Book - Text-Book of the Embryology of Man and Mammals

<centre>Text-Book of the Embryology of Man and Mammals

by Dr Oscar Hertwig (1892)

Professor extraordinarius of Anatomy and Comparative Anatomy, Director of the II. Anatomical Institute of the University of Berlin

TRANSLATED FROM THE THIRD GERMAN EDITION

BY

EDWARD L. MARK, PH.D.

Hersey Professor of Anatomy in Harvard University

with 339 figures in the text and 2 lithographic plates

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Translator's Preface

THE rapidly increasing recognition of the importance of Embryology in all morphological studies makes it desirable that the most valuable text-books upon the subject, in whatever language, be made available for those who are beginning its study. Although the English-reading student already has at command a number of text-books upon this subject, it is evident to any one familiar with HERTWIG'S Lehrbuch d?r Entwicklunysgeschichte des Me?ischen und der Wirbelthiere that thi> work covers the field of Vertebrate Embryology in a more complete and satisfactory way than any book heretofore published in English.

Two important objects to be accomplished in a text-book are : first, a clear and methodical exposition of the well-established facts of the science; and, secondly, such a presentation of unsettled questions as shall stimulate the reader to further inquiry and research. I believe it is far too common for the second of these aims to be overlooked. The present work fulfils both requirements in an eminent degree, and in its historical surveys exhibits an exceptional fairness of treatment, notwithstanding the author has been one of the foremost contestants in several of the fields reviewed. The summaries which follow the discussions of the several topics serve a useful purpose in directing attention to the more important conclusions drawn from each subject.

I have aimed to give a clear and accurate reproduction of the author's ideas ; while I have endeavored not always successfully to avoid awkward renderings and German idioms, I have preferred to err on the side of a too literal rather than a too liberal translation. There are a few points that demand a brief explanation. The German word Anlacje has heretofore been variously rendered into English by rudiment, origin, beginning, basis, foundation, etc., while some writers, recognising the inadequacy of any of these words to express the idea, have incorporated the German word itself in their English.

The Anlaye of a structure is its beginning or its undifferentiated state the object in a simple condition which is destined to be followed by a more complicated one. The use of rudiment in this sense is undesirable, because, in the interest of scientific accuracy, it is important to restrict its meaning, as in German, to a structure which is not destined to become more complicated, but which may have been, either ontogenetically or phylogenetically, even more highly developed than it now is. Origin and beginning are abstract terms, whereas A nlage is more frequently used in the concrete; basis and foundation (Grundlage) convey a wrong impression that of the substratum upon which the structure is erected. The need of a new word, which shall be used in the sense of Anlage, is evident. I suggest the adoption of an already existing word, -fundament, used at present only in a sense with which the proposed usage will not produce confusion. This word has been uniformly employed in the present translation, and the reader will see how readily and naturally it lends itself to this use. Fundament would thus bear the same relation to foundation that Anlage does to Grundlage.

I have also departed from authorised usage by sometimes employing for Bindeyeivebe and Stutzgewebe the term sustentative (in a mechanical sense) tissue, instead of connective tissue. My reason for this is the narrower meaning of connective as compared with sustentative.

In deference to a custom still followed in Human Anatomy, the author, in describing the relative positions of parts, has very generally used anterior and posterior for dorsal and ventral, etc. Instead of converting these expressions into terms which are independent of the temporary position of the organism, as I should have preferred, it has seemed better to indicate the direction by a bracketed word in those cases where a misunderstanding was most likely to occur. It has of course not been necessary to repeat this after each term of direction, but only after the first one of a series, the reader's attention being thus sufficiently directed to the matter to prevent any misconception.

The rapid advances in Embryology make it impossible for a book two years old to be a faithful reflection of the science of to-day in all its branches ; there are some topics in which even radical changes must be recognised. I have thought best, however, to reproduce the book as it left the hands of its author, and to content myself with calling the reader's attention to some of the topics in which the most important advances have been made, such as the metamerism of the head, and the plan and metamorphoses of the vessels of the visceral arches.

I am under very great obligations to my colleague, Dr. C. B. Davenport, for kind assistance and valuable criticism, but for which many defects of the translation would have been overlooked. I am also indebted to Drs. T. G. Lee, H. B. Ward, and W. McM. Woodworth for aid in reading portions of the proof.

E. L. MARK.

CAMBRIDGE, MASS.

Authors Preface to the First Edition

" Die Entwickelungsgeschichte 1st cler wahre Lichttrager fur Untersuchungen liber organische Kb'rper." C. E. v. BAER, "Ueber Entwickelungsgeschichte der Thiere " (Bd. L, S. 231).

THE Embryology of Animals, although one of the youngest shoots of morphological research, has, nevertheless, grown up in the course of sixty years, along with the cell-doctrine and that of the tissues, to a vigorous and stately tree. The comprehension of the structure of organisms has been extended in a high degree by numerous developmental investigations. The study of the human body has also derived great advantage from the same. In the newer anatomical textbooks (GrEGENBAUR, ScHWALBE) Embryology is receiving more and more attention in the description of the separate systems of organs. To what extent many things may be more clearly and attractively described in this manner is best shown by a comparison of the cles criptions of brain, eye, heart, etc., in the older and the more recent anatomical text-books.

Although it is generally recognised that Embryology constitutes " a foundation-stone of our comprehension of organic forms," nevertheless the attention which its importance warrants is not yet given to it ; it is especially true that it has not become as extensively as it should be a component of well-rounded medical and natural-history instruction, to which it is indispensable. The cause of this is perhaps in part to be sought in the fact that in student-circles the study of Embryology is often held to be especially difficult and a comprehension of it to be laborious. And thus many do not venture into this apparently obscure realm.

But ought the development of an organism to be really more difficult to comprehend than the complicated finished structure ? To a certain extent this was the case at a time when the most divergent and contradictory opinions prevailed concerning many of the most important processes of development, such as the formation of the germ-layers, the proto vertebrae, etc., which the lecturer had to take into account, and when many processes were not yet understood in their essence and their significance. But, thanks to the results of Comparative Embryology, the number of the unintelligible processes has been every year diminished, and in the same ratio the study of Embryology even for the beginner has been rendered easier.

At least, it is not in any way an essential feature of the process of development that it should be more difficult to understand than the structure of the completed form. For every development begins with a very simple condition, from which the more complicated is gradually derived and by which it is explained.

Inasmuch as I have for twelve years pursued the study of Embryology with especial interest, both in annually recurring academic lectures and in a series of scientific investigations, the desire has been awakened in me to acquire for Embryology a broader and more secure foundation in education, and to procure for it admission into larger circles of medical men and well-educated naturalists. As the result of this there has come into existence the book which is before us, in which the especial problem has been to make the complicated structure of the human body more intelligible through the knowledge of its development.

For the solution of this problem I have in the present text-book placed the comparative method of investigation in the foreground. I do not thereby find myself in any way in opposition to another direction of embryological research, which places the objective point in the physiological or mechanical explanation of the form of the animal body. Such a direction I hold to be fully warranted, and I believe that, instead of being opposed to a comparative-morphological direction, it can be of the most permanent value to it in the solution of its problems. One will find that I have here given full attention to the mechanico-physiological explanation of forms. Compare the sections on cell-division and Chapter IV., " General Discussion of the Principles of Development," in which the laws of unlike growth and the processes of the formation of folds and evaginations are treated.

In the presentation of the separate processes of development, in the main the important things only have been selected, the subsidiary left out, in order thus to make the introduction into embryological study easier. In the case of fundamental theories I have gone into their history extensively, because it is of great interest, and under certain circumstances operates as a stimulus, for one to see in what way the state of a scientific question for the time being has been attained. In pending controversial questions.

I have, it is true, employed chiefly as the foundation of my presentation the views which appear to me the most entitled to acceptance, but have not left immentioned opposing conceptions.

Numerous figures in the text, as well as some colored plates, will contribute materially to the easier comprehension of the various developmental processes.

I submit, then, this text-book to physicians and to students of medicine and the natural sciences, with the desire that it may promote and facilitate the study of Embryology in wider circles, and that it may thereby contribute to a deeper insight into the structure of our own bodies.

OSCAK HERTWIG. JENA, October 1886.

Authors Preface to the Second Edition

THE friendly reception which the " Text-book of the Embryology of Man and Mammals" has found, is an indication of the increased interest which this branch of Morphology now meets with.

Even more than a year ago, after the first part of the text-book appeared and while the second part was in the press, the necessity of preparing a second edition became evident.

In this edition fundamental changes have not been undertaken ; the text has, however, undergone an expansion in some places, owing to the attention given to several works which have recently appeared. This has been the case with the section on the first developmental processes of the egg (WEISMANX, BLOCHMANN) ; that on the origin of the vascular s}^stem (RABL, RUCKERT) ; that on the development of the foetal membranes (DuvAL, OSBORX) ; and that on the human placenta (KASTSCHEXKO, WALDEYER, HUGE).

As the second part of the text-book has just appeared, it has been possible to incorporate it in the second edition without alteration.

It has, furthermore, seemed to me expedient in the second edition to distribute at the ends of the several chapters the synopses of the literature, which in the first edition were brought together at the close of the whole work. Finally, there has been added an index of subjects, by which a more rapid orientation concerning the separate topics will be facilitated ; this will increase the usefulness of the work.

May the book in this form make for itself new friends, not only among students of medicine and the natural sciences, but also with all those who have a fondness for and a comprehension of studies in natural science.

OSCAR HERTWIG. JEXA, February 1888.

Authors Preface to the Third Edition

IN the two years which have elapsed since the appearance of the second edition of this text-book, our knowledge of the embryology of Vertebrates has experienced many important enrichments, thanks to the numerous investigations which are annually published. Therefore, as the problem of preparing a third edition of the text-book confronted me, I was compelled to make extensive changes in many places. Thus the second and third chapters, concerning the processes of fertilisation and cleavage of the egg, have undergone expansion, owing to the presentation of the important discoveries which have been made on the the egg of Ascaris megalocephala. I have given an entirely new wording to the ninth chapter on the development of connective substance and blood, also to the sections on the origin of the urinary organs and the development of the peripheral nervous system, and, finally, to the account of the development of the heart and the venous system. Also at other places one will often recognise the hand of improvement.

The third edition has been essentially improved by the addition of thirty new figures, which I have taken from the investigations of VAN BENEDEN, BOVERI, DUVAL, FLEMMING, HERMANN, His, BORN, GEGENBAUR, NAGEL, VAN WIJHE, GRAF SPEE, BONNET, and IVETBEL. Through the friendliness of Professor VAN BENEDEN I was also put in a position to employ for my text-book three figures out of his hitherto unpublished extensive work on the development of the germinal layers of the Rabbit. By means of the increase in the number of figures I hope that I have been able to render still easier the comprehension of many of the processes of development.

And so I close the preface to the third edition by expressing my thanks to all those who have rendered me friendly aid, and especially to the publisher, who in the further equipment of the text-book has met my wishes with the greatest willingness.

OSCAR HERTWIG. BERLIN, March 1890.