Chapter XI: Preface

The present Monograph is a reprint of a series of papers published in the _Journal of Anatomy and Physiology_ during the years 1876, 1877 and 1878. The successive parts were struck off as they appeared, so that the earlier pages of the work were in print fully two years ago. I trust the reader will find in this fact a sufficient excuse for a certain want of coherence, which is I fear observable, as well as for the omission of references to several recent publications. The first and second chapters would not have appeared in their present form had I been acquainted, at the time of writing them, with the researches which have since been published, on the behaviour of the germinal vesicle and on the division of nuclei. I may also call attention to the valuable papers of Prof. His[66] on the formation of the layers in Elasmobranchii, and of Prof. Kowalevsky[67] on the development of Amphioxus, to both of which I would certainly have referred, had it been possible for me to do so.

Footnote 66: _Zeitschrift f. Anat. u. Entwicklungsgeschichte_,
Bd. II.

Footnote 67: _Archiv f. Micr. Anat._ Bd. XIII.

Professor His deals mainly with the subjects treated of in Chapter III., and gives a description very similar to my own of the early stages of development. His interpretations of the observed changes are, however, very different from those at which I have arrived. Although this is not the place for a discussion of Prof. His's views, I may perhaps state that, in spite of the arguments he has brought forward in support of his position, I am still inclined to maintain the accuracy of my original account. The very striking paper on Amphioxus by Kowalevsky (the substance of which I understand to have been published in Russia at an earlier period) contains a confirmation of the views expressed in chapter VI. on the development of the mesoblast, and must be regarded as affording a conclusive demonstration, that in the case of Vertebrata the mesoblast has primitively the form of a pair of diverticula from the walls of the archenteron.

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The present Memoir, while differing essentially in scope and object from the two important treatises by Professors His[68] and Götte[69], which have recently appeared in Germany, has this much in common with them, that it deals monographically with the development of a single type: but here the resemblance ends. Both of these authors seek to establish, by a careful investigation of the development of a single species, the general plan of development of Vertebrates in general, if not of the whole animal kingdom. Both reject the theory of descent, as propounded by Mr Darwin, and offer completely fresh explanations of the phenomena of Embryology. Accepting, as I do, the principle of natural selection, I have had before me, in writing the Monograph, no such ambitious aim as the establishment of a completely new system of Morphology. My object will have been fully attained if I have succeeded in adding a few stones to the edifice, the foundations of which were laid by Mr Darwin in his work on the _Origin of Species_.

Footnote 68: _Erste Anlage des Wirbelthierleibes._

Footnote 69: _Entwicklungsgeschichte der Unke._

I may perhaps call attention to one or two special points in this work which seem to give promise of further results. The chapter on the Development of the Spinal and Cranial Nerves contains a modification of the previously accepted views on this subject, which may perhaps lead to a more satisfactory conception of the origin of nerves than has before been possible, and a more accurate account of the origin of the muscle-plates and vertebral column. The attempt to employ the embryological relations of the cephalic prolongations of the body-cavity, and of the cranial nerves, in the solution of the difficult problems of the Morphology of the head, may prove of use in the line of study so successfully cultivated by our great English Anatomist, Professor Huxley. Lastly, I venture to hope that my conclusions in reference to the relations of the sympathetic system and the suprarenal body, and to the development of the mesoblast, the notochord, the limbs, the heart, the venous system, and the excretory organs, are not unworthy of the attention of Morphologists.

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The masterly manner in which the systematic position of Elasmobranchii is discussed by Professor Gegenbaur, in the introduction to his Monograph on the Cranial Skeleton of the group, relieves me from the necessity of entering upon this complicated question. It is sufficient for my purpose that the Elasmobranch Fishes be regarded as forming one of the most primitive groups among Vertebrates, a view which finds ample confirmation in the importance of the results to which Prof. Gegenbaur and his pupils have been led in this branch of their investigations.

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Though I trust that the necessary references to previous contributions in the same department of enquiry have not been omitted, the 'literature of the subject' will nevertheless be found to occupy a far smaller share of space than is usual in works of a similar character. This is an intentional protest on my part against, what appears to me, the unreasonable amount of space so frequently occupied in this way. The pages devoted to the 'previous literature' only weary the reader, who is not wise enough to skip them, and involve a great and useless expenditure of time on the part of any writer, who is capable of something better than the compilation of abstracts.

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In conclusion, my best thanks are due to Drs Dohrn and Eisig for the uniformly kind manner in which they have forwarded my researches both at the Zoological Station in Naples, and after my return to England; and also to Mr Henry Lee and to the Manager and Directors of the Brighton Aquarium, who have always been ready to respond to my numerous demands on their liberality.

To my friend and former teacher Dr Michael Foster I tender my sincerest thanks for the never-failing advice and assistance which he has given throughout the whole course of the work.

TABLE OF CONTENTS.

CHAPTER I.

THE RIPE OVARIAN OVUM, pp. 213-221.

Structure of ripe ovum. Atrophy of germinal vesicle. The extrusion of its membrane and absorption of its contents. Oellacher's observations on the germinal vesicle. Götte's observations. Kleinenberg's observations. General conclusions on the fate of the germinal vesicle. Germinal disc.

CHAPTER II.

THE SEGMENTATION, pp. 222-245.

Appearance of impregnated germinal disc. Stage with two furrows. Stage with twenty-one segments. Structure of the sides of the furrows. Later stages of segmentation. Spindle-shaped nuclei. Their presence outside the blastoderm. Knobbed nuclei. Division of nuclei. Conclusion of segmentation. Nuclei of the yolk. Asymmetry of the segmented blastoderm. Comparison of Elasmobranch segmentation with that of other meroblastic ova. Literature of Elasmobranch segmentation.

CHAPTER III.

FORMATION OF THE LAYERS, pp. 246-285.

Division of blastoderm into two layers. Formation of segmentation cavity. Disappearance of cells from floor of segmentation cavity. Nuclei of yolk and of blastoderm. Formation of embryonic rim. Appearance of a layer of cells on the floor of the segmentation cavity. Formation of mesoblast. Formation of medullary groove. Disappearance of segmentation cavity. Comparison of segmentation cavity of Elasmobranchii with that of other types. Alimentary cavity. Formation of mesoblast in two lateral plates. Protoplasmic network of yolk. Summary. Nature of meroblastic ova. Comparison of Elasmobranch development with that of other types. Its relation to the Gastrula. Haeckel's views on vertebrate Gastrula. Their untenable nature. Comparison of primitive streak with blastopore. Literature.

CHAPTER IV.

GENERAL FEATURES OF THE ELASMOBRANCH EMBRYO AT SUCCESSIVE
STAGES, pp. 286-297.

Description of Stages A-Q. Enclosure of yolk by blastoderm. Relation of the anus of Rusconi to the blastopore.

CHAPTER V.

STAGES B-G, pp. 298-314.

_General features of the epiblast._--Original uniform constitution. Separation into lateral and central portions. _The medullary groove._--Its conversion into the medullary canal. _The mesoblast._--Its division into somatic and splanchnic layers. Formation of protovertebræ. The lateral plates. The caudal swellings. The formation of the body-cavity in the head. _The alimentary canal._--Its primitive constitution. The anus of Rusconi. Floor formed by yolk. Formation of cellular floor from cells formed around nuclei of the yolk. Communication behind of neural and alimentary canals. Its discovery by Kowalevsky. Its occurrence in other instances. _General features of the hypoblast._ _The notochord._--Its formation as a median thickening of the hypoblast. Possible interpretations to be put on this. Its occurrence in other instances.