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=The Development of the Chick - An Introduction to Embryology=
=The Development of the Chick - An Introduction to Embryology=
Frank R. Lillie
Professor In The University Op Chicago
Second Edition, Revised
New York
Henry Holt And Company
Copyright, 1908, 1919,
By  Henry Holt And Company
==Preface to First Edition==
==Preface to First Edition==

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Lillie FR. The development of the chick. (1919) Henry Holt And Company New York, New York.

Lille 1919: Introduction | Part 1 - 1 The Egg | 2 Development Prior to Laying | 3 Outline of development, orientation, chronology | 4 From Laying to Formation of first somite | 5 Head-fold to twelve somites | 6 From twelve to thirty-six somites | Part 2 - 7 External form of embryo and embryonic membranes | 8 Nervous system | 9 Organs of special sense | 10 Alimentary tract and appendages | 11 The body-cavities, mesenteries and septum transversum | 12 Later development of the vascular system | 13 Urinogenital system | 14 Skeleton | Appendix
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This historic 1908 book by Lille is an early chicken developmental textbook as an example of embryonic development.

1908 Edition - Lillie FR. The development of the chick. (1908) New York.

https://archive.org/details/developmentofchi00lillrich/page/n8 https://archive.org/details/developmentofchi00lill/page/n6 https://archive.org/details/developmentofchi00lillief/page/n6

1919 Edition - https://archive.org/details/developmentofchi1919lill/page/n6

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Chicken Links: Introduction | Chicken stages | Hamburger Hamilton Stages | Witschi Stages | Placodes | Category:Chicken
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1883 History of the Chick | 1900 Chicken Embryo Development Plates | 1904 X-Ray Effects | 1910 Somites | 1914 Primordial Germ Cells

1919 Lillie Textbook | 1920 Chick Early Embryology | 1933 Neural | 1939 Sternum | 1948 Limb | Movie 1961 | Historic Papers

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

The Development of the Chick - An Introduction to Embryology


Frank R. Lillie

Professor In The University Op Chicago

Second Edition, Revised

New York Henry Holt And Company


Copyright, 1908, 1919,

By Henry Holt And Company

Preface to First Edition

This book is a plain account of the development of the neverfailing resource of the embryologist, the chick. It has been necessary to fill certain gaps in our knowledge of the development of the chick by descriptions of other birds. But the account does not go beyond the class Aves, and it applies exclusively to the chick except where there is specific statement to the contrary. Projected chapters on the integument, muscular system, physiology of development, teratology, and history of the subject have been omitted, as the book seemed to be already sufficiently long. The account has been written directly from the material in almost every part, and it has involved some special investigations, particularly on the early development undertaken by Doctor Mary Blount and Doctor J. T. Patterson, to whom acknowledgments are due for permission to incorporate their results before full publication by the authors. As the book is meant for the use of beginners in embryology, references to authors are usually omitted except where the account is based directly on the description of a single investigator. A fairly full list of original sources is published as an appendix.

Figures borrowed from other publications are credited in the legends to the figures. The majority of the illustrations are from original preparations of the author: Figures 46, 48, 50, 51, 52, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 71, 72, 73, 74, 75, 99, 105 and 106 were drawn by Mr. K. Hayashi; the remainder of the original drawings were executed by Mr. Kenji Toda. The photographs in Figures 118, 119, 120, 168, 181, 182, 189, 194, 197, and 231 are the work of Mr. Willard C. Green. Some of the figures may be studied with advantage for points not described in the text.

Acknowledgments are also due my colleague, Professor W. L. Tower for much assistance, and to Doctor Rov L. Moodie for special work on the skeleton, and photographs of potash preparations reproduced in Figures 242, 246, 249 and 250.

The best introduction to the problems opened up by the study of embryology is a careful first-hand study of some one species. It is in this sense that the book may serve as an introduction to embryology, if its study is accompanied by careful laboratory work. In some respects it is fuller, and in others less complete, than other books with which it might be compared. On its comparative and experimental sides, embryology is the only key to the solution of some of the most fundamental problems of biology. The fact that comparative and experimental embryology receive bare mention is not due to any lack of appreciation of their interest and importance, but to the conviction that the beginner is not prepared to appreciate these problems at the start; to the belief that our teachers of embryology are competent to remedy omissions; and finally to the circumstance that no one book can, as a matter of fact, cover the entire field, except in the most superficial way.

The development before laying and the first three days of incubation are treated by stages as far as possible, and this matter constitutes Part I of the book. It involves the study of the origin of the primordia of most of the organs. The matter concerning the later development is classified by the organs concerned, which seems to be the only possible way, and this constitutes Part II. The first part is complete in itself, so far as it goes, and no doubt it will be the only part consulted by some students.

The attempt to present a consecutive account of the development of the form on which so many classics in the history of embryology have been based is no slight undertaking. The author can hardly hope that he has avoided omissions and errors, and he will be sincerely grateful to those who call such to his attention.



I. The Cell Theory . 1

II. The Recapitulation Theory 3

III. The Physiology of Development 6

IV. Embryonic Primordia and the Law of Genetic Restric tion 8

V. General Characters of Germ-cells 9

The Spermatozoon 9

The Ovum 10

Comparison of the Germ-cells 12

VI. Polarity and Organization of the Ovum .... 14

Part I The Early Development To The End Of The Third Day

Chapter I. The Egg

Chemical Composition of the Hen's Egg 20

Formation of the Egg 21

Abnormal Eggs 25

Ovogenesis 26

Chapter II. The Development Prior To Laying

I. Maturation 32

11. Fertilization 35

III. Cleavage of the Ovum

The Hen's Egg

The Pigeon's Egg

IV. Origin of the Periblastic Nuclei, Formation of the Germ-wall

V. Origin of the Ectoderm and Entoderm

Chapter III. Outline Of Development, Orientation, Chronology

Orientation 63

Chronology {Classification of Stages)

Tables of the Development of the Chick

Chapter IV. From Layixg To The Formatiox Of The First Somite

I. Structure of the Unincubated Blastoderm

II. The Primitive Streak

Total Views

Sections 74

The Head-process 80

hiterpretation of the Primitive Streak 83

III. The Mesoderm of the Opaque Area 86

IV. The Germ-wall 90

Chapter V. Head-Fold To Twelve Somites

(From about the twenty-first to the thirty-third hour of incubation) 91

I. Origin of the Head-fold 91

II. Formation of the Fore-gut 93

III. Origin of the Xeural Tube 95

The Medullary Plate 95

The Neural Groove and Folds 97

Primary Divisions of the Neural Tube 105

Origin of the Primary Divisions of the Embryonic Brain 108

IV. The Mesoblast 109

Primary Structure of the Sornites 11-4

The Nephrotome, or Intermediate Cell-mass (Middle

Plate) 114

The Lateral Plate 115

Development of the Body-cavity or Cadome 115

Mesoblast of the Head 116

Vascular System 117

Origin of the Heart 119

The Embryonic Blood-vessels 121

V. Description of an Embryo with 10 Somites .... 122

The Nervous System 124

Alimentary Canal 126

Vascular System 126

General 127

Zones of the Blastoderm 127

Chapter VI. From Twelve To Thirty-Six Somites

Thirty-Four To Sevexty-Two Hours . 130

I. Development of the External Form, and Turning of the Embryo 130

Separation of the Embryo from the Blastoderm . . . 130

The Turning of the Embryo and the Embryonic Flexures 133

II. Origin of the Embryonic Membranes 135

Origin of the Amnion and Chorion 135

The Yolk-sac 143

Origin of the Allantois 143

Summary of Later History of the Embryonic Membranes . 145

III. The Xervous System 147

The Brain 147

The Neural Crest and the Cranial and Spinal Ganglia 156

IV. The Organs of Special Sense (Eye, Ear, X'ose) . 164

The Eye ^ . 164

The Auditory Sac 168

The Nose (Olfactory Pits) 169

V. The Alimentary Canal and its Appendages . . . 170

The StomodoEum 173

The Pharynx and Visceral Arches 173

(Esophagus and Stomach 179

The Liver 179

The Pancreas 181

The Mid-Gut 181

Ancd Plate, Hind-gut, Post-anal gut and Allantois 182

VI. History of the Mesoderm 183

Somites 183

The Intermediate Cell-mass 190

The Vascular System 197

VII. The Body-cavity and Mesenteries 205

Part II The Forrth Day To Hatching, Organogeny, Development Of The Organs

Chapter VII. The Exterxal Form Of The Embryo Axd The Embryonic Membraxes

I. The External Form 211

General 211

Head 213

II. Embryonic Membranes . . . 216

General 216

The Allantois 220

The Yolk-sac 225

The Amnion 231

Hatching . . 232

Chapter VIII. The Nervous System

I. The Neuroblasts 233

The Medullary Neuroblasts 233

The Ganglionic Neuroblasts 236

II. The Development of the Spinal Cord 239

Central Canal and Fissures of the Cord 242

Neuroblasts, Commissures, and Fiber Tracts of the Cord . 244

III. The Development of the Brain 244

The Telencephalon 245

The Diencephalon 249

The Meseyicephalon 251

The Metencephalon 251

The Myelencephalon 252

Commissures of the Brain 252

IV. The Peripheral Nervous System . 252

The Spinal Nerves 252

The Cranial Nerves 261

Chapter IX. Organs Of Special Sense

I. The Eye 271

The Optic Cup 271

The Vitreous Humor 275

The Lens 276

Anterior Chamber and Cornea 278

The Choroid and Sclerotic Coats 279

The Eyelids and Conjunctival Sac 279

Choroid Fissure, Pecten and Optic Nerve 281

II. The Development of the Olfactory Organ . . . 285

III. The Development of the Ear 288

Development of the Otocyst and Associated Parts . . . 289

The Development of the Tubo-tyyn panic Cavity, External Auditory Meatus and Tympanum 297

Chapter X. The Alimentary Tract And Its Appendages

I. Mouth and Oral Cavity 301

Beak and Egg-tooth 302

The Tongue 305

Oral Glands 306

II. Derivatives of the Embryonic Pharynx 306

Fate of the Visceral Clefts 307

Thyroid 307

Visceral Pouches • • 307

The Thymus 308

Epithelial Vestiges 309

The Posthranchial Bodies 309

III. The (Esophagus, Stomach and Intestine .... 309

Oesophagus 312

Stomach 313

Large Intestine, Cloaca, and Anus 314

IV. The Development of the Liver and Pancreas , . . 319

The Liver 319

The Pancreas 323

V. The Respiratory Tract 325

Bronchi, Lungs and Air-sacs 325

The Laryngotracheal Groove 331

Chapter XI. The Body-Cavities, Mesenteries And Septum Transversum

I. The Separation of the Pericardial and Pleuroperi TONEAL Cavities 333

Septum Transversum 334

Closure of the Dorsal Opening of the Pericardium . . . 337

Estahlishment of Independent Pericardial Walls . . . 338

Derivatives of the Septum Transversum 339

II. Separation of Pleural and Peritoneal Cavities; Or igin OF THE Septum Pleuro-peritoneale . . . 340

III. The Mesenteries 342

The Dorsal Mesentery 342

The Origin of the Omentum 343

Origin of the Spleen 345

Chapter XII. The Later Development Of The Vascular System

I. The Heart 348

The Development of the External Form of the Heart . . 348

Division of the Cavities of the Heart 350

Fate of the Bulbus .357

The Sinus Ve?iosus 357

II. The Arterial System 358

The Aortic Arches 358

The Carotid Arch 361

The Subclavian Artery 362

The Aortic System 362

III. The Venous System ..... c .... . 363

The Anterior Vence Cavce 363

The Omphalomesenteric Veins 364

The Umbilical Veins 367

The System of the Inferior Vena Cava 368

IV. The Embryonic Circulation 372

Chapter XIII. The Urinogenital System

I. The Later History of the Mesonephros 378

II. The Development of the Metanephros or Permanent

Kidney 38-1:

The Metanephric Diverticulum 384

The Nephrogenous Tissue of the Metanephros . . . 387

III. The Organs of Reproduction • 390

Development of Ovary and Testis 391

Development of the Genital Ducts 401

IV. The Suprarenal Capsules 403

Origin of the Cortical Cords 405

Origin of the Medullary Cords 406

Chapter XIV. The Skeleton

I. General 407

II. The Vertebral Column 411

The Sclerotomes and Vertebral Segmentation .... 412

Membranous Stage of the Vertebrce 414

Chondrification 418

Atlas and Axis (Epistropheus) 420

Formation of Vertebral Articulations 421

Ossification 421

III. Development of the Ribs and Sternal Apparatus. . 424

IV. Development of the Skull 427

Development of the Cartilaginous or Primordial Cranium. 428

Ossification of the Skull 431

V. Appendicular Skeleton 434

The Fore-limb 434

The Skeleton of the Hind-limb 438


General Literature

Literature — Chapter I 443

Literature — Chapter II 444

Literature — Chapter III 44o

Literature — Chapters IV and V 44o

Literature — Chapter VII 447

Literature — Chapter VIII 449

Literature — Chapter IX 450

Literature — Chapter X 453

Literature — Chapter XI 4o/

Literature — Chapter XII 458

Literature — Chapter XIII 459

Literature — Chapter XIV 461



I. The Cell Theory

The fundamental basis of the general conceptions of embryology, as of other biological disciplines, is the cell theor3^ The organism is composed of innumerable vital units, the cells, each of which has its independent life. The life of the organism as a whole is a product of the combined activity of all the cells. New cells arise always by subdivision of pre-existing cells, and new generations of the organism from liberated cells of the parental body. The protozoa, however, have the grade of organization of single cells, and the daughter-cells arising by fission constitute at the same time new generations. In some metazoa new generations may arise asexually by a process of budding, as in Hydra, or of fission, as in some Turbellaria; such cases constitute exceptions to the rule that new generations arise from liberated cells of the parental body, but the rule holds without exception for all cases of sexual reproduction.

The body consists of various functional parts or organs; each of these again consists of various tissues, and the tissues are composed of specific kinds of cells. The reproductive organs, or gonads, are characterized by the production of germ-cells, ova in the female gonad or ovary, and spermatozoa in the male gonad or testis. However large the ovum may be, and in the hen it is the part of the egg known as the yolk, it is, nevertheless, a single cell at the time that it leaves the ovary in all animals. Similarly the spermatozoon is a single cell. An ovum and spermatozoon unite, in the manner to be described later, and constitute a single cell by fusion, the fertilized ovum or oosperm. This cell divides and forms two; each of the daughter-cells divides, making four, and the number of cells steadily increases by successive divisions of all daughter-cells, so that a large number of cells is rapidly produced. Organs are formed by successive and orderly differentiation among groups of these cells. Among these organs are the gonads, consisting of cells which trace a continuous lineage by cell-division back to the fertilized ovum, and which are capable of developing into ova or spermatozoa according to the sex of the individual.

The lives of successive generations are thus continuous because the series of germ-cells from which they arise shows no break in continuity. All other kinds of cells composing the body finally die. In view of this contrast the non-germinal cells of the body are known collectively as somatic cells. In some way the germcells of a species maintain very constant properties from generation to generation in spite of their enormous multiplication, and this furnishes the basis for hereditary resemblance.

The establishment of the fact that in all animals the ovum is a single cell, and that the cells of all tissues of the body are derived from it by a continuous process of cell-division, completes the outline of the cycle of the generations, and furnishes the basis for a complete theory of development. The full significance of this principle can only be appreciated by learning the condition of embryology before the establishment of the cell-theory in the eighteenth century. The history of our knowledge of the development of mammals is particularly instructive in this respect: some knowledge had been gained of the anatomy of the embryos, mostly relatively advanced, of a few^ mammals; but the origin of the embryo was entirely unknown; the ovum itself had not been discovered; the process of fertilization was not understood. In the knowledge of the cycle of generations there was a great gap, and the embryo was as much a mystery as if it had arisen by a direct act of creation. To be sure Harvey in 1651 had propounded the theorem, omne vivum ex ovo, but no one had ever seen the egg of a mammal, and there was no clear idea in the case of other forms what the egg signified.

In 1672, de Graaf (who died in 1673 at the age of 32) published a work, "de mulierum organis generationis inservientibus," in which he attempted to show that the vesicles seen on the surface of the ovaries contained the female reproductive material in bladder-like form. But he could not reconcile this view of the Graafian follicle with the fact that the earliest embryos discovered by him were smaller than the follicles. For this reason his views were opposed by Leeuwenhoek and Valisnieri; and the later researches of Haller and his pupil Kuhlemann seemed to establish a view which l^anished all possibility of a rational explanation of development, viz., that, in the highest group of animals (the mammalia) the embryo arose after fertilization out of formless fluids.

In 1827 V. Baer discovered the mammalian ovum within the Graafian follicle. But no correct interpretation of this discovery w^as possible until the establishment of the cell-theory by Theodore Schwann in 1839; Schwann concluded as the result of his investigations that there was one general principle for the formation of all organisms, namely, the formation of cells; that the cause of nutrition and growth resides not in the organism as a whole, but in the separate elementary parts, the cells." He recognized the ovum as a single cell and the germinal vesicle as its nucleus. But on account of his erroneous conception of the origin of cells as a kind of crystallization in a primordial substance, the cytoblastema, he was unable to form the conception of continuity of generations which is an essential part of the modern cell-theory.

Schwann's theory as regards the ovum was not at once accepted. Indeed, for a period of about twenty years some of the best investigators, notably Bischoff, opposed the view that the ovum is a single cell, and the so-called germinal vesicle its nucleus. It was not, indeed, until 1861 that Gegenbaur decisively demonstrated that the bird's ovimi is a single cell. Even after that it was maintained for a long time by His and his followers that all the cells were not derived from the ovum directly, but that certain tissues, notably the blood and connective tissues, were to be traced to maternal leucocytes that had migrated into the ovum while it was yet in the follicle. This view was decisively disproved in the course of time.

II. The Recapitulation Theory

Haeckel's formula, that the development of the indi\ddual repeats briefly the evolution of the species, or that ontogeny is a brief recapitulation of phylogeny, has been widely accepted by embryologists. It is based on a comparison between the embryonic development of the individual and the comparative anatomy of the phylum. The embryonic conditions of any set of organs of a higher species of a phylum resemble, in many essential particulars, conditions that are adult in lower species of the same phylum; and, moreover, the order of embryonic development of organs corresponds in general to the taxonomic order of organization of the same organs. As the taxonomic order is the order of evolution, Haeckel's generalization, which he called the fundamental law of biogenesis, w^ould appear to follow^ of necessity.

But it never happens that the embryo of any definite species resembles in its entirety the adult of a lower species, nor even the embryo of a lower species; its organization is specific at all stages from the ovum on, so that it is possible without any difficulty to recognize the order of animals to which a given embryo belongs, and more careful examination will usually enable one to assign its zoological position very closely.

If phylogeny be understood to be the succession of adult forms in the line of evolution, it cannot be said in any real sense that ontogeny is a brief recapitulation of phylogeny, for the embryo of a higher form is never like the adult of a lower form, though the anatomy of embryonic organs of higher species resembles in many particulars the anatomy of the homologous organs of the adult of the lower species. However, if w^e conceive that the whole life history is necessary for the definition of a species, we obtain a different basis for the recapitulation theory. The comparable units are then entire ontogenies, and these resemble one another in proportion to the nearness of relationship, just as the definitive structures do. The ontogeny is inherited no less than the adult characteristics, and is subject to precisely the same laws of modification and variation. Thus in nearly related species the ontogenies are very similar; in more distantly related species there is less resemblance, and in species from different classes the ontogenies are widely divergent in many respects.

From this it follows that inheritance of the life-history or ontogeny is the fundamental basis of the recapitulation theory. In the course of evolution terminal or late stages of the life history are modified more rapidly in a visible morphological sense, and earlier stages are more conservative in the same sense. Hence ancestral resemblances adhere incomparably longer to the embryo than to the adult. Ontogenies receive something from every stage of evolution, but they retain most of the previous ontogenetic forms, especially of the early stages, in each succeeding evolutionary stage; hence the appearance of recapitulation of the ancestral history.

Some of these considerations may be represented graphically as follows: let us take a species D that has an ontogeny A, B, C, D, and suppose that this species evolves successively into species E, F, G, H, etc. When evolution has progressed a step, to E, the characters of the species established develop directh' from the ovum, and are therefore, in some way, involved in the composition of the latter. All of the stages of the ontogeny leading up to E are modified, and we can indicate this in the ontogeny

1. A B C D of E as in line 2; similarly, when evolu 2. A^ B^ C^ D^ E tion has progressed to species F, seeing

3. A^ B2 C^ D2 E^ F that the characters of F now develop

4. A^ B^ C^ D^ E2 F^ G directly from the ovum, all the onto 5. A^ B^ C^ D^ E^ F^ G^ H genetic stages leading up to F are modified, line 3. And so on for each successive advance in evolution, lines 4 and 5. It will also be noticed that the terminal stage D of species 1, becomes a successively earlier ontogenetic stage of species 2, 3, 4, 5, etc., and moreover it does not recur in its pure form, but in the form D^ in species 2, D^ in species 3, etc. Now if the last five stages of the ontogeny of species 5 be examined, viz.^ D^ E^, F^, G^ H, it will be seen that they repeat the phylogeny of the adult stages D, E, F, G, H, but in a modified form.

This is in fact what the diagram shows; but it is an essential defect of the diagram that it is incapable of showing the character of the modifications of the ancestral conditions. Not only is each stage of the ancestral ontogenies modified with each phylogenetic advance, but the elements of organization of the ancestral stages are also dispersed so that no ancestral stage hangs together as a unit. The embryonic stages show as much proportional modification in the course of evolution as the adult, but this is not so obvious owing to the simpler and more generalized character of the embryonic stages.

The recapitulation theory as outlined above is obviousl}^ a corollary of the theory of organic descent; it was in fact developed in essentially its present form, soon after the publication of the Origin of Species," by Fritz Miiller and Ernst Haeckel. But the data on which it was based were known to the earlier embryologists; and Meckel, for instance, insisted very strongly on the resemblance between the ontogenetic and the taxonomic series (1821). V. Baer opposed Meckel's view that higher organisms pass through the definitive stages of the lower organisms, and formulated his conclusions on the subject in 1828 in the following

theses :

1. "The more general features of a large division of animals arise in the embryo earlier than the more special features."

2. " From the most general features of structure arise those that are less general, and so on until the most specific features arise."

3. "The embryo of any definite species tends away from the specific forms of other species instead of passing through them."

4. "Fundamentally, therefore, the embryo of any higher species is never like a lower species, but only like its embryo."

Some embryologists profess to prefer the laws of v. Baer to the recapitulation theory as a formulation of the actual facts. But it is obvious that the only possible explanation of the facts is found in the theory of descent, and that therefore they must be formulated in terms of this theory. The method of formulation will depend on the conception of the nature of the factors of organic evolution. Haeckel stated his theory in Lamarckian terms, which renders it inacceptable in many places to those who cannot accept the Lamarckian point of view. But as the basis of any theory of descent is heredity, and it must be recognized that ontogenies are inherited, the resemblance between the individual history and the phylogenetic history necessarily follows. If one holds, as does the present writer, that phylogenetic variations are germinal in their character, then one must admit that every phase of development of every part has two aspects, viz.: the modern, specific, or coenogenetic, and the ancestral or palingenetic aspect. The latter aspect may be more or less completely obscured in the course of evolution, but it can never entirely vanish because it is the original germ of the specific form acquired. It is not correct from this point of view to classify some features of development as coenogenetic and others as palingenetic, though it is obvious that some characters may exhibit the ancestral conditions in more apparent and others in less apparent form.

III. The Physiology of Development To explain how a germ possessed the potency of forming an adult, the prefor7nationists of the eighteenth century assumed that it contained a miniature adult, and that the process of development consisted essentially in enlargement and completion in detail of that which was already preformed. They solved the problem of development, therefore, by denying its existence: In the begininng the Creator had not only made all species of animals and plants in essentially their present forms, but had at the same time created the germs of all the generations that were ever to come into existence. The ovum of any species, therefore, contained encapsuled the germ of the next generation; this, likewise encapsuled, the germ of the generation next following, and so on to the predetermined end of the species. This was known as the doctrine of evolution or preformation. In opposition to this conception, those of the same period who believed in epigenesis maintained the apparent simplicity of the germ to be real, and development to be actual. But, as there was no conception of the continuity of generations, the adherents of this point of view had to assume the spontaneous generation of the embryo.

A great advance over the preformation theory of development was made in the modern theory of determinants. This conception, which forms the basis of Darwin's theory of pangenesis as well as of Weismann's germ-plasm theory of development, is, essentially, that all the diverse components of the organism are represented in the germ by distinct entities (pangens of Darwin, determinants of Weismann) which are germs of the parts that they represent, and which are so distributed in the process of development that they produce all the parts of the embryo in their proper sequence and relations. This is not the place to enter into the numerous and diverse variations of the determinant hypothesis. It was an advance over the preformation theory of development in so far as it was reconcilable with the cell and protoplasm theories of organization, but it has a real relationship to the preformation theory inasmuch as it denies the simplicity of the germ and avoids any real explanation of the modus operandi of development.

Development is as truly a physiological process as secretion, and as such is to be studied by similar methods, mainly experimental. The limits of pure observation without experiment are soon reached in the analysis of such a complex subject as the physiology of development; experiment then becomes necessary to push the analysis of the subject farther^ and to furnish the true interpretation of the observations. In some cases experiments have confirmed the physiological deductions of pure observation, and in many cases have decided between conflicting views. Not all embryological experiments, however, are essays in the direction of a physiologv of development; some are directed to the solution of morphological problems, as, for instance, the origin of the sheath cells of nerves, or the order of origin of somites, or the relation of the primitive streak to the embr3'o. Experimental embryology is, therefore, not synonymous with physiology of development.

Physiology of development must proceed from an investigation of the composition and properties of the germ-cells. It must investigate the role of cell-division in development, the factors that determine the location, origin, and properties of the primordia of organs, the laws that determine unequal growth, the conditions that determine the direction of differentiation, the influence of extraorganic conditions on the formation of the embryo, and the effects of the intraorganic environment, i.e., of component parts of the embryo on other parts (correlative differentiation). Each of these divisions of the subject includes numerous problems, which have attracted many investigators, so that the materials for a consistent exposition of the physiology of embryonic development are being rapidly accumulated. This direction of investigation is, however, one of the youngest of the biological disciplines. It will be seen how far it is removed from attempts to explain embryonic development by a single principle.

IV. Embryonic Primordia and the Law of Genetic Restriction

In the course of development the most general features of organization arise first, and those that are successively less general in the order of their specialization. For every structure, therefore, there is a period of emergence from something more general. The earliest discernible germ of any part or organ may be called its primordium. In this sense the ovum is the primordium of the individual, the ectoderm the primordium of all ectodermal structures, the medullary plate the primordium of the central and part of the peripheral nervous system, the first thickening of the ectoderm over the optic cup the primordium of the lens, etc. Primordia are, therefore, of all grades, and each arises from a primordium of a higher grade of generality.

The emergence of a primordium involves a limitation in two directions: (1) it is itself limited in a positive fashion by being restricted to a definite line of differentiation more special than the primordium from which it sprang, and (2) the latter is limited in a negative way by losing the capacity for producing another primordium of exactly the same sort. The advance of differentiation sets a limit in all cases, in the manners indicated, to subsequent differentiation, a principle that has been designated by Minot the law of genetic restriction.

This law has not been sufficiently investigated in an experimental fashion to demonstrate its universal validity, but enough is known to establish its general applicability. A very important property of primordia in many animals is their capacity for subdivision, each part retaining the potencies of the whole. Thus, for instance, in some animals two or several embrvos mav be produced from parts of one ovum. Similarly two or more limbs may be produced in some forms by subdividing a limbbud, etc.

V. General Character of Germ-cells

As already remarked the ovum and spermatozoon have the character of single cells in all animals. They are, however, specialized for the performance of their respective functions. The ovum is relatively large, inert, and usually rounded in form. Its size is due to the presence of a sufficient quantity of protoplasm to serve as the primordium of an embryo, and of a greater or less amount of yolk for its nutrition. The spermatozoon, on the other hand, is relatively minute and capable of locomotion. It contains no food substances, and only sufficient protoplasm to serve as transmitter of paternal qualities and for organs of locomotion.

The Spermatozoon. The spermatozoon (Fig. 1) is an elongated flagellated cell in which three main divisions are distinguished, viz., head (caput), neck (coUum) and tail (cauda). The head contains the nucleus, and the neck the centrosomes of the sperm mother-cell or spermatid. The tip of the head is often transformed into a perforatorium. Three parts may be recognized in the tail, viz., the connecting piece (pars conjunctionis) next to the neck, frequently called the middle piece, the main piece (pars principalis) and the end-piece or terminal filament (pars terminahs). The entire tail is traversed by an axial filament; in the region

of the connecting and main pieces the axial filament is surrounded by a protoplasmic sheath (involucrum) which may be variously modified in different animals. The end-piece is made up of the axial filament alone.

The Ovum, The ova of different phyla and classes of animals vary greatly in size, in organization, and in the nature of their envelopes. In considering these variations we shall limit ourselves to the vertebrates. Within the ovary the ovum receives two envelopes, viz., a primary envelope, the so-called vitelline membrane, which is supposed to be secreted by the ovum itself, and a secondary or follicular membrane, which is secreted by the follicular cells. (See Chap. I). Theoretically the distinction between vitelline membrane and follicular membrane (primary and secondary egg-membranes) is perfectly clear; but practically it is impossible in most cases to make such a distinction. Therefore the membrane that surrounds the ovarian ovum will be termed the vitelline membrane or zona radiata without reference to its theoretical mode of origin.

The ovum escapes from the ovary (ovulaeon from the vas tion) by rupture of the wall of the follicle, and, deferens, (After -^^ most vertebrates, is taken up by the oviduct

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through which it passes on its way to the exterior. Within the oviduct it may become surrounded by tertiary membranes secreted by the wall of the oviduct itself. Tertiary membranes are lacking in some vertebrates, in others they are of great importance. Thus in birds the albumen, the shellmembrane and the shell itself are tertiary membranes.

The principal differences to be emphasized in the ova of vertebrates are, however, in the amount and arrangement of the yolk contained within the ovum proper. All ova contain more or less yolk. In the case of mammals (excepting the monotremata: Ornithorhynchus, Echidna, etc., which have large ova) the yolk is scanty in amount, and quite uniformly distributed in the form of fine granules; the ovum is, therefore, relatively very small (mouse, 0.059 mm.; man, 0.17 mm.). Such ova are often termed alecithal, which means literally without yolk. In the literal sense, however, no ova are entirely alecithal, so that it will be better to use the term of Waldeyer, isolecithal. In the amphibia the yolk is much greater in amount and it is centered towards one pole of the ovum; the germinal vesicle (nucleus of the egg-cell), which occupies the center of the protoplasm of the ovum, is therefore displaced towards the opposite pole of the ovum. Such ova are termed telolecithal. In the ova of Selachia, reptiles and birds, the yolk is very much greater in amount and in consequence the protoplasm containing the germinal vesicle appears as a small disc, the germinal disc, on the surface of the huge yolk-mass.

Fig. 1. — Sperma tozoon of the pig

But no matter how large the ovum may become by deposition of yolk, its unicellular character is not altered. The deposition of yolk is simply a provision for the nutrition of the embryo. In the mammals the nutrition of the embryo is provided for by the placenta; therefore yolk may be dispensed with. In the absence of such provision the amount of yolk is a measure of the length of the embryonic period of development. In the amphibia, for instance, this is relatively brief, for the yolk is soon used up, and the larva must then depend on its own activities for its nutrition. Therefore the development involves a metamorphosis: the embryo is born in a very unfinished condition, as a larva (the tadpole in the case of amphibia), which must undergo an extensive metamorphosis to reach the adult condition. In the reptiles and birds, however, the amount of yolk is sufficient to carry the development through to a juvenile condition, before an extraneous food-supply is necessary. The metamorphosis, therefore, which takes place in free life in amphibia, goes on within the egg in reptiles and birds. The first form of development is known as larval, the second as foetal.

The amount and arrangement of yolk also influences very profoundly the form of the early stages of development. Ova are classified in this respect as holoblastic and meroblastic. Holoblastic ova are those in which the process of cell division (cleavage or segmentation of the ovum), with which development begins, involves the entire ovum. This occurs where the amount of the yolk is relatively small and where it is completely interpenetrated by sufficient protoplasm to carry the planes of division through the inert volk. But where the amount of yolk becomes very large, or where it is not interpenetrated sufficiently by the protoplasm, the division planes are confined to the protoplasmic portion of the ovum, and the yolk remains undivided. Such ova are known as meroblastic. In these ova the cellular part of the ovum forms a blastodisc (germinal disc) on the surface of the yolk. The ova of Amphioxus, Petromyzontidse, Ganoidea. Dipnoi, Amphibia, Marsupialia, and Placentalia are holoblastic; those of Myxinoidea, Teleostei, Selachia, Reptilia, Aves, and Monotremata are meroblastic.

It is obvious that transitional conditions between holoblastic and meroblastic ova may occur; such are in fact found among the ganoids. In Lepidosteus, for instance, the quantity of protoplasm in the lower hemisphere is so slight that the division planes form with extreme slowness. On the other hand, it should be emphasized that the distinction between holoblastic and meroblastic ova is not so much due to amount of yolk as to the definiteness of its separation from the protoplasm. Thus the ova of some teleosts, particularly of the viviparous forms described by Eigenmann, are many times smaller than the ova of Necturus or Cryptobranchus among amphibia. Yet the teleost ovum is meroblastic, because the protoplasm does not penetrate sufficiently into the yolk, and the amphibian ovum is holoblastic.

Comparison of the Germ-cells. Although it is not within the province of this book to enter fully into a cUscussion of this question, yet it should be pointed out that, in spite of the extreme differences in the structure of the germ-cells, they are exactly equivalent in hereditary potency, as is proved by the similar nature of reciprocal crosses. Their resemblances are in fact fundamental and their differences must be regarded as adaptations to secure their union. The comparative history of the germ-cells, that is a comparison of ovogenesis and spermatogenesis, brings out their fundamental similarity as germ-cells. In both the ovogenesis and spermatogenesis three periods are clearly distinguishable, viz. : a period of multiplication, a period of growth, and a period of maturation. In the period of multiplication the primordial germ-cells, known as ovogonia and spermatogonia are very similar in their morphological characters; both kinds are small, yolkless cells containing the typical or somatic number of chromosomes; they multiply rapidly by karyokinetic division.

At the end of this period multiplication ceases and the germcells increase in size (period of growth). They are now known as ovocytes and spermatocytes of the first generation. The growth of the ovocyte is much greater than that of the spermatocyte; deposition of yolk occurs in the ovocyte during this period, whereas in the spermatocyte no yolk is ever deposited, though mitochondria may simulate it in appearance. Another characteristic feature of the period of growth is the reduction of the number of chromosomes to one half of the typical number, w^hich takes place, according to the current conception, by union of the chromosomes in pairs (synapsis) forming one half of the somatic number of chromosomes, which are, however, bivalent and are known as tetrads.

At the end of the period of growth the ovocyte of the first generation is usually many times larger than the spermatocyte, owing mainly to the amount of yolk formed. But the tw^o kinds of cells are precisely alike in nuclear constitution. Then comes the period of maturation, which is the same in both kinds of cells with reference to the nuclear phenomena, but very different as regards the behavior of the cell-body. The maturation consists of two rapidly succeeding karyokinetic divisions: in the case of the spermatocyte the first division results in the formation of two similar cells, the spermatocytes of the second order, and the second maturation division divides each of these equally, forming two similar spermatids, so that four equal and similar spermatids arise from each spermatocyte of the first order. Each spermatid then differentiates into a single spermatozoon. In the case of the ovocyte of the first order, the first maturation division is exceedingly unequal; the smaller cell is known as the first polar bodv, but both cells are ovocvtes of the second order. The second maturation division usually involves only the large secondary ovocyte; it is as unequal as the first division and results in the formation of a second polar body. The division of the first polar body, where it occurs, is equal. Thus the net result of the maturation division of the ovum is the production of three cells (four if the first polar body divides), viz., the two (or three) polar bodies and the ovum. The size of the polar globules is usually so small that their elimination makes no appreciable difference in the size of the ovum proper, but they have, nevertheless, the same nuclear constitution as the ovum.

The mature ovum (ootid) and the polar bodies are the precise equivalent of the four spermatids, but whereas each of the latter becomes a functional spermatozoon, only the ovum on the female side is functional; the polar bodies lack the necessary protoplasm and yolk for development, and they therefore die. The polar bodies must be regarded as abortive ova; and a teleological explanation of the form of maturation of the ovum is afforded by the consideration that equal maturation divisions would reduce the amount of protoplasm and yolk in the products below the minimum desirable for perfect development.

Although the maturation divisions of the ovum and spermatozoon are so dissimilar externally, yet the nuclear phenomena are exactly alike. The net result of the maturation divisions is to produce definitive germ-cells containing one half of the somatic number of chromosomes owing to the reduction by pairing (synapsis) that occurs in both at the beginning of the period of growth. The somatic number is again restored when the sperm-nucleus and the egg-nucleus unite in fertilization. Questions of fundamental importance for the problems of heredity arise in connection with the phenomena of maturation and fertilization, but their consideration lies without the scope of the present book.

VI. Polarity and Organization of the Ovum Although the ovum is morphologically a single cell, yet, as the primordium of an individual, it has certain specific properties that predelineate or foreshadow the main structural features of the embryo. Polarity is the most general of these features: all the axes of the ovum are not similar, though they may be equal; there is one axis around which the development centers; the ends of this axis are known as the animal and the vegetative poles of the ovum, and the hemispheres in which they lie are named correspondingly. In telolecithal ova the yolk is centered in the vegetative hemisphere, the protoplasm in the animal hemisphere; even in ova which are called isolecithal there is a tendency for the yolk to be more abundant in the vegetative hemisphere. The polar globules are formed at the animal pole; hence their name; they often furnish the only clear indication of polarity before cleavage begins.

With reference to the heteropolar ovic axis a series of meridia may be defined, drawn from pole to pole over the surface; likewise an equator and a series of horizontal zones parallel to the equator. Thus directions on the surface of the ovum may be defined as meridional, equatorial, or oblique.

Cleavage takes place with reference to the axis of the ovum. Thus in holoblastic vertebrate ova the first and second cleavage planes are meridional, and the third usually equatorial. The mammalian ovum may form an exception to this rule, though little is known, as a matter of fact, about the polarity of the mammalian ovum. The cleavage of meroblastic ova takes place likewise with reference to the polarity (see Chap. II); and the location of the primary germ-layers is determined by the polarity.

Not only is the ovum heteropolar, but in many bilateral animals, and perhaps in all, it is bilaterally symmetrical before cleavage begins; that is to say, one of the meridional planes defines the longitudinal axis of the future embryo, and the direction of anterior and posterior ends is also predetermined in this meridian, so that halves of the egg corresponding to future right and left sides of the embryo may be distinguished. In the frog's egg the plane of symmetry is marked by a gray crescent that appears above the equator on the side of the egg that corresponds to the hinder end of the embryo. This crescent is bisected by the meridional plane of symmetry. In the hen's egg the plane of symmetry of the embryo appears on the surface of the yolk in a line at right angles to the axis of the shell, and the left side of the embryo is turned towards the broad end, the right side towards the narrow end of the shell. The same plane of symmetry must exist in the ovum prior to cleavage for reasons explained beyond, although there is no morphological differentiation in the ovum proper, i.e., the germinal disc or yolk, that indicates it.

This predelineation of embryonic axes within the unsegmented ovum has been interpreted physiologically as due to gradients in rate of metabolic processes along the embryonic axes (Child), which determine the locaUzation of the main developmental events.