The Cell in Development and Inheritance

This book is due on the date indicated below and is subject to an overdue fine as posted at the Circulation Desk. This volume is the outcome of a course of lectures, delivered at Columbia University in the winter of 1892-93, in which I endeavoured to give to an audience of general university students some account of recent advances in cellular biology, and more especially to trace the steps by which the problems of evolution have been reduced to problems of the cell. It was my first intention to publish these lectures in a simple and general form, in the hope of showing to wider circles how the varied and apparently heterogeneous cell- researches of the past twenty years have grown together in a coherent group, at the heart of which are a few elementary phe- nomena, and how these phenomena, easily intelligible even to those having no special knowledge of the subject, are related to the problems of development. Such a treatment was facilitated by the appearance, in 1893, of Oscar Hertwig's invaluable book on the cell, which brought together, in a form well designed for the use of special students, many of the more important results of modern cell-research. I am glad to acknowledge my debt to Hert- wig's book ; but it is proper to state that the present volume was fully sketched in its main outlines at the time the Zelle mid Gcwcbc appeared. Its completion was, however, long delayed by investiga- tions which I undertook in order to re-examine the history of the centrosomes in the fertilization of the ^gg, — a subject which had been thrown into such confusion by Fol's extraordinary account of the " Quadrille of Centres " in echinoderms that it seemed for a time impossible to form any definite conception of the cell in its relation to inheritance. By a fortunate coincidence the same task was inde- pendently undertaken, nearly at the same time, by several other investigators.

The concordant results of these researches led to a decisive overthrow of Fol's conclusions, and the way was thus cleared for a return to the earlier and juster views founded by Hertwig, Strasburger, and Van Beneden, and so lucidly and forcibly developed by Boveri. The rapid advance of discovery in the mean time has made it seem desirable to amplify the original plan of the work, in order to render it useful to students as well as to more general readers ; and to this end it has been found necessary to go over a considerable

part of the ground already so well covered by Hertwig.i This book does not, however, in any manner aim to be a treatise on general histology, or to give an exhaustive account of the cell. It has rather been my endeavour to consider, within moderate limits, those features of the cell that seem more important and suggestive to the student of development, and in some measure to trace the steps by which our present knowledge has been acquired. A work thus limited neces- sarily shows many gaps ; and some of these, especially on the botani- cal side, are, I fear, but too obvious. On its historical side, too, the subject could be traced only in its main outlines, and to many investigators of whose results I have made use it has been impossible to do full justice.

To the purely speculative side of the subject I do not desire to add more than is necessary to define some of the problems still to be solved ; for I am mindful of Blumenbach's remark that while Drelin- court rejected two hundred and sixty-two "groundless hypotheses" of development, ** nothing is more certain than that Drelincourt's own theory formed the two hundred and sixty-third." ^ i have no wish to add another to this list. And yet, even in a field where standpoints are so rapidly shifting and existing views are still so widely opposed, the conclusions of the individual observer may have a certain value if they point the way to further investigation of the facts. In this spirit I have endeavoured to examine some of the more important existing views, to trace them to their sources, and in some measure to give a critical estimate of their present standing, in the hope of finding suggestion for further research.

Every writer on the cell must find himself under a heavy obliga- tion to the works of Van Beneden, Oscar Hertwig, Flemming, Stras- burger, and Boveri ; and to the last-named author I have a special sense of gratitude. I am much indebted to my former student, Mr. A. P. Mathews, for calling my attention to the importance of the recent work of physiological chemists in its bearing on the problems of synthetic metabolism. The views developed in Chap- ter VII. have been considerably influenced by his suggestions, and this subject will be more fully treated by him in a forthcoming work ; but I have endeavoured as far as possible to avoid anticipating his own special conclusions. Among many others to whom I am indebted for kindly suggestion and advice, I must particularly mention my ever helpful friend. Professor Henry F. Osborn, and Professors J. E. Humphrey, T. H. Morgan, and F. S. Lee.

In copying so great a number of figures from the papers of other 1 Henneguy's Le(;ons sur la cellule is received, too late for further notice, as this volume is going through the press. ^ Allen Thomson. investigators, I must make a virtue of necessity. Many of the facts could not possibly have been illustrated by new figures equal in value to those of special workers in the various branches of cytological research, even had the necessary material and time been available. But, apart from this, modern cytology extends over so much debatable ground that no general work of permanent value can be written that does not aim at an objective historical treatment of the subject; and I believe that to this end the results of investigators should as far as practicable be set forth by means of their original figures. Those for which no acknowledgment is made are original or taken from my own earlier papers.

The arrangement of the literature lists is as follows. A general list of all the works referred to in the text is given at the end of the book (p. 449). These are arranged in alphabetical order, and are referred to in the text by name and date, according to Mark's con- venient system. In order, however, to indicate to students the more important references and partially to classify them, a short separate list is given at the end of each chapter. The chapter-lists include only a few selections from the general list, comprising especially works of a general character and those in which reviews of the special literature may be found.

Since the appearance of the first edition of this work, in 1896, the aspect of some of the most important questions with which it deals has materially changed, most notably in case of those that are f ocussed in the centrosome and involve the phenomena of cell-division and fertilization. This has necessitated a complete revision of the book, many sections having been entirely rewritten, while minor changes have been made on almost every page. In its first form, the work was compressed within limits too nar- row for a sufficiently critical treatment of many disputed subjects. It has therefore been considerably enlarged, and upwards of fifty new illustrations have been added. The endeavour has, however, still been made to keep the book within moderate Hmits, even at some cost of comprehensiveness ; and the present edition aims no more than did the first to cover the whole vast field of cellular biology. Its limita- tions are, as before, especially apparent in the field of botanical cytology. Here progress has been so rapid that, apart from the dif- ficulty experienced by a zoologist in the attempt to maintain a due sense of proportion in reviewing the subject, an adequate treatment would have required a separate volume. I have therefore, for the most part, considered the cytology of plants in an incidental way, endeavouring only to bring the more important phenomena into rela- tion with those more fully considered in the case of animals.

The steady and rapid expansion of the literature of the general subject renders increasingly difficult the historical form of treatment and the citation of specific authority in matters of detail. This plan has nevertheless still been followed as far as possible, despite the increased bulk of the book and the encumbrance of the text with references thus occasioned, in the hope that these disadvantages will be outweighed by increased usefulness of the work. I beg the reader to remember, however, that no approach to a complete history of cytology and experimental embryology could be attempted, save in a work of far greater proportions, and that it has been necessary

to pass by, or dismiss with very brief mention, many works to which space would gladly have been given. Recent research has yielded many new results of high interest, conspicuous among them the outcome of experiments on cell-division, fertilization, and regeneration ; and they have cleared up many special problems. Broadly viewed, however, the recent advance of discovery has not, in the author's opinion, tended to simplify our conceptions of cell-life, but has rather led to an emphasized sense of the diversity and complexity of its problems. " One is sometimes tempted to con- clude," was recently remarked by a well-known embryologist, " that every <t%g is a law unto itself ! " The jest, perhaps, embodies more of the truth than its author would seriously have maintained, express- ing, as it does, a growing appreciation of the intricacy of cell-phe- nomena, the difficulty of formulating their general aspects in simple terms, and the inadequacy of some of the working hypotheses that have been our guides. It is in the full recognition of such inade- quacy, when it exists, and of the danger of hasty generalization in a subject so rapidly moving as this, that our best hope of progress lies.

My best thanks are again due to many friends for helpful criti- cism, suggestion, and other aid ; and I am indebted to Professor Ulric Dahlgren for the beautiful preparation imperfectly represented by Fig. 59 (from a direct photograph); to F. Emil, E. M. Van Harlin- gen, and Dr. G. N. Calkins, for aid in the preparation of new illus- trations ; and to Messrs. Ginn & Co. for the electrotypes of Figs. 1 1, 12, and 1 88, from the Wood's Holl Biological Lectures for 1899.

Postscript. — Of especial importance for some of the discussions in Chapters I., V., and VII. are F'ischer's extensive work on protoplasm (see Literature, I.) and Strasburger's new researches on reduction (see Literature, V.), both received while this volume was in press and too late for more than a passing mention in the text. Historical Sketch of the Cell-theory; its Relation to the Evolution-theory. Earlier Views of Inheritance and Development. Discovery of the Germ-cells. Cell- division, Cleavage, and Development. Modern Theories of Inheritance. Lamarck,

(/7) Theory of Kil)rillar Contractility {b) Other Facts and Theories E. * Staining-reactions of the Germ-nuclei Literature, III. ..... D. Some Peculiarities of Reduction in the Insects . . . . . . -271 {a) Hypothesis of the Individuality of the Chromosomes (/;) Composition of the Chromosomes .... G. The Nucleus in Later Development H. The External Conditions of Development I. Development, Inheritance, and Metabolism . J. Preformation and Epigenesis. The Unknown Factor in Development Literature, IX.

General Literature- list Lndex of Authors . Index of Subjects . Group of cells, showing cytoplasm, nucleus, and centrosomes Living cells of salamander larva, showing fibrillar structure . Alveolar or foam-structure of protoplasm, according to BiitschU Structure of protoplasm in the echinoderm e^ Aster-formation in alveolar protoplasm . Nuclei from the crypts of Lieberkiihn . Special forms of nuclei .... Scattered nucleus in Trachelocerca Scattered nucleus in Bacteria and Flagellata Ciliated cells . Cells of amphibian pancreas

34. Mitosis in pollen-mother-cells of lily, according to Guignard 80. Ovarian eggs of spider, earthworm, ascidian, showing yolk-nucleus 84. P'ormation of the spermatozoon in Salamandra and Amphinma 99. Fertilization and persistence of centrosomes in Thalassema 146. Independence of chromosomes in fertilization of Cyclops 151. Artificially produced asters and centrosomes in echinoderms 152. Diagram of different types of centrosome and centrosphere 157. History of chromosomes in the germinal vesicle of sharks

" Jedes Thier erscheint ah eine Sunwie vitaler Einheiten, von denen jede den vollen Charakter des Lebens an sich tragt." \'lRCHO\v.i During the half-century that has elapsed since the enunciation of the cell-theory by Schleiden and Schwann, in 1838-39, it has become ever more clearly apparent that the key to all ultimate biological problems must, in the last analysis, be sought in the cell. It was the cell-theory that first brought the structure of plants and animals under one point of view, by revealing their common plan of organization. It was through the cell-theory that Kolliker, Remak, Nageli, and Hof- meister opened the way to an understanding of the nature of embryo- logical development, and the law of genetic continuity lying at the basis of inheritance. It was the cell-theory again which, in the hands of Goodsir, Virchow, and Max Schultze, inaugurated a new era in the history of physiology and pathology, by showing that all the various functions of the body, in health and in disease, are but the outward expression of cell-activities. And at a still later day it was through the cell-theory that Hertwig, Fol, Van Beneden, and Strasburgcr solved the long-standing riddle of the fertilization of the ^gg and the mechanism of hereditary transmission. No other biological generali- zation, save only the theory of organic evolution, has brought so many apparently diverse phenomena under a common point of view or has accomplished more for the unification of knowledge. The cell-theory must therefore be placed beside the evolution-theory as one of the foundation stones of modern biology.

And yet the historian of latter-day biology cannot fail to be struck with the fact that these two great generalizations, nearly related as they are, have been developed along widely different lines of research, and have only within a very recent period met upon a common ground. The theory of evolution originally grew out of the study of natural history, and it took definite shape long before the ultimate structure of living bodies was in any degree comprehended. The evolutionists of the Lamarckian period gave little heed to the finer details of internal organization. They were concerned mainly with the more

obvious characters of plants and animals — their forms, colours, habits, distribution, their anatom\' and embryonic development — and with the systems of classification based upon such characters ; and long afterward it was, in the main, the study of like characters with reference to their historical origin that led Darwin to his splen- did triumphs. The study of microscopical anatomy, on which the cell-theory was based, lay in a different field. It was begun and long carried forward with no thought of its bearing on the origin of living forms ; and even at the present day the fundamental problems of organization, with which the cell-theory deals, are far less accessible to historical inquiry than those suggested by the more obvious external characters of ])lants and animals. Only within a few years, indeed, has the ground been cleared for that close alliance between students of organic evolution and students of the cell, which forms so striking a feature of latter-day biology and is exerting so great an influ- ence on the direction of research. It has, therefore, only recently become possible adequately to formulate the great problems of devel- opment and heredity in the terms of cellular biology — indeed, we can as vet do little more than so formulate them. Yet the fact that these two great lines of research, both concerned with the deeper problems of life, yet having their beginnings so far apart, have at length converged to a meeting-point, is one of the more striking evidences of progress that modern biology has to show ; and it sufficiently justifies an attempt to treat the cell from the standpoint of the general student of development.

Let us at the outset briefly outline the cell-theory as thus regarded, and indicate the manner of its historical connection with the general problems of evolution.^ ^ Schleiden and Schwann are universally and justly recognized as the founders of the cell- theory; hut like every other great generalization the theory was based on a long series of earlier investigations l)eginniiig with the memorable microscopical researches of l.eeuwen- hoek, Mali)ighi, lIo(>ke, and (jrew in the second lialf of the seventeenth century.

Wolff, in the Theoria Cenerationis (1759), clearly recognized the "spheres" and "vesi- cles" composing the embryonic parts both of animals and of plants, though without grasping iheir real nature or mode of origin, and his conclusions were developed by the botanist Mirbel at the beginning of the i^resent century. Nearly at the same time (1805) Oken fore- shadowed the cell-theory in the form that it assumed with Schleiden and Schwann; but his conception of " Urschleim " and " Hlaschen " can hardly be regarded as more than a lucky guess. A still closer approximation to the truth is fuuntl in the works of 'ruri)in (1826), Meyen (1830), Raspail (1831), and Dutrochet (1837); '^"^^ these, like others of the same period, only paved the way for the real founders of the cell-theory. Among other immedi- ate predecessors f)r contemporaries of Schleiden and Schwann should be especially mentioned Robert Brown, Dujardin, Johannes Miiller, I'urkinje, Hugo von Mohl, Valentin, Unger, Nageli, and Henle. The significance of Schleiden's, and especially of Schwann's, work lies in the thorough and comprehensive way in which the problem was studied, the philosophic breadth with which the conclusions were developed, and the far-reaching influence which they exercised upon subsequent research. In this respect it is hardly too much to com- pare the Mikroikopische L'ntersiichnngcn with the Origin of Species.

During the past thirty years the theory of organic descent has been shown, by an overwhelming mass of evidence, to be the only tenable conception of the origin of diverse living forms, however we mav conceive the causes of the process. While the study of general zoology and botany has systematically set forth the results, and in a measure the method, of organic evolution, the study of microscopical Fig i._ A portion of the epidermis of a larval salamander {Amblystoma) as seen in slightly oblique horizontal section, enlarged 550 diameters. Most of the cells are polygonal m form, con- tain large nuclei, and are connected by delicate protoplasmic bridges. Above v is a branched, dark pigment-cell that has crept up from the deeper layers and lies between the epidermal ceLs. Three of the latter are undergoing division, the earliest stage {sp,rcme) at a, ^J l-';»«^r/»-'f ("^"^"^ figure in the anaphase) at b, showing the chromosomes, and a final stage {telophase), showing fission of the cell-body, to the right.

anatomy has shown us the nature of the material on which it has operated, demonstrating that the obvious characters ot plants and animals are but varving expressions of a subtle interior organization common to all. In its broader outlines the nature of this organiza- tion is now accurately determined; and the ''cell-theory," by which it is formulated, is, therefore, no longer of an inferential or hypo- thetical character, but a generalized statement of observed fact which may be outlined as follows : — *

In all the higher forms of life, whether plants or animals, the body may be resolved into a vast host of minute structural units known as cells, out of which, directly or indirectly, every part is built (Figs. 1,2). The substance of the skin, of the brain, of the blood, of the bones or muscles or any other tissue, is not homogeneous, as it appears to the unaided eye, but is shown by the microscope to be an aercrretrate composed of innumerable minute bodies, as if it were a

Fig. 2. — General view of cells in the growing root-tip of the onion, from a longitudinal section, enlarged 800 diameters. a. non-dividing cells, with chromatin-network and deeply stained nucleoli ; b. nuclei preparing for division (spireme-stage) ; <r. dividing cells showing mitotic figures; e. pair of daughter-cells shortly after division. colony or congeries of organisms more elementary than itself. The name cells given to these bodies by the early botanists, and ulti- mately adopted by nearly all students of microscopical anatomy, was not happily chosen ; for modern studies have shown that although the cell may assume the form of a hollow chamber, as the name indicates, this is not one of its characteristic or even usual features. Essentially the cell is a minute mass of protoplasm, a substance long since identified by Cohn, Leydig, Max Schultze, and De Bary as the essential active basis of the organism, afterward happily characterized

by Huxley as the *' physical basis of life," and at the present time universally recognized as the immediate substratum of all vital activity.^ Endlessly diversified in the details of their form and struc- ture, these protoplasmic masses nevertheless possess a characteristic type of organization common to them all; hence, in a certain sense, they may be regarded as elementary organic units out of which the body is compounded. This composite structure is, however, character-

Fig. 3. — Amcela Proteus, an animal consisting of a single naked cell, x 280. (From Sedgwick and Wilson's Biology.) n. The nucleus; iv.v. water-vacuoles ; c.v. contractile vacuole ; f.v. food-vacuole, istic of only the higher forms of life. Among the lowest forms at the base of the series are an immense number of microscopic plants and animals, famiUar examples of which are the bacteria, diatoms, rhizo- pods, and Infusoria, in which the entire body consists of a single cell (Fig. 3), of the same general type as those which in the higher multi- cellular forms are associated to form one organic whole. Structurally, therefore, the multicellular body is in a certain sense comparable with a colony or aggregation of the lower one-celled forms.- This com-

1 The word protoplasm is due to Purkinje (1840), who applied it to the formative sub- stance of the animal embryo and compared it with the granular material of vegetable "cambium." It was afterward independently used by \\. von Mohl (1846) to designate the contents of the plant-cell. The full physiological signiticance of protoplasm, its identity with the "sarcode" (Dujardin) of the unicellular forms, and its essential similarity in plants and animals, was first clearly placed in evidence through the classical works of Max Schultze and De Bary, beside which should be placed the earlier works of Dujardin, L nger, Nageli, and Mohl, and that of Cohn, Huxley, Virchow, Leydig, Brucke, Kuhne, and Beale.

2 This comparison must be taken with some reservation, as will appear beyond. parison is not less suggestive to the physiologist than to the mor- phologist. In the one-celled forms all of the vital functions are performed by a single cell. In the multicellular forms, on the other hand, these functions are not ecjualh- i)erformed b)- all the cells, but are in varving degree distributed among them, the cells thus falling into physiological groups or tissues, each ot which is especially de- voted to the performance of a specific function. Thus arises the "physiological division of labour" through which alone the highest development of vital activity becomes possible ; and thus the cell becomes a unit, not merely of structure, but also of function. luich bodilv function, and even the life of the organism as a whole, may thus in one sense be regarded as a resultant arising through the inte- gration of a vast number of cell-activities ; and it cannot be adequately investigated without the study of the individual cell-activities that lie at its root.^

The foregoing conceptions, founded by Schwann, and skilfully developed by Kolliker, Siebold, Virchow, and Haeckel, gave an im- pulse to anatomical and physiological investigation the force of which could hardly be overestimated; yet they did not for many years measurably affect the more speculative side of biological inquiry. The Origin of Species, published in 1859, scarcely mentions it; nor, with the important exception of the theory of pangenesis, did Darwin attempt at any later period, to bring it into any very definite relation to his views. The initial impulse to the investigations that brought the cell-theory into definite contact with the evolution-theory was given nearly twenty years after the Origin of Species, by researches on the early history of the germ-cells and the fertilization of the ovum. Begun in 1873-74 by Auerbach, Fol, and Butschli, and eagerly followed up b\- Oscar Hertwig, Van Beneden, Strasburger, and a host of later workers, these investigations raised wholly new questions regarding the mechanism of development and the role of the cell in hereditary transmission. Through them it became for the first time clearly apparent that the general problems of embryology, heredity, and evolution are indissolubly bound up with those of cell- structure, and can only be fully apprehended in the light of cytologi- cal research. As the most significant step in this direction, we may re£:ard the identification of the cell-nucleus as the vehicle of inheri-

1 Cf. pp. 58-61. " It is to the cell that the study of every bodily function sooner or later drives us. In the muscle-cell lies the problem of the heart-beat and that of muscular con- traction ; in the gland-cell reside the causes of secretion ; in the epithelial cell, in the white blood-cell, lies the problem of the absorption of food, and the secrets of the mind are hidden in the ganglion-cell. ... If then physiology is not to rest content with the mere extension of our knowledge regarding the gross activities of the human body, if it would seek a real explanation of the fundamental phenomena of life, it can only attain its end through the study of cell-physiology'''' (Verworn, Alkemeine Fhysiologie, p. 53, 1895).