AND</span> HAZEN</p> + </td> + <td class="it1p05">"The Development of the Coxal Gland, etc., of <i>Limulus + Polyphemus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page408">408</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Morphol.</i> Vol. 16. 1900</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PATTEN <span class="smaller">AND</span> REDENBAUGH</td> + <td class="it1p05">Studies on Limulus. II. "The Nervous System of <i>Limulus + Polyphemus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page314">314</a>, <a href="#page315">315</a>, <a + href="#page381">381</a>, <a href="#page382">382</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Morphol.</i> Vol. 16. 1900</td> + </tr> + <tr> + <td class="br pb05 it1p05">PERLIA</td> + <td class="pb05 it1p05">Quoted by Edinger</td> + <td class="bl vbm pb05"><a href="#page264">264</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">PICK</td> + <td class="pb05 it1p05"><span class="hid">Quo</span>"<span class="hid">ed by Ed</span>"<span + class="hid">nger</span></td> + <td class="bl vbm pb05"><a href="#page265">265</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PLATT</td> + <td class="it1p05">"A Contribution to the Morphology of the Vertebrate Head, based on a Study + of <i>Acanthias vulgaris</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page253">253</a>, <a href="#page265">265</a>-<a + href="#page267">267</a>, <a href="#page273">273</a>, <a href="#page274">274</a>, <a + href="#page279">279</a>, <a href="#page284">284</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. Morphol.</i> Vol. 5. 1891</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Fibres connecting the Central Nervous System and Chorda in Amphioxus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page443">443</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeig.</i> 1892</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">PRICE</td> + <td class="it1p05">"Development of the Excretory Organs of <i>Bdellostoma Stouti</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page394">394</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 10. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">RABL</td> + <td class="it1p05">"Ueber die Metamerie des Wirbelthierkopfes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page258">258</a>, <a + href="#page262">262</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Verhandl. der Anat. Gesellsch. Versamml. in Wien. Anat. + Anzeig.</i> 1892</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Die Entwicklung und Structur der Nebennieren bei den Vögeln"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. f. mikr. Anat.</i> Vol. 38. 1891</td> + </tr> + <tr> + <td class="br pb05 it1p05">RAMÓN Y. CAJAL</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page72">72</a>, <a href="#page465">465</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">RATHKE</td> + <td class="it1p05">"Anatomie des Querders"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page161">161</a>, <a href="#page169">169</a>, <a + href="#page304">304</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Naturforsch. Gesellsch. zu Dantzig.</i> Vol. 2. 1827</td> + </tr> + <tr> + <td class="br pb05 it1p05">REDENBAUGH</td> + <td class="pb05 it1p05"><i>See</i> Patten and Redenbaugh.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">REICHENBACH</td> + <td class="it1p05">"Entwicklungs-geschichte des Flusskrebses"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page98">98</a>-<a href="#page100">100</a>, <a + href="#page114">114</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Abhandl. d. Senckenbergischen Naturforsch. Gesellsch.</i> Vol. 14. + 1886.</td> + </tr> + <tr> + <td class="br pb05 it1p05">RETZIUS</td> + <td class="pb05 it1p05">'Biologische Untersuchungen.' Vol. 1. 1890. "Zur Kenntniss des + Nervensystem der Crustaceen"</td> + <td class="bl vbm pb05"><a href="#page20">20</a>, <a href="#page489">489</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">ROHON</td> + <td class="it1p05">Die Obersilurischen Fische von Oesel. 1st Theil. "Thyestidæ und + Tremataspidæ"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page32">32</a>, <a href="#page275">275</a>, <a + href="#page276">276</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mem. d. l'Acad. Imp. d. Sci. d. St. Petersbourg.</i> 7th Ser. Vol. + 38. 1892</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Weitere Mittheilungen über die Gattung <i>Thyestes</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a>-<a href="#page330">330</a>, <a + href="#page339">339</a>-<a href="#page341">341</a>, <a href="#page382">382</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Bull. d. l'Acad. d. St. Petersbourg.</i> 5th Ser. Vol. 4. + 1896</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">ROLPH</td> + <td class="it1p05">"Untersuchungen über den Bau des <i>Amphioxus lanceolatus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page444">444</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Morphol. Jahrbuch.</i> Vol. 2. 1887</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">RÜCKERT, J.</td> + <td class="it1p05">"Entwicklung der Excretionsorgane"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a>, <a href="#page393">393</a>, <a + href="#page400">400</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Merkel und Bonnet; Anat. Hefte.</i> Vol. 1. 1891.</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber die Entstehung der Excretionsorgane bei Selachiern"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page403">403</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Anatomie.</i> 1888</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">ST. HILAIRE</td> + <td class="it1p05">"Sur la Vertèbre"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page11">11</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>La Revue Encyclopédique.</i> 1822</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page513">{513}</span> + <p class="sp0">SAMASSA</p> + </td> + <td class="it1p05">"Bemerkungen über die Methode der Vergleichenden + Entwicklungsgeschichte"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page462">462</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Biol. Centralblatt.</i> Vol. 18. 1898</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCHAFFER</td> + <td class="it1p05">"Ueber das Knorpelige Skelett von Ammocœtes"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page126">126</a>-<a href="#page135">135</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 61. 1896</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber die Thymusanlage bei <i>Petromyzon Planeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page426">426</a>-<a href="#page428">428</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Sitzungsber. d. K. Akad d. Wiss. in Wien.</i> Vol. 103. 1894</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCHIMKÉWITSCH</td> + <td class="it1p05">"Sur la structure et sur la signification de l'Endosternite des + Arachnides"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page143">143</a>-<a href="#page145">145</a>, <a + href="#page342">342</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zool. Anzeig.</i> 1893</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Anatomie de l'Epeire"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page369">369</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ann. d. Sci. Nat.</i> Vol. 17. 1884</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCHMIDT</td> + <td class="it1p05">"Die Crustaceen-fauna der Eurypterenschichten von Rootziküll auf + Oesel"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page190">190</a>, <a href="#page191">191</a>, <a + href="#page236">236</a>, <a href="#page240">240</a>, <a href="#page329">329</a>, <a + href="#page341">341</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Mem. d'Acad. Imp. d. Sci. d. St. Petersbourg.</i> Vol. 31. + 1883</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCHMIEDEBERG</td> + <td class="it1p05">"Ueber die chemische Zusammensetzung des Knorpels"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page147">147</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arch. f. exper. Pathol. und Pharmak.</i> Vol. 28. 1891</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCHNEIDER, A.</td> + <td class="it1p05">'Beiträge zur Anatomie und Entwicklungsgeschichte der Wirbelthiere'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page128">128</a>, <a href="#page130">130</a>, <a + href="#page172">172</a>, <a href="#page195">195</a>, <a href="#page197">197</a>, <a + href="#page213">213</a>, <a href="#page310">310</a>, <a href="#page445">445</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Berlin. 1879</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCHNEIDER, G.</td> + <td class="it1p05">"Ueber phagocytäre Organe und Chloragogenzellen der Oligochæta"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page421">421</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 61. 1896</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SCOTT</td> + <td class="it1p05">"Notes on the Development of Petromyzon"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page42">42</a>, <a href="#page78">78</a>, <a + href="#page111">111</a>, <a href="#page112">112</a>, <a href="#page406">406</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Morphol.</i> Vol. 1. 1887</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SEDGWICK</td> + <td class="it1p05">"A Monograph of the Development of <i>Peripatus capensis</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page397">397</a>-<a href="#page400">400</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Studies from the Morphological Laboratory, Cambridge.</i> Vol. 4. + 1888</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Development of the Kidney in its Relation to the Wolffian Body in the + Chick"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 20. 1880</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Early Development of the Wolffian Duct and Anterior Wolffian Tubules in + the Chick; with some Remarks on the Vertebrate Excretory System"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page393">393</a>, <a href="#page394">394</a>, <a + href="#page400">400</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 21. 1881</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SEMON</td> + <td class="it1p05">"Das Excretionssystem der Myxinoiden"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page400">400</a>, <a + href="#page419">419</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Festschrift f. Gegenbaur.</i> Leipzig. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SEMPER</td> + <td class="it1p05">"Die Stammesverwandschaft der Wirbelthiere und Wirbellosen"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a>, <a + href="#page392">392</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Arbeit. a. d. Zool. Zoot. Inst. Würzburg.</i> Vol. 2. 1875</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Das Urinogenitalsystem der Plagiostomen und seine Bedeutung für die + übrigen Wirbelthiere"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a>, <a + href="#page392">392</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ibid.</i> Vol. 2. 1875</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SHELDON</td> + <td class="it1p05">"On the Development of <i>Peripatus Nova-Zealandiæ</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page400">400</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Studies from the Morphological Laboratory, Cambridge.</i> Vol. 4. + 1889</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page514">{514}</span> + <p class="sp0">SHERRINGTON</p> + </td> + <td class="it1p05">"On the Anatomical Constitution of the Nerves of Muscles"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page267">267</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 17. 1894. <i>Proc. of Physiol. + Soc.</i> June 23</td> + </tr> + <tr> + <td class="br pb05 it1p05">SHIPLEY</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page334">334</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"On some points in the Development of <i>Petromyzon fluviatilis</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page167">167</a>, <a href="#page305">305</a>, <a + href="#page378">378</a>, <a href="#page401">401</a>, <a href="#page405">405</a>, <a + href="#page406">406</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 27. 1887</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. SMIRNOW</td> + <td class="it1p05">"Ueber die Nervenendigungen in den Nieren der Säugethiere"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page477">477</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 19. 1901</td> + </tr> + <tr> + <td class="br pb05 it1p05">SMITH, ELLIOT</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page17">17</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SPANGENBERG</td> + <td class="it1p05">"Zur Kenntniss von <i>Branchipus stagnalis</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page396">396</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 25. 1875</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">SPENGEL</td> + <td class="it1p05">'Die Enteropneusten'</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page494">494</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">Berlin. 1893</td> + </tr> + <tr> + <td class="br pb05 it1p05">STARR</td> + <td class="pb05 it1p05">Quoted by Edinger</td> + <td class="bl vbm pb05"><a href="#page265">265</a>, <a href="#page266">266</a></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">STUDNIÇKA</td> + <td class="it1p05">"Sur les organes pariétaux de <i>Petromyzon Planeri</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page80">80</a>, <a href="#page81">81</a>, <a + href="#page86">86</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Sitzungsber. d. K. Gesell. d. Wiss. in Prag.</i> 1893</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Ueber den feineren Bau der Parietalorgane von <i>Petromyzon + marinus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page81">81</a>, <a href="#page86">86</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Sitzungsber. d. K. böhmischen Gesell. d. Wiss. Prag.</i> 1899</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">TAKAMINE</td> + <td class="it1p05">"The Isolation of the Active Principle of the Supra-renal Gland"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page423">423</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 27. <i>Proc. of Physiol. Soc.</i>, + Dec. 14, 1901</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">TARNANI</td> + <td class="it1p05">"On the Anatomy of the Thelyphonides"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page190">190</a>, <a href="#page206">206</a>-<a + href="#page208">208</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Revue des Sciences Naturelles, St. Petersbourg.</i> 1890</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Die genitalen Organe der Thelyphonus"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page190">190</a>, <a href="#page206">206</a>-<a + href="#page208">208</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Biol. Centralblatt.</i> Vol. 9. 1889</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">TRAQUAIR</td> + <td class="it1p05">"Report on Fossil Fishes collected by the Geological Survey of Scotland in + the Silurian Rocks of the South of Scotland"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page343">343</a>-<a href="#page345">345</a>, <a + href="#page350">350</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Roy. Soc., Edin.</i> Vol. 39. 1899</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">VIALLANES</td> + <td class="it1p05">"Contribution à l'histologie du système nerveux des Invertébrés; la lame + ganglionnaire de la Langouste"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page100">100</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Ann. Sci. Nat.</i> Vol. 13</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">VINCENT, SWALE</td> + <td class="it1p05">"The Carotid Gland of Mammalia and its Relation to the Supra-renal Capsule, + with some Remarks upon Internal Secretion and the <span class="correction" + title="Original reads 'Phyogeny'.">Phylogeny</span> of the latter Organ"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 18. 1900</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Contributions to the Comparative Anatomy and Histology of the Supra-renal + Capsules"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 14. 1897</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">VIRCHOW</td> + <td class="it1p05">"Transformation and Descent"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page479">479</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Journ. of Path. and Bacter.</i> Vol. 1. 1893</td> + </tr> + <tr> + <td class="br pb05 it1p05">VOGT</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page258">258</a></td> + </tr> + <tr> + <td class="br pb05 it1p05">VÖLCKERS</td> + <td class="pb05 it1p05"><i>See</i> Hensen and Völckers.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td class="br pb05 it1p05">WAGNER</td> + <td class="pb05 it1p05">Quoted by Gaubert</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05"><span class="pagenum" id="page515">{515}</span> + <p class="sp0">WEISS</p> + </td> + <td class="it1p05">"Excretory Tubules in <i>Amphioxus Lanceolatus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page426">426</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 31. 1890</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">WELDON</td> + <td class="it1p05">"On the Supra-renal Bodies of Vertebrates"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page420">420</a>, <a href="#page424">424</a>, <a + href="#page429">429</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 25. 1885</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Note on the Origin of the Supra-renal Bodies in Vertebrates"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Proc. Roy. Soc.</i> Vol. 37. 1884</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">WHEELER</td> + <td class="it1p05">"Development of the Urino-genital Organs of the Lamprey"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page402">402</a>, <a + href="#page405">405</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 13. 1899</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">v. WIJHE</td> + <td class="it1p05">"Ueber die Mesodermsegmente des Rumpfes und die Entwicklung des + Excretionsystems bei Selachiern"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page155">155</a>-<a href="#page157">157</a>, <a + href="#page172">172</a>, <a href="#page173">173</a>, <a href="#page188">188</a>, <a + href="#page234">234</a>, <a href="#page258">258</a>, <a href="#page260">260</a>, <a + href="#page262">262</a>, <a href="#page263">263</a>, <a href="#page266">266</a>, <a + href="#page273">273</a>, <a href="#page280">280</a>, <a href="#page308">308</a>, <a + href="#page390">390</a>-<a href="#page393">393</a>, <a href="#page397">397</a>, <a + href="#page400">400</a>, <a href="#page406">406</a>-<a href="#page408">408</a>, <a + href="#page412">412</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 33. 1889</td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">"Beiträge zur Anatomie der Kopfregion des <i>Amphioxus + lanceolatus</i>"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page410">410</a>, <a href="#page426">426</a>-<a + href="#page428">428</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Petrus Camper. Deel.</i> 1; <i>Aflevering.</i> 2</td> + </tr> + <tr> + <td class="br pb05 it1p05">WILLEY</td> + <td class="pb05 it1p05"><i>See</i> Lankester and Willey.</td> + <td class="bl pb05"></td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">WOLFF</td> + <td class="it1p05">"Die Cuticula der Wirbelthierepidermis"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page302">302</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Jen. Zeitsch. f. Naturwissenschaft.</i> Vol. 23. 1889</td> + </tr> + <tr> + <td rowspan="2" class="br it1p05">WOODWARD, H.</td> + <td class="it1p05">"A Monograph of the British Fossil Crustacea, belonging to the order + Merostomata"</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page235">235</a>-<a href="#page240">240</a>, <a + href="#page249">249</a>, <a href="#page251">251</a>, <a href="#page275">275</a></td> + </tr> + <tr> + <td class="ar pb05 pr05"><i>Palæontographical Society.</i> 1878</td> + </tr> + <tr> + <td class="br pb05 it1p05">WOODWARD, SMITH</td> + <td class="pb05"></td> + <td class="bl vbm pb05"><a href="#page339">339</a></td> + </tr> + <tr> + <td rowspan="2" class="br"></td> + <td class="it1p05">'Catalogue of Fossil Fishes in the British Museum.' Part II.</td> + <td rowspan="2" class="bl vbm pb05"><a href="#page29">29</a>, <a href="#page326">326</a>, <a + href="#page327">327</a>, <a href="#page344">344</a>, <a href="#page349">349</a>, <a + href="#page351">351</a></td> + </tr> + <tr> + <td class="ar pb05 pr05">London. 1891</td> + </tr> + <tr> + <td class="br pb05 it1p05">v. ZITTEL</td> + <td class="pb05 it1p05">Handbuch der Palæontologie</td> + <td class="bl vbm pb05"><a href="#page190">190</a></td> + </tr> + </table> + + <div><span class="pagenum" id="page517">{517}</span></div> + + <p class="ac" style="margin-bottom:2.9ex;"><span class="larger">GENERAL INDEX</span></p> + + <p class="sp3 ac">[<i>The numbers in dark type refer to illustrations</i>]</p> + + <div class="poem sp3"> + <p>Acilius larva, eye of, <b><a href="#page78">78</a></b>, <b><a href="#page83">83</a></b></p> + <p>Acromegaly, <a href="#page425">425</a></p> + <p>Actinotrocha, <a href="#page438">438</a></p> + <p>Addison's disease, <a href="#page423">423</a></p> + <p>Adelopthalmus, <a href="#page249">249</a></p> + <p>Adrenalin, <a href="#page423">423</a>, <a href="#page491">491</a></p> + <p>Adrenals, <a href="#page423">423</a>, <a href="#page491">491</a></p> + <p>Agnathostomatous fishes, <a href="#page29">29</a>, <a href="#page343">343</a></p> + <p>Alimentary canal, <a href="#page433">433</a></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + Ammocœtes, <a href="#page168">168</a>, <a href="#page405">405</a>, <a + href="#page445">445</a></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + invertebrate, compared to tube of central nervous system of vertebrate, <a + href="#page43">43</a>, <a href="#page433">433</a></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + innervation of, <a href="#page447">447</a></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + origin of, <a href="#page444">444</a></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + position of vertebrate and invertebrate, <b><a href="#page10">10</a></b></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + possibility of formation of new, <a href="#page58">58</a></p> + <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span> + relationship between notochord and, <a href="#page434">434</a></p> + <p>Ammocœtes, <b><a href="#page32">32</a></b>, <b><a href="#page245">245</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> an ancestral type, <a + href="#page35">35</a>, <a href="#page309">309</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> alimentary canal, <a + href="#page168">168</a>, <a href="#page405">405</a>, <a href="#page445">445</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> auditory organ, <a + href="#page378">378</a>, <b><a href="#page379">379</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> brain, <a + href="#page39">39</a>, <b><a href="#page40">40</a></b>, <b><a href="#page41">41</a></b>, <a + href="#page45">45</a>, <b><a href="#page46">46</a></b>, <a href="#page48">48</a>, <b><a + href="#page54">54</a></b>, <a href="#page61">61</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> branchial appendages, <a + href="#page161">161</a>, <b><a href="#page162">162</a></b>, <b><a href="#page163">163</a></b>, + <b><a href="#page164">164</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> basket-work, <b><a href="#page126">126</a></b>, <b><a + href="#page128">128</a></b>, <b><a href="#page296">296</a></b>, <a href="#page331">331</a>, + <b><a href="#page335">335</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> chamber, <a href="#page161">161</a>, <a href="#page168">168</a>, <b><a + href="#page162">162</a></b>, <b><a href="#page163">163</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> circulation in Limulus and, <a href="#page174">174</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> diaphragms, <a href="#page161">161</a>, <a href="#page167">167</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> lamellæ, <b><a href="#page175">175</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> muscles, <a href="#page171">171</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> nerves, <a href="#page164">164</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, bra</span>"<span + class="hid">chial</span> segments, <b><a href="#page178">178</a></b>, <b><a + href="#page312">312</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> cartilage, hard, <a + href="#page133">133</a>, <b><a href="#page133">133</a></b>, <b><a href="#page293">293</a></b>, + <b><a href="#page294">294</a></b>, <a href="#page377">377</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, cart</span>"<span + class="hid">lage,</span> muco, <a href="#page130">130</a>, <b><a href="#page131">131</a></b>, <a + href="#page291">291</a>, <b><a href="#page293">293</a></b>, <b><a href="#page294">294</a></b>, + <b><a href="#page296">296</a></b>, <b><a href="#page330">330</a></b>, <a + href="#page331">331</a>, <b><a href="#page333">333</a></b>, <b><a href="#page334">334</a></b>, + <b><a href="#page335">335</a></b>, <b><a href="#page338">338</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, cart</span>"<span + class="hid">lage,</span> soft, <a href="#page129">129</a>, <b><a href="#page130">130</a></b>, + <b><a href="#page293">293</a></b>, <b><a href="#page294">294</a></b>, <b><a + href="#page296">296</a></b>, <b><a href="#page335">335</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> degeneracy, evidence of, + <a href="#page59">59</a>, <a href="#page94">94</a>, <a href="#page343">343</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> development, <a + href="#page228">228</a>, <a href="#page458">458</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> digestion, <a + href="#page58">58</a>, <a href="#page442">442</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> epithelial cells of + gills, <b><a href="#page214">214</a></b></p> + <p><span class="pagenum" id="page518">{518}</span><span class="hid">Ammo</span>"<span + class="hid">œtes,</span> epithelial cells of skin, <b><a href="#page347">347</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, epit</span>"<span + class="hid">elial</span> pits, <a href="#page173">173</a>, <b><a href="#page200">200</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> eye, <a + href="#page93">93</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, e</span>"<span + class="hid">e,</span> muscles, <a href="#page267">267</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, e</span>"<span + class="hid">e,</span> median or pineal, <a href="#page63">63</a>, <a href="#page75">75</a>, <a + href="#page76">76</a>, <b><a href="#page77">77</a></b>, <b><a href="#page78">78</a></b>, <a + href="#page80">80</a>, <b><a href="#page85">85</a></b>, <a href="#page86">86</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, e</span>"<span class="hid">e, + me</span>"<span class="hid">ian or pi</span>"<span class="hid">eal</span> left, <a + href="#page78">78</a>, <b><a href="#page79">79</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> fat-column, <a + href="#page181">181</a>, <b><a href="#page182">182</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, f</span>"<span + class="hid">t</span> in degenerated muco-cartilage, <b><a href="#page333">333</a></b>, <b><a + href="#page334">334</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> ganglia in embryo, <b><a + href="#page229">229</a></b>, <b><a href="#page283">283</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> gland-tissue round the + brain, <a href="#page209">209</a>, <b><a href="#page210">210</a></b>, <b><a + href="#page379">379</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> head-region, <b><a + href="#page128">128</a></b>, <b><a href="#page162">162</a></b>, <b><a + href="#page163">163</a></b>, <b><a href="#page193">193</a></b>, <b><a + href="#page293">293</a></b>, <b><a href="#page294">294</a></b>, <b><a + href="#page296">296</a></b>, <b><a href="#page298">298</a></b>, <b><a + href="#page335">335</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> head-shield, <a + href="#page329">329</a>, <a href="#page331">331</a>, <b><a href="#page338">338</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> liver, <a + href="#page442">442</a>, <a href="#page452">452</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> lymphatic glandular + tissue, <a href="#page426">426</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> Müllerian fibres, <a + href="#page489">489</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> muscles, eye, <a + href="#page173">173</a>, <a href="#page267">267</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, mu</span>"<span + class="hid">cles,</span> lip, lower, <a href="#page297">297</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, mu</span>"<span + class="hid">cles, li</span>"<span class="hid">,|</span> upper, <a href="#page305">305</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, mu</span>"<span + class="hid">cles,|</span> respiratory, <a href="#page171">171</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, mu</span>"<span + class="hid">cles,|</span> somatic, <a href="#page332">332</a>, <a href="#page336">336</a>, <a + href="#page409">409</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, mu</span>"<span + class="hid">cles,|</span> tubular, <a href="#page173">173</a>, <a href="#page298">298</a>, <b><a + href="#page309">309</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> nerves, cranial, <b><a + href="#page141">141</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, ne</span>"<span + class="hid">ves,</span> facial, <a href="#page186">186</a>, <b><a + href="#page311">311</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, ne</span>"<span + class="hid">ves,</span> glossopharyngeal, <a href="#page186">186</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, ne</span>"<span + class="hid">ves,</span> optic, <a href="#page105">105</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, ne</span>"<span + class="hid">ves,</span> trigeminal, <a href="#page282">282</a>, <a href="#page288">288</a>, + <b><a href="#page288">288</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, ne</span>"<span + class="hid">ves,</span> vagus, <a href="#page153">153</a>, <a href="#page173">173</a>, <a + href="#page186">186</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> nerve-fibres, medullation + of, <a href="#page20">20</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> notochord, <b><a + href="#page182">182</a></b>, <a href="#page435">435</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> olfactory tube, <a + href="#page219">219</a>, <a href="#page225">225</a>, <b><a href="#page227">227</a></b>, <a + href="#page317">317</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> oral chamber, <a + href="#page317">317</a>, <a href="#page243">243</a>, <a href="#page287">287</a>, <a + href="#page458">458</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> parasitism, <a + href="#page60">60</a>, <a href="#page286">286</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> pituitary, <b><a + href="#page321">321</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> prosomatic region, <a + href="#page243">243</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> pronephric duct, <a + href="#page402">402</a>, <a href="#page405">405</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> relationship to + Ostracodermata, <a href="#page326">326</a>, <a href="#page338">338</a>, <a + href="#page344">344</a>, <a href="#page414">414</a>, <a href="#page416">416</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> retina, <a + href="#page93">93</a>, <b><a href="#page111">111</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> skin, <a + href="#page58">58</a>, <a href="#page346">346</a>, <b><a href="#page348">348</a></b>, <a + href="#page442">442</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> skeleton, <a + href="#page125">125</a>, <b><a href="#page126">126</a></b>, <a href="#page132">132</a>, <a + href="#page291">291</a>, <b><a href="#page296">296</a></b>, <b><a + href="#page335">335</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> segments, comparison with + segments of Eurypterus, <a href="#page323">323</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, segm</span>"<span + class="hid">nts,</span> facial, <b><a href="#page201">201</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, segm</span>"<span + class="hid">nts,</span> hyoid, <a href="#page186">186</a>, <b><a href="#page201">201</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes, segm</span>"<span + class="hid">nts,</span> prosomatic, <a href="#page286">286</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> septa between myomeres, + <b><a href="#page416">416</a></b></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> tentacles of upper lip, + <a href="#page303">303</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> test, biological, to show + relationship with Limulus, <a href="#page493">493</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> thyroid, <a + href="#page192">192</a>, <b><a href="#page194">194</a></b>, <b><a href="#page196">196</a></b>, + <b><a href="#page205">205</a></b>, <a href="#page213">213</a>, <a href="#page430">430</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> transformation, <a + href="#page18">18</a>, <a href="#page59">59</a>, <a href="#page125">125</a>, <a + href="#page168">168</a>, <a href="#page193">193</a>, <a href="#page199">199</a>, <a + href="#page200">200</a>, <a href="#page220">220</a>, <a href="#page227">227</a>, <a + href="#page228">228</a>, <a href="#page287">287</a>, <a href="#page291">291</a>, <a + href="#page304">304</a>, <a href="#page307">307</a>, <a href="#page309">309</a>, <a + href="#page331">331</a>, <a href="#page336">336</a>, <a href="#page347">347</a>, <a + href="#page349">349</a>, <a href="#page389">389</a>, <a href="#page445">445</a></p> + <p><span class="hid">Ammo</span>"<span class="hid">œtes,</span> velum, <a + href="#page228">228</a>, <a href="#page289">289</a>, <a href="#page298">298</a>, <b><a + href="#page302">302</a></b></p> + <p>Amœbocytes, <a href="#page473">473</a></p> + <p>Amphibia, <a href="#page23">23</a>, <a href="#page345">345</a></p> + <p><span class="pagenum" id="page519">{519}</span>Amphioxus, <a href="#page33">33</a>, <a + href="#page407">407</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> atrial cavity, <a + href="#page409">409</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> branchial nephric glands, <a + href="#page426">426</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> endostyle, <a + href="#page198">198</a>, <a href="#page212">212</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> excretory organs, <a + href="#page389">389</a>, <a href="#page395">395</a>, <a href="#page477">477</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> neuropore, <a + href="#page220">220</a>, <a href="#page457">457</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> notochord, <a + href="#page435">435</a>, <a href="#page436">436</a>, <a href="#page443">443</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> pleural folds, <b><a + href="#page495">495</a></b></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> septa between myomeres, <b><a + href="#page416">416</a></b></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> somatic muscles, <a + href="#page409">409</a></p> + <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> yolk, <a + href="#page485">485</a></p> + <p>Androctonus, <a href="#page53">53</a>, <b><a href="#page54">54</a></b>, <b><a + href="#page372">372</a></b>, <a href="#page423">423</a></p> + <p>Annelids, lateral sense-organs, <a href="#page357">357</a>, <a href="#page367">367</a></p> + <p><span class="hid">Ann</span>"<span class="hid">lids,</span> nephric organs, <a + href="#page390">390</a></p> + <p><span class="hid">Ann</span>"<span class="hid">lids,</span> rigin of Arthropods from, <a + href="#page395">395</a></p> + <p><span class="hid">Ann</span>"<span class="hid">lids,</span> parapodal ganglia, <a + href="#page283">283</a></p> + <p><span class="hid">Ann</span>"<span class="hid">lids,</span> phagocytic glands, <a + href="#page421">421</a></p> + <p>Anthozoa, <a href="#page474">474</a></p> + <p>Antiarcha, <a href="#page29">29</a>, <a href="#page326">326</a>, <a + href="#page343">343</a></p> + <p>Antibody, <a href="#page492">492</a></p> + <p>Antitoxin, <a href="#page492">492</a></p> + <p>Anus, <a href="#page43">43</a>, <a href="#page457">457</a></p> + <p>Aponeuroses, <a href="#page327">327</a>, <a href="#page342">342</a>, <a + href="#page414">414</a></p> + <p>Apparatus, auditory, <a href="#page355">355</a></p> + <p><span class="hid">Appa</span>"<span class="hid">rtus,</span> dioptric, <a + href="#page83">83</a></p> + <p><span class="hid">Appa</span>"<span class="hid">rtus,</span> respiratory, <a + href="#page148">148</a></p> + <p><span class="hid">Appa</span>"<span class="hid">rtus,</span> suctorial, of Petromyzon, <a + href="#page287">287</a></p> + <p>Appendages, branchial, of Ammocœtes, <a href="#page161">161</a>, <b><a + href="#page162">162</a></b>, <b><a href="#page163">163</a></b>, <b><a + href="#page164">164</a></b></p> + <p><span class="hid">Appe</span>"<span class="hid">dages, bra</span>"<span class="hid">chial, + of</span> Limulus, <b><a href="#page164">164</a></b></p> + <p><span class="hid">Appe</span>"<span class="hid">dages, bra</span>"<span class="hid">chial, + of</span> internal, <a href="#page149">149</a></p> + <p><span class="hid">Appe</span>"<span class="hid">dages,</span> derivation of suctorial + apparatus of Petromyzon from, <a href="#page290">290</a></p> + <p><span class="hid">Appe</span>"<span class="hid">dages,</span> disappearance of, in + transformation of Arthropod into Vertebrate, <a href="#page386">386</a>, <b><a + href="#page413">413</a></b></p> + <p><span class="hid">Appe</span>"<span class="hid">dages,</span> evidence of, in prosomatic + region of ancient fishes, <a href="#page342">342</a></p> + <p><span class="hid">Appe</span>"<span class="hid">dages,</span> muscles, in Limulus and + Scorpion, <a href="#page247">247</a></p> + <p><span class="hid">Appe</span>"<span class="hid">dages,</span> prosomatic, of Gigantostraca, + <a href="#page234">234</a></p> + <p><span class="hid">Appe</span>"<span class="hid">dages,</span> Trilobites, <a + href="#page351">351</a></p> + <p>Apus, <b><a href="#page28">28</a></b>, <a href="#page137">137</a>, <a + href="#page436">436</a>, <b><a href="#page437">437</a></b></p> + <p>Arachnids, eyes, <a href="#page75">75</a>, <a href="#page87">87</a></p> + <p><span class="hid">Arac</span>"<span class="hid">nids,</span> diverticula of stomach, <a + href="#page109">109</a></p> + <p><span class="hid">Arac</span>"<span class="hid">nids,</span> lyriform organs, <a + href="#page364">364</a>, <a href="#page368">368</a></p> + <p><span class="hid">Arac</span>"<span class="hid">nids,</span> segmental excretory organs, <a + href="#page423">423</a></p> + <p>Archæocytes, <a href="#page473">473</a></p> + <p>Artemia, <i>v.</i> Branchipus</p> + <p>Arthropleura, <a href="#page249">249</a></p> + <p>Arthropoda, arrangement of organs, <b><a href="#page10">10</a></b></p> + <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> evolution, <a + href="#page11">11</a></p> + <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> excretory organs, <a + href="#page396">396</a>, <a href="#page418">418</a></p> + <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> eyes, <a href="#page75">75</a>, + <a href="#page89">89</a></p> + <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> giant-fibres, <a + href="#page489">489</a></p> + <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> musculature, <a + href="#page411">411</a></p> + <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> olfactory organs, <a + href="#page220">220</a></p> + <p><span class="pagenum" id="page520">{520}</span><span class="hid">Arthr</span>"<span + class="hid">poda,</span> resemblance to ancient fishes, <a href="#page29">29</a></p> + <p>Astacus, brain, <b><a href="#page54">54</a></b></p> + <p><span class="hid">Ast</span>"<span class="hid">cus,</span> digestive ferment in cells lining + the carapace, <a href="#page442">442</a></p> + <p><span class="hid">Ast</span>"<span class="hid">cus,</span> optic chiasma, <a + href="#page101">101</a></p> + <p><span class="hid">Ast</span>"<span class="hid">cus,</span> optic stalk, <a + href="#page91">91</a></p> + <p><span class="hid">Ast</span>"<span class="hid">cus,</span> etina, <a + href="#page98">98</a></p> + <p>Asterolepis, <a href="#page326">326</a>, <a href="#page342">342</a></p> + <p>Atrium, <a href="#page410">410</a></p> + <p>Auchenaspis (Thyestes), <b><a href="#page30">30</a></b>, <b><a href="#page31">31</a></b>, <a + href="#page75">75</a>, <a href="#page275">275</a>, <a href="#page326">326</a>, <a + href="#page327">327</a>, <b><a href="#page328">328</a></b>, <b><a + href="#page338">338</a></b></p> + <p>Auditory apparatus, <a href="#page355">355</a></p> + <p>Auerbach, plexus of, <a href="#page447">447</a></p> + <p>Aurelia, <a href="#page475">475</a></p> + <p>Autonomic nerves, <a href="#page3">3</a></p> + <p class="stanza">Balanoglossus, <a href="#page12">12</a>, <b><a href="#page12">12</a></b>, <a + href="#page433">433</a>, <a href="#page438">438</a>, <a href="#page494">494</a></p> + <p>Bdellostoma, <a href="#page394">394</a>, <a href="#page405">405</a></p> + <p>Belinurus, <a href="#page24">24</a>, <a href="#page249">249</a>, <a + href="#page351">351</a></p> + <p>Bird, rhomboidal sinus, <b><a href="#page46">46</a></b></p> + <p>Bladder, <a href="#page449">449</a></p> + <p><span class="hid">Bla</span>"<span class="hid">der,</span> swim, <a + href="#page148">148</a></p> + <p>Blastula, <a href="#page459">459</a>, <a href="#page471">471</a>, <a + href="#page473">473</a></p> + <p>Blood, <a href="#page463">463</a>, <a href="#page472">472</a>, <a href="#page474">474</a></p> + <p><span class="hid">Bl</span>"<span class="hid">od,</span> circulation, in Ammocœtes and + Limulus, <a href="#page174">174</a></p> + <p><span class="hid">Bl</span>"<span class="hid">od,</span> secretion of ductless glands into, + <a href="#page418">418</a></p> + <p>Bothriolepis, <a href="#page29">29</a>, <b><a href="#page32">32</a></b>, <a + href="#page239">239</a>, <a href="#page326">326</a>, <a href="#page351">351</a>, <a + href="#page450">450</a></p> + <p>Bone, <a href="#page344">344</a>, <a href="#page474">474</a>, <a href="#page481">481</a></p> + <p>Brain, Ammocœtes and Arthropod, <b><a href="#page54">54</a></b>, <a + href="#page61">61</a></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> and brain-case of Ammocœtes, + <b><a href="#page40">40</a></b>, <b><a href="#page41">41</a></b>, <b><a + href="#page46">46</a></b>, <a href="#page209">209</a></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> caudal, of Thelyphonus, <a + href="#page450">450</a></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> epithelial lining of, <a + href="#page38">38</a></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> roof, <a href="#page39">39</a></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> Sphæroma serratum, <b><a + href="#page62">62</a></b>, <b><a href="#page90">90</a></b></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> Thelyphonus, <b><a + href="#page56">56</a></b></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> ventricles, <a + href="#page4">4</a></p> + <p><span class="hid">Br</span>"<span class="hid">in,</span> vesicles, <a + href="#page48">48</a></p> + <p>Branchial basket-work of Ammocœtes, <b><a href="#page126">126</a></b>, <b><a + href="#page128">128</a></b>, <b><a href="#page296">296</a></b>, <a href="#page331">331</a>, + <b><a href="#page335">335</a></b></p> + <p>Branchipus, <b><a href="#page28">28</a></b></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> brain, <a + href="#page51">51</a>, <b><a href="#page54">54</a></b></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> eyes, lateral, <a + href="#page88">88</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus, e</span>"<span class="hid">es, + lat</span>"<span class="hid">ral,</span> retina of, <b><a href="#page91">91</a></b>, <a + href="#page97">97</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus, e</span>"<span class="hid">es,</span> + median, <a href="#page75">75</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> excretory organs, <a + href="#page396">396</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> (Artemia) diverticula of gut + and retinal ganglion, <b><a href="#page110">110</a></b>, <b><a href="#page111">111</a></b>, <a + href="#page113">113</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> nerves of appendages, <a + href="#page157">157</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> segmentation, <a + href="#page159">159</a></p> + <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> resemblance to Trilobite, <a + href="#page436">436</a></p> + <p>Bunodes, <a href="#page24">24</a>, <b><a href="#page30">30</a></b>, <a + href="#page249">249</a>, <b><a href="#page341">341</a></b>, <a href="#page351">351</a>, <a + href="#page414">414</a></p> + <p>Bundle of Meynert, <a href="#page48">48</a>, <a href="#page77">77</a></p> + <p>Bundles, posterior longitudinal, <a href="#page489">489</a></p> + <p>Buthus, muscles, <a href="#page270">270</a></p> + <p class="stanza">Calcification in aponeuroses of Cephalaspis, <a href="#page414">414</a></p> + <p><span class="pagenum" id="page521">{521}</span><span class="hid">Calcifi</span>"<span + class="hid">ation</span> cartilage, <a href="#page140">140</a>,<a href="#page330">330</a></p> + <p><span class="hid">Calcifi</span>"<span class="hid">ation</span> successive layers of the + skin, <a href="#page348">348</a></p> + <p>Camerostome, <a href="#page221">221</a>, <b><a href="#page222">222</a></b>, <b><a + href="#page223">223</a></b>, <a href="#page224">224</a>, <a href="#page241">241</a>, <a + href="#page271">271</a></p> + <p>Canal, alimentary, formation of vertebrate, <a href="#page58">58</a>, <a + href="#page433">433</a>, <a href="#page446">446</a></p> + <p><span class="hid">Ca</span>"<span class="hid">al, alime</span>"<span class="hid">tary,</span> + innervation, <a href="#page447">447</a></p> + <p><span class="hid">Ca</span>"<span class="hid">al, alime</span>"<span class="hid">tary,</span> + relationships between notochord and, <a href="#page434">434</a></p> + <p><span class="hid">Ca</span>"<span class="hid">al, alime</span>"<span class="hid">tary,</span> + origin, <a href="#page444">444</a></p> + <p><span class="hid">Ca</span>"<span class="hid">al,</span> Haversian, <a + href="#page329">329</a></p> + <p><span class="hid">Ca</span>"<span class="hid">al,</span> central, of spinal cord, <a + href="#page405">405</a>, <a href="#page439">439</a>, <a href="#page455">455</a></p> + <p><span class="hid">Ca</span>"<span class="hid">al,</span> spinal, <b><a + href="#page182">182</a></b></p> + <p>Capsule, auditory, <a href="#page377">377</a>, <b><a href="#page379">379</a></b></p> + <p>Cartilage Ammocœtes, muco, <a href="#page127">127</a>, <a href="#page130">130</a>, + <b><a href="#page131">131</a></b>, <a href="#page200">200</a>, <a href="#page291">291</a>, <a + href="#page303">303</a>, <a href="#page330">330</a>, <b><a href="#page333">333</a></b>, <b><a + href="#page334">334</a></b>, <a href="#page344">344</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge Ammo</span>"<span + class="hid">œtes,</span> hard, <a href="#page133">133</a>, <b><a + href="#page133">133</a></b>, <a href="#page377">377</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge Ammo</span>"<span + class="hid">œtes,</span> soft, <a href="#page126">126</a>, <a href="#page129">129</a>, + <b><a href="#page130">130</a></b></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge Ammo</span>"<span + class="hid">œtes,</span> spinal cartilages, <a href="#page414">414</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge</span> Hypoctonus, <b><a + href="#page133">133</a></b>, <a href="#page142">142</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge</span> Limulus, hard, <a + href="#page142">142</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge Lim</span>"<span class="hid">lus,</span> + muco, <a href="#page139">139</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge Lim</span>"<span class="hid">lus,</span> + soft, <a href="#page20">20</a>, <b><a href="#page130">130</a></b>, <a + href="#page137">137</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge</span> origin, <a + href="#page474">474</a>, <a href="#page481">481</a></p> + <p><span class="hid">Cart</span>"<span class="hid">ilge</span> staining reactions, <a + href="#page131">131</a>, <a href="#page133">133</a>, <a href="#page139">139</a>, <a + href="#page330">330</a>, <a href="#page336">336</a></p> + <p>Cavity, atrial, <a href="#page409">409</a>, <b><a href="#page413">413</a></b></p> + <p><span class="hid">Cai</span>"<span class="hid">ty,</span> cœlomic, <a + href="#page167">167</a>, <a href="#page251">251</a>, <a href="#page266">266</a>, <a + href="#page320">320</a>, <a href="#page389">389</a>, <b><a href="#page391">391</a></b>, <b><a + href="#page408">408</a></b>, <a href="#page422">422</a>, <a href="#page430">430</a>, <a + href="#page472">472</a></p> + <p>Cells, free-living, <a href="#page463">463</a></p> + <p>Centre, vaso-motor, <a href="#page468">468</a></p> + <p>Cephalaspis, diverticula of gut, <a href="#page109">109</a></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> eyes, lateral, <a + href="#page75">75</a>, <a href="#page275">275</a></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis, e</span>"<span class="hid">es,</span> + median, <a href="#page75">75</a></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> head-shield, <a + href="#page327">327</a>, <b><a href="#page328">328</a></b>, <b><a href="#page330">330</a></b>, + <b><a href="#page338">338</a></b></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> muscles on head-shield, <b><a + href="#page269">269</a></b></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> resemblance to + Ammocœtes, <a href="#page145">145</a>, <a href="#page291">291</a>, <a + href="#page326">326</a>, <a href="#page329">329</a>, <b><a href="#page338">338</a></b>, <a + href="#page348">348</a>, <a href="#page414">414</a></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis, resem</span>"<span class="hid">lance + to</span> Arthropod, <a href="#page29">29</a></p> + <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> segmentation, <a + href="#page339">339</a></p> + <p>Ceratodus, <a href="#page148">148</a></p> + <p>Cephalization, <a href="#page51">51</a></p> + <p>Cephalodiscus, <a href="#page438">438</a></p> + <p>Cephalopod, <a href="#page23">23</a></p> + <p>Cerebellum, <a href="#page47">47</a>, <b><a href="#page50">50</a></b></p> + <p>Chætopoda, <a href="#page395">395</a></p> + <p>Chamber, oral, of Ammocœtes, <a href="#page243">243</a>, <a href="#page287">287</a>, + <a href="#page458">458</a></p> + <p>Cheliceræ, <a href="#page235">235</a></p> + <p>Chiasma, optic, <a href="#page101">101</a></p> + <p>Chilaria, <a href="#page235">235</a>, <a href="#page238">238</a>, <a href="#page291">291</a>, + <a href="#page301">301</a>, <a href="#page458">458</a></p> + <p>Chitin, <a href="#page85">85</a>, <a href="#page119">119</a>, <a href="#page139">139</a>, <a + href="#page205">205</a>, <a href="#page206">206</a>, <a href="#page302">302</a>, <a + href="#page329">329</a>, <a href="#page346">346</a>, <a href="#page359">359</a>, <a + href="#page440">440</a>, <a href="#page443">443</a></p> + <p>Cilia, <a href="#page206">206</a></p> + <p>Circulation, branchial, <a href="#page174">174</a></p> + <p>Cirri, <a href="#page357">357</a></p> + <p>Clarke's column, <a href="#page467">467</a></p> + <p>Clepsine, nephridial glands, <a href="#page423">423</a></p> + <p>Cochlea, <a href="#page378">378</a></p> + <p>Cœlenterata, <a href="#page465">465</a>, <a href="#page472">472</a></p> + <p>Cœlolepidæ, <a href="#page344">344</a></p> + <p><span class="pagenum" id="page522">{522}</span>Cœlom, <a href="#page167">167</a>, <a + href="#page251">251</a>, <a href="#page400">400</a>, <a href="#page472">472</a>, <a + href="#page481">481</a></p> + <p>Cœlomata, <a href="#page472">472</a></p> + <p>Cœlomocœla, <a href="#page472">472</a>, <a href="#page475">475</a></p> + <p>Cœlomostomes, <a href="#page477">477</a>, <a href="#page481">481</a></p> + <p>Colleneytes, <a href="#page474">474</a></p> + <p>Commissure, anterior, <a href="#page49">49</a></p> + <p><span class="hid">Comm</span>"<span class="hid">ssure,</span> œsophageal, <a + href="#page14">14</a></p> + <p><span class="hid">Comm</span>"<span class="hid">ssure,</span> posterior, <a + href="#page48">48</a>, <a href="#page280">280</a></p> + <p>Comparison of brains of Ammocœtes and Arthropod, <a href="#page61">61</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of br</span>"<span class="hid">ins + of</span> invertebrate from Branchipus to Ammocœtes, <b><a href="#page54">54</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of br</span>"<span class="hid">ins + of</span> vertebrate, <b><a href="#page40">40</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> branchial circulation in + Ammocœtes and Limulus, <a href="#page174">174</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of bran</span>"<span + class="hid">hial</span> lamellæ of Scorpion and Ammocœtes, <b><a + href="#page175">175</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of bran</span>"<span + class="hid">hial</span> segments of Ammocœtes and Petromyzon, <b><a + href="#page169">169</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> Cephalaspidian and + Palæostracan fish, <a href="#page31">31</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> Cœlom of Peripatus and + Vertebrate, <a href="#page400">400</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> dermal covering of Pteraspis + with chitin of Limulus or dentine of fish scales, <a href="#page346">346</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> entosternite or plastron of + Limulus with trabeculæ of Ammocœtes, <a href="#page145">145</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> excretory organs of + vertebrates and invertebrates, <a href="#page389">389</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> gut of Arthropod and tube of + central nervous system of Vertebrate, <a href="#page43">43</a>, <a href="#page244">244</a>, <a + href="#page433">433</a>, <a href="#page440">440</a>, <a href="#page455">455</a>, <a + href="#page457">457</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> head-shield of Cephalaspis + and Ammocœtes, <a href="#page291">291</a>, <a href="#page329">329</a>, <b><a + href="#page338">338</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> hypophysial tube with + olfactory tube of Arthropod ancestor, <a href="#page229">229</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of hypop</span>"<span class="hid">ysial + t</span>"<span class="hid">be</span> with position of palæostoma, <a href="#page317">317</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> mesosomatic region of + Ammocœtes and Eurypterus, <a href="#page192">192</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> muscles, branchial, of + Ammocœtes and appendage muscles of Scorpion, <a href="#page171">171</a>, <a + href="#page447">447</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of mu</span>"<span + class="hid">cles,</span> eye, of Vertebrate with dorso-ventral muscles of Scorpion, <a + href="#page267">267</a>, <a href="#page272">272</a>, <a href="#page459">459</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of mu</span>"<span + class="hid">cles,</span> of oral chamber of Ammocœtes and prosomatic musculature of + Limulus, <a href="#page247">247</a>, <a href="#page447">447</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of mu</span>"<span + class="hid">cles,</span> longitudinal body-muscles of Vertebrate and dorsal longitudinal muscles + of Arthropod, <a href="#page411">411</a>, <a href="#page447">447</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> nerves, appendage of Limulus + and Branchipus to lateral root system of Vertebrate, <a href="#page157">157</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of ne</span>"<span + class="hid">ves,</span> cranial and spinal segmental, <a href="#page152">152</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> nervous systems of Vertebrate + and Arthropod, <a href="#page36">36</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> pineal gland of vertebrates + and median eyes of Arthropod, <a href="#page63">63</a>, <a href="#page456">456</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> pituitary body and coxal + glands, <a href="#page246">246</a>, <a href="#page319">319</a>, <b><a + href="#page321">321</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> prosoma and mesosoma of + Limulus and Ammocœtes, <b><a href="#page140">140</a></b>, <b><a + href="#page141">141</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> prosomatic region of + Ammocœtes and Eurypterus, <a href="#page244">244</a>, <b><a + href="#page333">333</a></b></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> retina in Ammocœtes + and Musca, <a href="#page97">97</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of re</span>"<span + class="hid">ina</span> compound in Arthropod and Vertebrate, <a href="#page87">87</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> skeleton of Limulus and + Ammocœtes, <b><a href="#page126">126</a></b>, <a href="#page136">136</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> sense-organs of Arthropod + appendages with auditory organs of Vertebrate, <a href="#page375">375</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of</span> thyroid with endostyle, <a + href="#page198">198</a></p> + <p><span class="hid">Comp</span>"<span class="hid">rison of thy</span>"<span class="hid">oid + w</span>"<span class="hid">th</span> uterus of Scorpion, <b><a href="#page205">205</a></b></p> + <p>Corneagen, <a href="#page69">69</a></p> + <p>Corpora quadrigemina, <a href="#page47">47</a></p> + <p><span class="pagenum" id="page523">{523}</span>Corpuscles, Pacinian, Herbst, Grandry, etc., + <a href="#page470">470</a></p> + <p>Coxal glands, <a href="#page242">242</a>, <a href="#page246">246</a>, <a + href="#page319">319</a>, <b><a href="#page321">321</a></b>, <a href="#page389">389</a>, <a + href="#page398">398</a>, <a href="#page403">403</a>, <a href="#page429">429</a></p> + <p>Cranium, <a href="#page121">121</a>, <a href="#page145">145</a>, <a + href="#page339">339</a></p> + <p>Crayfish, <a href="#page442">442</a>, <a href="#page489">489</a></p> + <p>Crest, neural, <a href="#page281">281</a></p> + <p>Cromatophores of frog, <a href="#page470">470</a></p> + <p>Crura cerebri, <a href="#page14">14</a></p> + <p>Crustacea, first appearance, <a href="#page27">27</a></p> + <p><span class="hid">Crust</span>"<span class="hid">cea,</span> eyes, <a href="#page76">76</a>, + <a href="#page87">87</a></p> + <p><span class="hid">Crust</span>"<span class="hid">cea,</span> retina, <b><a + href="#page100">100</a></b></p> + <p><span class="hid">Crust</span>"<span class="hid">cea,</span> segmental glands, <a + href="#page422">422</a></p> + <p>Ctenophora, <a href="#page474">474</a></p> + <p>Cyathaspis, <a href="#page29">29</a>, <a href="#page326">326</a>, <b><a + href="#page340">340</a></b>, <a href="#page343">343</a></p> + <p>Cyclostomata, <a href="#page165">165</a>, <a href="#page229">229</a>, <a + href="#page343">343</a>, <a href="#page353">353</a>, <a href="#page424">424</a></p> + <p>Cysts, <a href="#page50">50</a></p> + <p class="stanza">Daphnia, <a href="#page112">112</a></p> + <p>Degeneration, <a href="#page17">17</a>, <a href="#page19">19</a>, <a href="#page59">59</a>, + <a href="#page74">74</a>, <a href="#page78">78</a>, <a href="#page94">94</a>, <a + href="#page107">107</a>, <a href="#page212">212</a>, <a href="#page309">309</a>, <a + href="#page333">333</a>, <a href="#page336">336</a>, <a href="#page343">343</a></p> + <p>Deiters' nucleus, <a href="#page489">489</a></p> + <p>Dendrites, <a href="#page72">72</a></p> + <p>Development, parallel, <a href="#page497">497</a></p> + <p><span class="hid">Devel</span>"<span class="hid">pment,</span> of two types of eye, <a + href="#page73">73</a></p> + <p><span class="hid">Devel</span>"<span class="hid">pment,</span> vertebrate retina, <a + href="#page101">101</a></p> + <p>Diaphragms, <a href="#page161">161</a>, <a href="#page167">167</a></p> + <p>Didymaspis, <a href="#page327">327</a>, <b><a href="#page338">338</a></b></p> + <p>Digestion, <a href="#page441">441</a></p> + <p>Dinosaurs, <a href="#page17">17</a></p> + <p>Dipnoans, <a href="#page23">23</a>, <a href="#page45">45</a>, <a href="#page148">148</a></p> + <p>Diptera, <a href="#page89">89</a>, <a href="#page369">369</a></p> + <p>Diverticula, optic, <a href="#page102">102</a></p> + <p>Dogfish, skull, <b><a href="#page121">121</a></b>, <b><a href="#page123">123</a></b></p> + <p>Drepanaspis, <a href="#page344">344</a>, <b><a href="#page345">345</a></b>, <a + href="#page450">450</a></p> + <p>Drepanopterus Bembycoides, <a href="#page238">238</a></p> + <p class="stanza">Ectognath, <a href="#page238">238</a>, <a href="#page242">242</a>, <a + href="#page271">271</a>, <a href="#page304">304</a>, <a href="#page342">342</a>, <b><a + href="#page381">381</a></b></p> + <p>Eel, <a href="#page488">488</a></p> + <p>Elasmobranchs, <a href="#page23">23</a>, <a href="#page343">343</a>, <a + href="#page423">423</a></p> + <p>Elastin, <a href="#page435">435</a></p> + <p>Embryo, head of dogfish, <b><a href="#page121">121</a></b>, <b><a + href="#page123">123</a></b></p> + <p><span class="hid">Em</span>"<span class="hid">ryo,</span> skull of pig, <b><a + href="#page121">121</a></b></p> + <p>Embryology, principles of, <a href="#page455">455</a></p> + <p>Encepalomeres, <a href="#page262">262</a></p> + <p>Endognath, <a href="#page238">238</a>, <a href="#page271">271</a>, <a + href="#page304">304</a>, <b><a href="#page381">381</a></b></p> + <p>Endostoma, <a href="#page241">241</a>, <a href="#page306">306</a></p> + <p>Endostyle, <a href="#page198">198</a>, <a href="#page212">212</a></p> + <p>Entapophysis of Limulus, <a href="#page139">139</a></p> + <p>Enterocœla, <a href="#page472">472</a></p> + <p>Enteropneusta, <a href="#page438">438</a>, <a href="#page494">494</a></p> + <p>Entochondrites, <a href="#page377">377</a></p> + <p>Entosclerite, <a href="#page222">222</a>, <a href="#page271">271</a></p> + <p>Entosternite, <b><a href="#page143">143</a></b></p> + <p>Epiblast, <a href="#page444">444</a>, <a href="#page445">445</a>, <a + href="#page459">459</a></p> + <p><span class="pagenum" id="page524">{524}</span>Epithelium cells of Ammocœtes, <b><a + href="#page347">347</a></b></p> + <p><span class="hid">Epith</span>"<span class="hid">lium</span> of central nervous system of + vertebrates, <a href="#page38">38</a>, <a href="#page457">457</a></p> + <p><span class="hid">Epith</span>"<span class="hid">lium of</span> cœlomic spaces in + annelids, <a href="#page421">421</a></p> + <p><span class="hid">Epith</span>"<span class="hid">lium of</span> optic diverticula, <a + href="#page103">103</a></p> + <p><span class="hid">Epith</span>"<span class="hid">lium of</span> peritoneal, pleural, and + pericardial cavities, <a href="#page477">477</a></p> + <p><span class="hid">Epith</span>"<span class="hid">lium of</span> velum of Ammocœtes, <a + href="#page301">301</a>, <b><a href="#page302">302</a></b></p> + <p>Equilibration, <a href="#page358">358</a></p> + <p>Eukeraspis, <a href="#page326">326</a></p> + <p>Eurypterus, <b><a href="#page26">26</a></b>, <b><a href="#page150">150</a></b>, <b><a + href="#page191">191</a></b>, <b><a href="#page237">237</a></b></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> appendages, <a + href="#page150">150</a>, <a href="#page236">236</a>, <b><a href="#page237">237</a></b></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> classification, <a + href="#page249">249</a></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> comparison with + Ammocœtes, <a href="#page170">170</a>, <a href="#page323">323</a></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> diagram of sagittal median + section, <b><a href="#page240">240</a></b>, <b><a href="#page245">245</a></b></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> endostoma, <a + href="#page241">241</a>, <a href="#page306">306</a></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> eyes, <a + href="#page275">275</a></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> mesosomatic segments, <b><a + href="#page192">192</a></b></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> muscles of carapace, <a + href="#page269">269</a></p> + <p><span class="hid">Eury</span>"<span class="hid">terus,</span> operculum, <b><a + href="#page150">150</a></b>, <b><a href="#page190">190</a></b>, <a href="#page212">212</a></p> + <p>Evidence of alimentary canal, innervation, <a href="#page446">446</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> auditory apparatus and lateral + line organs, <a href="#page355">355</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> cœlomic cavities in + Limulus, <a href="#page251">251</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> degeneracy in Ammocœtes, + <a href="#page59">59</a>, <a href="#page94">94</a>, <a href="#page343">343</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> embryology, cartilage, <a + href="#page20">20</a>, <a href="#page129">129</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> eye-muscles, <a href="#page263">263</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> excretory organs, <a href="#page390">390</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> heart, <a href="#page179">179</a>, <a href="#page451">451</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> nervous system, central, cerebral vesicles, <a href="#page48">48</a>, + <a href="#page458">458</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy, + ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span + class="hid">tral,</span> epithelial tube, <a href="#page37">37</a>, <a href="#page42">42</a>, <a + href="#page102">102</a>, <a href="#page244">244</a>, <a href="#page433">433</a>, <a + href="#page455">455</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy, + ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span + class="hid">tral,</span> neurenteric canal, <a href="#page37">37</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy, + ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span + class="hid">tral,</span> neuropore, <a href="#page220">220</a>, <a href="#page457">457</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy, + ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span + class="hid">tral,</span> optic diverticula, <a href="#page102">102</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy, + ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span + class="hid">tral,</span> spinal cord, <a href="#page46">46</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> oral chamber, <a href="#page228">228</a>, <a href="#page242">242</a>, + <a href="#page243">243</a>, <a href="#page290">290</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> olfactory organ, <a href="#page220">220</a>, <b><a + href="#page227">227</a></b></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> palæostoma or old mouth, <a href="#page317">317</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> pineal or median eyes, <a href="#page15">15</a>, <a + href="#page63">63</a>, <a href="#page74">74</a>, <a href="#page456">456</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> pituitary body and coxal glands, <a href="#page246">246</a>, <a + href="#page319">319</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> thyroid, <a href="#page192">192</a>, <b><a + href="#page194">194</a></b></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> segmentation, double, of head, <a href="#page157">157</a>, <a + href="#page234">234</a>, <a href="#page258">258</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span + class="hid">logy,</span> skeleton, cranial, <a href="#page120">120</a>, <a + href="#page153">153</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> nervous system, central, <a + href="#page8">8</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> notochord, origin from + segmented region, <a href="#page443">443</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> olfactory apparatus, <a + href="#page218">218</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> organs of vision, <a + href="#page68">68</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> palæontology, <a + href="#page20">20</a>, <a href="#page497">497</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> pineal or median eyes, <a + href="#page74">74</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> prosomatic musculature, <a + href="#page247">247</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> respiratory apparatus, <a + href="#page148">148</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> segmentation in head-shield, <a + href="#page339">339</a></p> + <p><span class="hid">Evide</span>"<span class="hid">ce of</span> skeleton, <a + href="#page119">119</a></p> + <p>Evolution, <a href="#page8">8</a>, <a href="#page15">15</a>, <a href="#page20">20</a>, <a + href="#page149">149</a>, <a href="#page482">482</a>, <a href="#page497">497</a></p> + <p><span class="pagenum" id="page525">{525}</span><span class="hid">Evol</span>"<span + class="hid">tion,</span> of brain in brain-case, <a href="#page210">210</a></p> + <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> cranium of Vertebrate, <a + href="#page342">342</a></p> + <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> excretory organs, <a + href="#page389">389</a></p> + <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> eye of Vertebrate, <a + href="#page114">114</a></p> + <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> nervous system, central, <a + href="#page34">34</a></p> + <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> tissues, <a + href="#page19">19</a></p> + <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> Vertebrate from Balanoglossus + and Amphioxus, <a href="#page33">33</a></p> + <p>Eyes, <a href="#page68">68</a></p> + <p><span class="hid">Ey</span>"<span class="hid">s,</span> lateral, <a href="#page87">87</a>, + <b><a href="#page105">105</a></b>, <b><a href="#page108">108</a></b></p> + <p><span class="hid">Ey</span>"<span class="hid">s,</span> median or pineal, <a + href="#page74">74</a>, <b><a href="#page77">77</a></b>, <b><a href="#page78">78</a></b>, <b><a + href="#page79">79</a></b></p> + <p class="stanza">Fat-cells in muco-cartilage, <a href="#page332">332</a></p> + <p>Fat-column of Ammocœtes, <a href="#page181">181</a>, <b><a + href="#page182">182</a></b></p> + <p>Fibres, Mauthnerian, <a href="#page488">488</a></p> + <p><span class="hid">Fib</span>"<span class="hid">rs,</span> Müllerian, of Ammocœtes + central nervous system, <a href="#page489">489</a></p> + <p><span class="hid">Fib</span>"<span class="hid">rs, Müll</span>"<span class="hid">rian, + of</span> retina, <a href="#page96">96</a>, <a href="#page107">107</a></p> + <p>Fishes, classification, <a href="#page218">218</a></p> + <p><span class="hid">Fis</span>"<span class="hid">es,</span> ancient, classification, <a + href="#page326">326</a>, <a href="#page343">343</a></p> + <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span> + cloacal region, <a href="#page450">450</a></p> + <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span> + dominance, <a href="#page23">23</a></p> + <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span> + eyes, <a href="#page75">75</a></p> + <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span> + head-shields. <i>See</i> Head-shields</p> + <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span> + pleural folds, <a href="#page414">414</a></p> + <p>Fissure, posterior, <a href="#page43">43</a></p> + <p>Fittest, survival of, <a href="#page16">16</a>, <a href="#page34">34</a></p> + <p>Flabellum, <a href="#page359">359</a>, <b><a href="#page360">360</a></b>, <b><a + href="#page362">362</a></b>, <b><a href="#page363">363</a></b>, <b><a + href="#page366">366</a></b></p> + <p>Folds, pleural, <a href="#page410">410</a>, <a href="#page414">414</a></p> + <p>Function of auditory organ, double, <a href="#page358">358</a></p> + <p><span class="hid">Funt</span>"<span class="hid">ion of</span> lateral line sense-organs, <a + href="#page357">357</a></p> + <p><span class="hid">Funt</span>"<span class="hid">ion of</span> nerves, <a + href="#page448">448</a></p> + <p><span class="hid">Funt</span>"<span class="hid">ion of</span> thyroid, <a + href="#page212">212</a>, <a href="#page215">215</a></p> + <p>Fusion of ganglia, <a href="#page52">52</a></p> + <p class="stanza">Galeodes, <a href="#page230">230</a></p> + <p><span class="hid">Gale</span>"<span class="hid">des,</span> brain, and camerostome, <b><a + href="#page222">222</a></b>, <b><a href="#page223">223</a></b></p> + <p><span class="hid">Gale</span>"<span class="hid">des,</span> primordial cranium, <a + href="#page341">341</a></p> + <p><span class="hid">Gale</span>"<span class="hid">des,</span> racquet-organs, <a + href="#page369">369</a>, <a href="#page375">375</a></p> + <p>Ganglia, infraœsophageal, <a href="#page4">4</a>, <a href="#page12">12</a>, <a + href="#page14">14</a>, <a href="#page51">51</a>, <a href="#page221">221</a></p> + <p><span class="hid">Ganl</span>"<span class="hid">ia,</span> supraœsophageal, <a + href="#page4">4</a>, <a href="#page12">12</a>, <a href="#page14">14</a>, <a + href="#page49">49</a>, <a href="#page52">52</a>, <a href="#page221">221</a>, <a + href="#page225">225</a></p> + <p><span class="hid">Ganl</span>"<span class="hid">ia,</span> origin of, of cranial and spinal + nerves, <a href="#page281">281</a></p> + <p>Ganglion, epibranchial, <a href="#page164">164</a>, <a href="#page282">282</a></p> + <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> habenulæ, <a + href="#page48">48</a>, <a href="#page78">78</a></p> + <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> optic of retina, <b><a + href="#page72">72</a></b>, <a href="#page89">89</a>, <a href="#page97">97</a></p> + <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> of posterior root, <a + href="#page466">466</a></p> + <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> cells of sympathetic system, <a + href="#page424">424</a>, <a href="#page428">428</a>, <a href="#page448">448</a></p> + <p>Ganoids, <a href="#page23">23</a>, <a href="#page345">345</a></p> + <p>Gastrula theory, <a href="#page165">165</a>, <a href="#page459">459</a></p> + <p>Genital corpuscles, <a href="#page470">470</a></p> + <p>Geological record, <a href="#page20">20</a></p> + <p><span class="hid">Geol</span>"<span class="hid">gical</span> strata, <b><a + href="#page22">22</a></b></p> + <p>Geotria australis, <a href="#page80">80</a></p> + <p>Germ-band, <a href="#page482">482</a></p> + <p><span class="pagenum" id="page526">{526}</span>Germ-cells, <a href="#page471">471</a></p> + <p>Giant-fibres, <a href="#page489">489</a></p> + <p>Gigantostraca, <a href="#page25">25</a>, <a href="#page234">234</a></p> + <p>Gills, <a href="#page148">148</a>, <a href="#page161">161</a>, <a href="#page185">185</a>, <a + href="#page214">214</a>, <a href="#page494">494</a></p> + <p>Glabellum, <a href="#page339">339</a></p> + <p>Glands, carotid, <a href="#page427">427</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> coxal, <a href="#page242">242</a>, + <a href="#page246">246</a>, <a href="#page319">319</a>, <b><a href="#page321">321</a></b>, <a + href="#page425">425</a>, <a href="#page429">429</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> ductless, <a + href="#page418">418</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> generative, of Limulus, <a + href="#page209">209</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> internal secretion of, <a + href="#page214">214</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> lymphatic, <a + href="#page418">418</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> pineal, <a href="#page15">15</a>, + <a href="#page63">63</a>, <a href="#page75">75</a>, <a href="#page456">456</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> pituitary, <a + href="#page244">244</a>, <a href="#page246">246</a>, <a href="#page319">319</a>, <a + href="#page425">425</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> segmental, of Crustacea, <a + href="#page422">422</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> submaxillary, <a + href="#page466">466</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> sweat, <a + href="#page448">448</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> thymus, <a + href="#page425">425</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> thyroid, of Ammocœtes, <a + href="#page193">193</a>, <b><a href="#page194">194</a></b>, <b><a href="#page196">196</a></b>, + <b><a href="#page201">201</a></b>, <a href="#page205">205</a>, <a href="#page429">429</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> tissue round brain of + Ammocœtes, <a href="#page209">209</a>, <a href="#page379">379</a></p> + <p><span class="hid">Gla</span>"<span class="hid">ds,</span> uterine, of Scorpion, <a + href="#page202">202</a>, <b><a href="#page203">203</a></b>, <b><a href="#page204">204</a></b>, + <b><a href="#page205">205</a></b></p> + <p>Gnathostomata, <a href="#page60">60</a>, <a href="#page343">343</a></p> + <p>Goblet, <a href="#page359">359</a>, <b><a href="#page360">360</a></b>, <b><a + href="#page373">373</a></b></p> + <p>Goitre, <a href="#page215">215</a></p> + <p>Gonad, <a href="#page475">475</a>, <a href="#page479">479</a></p> + <p>Gonocœle, <a href="#page475">475</a>, <a href="#page481">481</a></p> + <p>Grooves, ciliated, <a href="#page188">188</a>, <a href="#page197">197</a>, <a + href="#page212">212</a></p> + <p><span class="hid">Gro</span>"<span class="hid">ves,</span> hyper-pharyngeal of Amphioxus, <a + href="#page410">410</a></p> + <p><span class="hid">Gro</span>"<span class="hid">ves,</span> ventral, of apus and trilobites, + <a href="#page436">436</a></p> + <p>Gymnophiona, <a href="#page393">393</a></p> + <p class="stanza">Hæmocytes, <a href="#page472">472</a></p> + <p>Head of embryo dogfish, <b><a href="#page121">121</a></b>, <b><a + href="#page123">123</a></b></p> + <p>Head-shield, dorsal, of Ammocœtes, <b><a href="#page330">330</a></b>, <a + href="#page331">331</a>, <a href="#page338">338</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Auchenaspis, <a href="#page29">29</a>, <b><a href="#page31">31</a></b>, <b><a + href="#page338">338</a></b></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Cephalaspis, <a href="#page327">327</a>, <b><a href="#page328">328</a></b>, <b><a + href="#page330">330</a></b>, <b><a href="#page338">338</a></b>, <a href="#page348">348</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Cyathaspis, <b><a href="#page340">340</a></b></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Didymaspis, <b><a href="#page338">338</a></b></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> evidence of segmentation, <a href="#page339">339</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Keraspis, <b><a href="#page328">328</a></b></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Ostreostraci, <a href="#page327">327</a>, <a href="#page348">348</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Palæostracan, <a href="#page348">348</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Pteraspis, <a href="#page29">29</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al, + of</span> Thyestes, <a href="#page29">29</a>, <b><a href="#page31">31</a></b>, <a + href="#page327">327</a>, <a href="#page332">332</a>, <a href="#page338">338</a>, <a + href="#page340">340</a>, <b><a href="#page341">341</a></b>, <a href="#page348">348</a></p> + <p><span class="hid">Head-s</span>"<span class="hid">ield,</span> ventral, Scaphaspis, <a + href="#page349">349</a></p> + <p>Heart, nerves, <a href="#page2">2</a>, <a href="#page447">447</a></p> + <p><span class="hid">Her</span>"<span class="hid">t,</span> origin of vertebrate, <a + href="#page179">179</a>, <a href="#page451">451</a>, <a href="#page459">459</a></p> + <p><span class="hid">Her</span>"<span class="hid">t,</span> relative position in vertebrate and + invertebrate, <a href="#page175">175</a></p> + <p><span class="hid">Her</span>"<span class="hid">t,</span> veins forming vertebrate, <b><a + href="#page180">180</a></b></p> + <p>Hemiaspis, <a href="#page24">24</a>, <b><a href="#page25">25</a></b>, <a + href="#page249">249</a>, <b><a href="#page250">250</a></b>, <a href="#page351">351</a>, <a + href="#page414">414</a></p> + <p>Hemispheres, cerebral, <a href="#page47">47</a></p> + <p>Hepatopancreas of Ammocœtes, <a href="#page452">452</a></p> + <p><span class="hid">Hepatop</span>"<span class="hid">ncreas of</span> Limulus, <a + href="#page211">211</a></p> + <p><span class="pagenum" id="page527">{527}</span>Heterostraci, <a href="#page29">29</a>, <a + href="#page275">275</a>, <a href="#page326">326</a>, <a href="#page343">343</a></p> + <p>Hirudinea, <a href="#page478">478</a></p> + <p>Histolysis in transformation of the lamprey, <a href="#page59">59</a></p> + <p>Homology of branchial region of vertebrate and invertebrate, <a href="#page149">149</a></p> + <p><span class="hid">Homol</span>"<span class="hid">gy of</span> ductless glands and nephridial + organs, <a href="#page418">418</a></p> + <p><span class="hid">Homol</span>"<span class="hid">gy of</span> external genital ducts of + arthropods and nephridia of annelids, <a href="#page429">429</a></p> + <p><span class="hid">Homol</span>"<span class="hid">gy of</span> germinal layers in all <span + class="correction" title="Original reads 'Metozoa'">Metazoa</span>, <a + href="#page459">459</a></p> + <p><span class="hid">Homol</span>"<span class="hid">gy of</span> pituitary body of + Ammocœtes and coxal glands of Limulus, <a href="#page319">319</a></p> + <p><span class="hid">Homol</span>"<span class="hid">gy of</span> tubular muscles of + Ammocœtes and veno-pericardial muscles of Limulus, <a href="#page309">309</a></p> + <p><span class="hid">Homol</span>"<span class="hid">gy of</span> ventral aorta of vertebrate and + longitudinal venous sinuses of Limulus, <a href="#page178">178</a></p> + <p>Hydra, <a href="#page441">441</a>, <a href="#page465">465</a>, <a href="#page472">472</a>, <a + href="#page476">476</a></p> + <p>Hydrophilus larva, eye, <b><a href="#page84">84</a></b></p> + <p>Hyoid segment in Ammocœtes, <a href="#page186">186</a>, <a href="#page267">267</a></p> + <p>Hypoblast, <a href="#page434">434</a>, <a href="#page438">438</a>, <a + href="#page444">444</a>, <a href="#page445">445</a>, <a href="#page459">459</a></p> + <p>Hypoctonus, cartilage cells in entosternite, <b><a href="#page133">133</a></b></p> + <p><span class="hid">Hypo</span>"<span class="hid">tonus,</span> operculum, <a + href="#page189">189</a>, <a href="#page207">207</a></p> + <p>Hypogastric plexus, <a href="#page3">3</a></p> + <p>Hypogeophis, <a href="#page393">393</a></p> + <p>Hypophysis, <a href="#page229">229</a>, <a href="#page244">244</a>, <a + href="#page317">317</a>, <b><a href="#page318">318</a></b>, <b><a + href="#page340">340</a></b></p> + <p class="stanza">Infundibulum, position, <a href="#page122">122</a>,<a + href="#page132">132</a></p> + <p><span class="hid">Infun</span>"<span class="hid">ibulum,</span> tube, the ancestral + œsophagus, <a href="#page4">4</a>, <a href="#page37">37</a>, <a href="#page244">244</a>, + <a href="#page318">318</a></p> + <p><span class="hid">Infun</span>"<span class="hid">ibulum, tu</span>"<span + class="hid">e,</span> relation to neural canal, <a href="#page14">14</a>, <a + href="#page36">36</a>, <b><a href="#page318">318</a></b>, <b><a href="#page440">440</a></b>, <a + href="#page457">457</a></p> + <p><span class="hid">Infun</span>"<span class="hid">ibulum, tu</span>"<span class="hid">e, + relati</span>"<span class="hid">n to</span> notochord, <b><a href="#page318">318</a></b>, <a + href="#page435">435</a>,<a href="#page440">440</a></p> + <p><span class="hid">Infun</span>"<span class="hid">ibulum, tu</span>"<span class="hid">e, + relati</span>"<span class="hid">n to</span> olfactory tube, <a href="#page220">220</a>, <a + href="#page228">228</a>, <b><a href="#page318">318</a></b>, <a href="#page340">340</a></p> + <p>Insects, chordotonal organs, <a href="#page364">364</a>, <b><a + href="#page370">370</a></b></p> + <p>Invertebrate, heart, <a href="#page175">175</a>, <a href="#page179">179</a></p> + <p><span class="hid">Invert</span>"<span class="hid">brate,</span> excretory organs, <a + href="#page418">418</a></p> + <p><span class="hid">Invert</span>"<span class="hid">brate,</span> nervous system, <b><a + href="#page13">13</a></b>, <b><a href="#page54">54</a></b></p> + <p><span class="hid">Invert</span>"<span class="hid">brate,</span> segmental nerves, <a + href="#page152">152</a></p> + <p class="stanza">Keraspis, <a href="#page75">75</a>, <b><a href="#page328">328</a></b>, <a + href="#page338">338</a></p> + <p>Kidney, <a href="#page420">420</a>, <a href="#page459">459</a>, <a + href="#page476">476</a></p> + <p><span class="hid">Kid</span>"<span class="hid">ey,</span> nerves, <a + href="#page477">477</a></p> + <p>King-crab, <i>v.</i> Limulus</p> + <p class="stanza">Labyrinthodont, <a href="#page21">21</a>, <a href="#page28">28</a></p> + <p>Lamina terminalis, <a href="#page49">49</a></p> + <p>Lamprey, <i>v.</i> Ammocœtes and Petromyzon</p> + <p>Larva, <i>v.</i> Transformation of the Lamprey</p> + <p>Lateral line system, <a href="#page261">261</a>, <a href="#page355">355</a>, <a + href="#page411">411</a>, <a href="#page470">470</a></p> + <p>Law of Progress, <a href="#page19">19</a></p> + <p><span class="hid">Lw</span>"<span class="hid"> of</span> Recapitulation, <a + href="#page434">434</a>, <a href="#page456">456</a>, <a href="#page498">498</a></p> + <p>Layer, germinal, <a href="#page459">459</a></p> + <p><span class="hid">Lay</span>"<span class="hid">r,</span> laminated, <a + href="#page347">347</a>, <b><a href="#page348">348</a></b></p> + <p>Leech, <a href="#page421">421</a></p> + <p>Lens, formation, <a href="#page83">83</a>, <a href="#page115">115</a></p> + <p>Lepidosiren, <a href="#page148">148</a>, <a href="#page461">461</a>, <a + href="#page466">466</a></p> + <p>Limulus or king-crab, <b><a href="#page25">25</a></b>, <b><a href="#page140">140</a></b>, + <b><a href="#page236">236</a></b>, <b><a href="#page240">240</a></b></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> appendages, branchial, <b><a + href="#page138">138</a></b>, <b><a href="#page164">164</a></b>, <b><a + href="#page175">175</a></b></p> + <p><span class="pagenum" id="page528">{528}</span><span class="hid">Liml</span>"<span + class="hid">us</span> appendages, prosomatic, <b><a href="#page381">381</a></b></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> brain, <b><a + href="#page54">54</a></b></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> circulation, <a + href="#page174">174</a>, <a href="#page176">176</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> classification, <a + href="#page26">26</a>, <a href="#page249">249</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> cœlomic cavities, <a + href="#page252">252</a>, <a href="#page328">328</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> coxal glands, <a + href="#page321">321</a>, <a href="#page389">389</a>, <a href="#page397">397</a>, <a + href="#page403">403</a>, <a href="#page429">429</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> eyes, median, <a + href="#page62">62</a>, <a href="#page74">74</a>, <a href="#page81">81</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> entosternite or plastron, <a + href="#page142">142</a>, <b><a href="#page143">143</a></b></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> flabellum, <b><a + href="#page360">360</a></b>, <b><a href="#page362">362</a></b>, <b><a + href="#page363">363</a></b>, <a href="#page380">380</a>, <b><a href="#page381">381</a></b></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> generative organs and ducts, <b><a + href="#page189">189</a></b>, <a href="#page202">202</a>, <a href="#page208">208</a>, <a + href="#page209">209</a>, <a href="#page380">380</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> heart, <b><a + href="#page180">180</a></b></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> musculature, branchial, <a + href="#page170">170</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us muscl</span>"<span + class="hid">ature,</span> prosomatic, <a href="#page247">247</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us muscl</span>"<span + class="hid">ature,</span> veno-pericardial, <a href="#page177">177</a>, <a + href="#page297">297</a>, <a href="#page309">309</a>, <a href="#page313">313</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> nerves, appendage, <b><a + href="#page140">140</a></b>, <a href="#page157">157</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us ner</span>"<span class="hid">es,</span> + cardiac, <a href="#page314">314</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us ner</span>"<span class="hid">es,</span> + segmental, tripartite division of, <a href="#page157">157</a>, <a href="#page235">235</a>, <a + href="#page267">267</a>, <a href="#page355">355</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> segments, branchial, <a + href="#page152">152</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us segm</span>"<span class="hid">nts,</span> + first mesosomatic, <a href="#page188">188</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us segm</span>"<span class="hid">nts,</span> + prosomatic, <a href="#page233">233</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> operculum, <b><a + href="#page189">189</a></b>, <a href="#page202">202</a>, <a href="#page235">235</a>, <a + href="#page295">295</a></p> + <p><span class="hid">Liml</span>"<span class="hid">us</span> sense-organs, poriferous, of + appendages, <a href="#page359">359</a></p> + <p>Lip, lower, of Ammocœtes, <a href="#page246">246</a>, <a href="#page289">289</a>, <a + href="#page297">297</a>, <a href="#page458">458</a></p> + <p><span class="hid">Li</span>"<span class="hid">,</span> upper, <span class="hid">of + Am</span>"<span class="hid">ocœtes,</span> <a href="#page228">228</a>, <a + href="#page243">243</a>, <a href="#page303">303</a>, <a href="#page336">336</a></p> + <p>Liver, Ammocœtes, <a href="#page452">452</a></p> + <p><span class="hid">Liv</span>"<span class="hid">r,</span> Limulus, <a href="#page209">209</a>, + <a href="#page211">211</a></p> + <p>Lizard, pineal eye, <a href="#page80">80</a></p> + <p><span class="hid">Liz</span>"<span class="hid">rd,</span> suprarenals, <a + href="#page424">424</a></p> + <p><span class="hid">Liz</span>"<span class="hid">rd,</span> tail, <a href="#page50">50</a></p> + <p>Lobes, optic, <a href="#page101">101</a></p> + <p>Lobster, <a href="#page489">489</a></p> + <p>Lungs, <a href="#page148">148</a></p> + <p>Lung-books of scorpions, <a href="#page150">150</a></p> + <p>Lymph, <a href="#page474">474</a></p> + <p>Lymph-corpuscles, <a href="#page463">463</a>, <a href="#page490">490</a></p> + <p>Lymphocytes, <a href="#page472">472</a></p> + <p class="stanza">Malapterurus, <a href="#page470">470</a></p> + <p>Mammal, dominance of, <a href="#page21">21</a></p> + <p>Man, dominance of, <a href="#page17">17</a></p> + <p>Marsipobranchs, <a href="#page23">23</a>, <a href="#page35">35</a></p> + <p>Medullation of nerve-fibres, <a href="#page20">20</a>, <a href="#page267">267</a>, <a + href="#page467">467</a>, <a href="#page477">477</a></p> + <p>Membranes, basement, <a href="#page436">436</a></p> + <p>Meroblastic egg, <a href="#page485">485</a></p> + <p>Merostomata, <a href="#page25">25</a>, <a href="#page249">249</a>, <a + href="#page321">321</a></p> + <p>Mesencepalon, <a href="#page48">48</a></p> + <p>Mesoblast, <a href="#page444">444</a>, <a href="#page455">455</a>, <a + href="#page459">459</a></p> + <p>Mesoglœa, <a href="#page474">474</a></p> + <p>Mesonephros, <a href="#page389">389</a>, <a href="#page400">400</a>, <a + href="#page424">424</a>, <a href="#page429">429</a></p> + <p>Mesosoma, <a href="#page52">52</a></p> + <p>Mesothelium, <a href="#page472">472</a>, <a href="#page477">477</a></p> + <p>Metanephros, <a href="#page389">389</a></p> + <p><span class="pagenum" id="page529">{529}</span>Metasoma, <a href="#page52">52</a>, <a + href="#page387">387</a>, <a href="#page411">411</a></p> + <p>Metastoma, <a href="#page239">239</a>, <a href="#page246">246</a>, <a + href="#page272">272</a>, <a href="#page289">289</a>, <a href="#page342">342</a>, <a + href="#page458">458</a></p> + <p>Metazoa, <a href="#page444">444</a>, <a href="#page459">459</a>, <a href="#page471">471</a>, + <a href="#page472">472</a></p> + <p>Meynert's bundle, <a href="#page48">48</a>, <a href="#page77">77</a></p> + <p>Mollusca, dominance of, <a href="#page23">23</a></p> + <p>Mouth, old, or palæostoma, <a href="#page14">14</a>, <a href="#page317">317</a>, <a + href="#page322">322</a>, <a href="#page440">440</a>, <a href="#page458">458</a></p> + <p><span class="hid">Mo</span>"<span class="hid">th,</span> vertebrate, <a + href="#page317">317</a></p> + <p>Muco-cartilage, <i>v.</i> Cartilage</p> + <p>Muscles, antagonistic, <a href="#page447">447</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> branchial, <a + href="#page170">170</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> connection of, with central + nervous system, <a href="#page464">464</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> eye, and their nerves, <a + href="#page263">263</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> prosomatic, <a + href="#page243">243</a>, <a href="#page247">247</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> phylogeny of origin of skeletal, + <a href="#page478">478</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> rudimentary, in Ammocœtes, + <a href="#page289">289</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> somatic trunk, origin of, <a + href="#page406">406</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> striated, <a + href="#page20">20</a>, <a href="#page155">155</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> tubular, of Ammocœtes, <a + href="#page309">309</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> unstriped, <a + href="#page20">20</a>, <a href="#page447">447</a>, <a href="#page491">491</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> visceral and parietal, <a + href="#page155">155</a>, <a href="#page172">172</a></p> + <p><span class="hid">Mus</span>"<span class="hid">les,</span> veno-pericardial of Limulus and + Scorpion, <a href="#page177">177</a>, <a href="#page297">297</a>, <a href="#page309">309</a></p> + <p>Muscle-spindles, <a href="#page267">267</a></p> + <p>Mygalidæ, stomach, <a href="#page109">109</a></p> + <p><span class="hid">Mygl</span>"<span class="hid">idæ,</span> segmentation, <a + href="#page249">249</a>, <a href="#page306">306</a></p> + <p>Myomeres, <a href="#page262">262</a>, <a href="#page337">337</a>, <a href="#page414">414</a>, + <a href="#page479">479</a></p> + <p>Myotomes, <a href="#page332">332</a>, <a href="#page337">337</a>, <a href="#page338">338</a>, + <b><a href="#page391">391</a></b>, <a href="#page407">407</a>, <b><a + href="#page408">408</a></b></p> + <p>Mysis, eyes, <a href="#page100">100</a></p> + <p><span class="hid">My</span>"<span class="hid">is,</span> ductless glands, <a + href="#page422">422</a></p> + <p>Myxine, <a href="#page220">220</a>, <a href="#page392">392</a>, <a href="#page402">402</a>, + <a href="#page419">419</a></p> + <p class="stanza">Nebalia, <a href="#page144">144</a>, <a href="#page422">422</a></p> + <p>Nemertina, <a href="#page475">475</a></p> + <p>Nephridia, <a href="#page395">395</a>, <a href="#page421">421</a>, <a + href="#page429">429</a></p> + <p>Nephrocœle, <a href="#page430">430</a></p> + <p>Nephrotome, <a href="#page393">393</a></p> + <p>Nerves, abducens, <a href="#page155">155</a>, <a href="#page263">263</a>, <a + href="#page266">266</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> auditory, <a + href="#page356">356</a>, <a href="#page376">376</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> autonomic, <a + href="#page3">3</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> facial, <a href="#page155">155</a>, + <a href="#page156">156</a>, <a href="#page186">186</a>, <a href="#page188">188</a>, <a + href="#page192">192</a>, <b><a href="#page311">311</a></b>, <a href="#page356">356</a>, <a + href="#page378">378</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es, fa</span>"<span class="hid">ial,</span> + ramus branchialis profundus, <a href="#page311">311</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> to flabellum, in Limulus, <a + href="#page361">361</a>, <a href="#page375">375</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> glossopharyngeal, <a + href="#page155">155</a>, <a href="#page156">156</a>, <a href="#page186">186</a>, <a + href="#page356">356</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> hypoglossal, <a + href="#page156">156</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> inhibitory, <a + href="#page447">447</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> inedullation of, <a + href="#page20">20</a>, <a href="#page267">267</a>, <a href="#page467">467</a>, <a + href="#page477">477</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> occulomotor, <a + href="#page155">155</a>, <a href="#page234">234</a>, <a href="#page263">263</a>, <a + href="#page274">274</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> olfactory, <a + href="#page229">229</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> optic, <a href="#page101">101</a>, + <a href="#page104">104</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es, ot</span>"<span class="hid">ic,</span> of + pineal eye, <a href="#page79">79</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> origin of ganglia of cranial and + spinal, <a href="#page281">281</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> to pecten of Scorpion, <a + href="#page375">375</a>, <a href="#page376">376</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> preganglionic, <a + href="#page2">2</a></p> + <p><span class="pagenum" id="page530">{530}</span><span class="hid">Ner</span>"<span + class="hid">es,</span> of prosoma in Limulus, <a href="#page235">235</a>, <a + href="#page355">355</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> regeneration of, <a + href="#page469">469</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> roots, of Limulus, <a + href="#page157">157</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> sacral, <a + href="#page448">448</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> segmental, <a + href="#page152">152</a>, <a href="#page156">156</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> segmental nature of cranial, <a + href="#page259">259</a>, <a href="#page411">411</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> spinal, absence of lateral roots + in, <a href="#page388">388</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> spinal accessory, <a + href="#page154">154</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> trigeminal, <a + href="#page151">151</a>, <a href="#page155">155</a>, <a href="#page156">156</a>, <a + href="#page234">234</a>, <a href="#page243">243</a>, <a href="#page257">257</a>, <a + href="#page279">279</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es, trigm</span>"<span + class="hid">inal,</span> motor nucleus of, <a href="#page280">280</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es, trigm</span>"<span + class="hid">inal,</span> of Ammocœtes, <b><a href="#page288">288</a></b></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> tripartite arrangement of cranial + nerves, <a href="#page154">154</a>, <a href="#page157">157</a>, <a href="#page235">235</a>, <a + href="#page267">267</a>, <a href="#page355">355</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> trochlear, <a + href="#page48">48</a>, <a href="#page155">155</a>, <a href="#page234">234</a>, <a + href="#page263">263</a>, <a href="#page276">276</a></p> + <p><span class="hid">Ner</span>"<span class="hid">es,</span> vagus, <a href="#page151">151</a>, + <a href="#page154">154</a>, <a href="#page156">156</a>, <a href="#page173">173</a>, <a + href="#page186">186</a>, <a href="#page356">356</a>, <a href="#page447">447</a>, <a + href="#page449">449</a></p> + <p>Nervous system, central, comparison of Vertebrate and Arthropod, <a href="#page36">36</a>, <a + href="#page457">457</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> connection of, with muscular and epithelial tissues, <a + href="#page464">464</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span class="hid">tral, + conne</span>"<span class="hid">tion of,</span> with retina, <a href="#page71">71</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> disease of, <a href="#page50">50</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> evidence of, <a href="#page8">8</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> evolution of, <a href="#page34">34</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> importance of, <a href="#page16">16</a>, <a href="#page463">463</a>, <a + href="#page482">482</a>, <a href="#page498">498</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> invertebrate, <b><a href="#page10">10</a></b>, <b><a + href="#page13">13</a></b>, <b><a href="#page54">54</a></b></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> origin of, <a href="#page480">480</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> relation of germ-band to, <a href="#page483">483</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> segmentation of vertebrate, <a href="#page51">51</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> tube of, <a href="#page36">36</a>-<a href="#page51">51</a>, <a + href="#page102">102</a>, <a href="#page211">211</a>, <a href="#page433">433</a>, <a + href="#page455">455</a>, <a href="#page457">457</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span + class="hid">tral,</span> vertebrate, <b><a href="#page10">10</a></b>, <b><a + href="#page13">13</a></b>, <b><a href="#page40">40</a></b>, <a href="#page41">41</a>, <a + href="#page152">152</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem,</span> enteric, <a + href="#page447">447</a></p> + <p><span class="hid">Nervous </span>"<span class="hid">ystem,</span> sympathetic, <a + href="#page2">2</a>, <a href="#page424">424</a>, <a href="#page428">428</a>, <a + href="#page448">448</a>, <a href="#page491">491</a></p> + <p>Neurenteric canal, <a href="#page37">37</a></p> + <p>Neuroblast, <a href="#page465">465</a></p> + <p>Neuromeres, <a href="#page55">55</a>, <a href="#page247">247</a>, <a href="#page262">262</a>, + <a href="#page312">312</a>, <a href="#page316">316</a></p> + <p>Neurones, <a href="#page72">72</a>, <a href="#page92">92</a>, <a href="#page465">465</a></p> + <p>Neuropil, <a href="#page71">71</a>, <a href="#page91">91</a></p> + <p>Neuropore, <a href="#page220">220</a>, <a href="#page457">457</a></p> + <p>Nose, <a href="#page219">219</a></p> + <p><span class="hid">No</span>"<span class="hid">e,</span> of Osteostraci, <a + href="#page329">329</a>, <a href="#page352">352</a>, <a href="#page458">458</a></p> + <p>Notochord, <a href="#page120">120</a>, <a href="#page122">122</a>, <a + href="#page180">180</a>, <b><a href="#page181">181</a></b>, <a href="#page220">220</a>, <a + href="#page244">244</a>, <a href="#page295">295</a>, <a href="#page318">318</a>, <a + href="#page405">405</a>, <a href="#page417">417</a>, <a href="#page433">433</a>, <b><a + href="#page436">436</a></b>, <a href="#page494">494</a></p> + <p class="stanza">Ocelli, <a href="#page70">70</a></p> + <p>Œsophagus of Ammocœtes, <a href="#page405">405</a></p> + <p><span class="hid">Œsop</span>"<span class="hid">agus of</span> Arthropod, compared to + tube of infundibulum, <a href="#page4">4</a>, <a href="#page244">244</a>, <a + href="#page440">440</a></p> + <p>Olfactory apparatus, evidence of the, <a href="#page218">218</a></p> + <p><span class="hid">Olfa</span>"<span class="hid">tory</span> organs of the Scorpion group, <a + href="#page220">220</a></p> + <p><span class="hid">Olfa</span>"<span class="hid">tory</span> tube of Ammocœtes, <a + href="#page219">219</a>, <a href="#page225">225</a>, <a href="#page244">244</a>, <a + href="#page317">317</a></p> + <p>Oligochæta, <a href="#page421">421</a>, <a href="#page478">478</a></p> + <p>Operculum of Eurypterus, <b><a href="#page191">191</a></b>, <a href="#page212">212</a>, <a + href="#page291">291</a></p> + <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Limulus, <b><a + href="#page189">189</a></b>, <a href="#page202">202</a>, <a href="#page235">235</a>, <a + href="#page295">295</a></p> + <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Phrynus, <a + href="#page191">191</a></p> + <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Scorpion, <b><a + href="#page189">189</a></b>, <a href="#page206">206</a>, <a href="#page212">212</a>, <b><a + href="#page372">372</a></b></p> + <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Thelyphonus, <a + href="#page189">189</a>, <b><a href="#page190">190</a></b>, <a href="#page206">206</a></p> + <p>Organs, arrangement of, <a href="#page10">10</a></p> + <p><span class="pagenum" id="page531">{531}</span><span class="hid">Org</span>"<span + class="hid">ns,</span> auditory, of arachnids and Insects, <a href="#page368">368</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> branchial, innervation of + vertebrate, <a href="#page151">151</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns, bran</span>"<span class="hid">hial,</span> + sense-organs of embryo vertebrate, <a href="#page261">261</a>, <a href="#page281">281</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> chordotonal, of insects, <a + href="#page364">364</a>, <a href="#page369">369</a>, <b><a href="#page370">370</a></b></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> electric, <a + href="#page470">470</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> generative, of Limulus, <a + href="#page208">208</a>, <a href="#page209">209</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns, gener</span>"<span + class="hid">tive,</span> connection between Thyroid gland and, <a href="#page215">215</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> genital, of sea-scorpions, <a + href="#page206">206</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> lateral line, <a + href="#page355">355</a>, <a href="#page411">411</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> lyriform, of arachnids, <a + href="#page364">364</a>, <a href="#page369">369</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> olfactory, of Scorpion group, <a + href="#page220">220</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> phagocytic, <a + href="#page420">420</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> racquet, of Galeodes, <a + href="#page369">369</a>, <a href="#page375">375</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> segmental excretory, <a + href="#page389">389</a>, <b><a href="#page391">391</a></b>, <b><a href="#page408">408</a></b>, + <a href="#page418">418</a>, <a href="#page459">459</a>, <a href="#page477">477</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> sense, of appendages of Limulus, <a + href="#page358">358</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> vestigial, <a + href="#page456">456</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> of vision, evidence of, <a + href="#page68">68</a></p> + <p><span class="hid">Org</span>"<span class="hid">ns,</span> vital, <a href="#page57">57</a></p> + <p>Origin of alimentary canal, <a href="#page444">444</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> arthropods from annelids, <a + href="#page395">395</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> atrial cavity, <a + href="#page409">409</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> auditory capsules and + parachordals, <a href="#page377">377</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> cœlom, <a + href="#page475">475</a>, <a href="#page481">481</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> ductless glands, <a + href="#page428">428</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> free cells, <a + href="#page472">472</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> heart of vertebrate, <a + href="#page179">179</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> lateral line organs, <a + href="#page356">356</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> muscles, <a + href="#page478">478</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> musculature, branchial, <a + href="#page170">170</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of muscul</span>"<span + class="hid">ture,</span> somatic trunk, <a href="#page406">406</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> nervous system, central, <a + href="#page480">480</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> notochord, <a + href="#page434">434</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> segmental excretory organs, <a + href="#page389">389</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> skeleton of vertebrates, <a + href="#page119">119</a></p> + <p><span class="hid">Ori</span>"<span class="hid">in of</span> vertebrates, <a + href="#page9">9</a>, <a href="#page36">36</a>, <a href="#page351">351</a>, <a + href="#page433">433</a>, <a href="#page493">493</a></p> + <p>Ostracodermata, <a href="#page326">326</a>, <a href="#page343">343</a></p> + <p>Osteostraci, <a href="#page29">29</a>, <a href="#page75">75</a>, <a href="#page275">275</a>, + <a href="#page326">326</a>, <a href="#page343">343</a></p> + <p>Otoliths, <a href="#page378">378</a></p> + <p>Ovum, <a href="#page473">473</a></p> + <p class="stanza">Pacinian bodies, <a href="#page470">470</a>, <a href="#page477">477</a></p> + <p>Palæmon, <a href="#page20">20</a>, <a href="#page422">422</a></p> + <p>Palæontology, evidence of, <a href="#page20">20</a>, <a href="#page497">497</a></p> + <p>Palæostoma, <a href="#page317">317</a></p> + <p>Palæostraca, <a href="#page27">27</a>, <a href="#page396">396</a></p> + <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> median eyes, <a + href="#page74">74</a></p> + <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> mesosomatic appendages, <a + href="#page188">188</a></p> + <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> olfactory organs, <a + href="#page221">221</a></p> + <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> segments, compared to + Ammocœtes, <a href="#page308">308</a></p> + <p>Pantopoda, glands, <a href="#page423">423</a></p> + <p>Parachordals, <a href="#page121">121</a>, <a href="#page132">132</a>, <a + href="#page377">377</a></p> + <p>Parapodia, <a href="#page357">357</a></p> + <p><span class="pagenum" id="page532">{532}</span>Parapodopsis, foot glands, <a + href="#page422">422</a></p> + <p>Parathymus, <a href="#page427">427</a></p> + <p>Parathyroids, <a href="#page427">427</a></p> + <p>Parietal organ, <a href="#page76">76</a></p> + <p>Pecten of scorpion, <a href="#page114">114</a>, <a href="#page359">359</a>, <a + href="#page366">366</a>, <a href="#page371">371</a>, <b><a href="#page372">372</a></b>, <b><a + href="#page373">373</a></b>, <b><a href="#page374">374</a></b></p> + <p>Pedipalpi, <a href="#page190">190</a></p> + <p>Periblast, <a href="#page471">471</a></p> + <p>Peripatus, <a href="#page396">396</a>, <b><a href="#page399">399</a></b>, <a + href="#page400">400</a>, <a href="#page411">411</a>, <a href="#page421">421</a>, <a + href="#page429">429</a></p> + <p>Petromyzon, alimentary canal, <a href="#page405">405</a>, <a href="#page445">445</a></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> auditory organ, <a + href="#page378">378</a></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> branchial segments, <b><a + href="#page169">169</a></b></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> life-history, <a + href="#page59">59</a></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> olfactory tube, <a + href="#page219">219</a>, <b><a href="#page226">226</a></b></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> pronephric duct, <a + href="#page402">402</a></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> retina and optic nerve, <b><a + href="#page95">95</a></b></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> skeleton, <b><a + href="#page125">125</a></b></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> suctorial apparatus, <a + href="#page287">287</a>, <a href="#page304">304</a></p> + <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> transformation, <i>v.</i> + Transformation of the Lamprey</p> + <p>Phagocytes, <a href="#page420">420</a>, <a href="#page471">471</a></p> + <p>Pharynx of Amphioxus, <a href="#page410">410</a></p> + <p><span class="hid">Phar</span>"<span class="hid">nx of</span> Vertebrate, <a + href="#page440">440</a></p> + <p>Phoronis, <a href="#page439">439</a></p> + <p>Phrynus, brain, <a href="#page53">53</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> caudal brain, <a + href="#page450">450</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> carapace and carapace removed, + <b><a href="#page250">250</a></b></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> cœcal diverticula, <a + href="#page109">109</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> evidence of segmentation of + carapace, <a href="#page249">249</a>, <b><a href="#page250">250</a></b>, <a + href="#page341">341</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> operculum, <a + href="#page191">191</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> prosomatic appendages, <a + href="#page306">306</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> crossing of dorso-ventral muscles, + <a href="#page271">271</a>, <a href="#page277">277</a></p> + <p><span class="hid">Phry</span>"<span class="hid">us,</span> stridulating apparatus, <a + href="#page368">368</a></p> + <p>Phyllodoce, <a href="#page395">395</a></p> + <p>Phyllopoda, <a href="#page321">321</a></p> + <p>Pigment, in Ammocœtes, in position of atrial cavity, <a href="#page412">412</a></p> + <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> epithelial lining of central + nervous system, <a href="#page43">43</a>, <a href="#page457">457</a></p> + <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> choroid of vertebrate eye, <a + href="#page104">104</a>, <a href="#page107">107</a></p> + <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> between glandular cells round + brain of Ammocœtes, <a href="#page211">211</a>, <a href="#page379">379</a></p> + <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> tapetal layer of retina, <a + href="#page70">70</a></p> + <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> white, of right pineal eye of + Lamprey, <a href="#page76">76</a>, <a href="#page80">80</a></p> + <p>Pineal body, <a href="#page14">14</a>, <a href="#page15">15</a></p> + <p><span class="hid">Pin</span>"<span class="hid">al</span> eyes, <a href="#page74">74</a>, <a + href="#page233">233</a>, <a href="#page244">244</a></p> + <p><span class="hid">Pin</span>"<span class="hid">al ey</span>"<span class="hid">s,</span> of + Ammocœtes, <a href="#page80">80</a>, <b><a href="#page78">78</a></b>, <b><a + href="#page85">85</a></b></p> + <p><span class="hid">Pin</span>"<span class="hid">al</span> gland, <a href="#page63">63</a>, <a + href="#page75">75</a>, <a href="#page456">456</a></p> + <p>Pits, epithelial, of diaphragms in Ammocœtes, <a href="#page164">164</a></p> + <p><span class="hid">Pi</span>"<span class="hid">t, epith</span>"<span class="hid">lial, + of</span> skin in Ammocœtes, <a href="#page173">173</a>, <b><a + href="#page200">200</a></b></p> + <p>Pituitary body, <a href="#page244">244</a>, <a href="#page246">246</a>, <a + href="#page319">319</a>, <b><a href="#page321">321</a></b>, <a href="#page425">425</a>, <a + href="#page430">430</a></p> + <p>Plasma-cells, <a href="#page471">471</a></p> + <p>Plakodes, <a href="#page283">283</a></p> + <p>Planarians, <a href="#page475">475</a></p> + <p>Plastron, formation of cranial walls from the, <a href="#page86">86</a>, <a + href="#page322">322</a>, <a href="#page341">341</a></p> + <p><span class="hid">Plat</span>"<span class="hid">ron,</span> of Limulus, <a + href="#page136">136</a>, <a href="#page142">142</a>, <b><a href="#page143">143</a></b></p> + <p><span class="hid">Plat</span>"<span class="hid">ron,</span> Palæostracan, compared to + trabeculæ of Ammocœtes, <a href="#page145">145</a>, <a href="#page377">377</a></p> + <p><span class="pagenum" id="page533">{533}</span><span class="hid">Plat</span>"<span + class="hid">ron,</span> muscles attached to the, <a href="#page270">270</a></p> + <p><span class="hid">Plat</span>"<span class="hid">ron,</span> of Thelyphonus, <b><a + href="#page143">143</a></b></p> + <p>Platyhelmia, <a href="#page475">475</a></p> + <p>Pleuron, <a href="#page410">410</a>, <a href="#page415">415</a></p> + <p>Plexus, of Auerbach, <a href="#page447">447</a></p> + <p><span class="hid">Plex</span>"<span class="hid">s, of</span> choroid, <a + href="#page38">38</a>, <a href="#page45">45</a>, <a href="#page49">49</a>, <a + href="#page103">103</a></p> + <p><span class="hid">Plex</span>"<span class="hid">s, of</span> hypogastric, <a + href="#page3">3</a></p> + <p>Polychæta, <a href="#page357">357</a>, <a href="#page395">395</a></p> + <p>Pores, abdominal, <a href="#page430">430</a></p> + <p>Porifera, <a href="#page473">473</a></p> + <p>Pouch, formation of gill, <a href="#page165">165</a>, <b><a href="#page166">166</a></b></p> + <p>Prestwichia, <a href="#page24">24</a>, <b><a href="#page25">25</a></b>, <a + href="#page249">249</a>, <a href="#page351">351</a></p> + <p>Principle of concentration and cephalization, <a href="#page51">51</a></p> + <p><span class="hid">Prini</span>"<span class="hid">ple of</span> embryology, <a + href="#page455">455</a></p> + <p>Pristiurus, <a href="#page424">424</a></p> + <p>Progress, law of, <a href="#page19">19</a></p> + <p><span class="hid">Progr</span>"<span class="hid">ss,</span> result of, <a + href="#page56">56</a></p> + <p>Pronephros, <a href="#page389">389</a>, <a href="#page397">397</a>, <a + href="#page419">419</a>, <a href="#page424">424</a>, <a href="#page449">449</a></p> + <p>Prosencephalon, <a href="#page48">48</a></p> + <p>Prosoma, <a href="#page52">52</a></p> + <p>Protopterus, <a href="#page148">148</a></p> + <p>Protostraca, <a href="#page27">27</a>, <a href="#page396">396</a>, <a + href="#page417">417</a></p> + <p><span class="hid">Proto</span>"<span class="hid">traca,</span> dominance of, <a + href="#page28">28</a></p> + <p>Protozoa, <a href="#page166">166</a>, <a href="#page479">479</a></p> + <p>Pseudoniscus, <b><a href="#page25">25</a></b>, <a href="#page249">249</a></p> + <p>Pteraspis, <a href="#page29">29</a>, <b><a href="#page30">30</a></b>, <a + href="#page275">275</a>, <a href="#page326">326</a>, <a href="#page343">343</a>, <a + href="#page344">344</a>, <b><a href="#page350">350</a></b></p> + <p>Pterichthys, <a href="#page29">29</a>, <b><a href="#page31">31</a></b>, <a + href="#page239">239</a>, <a href="#page326">326</a>, <a href="#page351">351</a></p> + <p>Pterygoid, pedicle of, <a href="#page295">295</a></p> + <p>Pterygotus, <a href="#page25">25</a>, <b><a href="#page27">27</a></b>, <a + href="#page56">56</a>, <a href="#page170">170</a>, <a href="#page191">191</a>, <a + href="#page221">221</a>, <a href="#page235">235</a>, <a href="#page238">238</a>, <a + href="#page249">249</a>, <a href="#page276">276</a></p> + <p>Ptychodera, <a href="#page494">494</a>, <b><a href="#page495">495</a></b></p> + <p class="stanza">Ramus branchialis profundus of facial nerve, <a href="#page311">311</a></p> + <p><span class="hid">Ra</span>"<span class="hid">ms</span> communicans, <a href="#page2">2</a>, + <a href="#page3">3</a></p> + <p>Raphe, <a href="#page46">46</a></p> + <p>Recapitulation, law of, <a href="#page434">434</a>, <a href="#page456">456</a>, <a + href="#page498">498</a></p> + <p>Regeneration of nerves, <a href="#page469">469</a></p> + <p>Reptiles, dominance of, <a href="#page21">21</a></p> + <p>Retina, compound, <a href="#page71">71</a></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> development of, <a + href="#page101">101</a></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> inversion of, in Vertebrates, <a + href="#page114">114</a></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> inverted, <b><a + href="#page70">70</a></b></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> layers of compound, <a + href="#page73">73</a></p> + <p><span class="hid">Ret</span>"<span class="hid">ia, lay</span>"<span class="hid">rs of</span> + in Crustacean eye, <b><a href="#page100">100</a></b></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> of lateral eye of Ammocœtes, + <a href="#page93">93</a>, <b><a href="#page95">95</a></b>, <b><a href="#page111">111</a></b></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> Musca, <b><a + href="#page89">89</a></b></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> Pecten and Spondylus, <a + href="#page114">114</a></p> + <p><span class="hid">Ret</span>"<span class="hid">ia,</span> upright compound, <b><a + href="#page72">72</a></b></p> + <p><span class="hid">Ret</span>"<span class="hid">ia, upri</span>"<span class="hid">ht</span> + simple, <b><a href="#page69">69</a></b></p> + <p>Rhabdites, <a href="#page69">69</a>, <a href="#page81">81</a></p> + <p class="stanza">Saccus vasculosus, <a href="#page244">244</a>, <a href="#page322">322</a></p> + <p>Scales, <a href="#page345">345</a></p> + <p><span class="pagenum" id="page534">{534}</span>Scaphaspis, <a href="#page349">349</a></p> + <p>Schwann, sheath of, <a href="#page469">469</a></p> + <p>Sclerotomes, <a href="#page388">388</a></p> + <p>Scorpion, brain, <b><a href="#page54">54</a></b></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> branchial lamellæ, <b><a + href="#page175">175</a></b></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> development, <a + href="#page482">482</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> entochondrites, <a + href="#page377">377</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> excretory organs, <a + href="#page397">397</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> eyes, <a + href="#page75">75</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> lung-books, <a + href="#page150">150</a>, <a href="#page170">170</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> lymphatic glands, <a + href="#page423">423</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> muscles, oblique, <b><a + href="#page278">278</a></b></p> + <p><span class="hid">Scor</span>"<span class="hid">ion, mus</span>"<span class="hid">les,</span> + recti, <a href="#page271">271</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion, mus</span>"<span class="hid">les,</span> + respiration, <a href="#page171">171</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion, mus</span>"<span class="hid">les,</span> + veno-pericardial, <a href="#page177">177</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> muscular system, <a + href="#page247">247</a>, <a href="#page268">268</a>, <b><a href="#page269">269</a></b></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> nerves to Cheliceræ, <a + href="#page237">237</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> olfactory organs, <a + href="#page220">220</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> operculum of male, <b><a + href="#page189">189</a></b>, <a href="#page206">206</a>, <a href="#page212">212</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> pecten, <a + href="#page359">359</a>, <a href="#page366">366</a>, <a href="#page371">371</a>, <a + href="#page373">373</a>, <a href="#page374">374</a>, <a href="#page377">377</a></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> under surface, <b><a + href="#page372">372</a></b></p> + <p><span class="hid">Scor</span>"<span class="hid">ion,</span> uterus, <b><a + href="#page189">189</a></b>, <a href="#page202">202</a>, <b><a href="#page203">203</a></b>, + <b><a href="#page204">204</a></b>, <b><a href="#page205">205</a></b>, <a + href="#page212">212</a></p> + <p>Sea-scorpions, <a href="#page25">25</a>, <b><a href="#page26">26</a></b>, <b><a + href="#page27">27</a></b>, <a href="#page56">56</a>, <a href="#page150">150</a>, <a + href="#page170">170</a>, <a href="#page191">191</a>, <a href="#page208">208</a>, <a + href="#page221">221</a>, <a href="#page232">232</a>, <a href="#page235">235</a>, <a + href="#page241">241</a>, <a href="#page349">349</a>, <a href="#page359">359</a></p> + <p>Segmentation, branchiomeric, <a href="#page124">124</a></p> + <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> body-muscles in vertebrate, + <a href="#page388">388</a></p> + <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> eye-muscles, <a + href="#page248">248</a></p> + <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> of head, double, <a + href="#page155">155</a>, <a href="#page157">157</a>, <a href="#page173">173</a>, <a + href="#page234">234</a>, <a href="#page258">258</a>, <a href="#page411">411</a>, <a + href="#page459">459</a></p> + <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> of head-shield, <a + href="#page339">339</a></p> + <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> history of cranial, <a + href="#page258">258</a></p> + <p>Segments, branchial of Ammocœtes, <a href="#page161">161</a>, <b><a + href="#page178">178</a></b>, <a href="#page186">186</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> hyoid, in Ammocœtes, + double, <a href="#page186">186</a>, <b><a href="#page201">201</a></b>, <a + href="#page267">267</a>, <a href="#page300">300</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> innervation of branchial, <a + href="#page151">151</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> first mesosomatic, in Limulus and + its allies, <a href="#page188">188</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> mesosomatic, of Eurypterus, <b><a + href="#page192">192</a></b></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> prosomatic of Limulus and its + allies, <a href="#page233">233</a>, <a href="#page249">249</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts, prosm</span>"<span class="hid">atic + of</span> Ammocœtes, <a href="#page286">286</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> of spinal region of Vertebrates, + <a href="#page388">388</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> of trigeminal nerve-group, <a + href="#page257">257</a>, <a href="#page279">279</a></p> + <p><span class="hid">Segm</span>"<span class="hid">nts,</span> tubular muscles of hyoid, <a + href="#page299">299</a></p> + <p>Sense-organs of Amphioxus, <a href="#page34">34</a></p> + <p><span class="hid">Sense-o</span>"<span class="hid">rans of</span> branchial, of Limulus, <a + href="#page359">359</a>, <b><a href="#page360">360</a></b></p> + <p><span class="hid">Sense-o</span>"<span class="hid">rans of</span> lateral, of Annelids, <a + href="#page357">357</a>, <a href="#page367">367</a></p> + <p><span class="hid">Sense-o</span>"<span class="hid">rans of</span> lateral-line system, <a + href="#page356">356</a>, <a href="#page411">411</a>, <a href="#page470">470</a></p> + <p>Serum, <a href="#page492">492</a></p> + <p>Significance of the optic diverticula, <a href="#page102">102</a></p> + <p>Silurus, <a href="#page488">488</a></p> + <p>Sinus, longitudinal venous, of Limulus, <a href="#page176">176</a>, <a + href="#page312">312</a>, <a href="#page451">451</a></p> + <p><span class="hid">Sin</span>"<span class="hid">s,</span> rhomboidal of bird, <b><a + href="#page46">46</a></b></p> + <p>Skeleton, Ammocœtes, <b><a href="#page126">126</a></b>, <b><a + href="#page296">296</a></b>, <b><a href="#page335">335</a></b></p> + <p><span class="hid">Skel</span>"<span class="hid">ton, Amm</span>"<span + class="hid">cœtes,</span> branchial, <a href="#page126">126</a>, <b><a + href="#page126">126</a></b></p> + <p><span class="hid">Skel</span>"<span class="hid">ton, Amm</span>"<span + class="hid">cœtes,</span> basi-cranial, <a href="#page132">132</a></p> + <p><span class="hid">Skel</span>"<span class="hid">ton, Amm</span>"<span + class="hid">cœtes,</span> muco-cartilaginous, <a href="#page291">291</a>, <b><a + href="#page296">296</a></b>, <a href="#page330">330</a>, <a href="#page331">331</a></p> + <p><span class="pagenum" id="page535">{535}</span><span class="hid">Skel</span>"<span + class="hid">ton,</span> aponeurotic, <a href="#page414">414</a></p> + <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Cephalaspis, <a + href="#page414">414</a>, <b><a href="#page415">415</a></b></p> + <p><span class="hid">Skel</span>"<span class="hid">ton,</span> evidence of the, <a + href="#page119">119</a></p> + <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Limulus, cartilaginous, <b><a + href="#page126">126</a></b>, <a href="#page136">136</a></p> + <p><span class="hid">Skel</span>"<span class="hid">ton, Lim</span>"<span class="hid">lus,</span> + mesosomatic, <a href="#page137">137</a></p> + <p><span class="hid">Skel</span>"<span class="hid">ton, Lim</span>"<span class="hid">lus,</span> + prosomatic, <a href="#page142">142</a></p> + <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Petromyzon, <b><a + href="#page125">125</a></b></p> + <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Vertebrate, commencement of bony, + <a href="#page120">120</a>, <a href="#page121">121</a></p> + <p>Skin, digestive power of cells of, in Ammocœtes, <a href="#page58">58</a>, <a + href="#page442">442</a></p> + <p><span class="hid">Si</span>"<span class="hid">n,</span> of Ammocœtes, <a + href="#page346">346</a></p> + <p><span class="hid">Si</span>"<span class="hid">n,</span> nerves of, <a + href="#page448">448</a></p> + <p>Skull of dogfish, <b><a href="#page123">123</a></b></p> + <p><span class="hid">Skl</span>"<span class="hid">l of</span> pig-embryo, <b><a + href="#page121">121</a></b></p> + <p>Slimonia, <b><a href="#page27">27</a>,</b> <a href="#page56">56</a>, <a + href="#page170">170</a>, <a href="#page235">235</a>, <a href="#page238">238</a>, <a + href="#page249">249</a>, <a href="#page276">276</a>, <a href="#page303">303</a></p> + <p>Solenocytes, <a href="#page395">395</a>, <a href="#page477">477</a></p> + <p>Solpugidæ, <a href="#page109">109</a></p> + <p>Sphæroma serratum, brain, <b><a href="#page62">62</a></b>, <b><a href="#page90">90</a></b>, + <a href="#page101">101</a>, <a href="#page225">225</a></p> + <p>Spiders, eyes, <a href="#page75">75</a></p> + <p><span class="hid">Spi</span>"<span class="hid">ers,</span> stomach, <a + href="#page109">109</a></p> + <p>Spina bifida, <a href="#page50">50</a></p> + <p>Spinal cord, difference between brain and, <a href="#page45">45</a></p> + <p><span class="hid">Spi</span>"<span class="hid">al cr</span>"<span class="hid">d,</span> + region of, <a href="#page385">385</a></p> + <p><span class="hid">Spi</span>"<span class="hid">al cr</span>"<span class="hid">d,</span> + termination in bird-embryo, <a href="#page51">51</a></p> + <p>Spondylus, retina of, <a href="#page114">114</a></p> + <p>Squilla, eyes, <a href="#page100">100</a></p> + <p><span class="hid">Sqi</span>"<span class="hid">lla,</span> glands, <a + href="#page422">422</a></p> + <p>Stomach, cephalic, <a href="#page4">4</a>, <a href="#page43">43</a>, <a + href="#page102">102</a>, <a href="#page244">244</a></p> + <p>Stylonurus Lagani, <b><a href="#page27">27</a></b>, <a href="#page235">235</a>, <a + href="#page239">239</a>, <a href="#page249">249</a></p> + <p>Substantia gelatinosa Rolandi, <a href="#page44">44</a></p> + <p>Suprarenal body, <a href="#page423">423</a></p> + <p>Surfaces, dorsal and ventral, <a href="#page11">11</a></p> + <p><span class="hid">Surf</span>"<span class="hid">ces,</span> reversal of, <a + href="#page15">15</a>, <a href="#page29">29</a>, <a href="#page36">36</a>, <a + href="#page87">87</a>, <a href="#page175">175</a>, <a href="#page352">352</a>, <a + href="#page433">433</a>, <a href="#page484">484</a></p> + <p>Synapse, <a href="#page72">72</a></p> + <p>Syncytium, <a href="#page464">464</a>, <a href="#page471">471</a>, <a + href="#page479">479</a></p> + <p class="stanza">Tail of lizards, <a href="#page50">50</a></p> + <p>Tapetum, <a href="#page69">69</a></p> + <p>Teleosteans, <a href="#page23">23</a>, <a href="#page345">345</a>, <a + href="#page420">420</a>, <a href="#page424">424</a></p> + <p>Tendon-organs, <a href="#page470">470</a></p> + <p>Tentacles of Ammocœtes, <a href="#page246">246</a>, <a href="#page289">289</a>, <a + href="#page303">303</a></p> + <p>Tergo-coxal muscles, <a href="#page247">247</a></p> + <p>Test, biological, of relationship of animals, <a href="#page492">492</a></p> + <p>Thalainencephalon, <a href="#page48">48</a></p> + <p>Thelodus, <a href="#page344">344</a></p> + <p>Thelyphonus, <b><a href="#page231">231</a></b></p> + <p><span class="hid">Thely</span>"<span class="hid">honus,</span> brain, <a + href="#page53">53</a>, <b><a href="#page54">54</a></b>, <b><a href="#page56">56</a></b>, <b><a + href="#page224">224</a></b></p> + <p><span class="hid">Thely</span>"<span class="hid">honus, br</span>"<span + class="hid">in,</span> caudal, <a href="#page450">450</a></p> + <p><span class="hid">Thely</span>"<span class="hid">honus,</span> cœcal diverticula, <a + href="#page109">109</a></p> + <p><span class="hid">Thely</span>"<span class="hid">honus,</span> entosternite, <b><a + href="#page143">143</a></b></p> + <p><span class="hid">Thely</span>"<span class="hid">honus,</span> genital organs, <a + href="#page206">206</a></p> + <p><span class="hid">Thely</span>"<span class="hid">honus,</span> lyriform organs, <a + href="#page368">368</a></p> + <p><span class="hid">Thely</span>"<span class="hid">honus,</span> olfactory passage, <b><a + href="#page226">226</a></b>, <a href="#page306">306</a></p> + <p><span class="pagenum" id="page536">{536}</span><span class="hid">Thely</span>"<span + class="hid">honus,</span> operculum, <a href="#page189">189</a>, <b><a + href="#page190">190</a></b>, <a href="#page206">206</a>, <b><a href="#page207">207</a></b></p> + <p>Theory, gastræa, <a href="#page444">444</a>, <a href="#page461">461</a></p> + <p>Theories of the origin of vertebrates, <a href="#page9">9</a>, <a href="#page411">411</a>, <a + href="#page433">433</a>, <a href="#page457">457</a></p> + <p>Thionin reaction, <a href="#page131">131</a>, <a href="#page139">139</a>, <a + href="#page213">213</a>, <a href="#page330">330</a>, <a href="#page336">336</a></p> + <p>Throat, formation of, <a href="#page179">179</a></p> + <p>Thyestes, <b><a href="#page30">30</a></b>, <b><a href="#page31">31</a></b>, <a + href="#page275">275</a>, <a href="#page326">326</a>, <b><a href="#page328">328</a></b>, <b><a + href="#page329">329</a></b>, <a href="#page339">339</a>, <b><a href="#page340">340</a></b>, + <b><a href="#page341">341</a></b></p> + <p>Thymus, <a href="#page425">425</a>, <a href="#page430">430</a></p> + <p>Thyroid gland of Ammocœtes, <a href="#page61">61</a>, <a href="#page127">127</a>, <a + href="#page192">192</a>, <b><a href="#page194">194</a></b>, <b><a href="#page196">196</a></b>, + <a href="#page429">429</a>, <a href="#page459">459</a></p> + <p><span class="hid">Thyr</span>"<span class="hid">id gl</span>"<span class="hid">nd</span> + evidence of the, <a href="#page185">185</a></p> + <p><span class="hid">Thyr</span>"<span class="hid">id gl</span>"<span class="hid">nd</span> + function of, in Ammocœtes, <a href="#page213">213</a></p> + <p>Tissues, connective, <a href="#page471">471</a>, <a href="#page474">474</a>, <a + href="#page481">481</a></p> + <p><span class="hid">Tiss</span>"<span class="hid">es,</span> evolution of, <a + href="#page19">19</a></p> + <p><span class="hid">Tiss</span>"<span class="hid">es,</span> notochordal, <a + href="#page435">435</a></p> + <p><span class="hid">Tiss</span>"<span class="hid">es,</span> two groups of, <a + href="#page463">463</a></p> + <p>Tongue of Ammocœtes, <a href="#page246">246</a>, <a href="#page303">303</a></p> + <p>Tonsils, <a href="#page427">427</a>, <a href="#page430">430</a></p> + <p>Torpedo, <a href="#page262">262</a>, <a href="#page392">392</a>, <a + href="#page470">470</a></p> + <p>Trabeculæ, <a href="#page121">121</a>, <a href="#page132">132</a>, <a + href="#page133">133</a>, <a href="#page145">145</a>, <a href="#page277">277</a>, <a + href="#page295">295</a>, <a href="#page377">377</a></p> + <p>Transformation of the Lamprey, <a href="#page18">18</a>, <a href="#page35">35</a>, <a + href="#page59">59</a>, <a href="#page61">61</a>, <a href="#page125">125</a>, <a + href="#page168">168</a>, <a href="#page193">193</a>, <a href="#page199">199</a>, <a + href="#page200">200</a>, <a href="#page220">220</a>, <a href="#page227">227</a>, <a + href="#page228">228</a>, <a href="#page287">287</a>, <a href="#page291">291</a>, <a + href="#page304">304</a>, <a href="#page307">307</a>, <a href="#page309">309</a>, <a + href="#page331">331</a>, <a href="#page336">336</a>, <a href="#page347">347</a>, <a + href="#page349">349</a>, <a href="#page389">389</a>, <a href="#page445">445</a></p> + <p>Tremataspis, <b><a href="#page32">32</a></b>, <a href="#page75">75</a>, <b><a + href="#page275">275</a></b>, <a href="#page326">326</a>, <a href="#page351">351</a>, <b><a + href="#page352">352</a></b></p> + <p>Trilobites, <a href="#page24">24</a>, <b><a href="#page25">25</a></b>, <b><a + href="#page26">26</a></b>, <b><a href="#page437">437</a></b></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> appendages, <a + href="#page351">351</a>, <b><a href="#page437">437</a></b></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> diagram of section through a + trilobite-like animal, <b><a href="#page413">413</a></b></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> dominance of, <a + href="#page26">26</a></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> excretory organs, <a + href="#page396">396</a></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> eyes, <a href="#page74">74</a>, + <a href="#page88">88</a></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> glabellum, <a + href="#page339">339</a></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> relations of, <a + href="#page249">249</a>, <a href="#page283">283</a></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> respiratory apparatus, <a + href="#page170">170</a></p> + <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> ventral surface, <b><a + href="#page437">437</a></b></p> + <p>Tube of central nervous system, <a href="#page37">37</a>, <a href="#page38">38</a>, <a + href="#page42">42</a>, <a href="#page102">102</a>, <a href="#page211">211</a>, <a + href="#page433">433</a>, <a href="#page455">455</a>, <a href="#page457">457</a></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> from IVth ventricle to surface of + brain in Ammocœtes, <a href="#page209">209</a></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> Fallopian, <a + href="#page431">431</a></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> hypophysial, <a + href="#page229">229</a>, <a href="#page244">244</a>, <a href="#page317">317</a>, <a + href="#page440">440</a></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> meeting of four tubes in vertebrate, + <b><a href="#page318">318</a></b>, <b><a href="#page440">440</a></b></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> notochord originally a, <a + href="#page436">436</a>, <a href="#page440">440</a></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> olfactory, of Ammocœtes, <a + href="#page219">219</a>, <a href="#page225">225</a>, <a href="#page317">317</a>, <a + href="#page440">440</a></p> + <p><span class="hid">Tu</span>"<span class="hid">e</span> unsegmented, in segmented animal, <a + href="#page439">439</a></p> + <p>Tunicata, <a href="#page16">16</a></p> + <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> budding of, <a + href="#page441">441</a></p> + <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> degeneration, <a + href="#page12">12</a>, <a href="#page17">17</a>, <a href="#page19">19</a>, <a + href="#page60">60</a></p> + <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> endostyle, <a + href="#page198">198</a>, <a href="#page212">212</a></p> + <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> hypophysis, <a + href="#page425">425</a></p> + <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> notochord, <a + href="#page438">438</a></p> + <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> position of, <a + href="#page494">494</a></p> + <p class="stanza">Unit, appendage, in non-branchial segments, <a href="#page185">185</a></p> + <p><span class="hid">Ui</span>"<span class="hid">t,</span> branchial, <a + href="#page161">161</a>, <a href="#page165">165</a>, <a href="#page168">168</a>, <a + href="#page185">185</a></p> + <p>Ureters, nerves of, <a href="#page448">448</a></p> + <p>Uterus of Scorpion group, <a href="#page189">189</a>, <a href="#page202">202</a>, <b><a + href="#page203">203</a></b>, <b><a href="#page204">204</a></b>, <b><a + href="#page205">205</a></b>, <a href="#page214">214</a></p> + <p><span class="hid">Utr</span>"<span class="hid">us</span> vertebrate, nerves of, <a + href="#page448">448</a></p> + <p class="stanza"><span class="pagenum" id="page537">{537}</span>Valve, ileo-colic, <a + href="#page449">449</a></p> + <p><span class="hid">Val</span>"<span class="hid">e,</span> of Vieussens, <a + href="#page48">48</a></p> + <p>Variation in dominant races, <a href="#page21">21</a>, <a href="#page88">88</a></p> + <p><span class="hid">Vari</span>"<span class="hid">tion</span> meristic, in spinal nerves, <a + href="#page154">154</a>, <a href="#page387">387</a></p> + <p>Veins, forming vertebrate heart, <b><a href="#page180">180</a></b></p> + <p>Velum, <a href="#page228">228</a>, <a href="#page289">289</a>, <a href="#page298">298</a>, + <b><a href="#page302">302</a></b></p> + <p>Vertebrates, alimentary canal, innervation of, <a href="#page446">446</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> atrial cavity, <a + href="#page410">410</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> auditory apparatus and + lateral-line system, <a href="#page356">356</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> body-cavity, <a + href="#page401">401</a>, <a href="#page430">430</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> brains, <b><a + href="#page40">40</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> branchial organs, <a + href="#page151">151</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> cœlomic cavities in + head region, <a href="#page251">251</a>, <a href="#page266">266</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> cranium, evolution of, <a + href="#page342">342</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> egg of, <a + href="#page483">483</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> evolution of, <a + href="#page11">11</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> excretory organs, <a + href="#page389">389</a>, <b><a href="#page391">391</a></b>, <b><a + href="#page408">408</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> glands, ductless, <a + href="#page418">418</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates, gla</span>"<span + class="hid">ds,</span> internal secretion of, <a href="#page215">215</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> heart, <a + href="#page175">175</a>, <a href="#page179">179</a>, <b><a href="#page180">180</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> muscles, evidence of + segmentation of eye, <a href="#page248">248</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates, mus</span>"<span + class="hid">les,</span> oblique, <b><a href="#page278">278</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates, mus</span>"<span + class="hid">les,</span> origin of somatic trunk, <a href="#page406">406</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> nervous system, central, <b><a + href="#page13">13</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> nerves, segmental, <a + href="#page152">152</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> notochord and gut, <a + href="#page434">434</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> organs of, <b><a + href="#page10">10</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> origin of, <a + href="#page9">9</a>, <a href="#page411">411</a>, <a href="#page433">433</a>, <a + href="#page457">457</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> segments, prosomatic, <a + href="#page257">257</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> skeleton, commencement of + bony, <a href="#page120">120</a>, <a href="#page458">458</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> spinal cord and medulla + oblongata, <b><a href="#page44">44</a></b></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> spinal region, <a + href="#page385">385</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> thyroid, connection between + generative organs and, <a href="#page215">215</a></p> + <p><span class="hid">Verte</span>"<span class="hid">rates,</span> tubes, meeting of four, <b><a + href="#page318">318</a></b>, <b><a href="#page440">440</a></b></p> + <p>Vesicles, cerebral, formation of, <a href="#page48">48</a>, <a href="#page458">458</a></p> + <p>Vitellophags, <a href="#page471">471</a>, <a href="#page483">483</a></p> + <p>Volvox, <a href="#page479">479</a></p> + <p class="stanza">Wolffian body, <a href="#page390">390</a></p> + <p class="stanza">Xiphosura, <a href="#page24">24</a>, <b><a href="#page26">26</a></b>, <a + href="#page249">249</a></p> + <p class="stanza">Yolk, <a href="#page482">482</a></p> + </div> + + <p class="sp4 ac">THE END</p> + + <div><span class="pagenum" id="page538">{538}</span></div> + + <p class="ac" style="margin-bottom:1.5ex;"><span class="x-larger">Notes.</span></p> + + <div class="foot"> + <a class="fnote" id="Nt_1" href="#NtA_1">[1]</a> + <p>N.B.—In addition to the nerves mentioned, C. Bell included, in his respiratory system + of nerves, the fourth nerve or trochlearis, the phrenic and the external respiratory of + Bell.</p> + </div> + + <div class="foot"> + <a class="fnote" id="Nt_2" href="#NtA_2">[2]</a> + <p>"The Origin of Vertebrates, deduced from the Study of Ammocœtes." Part X., "The Origin + of the Auditory Organ: the Meaning of the VIIIth Cranial Nerve." <i>Journ. Anat. and + Physiol.</i>, vol. 36, 1902.</p> + </div> + +<div>*** END OF THE PROJECT GUTENBERG EBOOK 44000 ***</div> +</body> +</html> diff --git a/44000-h/images/fig001.png b/44000-h/images/fig001.png Binary files differnew file mode 100644 index 0000000..0ea2c39 --- /dev/null +++ b/44000-h/images/fig001.png diff --git a/44000-h/images/fig002.png b/44000-h/images/fig002.png Binary files differnew file mode 100644 index 0000000..c68b1c2 --- /dev/null +++ b/44000-h/images/fig002.png diff --git a/44000-h/images/fig003.png b/44000-h/images/fig003.png Binary files differnew file mode 100644 index 0000000..95e5c20 --- /dev/null +++ b/44000-h/images/fig003.png diff --git a/44000-h/images/fig004.png b/44000-h/images/fig004.png Binary files differnew file mode 100644 index 0000000..31132e0 --- /dev/null +++ b/44000-h/images/fig004.png diff --git a/44000-h/images/fig005.png b/44000-h/images/fig005.png Binary files differnew file mode 100644 index 0000000..e9dc38a --- /dev/null +++ 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