Read Ebook: The Origin of Vertebrates by Gaskell Walter Holbrook

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Ebook has 1354 lines and 238095 words, and 28 pages

PAGE INTRODUCTION 1

THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM

Theories of the origin of vertebrates--Importance of the central nervous system--Evolution of tissues--Evidence of Palaeontology-- Reasons for choosing Ammocoetes 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 Ammocoetes compared with brain of arthropod--Summary 8

THE EVIDENCE OF THE ORGANS OF VISION

Different kinds of eye--Simple and compound retinas--Upright and inverted retinas--Median eyes--Median or pineal eyes of Ammocoetes 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 68

THE EVIDENCE OF THE SKELETON

The bony and cartilaginous skeleton considered, not the notochord-- Nature of the earliest cartilaginous skeleton--The mesosomatic skeleton of Ammocoetes; its topographical arrangement, its structure, its origin in muco-cartilage--The prosomatic skeleton of Ammocoetes; the trabeculae and parachordals, their structure, their origin in white fibrous tissue--The mesosomatic skeleton of Limulus compared with that of Ammocoetes; similarity of position, of structure, of origin in muco-cartilage--The prosomatic skeleton of Limulus; the entosternite, or plastron, compared with the trabeculae of Ammocoetes; similarity of position, of structure, of origin in fibrous tissue--Summary 119

THE EVIDENCE OF THE RESPIRATORY APPARATUS

Branchiae 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 Ammocoetes--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 148

THE EVIDENCE OF THE THYROID GLAND

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 Ammocoetes-- Its structure--Its openings--The nature of the thyroid segment--The uterus of the scorpion--Its glands--Comparison with the thyroid gland of Ammocoetes--Cephalic generative glands of Limulus-- Interpretation of glandular tissue filling up the brain-case of Ammocoetes--Function of thyroid gland--Relation of thyroid gland to sexual functions--Summary 185

THE EVIDENCE OF THE OLFACTORY APPARATUS

Fishes divided into Amphirhinae and Monorhinae--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 antennae--Its termination at the true mouth--Comparison with the olfactory tube of Ammocoetes--Origin of the nasal tube of Ammocoetes from the tube of the hypophysis-- Direct comparison of the hypophysial tube with the olfactory tube of the scorpion group--Summary 218

THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES

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 Ammocoetes-- Prosomatic segmentation shown by marks on carapace--Evidence of coelomic cavities in Limulus--Summary 233

THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP

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 257

THE PROSOMATIC SEGMENTS OF AMMOCOETES

The prosomatic region in Ammocoetes--The suctorial apparatus of the adult Petromyzon--Its origin in Ammocoetes--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 palaeostoma--The pituitary gland--Its comparison with the coxal gland of Limulus--Summary 286

THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE OSTRACODERMATA

The nose of the Osteostraci--Comparison of head-shield of Ammocoetes and of Cephalaspis--Ammocoetes 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 326

THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE

THE REGION OF THE SPINAL CORD

Difference between cranial and spinal regions--Absence of lateral root--Meristic variation--Segmentation of coelom--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 coelom of Peripatus and of vertebrate--Pronephric organs compared to coxal glands--Origin of vertebrate body-cavity --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 Cephalaspidae 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 385

THE NOTOCHORD AND ALIMENTARY CANAL

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 Ammocoetes--Formation of new gut in Ammocoetes 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 433

THE PRINCIPLES OF EMBRYOLOGY

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 coelom--Formation of neural canal--Gastrula of Amphioxus and of Lucifer--Summary 455

FINAL REMARKS

Problems requiring investigation--

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.

Criticisms of Balanoglossus theory--Theory of parallel development-- Importance of the theory advocated in this book for all problems of Evolution 488

GENERAL INDEX 517

THE

ORIGIN OF VERTEBRATES

In former days it was possible for a man like Johannes M?ller 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.

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.

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.

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, both cranial and spinal, and also the original segmental character of this part of the nervous system.

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 oesophagus, there in the cranial segmental nerves the infraoesophageal ganglia, there in the cerebral hemispheres and optic and olfactory nerves the supraoesophageal ganglia, there in the spinal cord the ventral chain of ganglia. But if the infundibulum was the old oesophagus, what then? The old oesophagus 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.

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.

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 forward in this book, and to the latter for his great kindness in undertaking the laborious task of collecting the proofs.

Theories of the origin of vertebrates.--Importance of the central nervous system.--Evolution of tissues.--Evidence of Palaeontology.--Reasons for choosing Ammocoetes 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 Ammocoetes compared with brain of arthropod.--Summary.

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.

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.

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; thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Coelenterata, 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.

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 palaeontology 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.

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.

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.

THEORIES OF THE ORIGIN OF VERTEBRATES.

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?

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