Heredity and environment in the development of men

Front matter

CORNELL

UNIVERSITY

LIBRARY

Preface To First Edition

The origin of species was probably the greatest biological problem of the past century ; the origin of individuals is the greatest biological subject of the present one. The many inconclusive attempts to determine just how species arose led naturally to a renewed study of the processes by which individuals come into existence, for it seems probable that the principles and causes of the development of individuals will be found to apply also to the evolution of races. As the doctrine of evolution wrought great change in prevalent beliefs regarding the origin and past history of man, so present studies of development are changing opinions as to the personality of man and the possibilities of improving the race. The doctrine of evolution was largely of theoretical significance, the phenomena of development are of the greatest practical importance ; indeed there is probably no other subject of such vast importance to mankind as the knowl-

Preface

edge of and the control over heredity and development. Within recent years the experimental study of heredity and development has led to a new epoch in our knowledge of these subjects, and it does not seem unreasonable to suppose that in time it will produce a better breed of men.

The lectures which compose this volume were given at Northwestern University in February, 1914, on the Norman W. Harris Foundation and were afterward repeated at Princeton University. I gladly take this opportunity of expressing to the faculties, students and friends of both institutions my deep appreciation of their interest and courtesy. In attempting to present to a general audience the results of recent studies on heredity and development, with special reference to their application to man, the author has had to choose between simplicity and sufficiency of statement, between apparent dogmatism and scientific caution, between a popular and a scientific presentation. These are hard alternatives, but the first duty of a lecturer is to address his audience and to make his subject plain and interesting, if he can.

PREFACE

rather than to talk to the scientific gallery over the heads of the audience. In preparing the lectures for publication it has not been possible to avoid the technical treatment of certain subjects, but in the main the lectures are still addressed to the audience rather than to the scientific gallery. Unfortunately biology is still a strange subject to many intelligent people and its terminology is rather terrifying to the uninitiated ; but it is hoped that the glossary at the end of the volume may rob these unfamiliar terms of many of their terrors.

The first three chapters of this book appeared in Popular Science Monthly, June to November 1914, by previous arrangement with the Princeton University Press, and a portion of the last chapter was first published in Science in January 1913. The illustrations are from many sources: Figures 9, 10, 19, 20, 22, 23, 26- 29, 35, 40-43, 51-54, 58, 72-75, 77, 83 are original; Figures 1-8, 11-18, 21, 24, 25, 30-34, 36- 39, 44-47, 49, 55-57, 59-62, 70 were redrawn with more or less modification from original sources which are indicated in every case; the

PREFACE

remaining figures, namely Figures 48, 50, 63- 69, 71, 76, 78-80, 84-96 were copied with little or no modification from vai-ious papers, photographs and books, the original source being given in each case. The writer wishes to express his obligation to all authors and publishers upon whose works he has drawn, and he thanks especially the Columbia University Press for permission to use Figures 48 and 50, and the Open Court Publishing Company for Figures 84-87,

I take this opportunity of thanking Dr. W. E. Castle and Dr. J. H. McGregor for the use of photographs which are reproduced in Figures 81, 82 and 96 ; and I wish especially to thank my assistant, Marguerite Ruddiman, for her aid in preparing figures and manuscript for publication.

Princeton, December, 1914 viii

Preface To Second Edition

The interest of the pubHc and the kindness of the publishers have made possible a revised edition of this book within a year after its first publication. This has permitted the rewriting of certain passages which were not well expressed and the introduction of considerable new material which will serve, it is hoped, to further elucidate some of the questions discussed, as well as to give results of more recent work. In particular Figs. 4, 5, 9, 10, 18, 19 have been added and besides minor corrections and additions to all the chapters considerable changes have been made in Chapter III, section on Maternal Inheritance, Chapter IV section on Inheritance or Non- Inheritance of Acquired Characters, and Chapter V sections on Artificial Selection, Origin of Mutations, Past Evolution of Man, and Eugenics. January, 1916

Puklisher's Note. — In the second edition, second printing, August, 1917, tlie text was corrected by the author so as to embody some of the results of the latest investigations in this subject.

Introduction

A. PHENOMENA OF DEVELOPMENT

I. Development of the Body

1. The Germ Cells

2. Fertilization

3. Cleavage

4. Embryogeny

5. Organogeny

6. Oviparity and Viviparity

7. Development of Functions

II. Development of the Mind

1. Sensitivity

2. TropismSj Reflexes, Instincts

3. Memory

4. Intellect, Reason

5. Will

6. Consciousness

B. FACTORS OF DEVELOPMENT

1. Preformation

2. Epigenesis

3. Endogenesis and Epigenesis

Chapter II. Cellular Basis Of Heredity And Development

A. INTRODUCTORY

1. Definitions

2. The Transmission Hypothesis

3. Germinal Continuity and Somatic Discontinuity

4. The Units of Living Matter

5. Heredity and Development

B. THE GERM CELLS

1. Fertilization

2. Cleavage and Differentiation

3. The Origin of the Sex Cells

a. The Division Period

b. The Growth Period

c. The Maturation Period

4. Sex Determination

C. THE MECHANISM OF HEREDITY

I. The Specificity of Germ Cells II. Correlations between Germinal and Somatic Organization

1. Nuclear Correlations

2. Cytoplasmic Correlations

a. Polarity

b. Symmetry

c. Inverse Symmetry

d. Localization Pattern

D. THE MECHANISM OF DEVELOPMENT

1. The Formation of Different Substances

Z. Segregation and Isolation of Different Substances in Cells a. By Protoplasmic Movements b. By Differential Cell Divisions

Chapter III. Phenomena Of Inheritance

A. OBSERVATIONS ON INHERITANCE Individuals and their Characters Hereditary Resemblances and Differences

I. Hereditary Resemblaxces

1. Racial Characters

2. Individual Characters

a. Morphological Features

b. Teratological and Pathological Peculiarities

c. Physiological Peculiarities

d. Psychological Characters II. Hereditary Differences

1. New Combinations of Characters

2. New Characters or Mutations

3. Mutations and Fluctuations

4. Every Individual Unique

B. STATISTICAL STUDY OF INHERITANCE

1. Galton's "Law of Ancestral Inheritance"

2. Galton's "Law of Filial Regression"

C. EXPERIMENTAL STUDY OF INHERI- TANCE

I. Mbndelibm

1. Results of Crossing Individuals with

one pair of contrasting characters xiii

Other Mendelian Ratios

2. Results of Crossing Individuals with more than one pair of contrasting characters

Dihybrids and Trihybrids

3. Inheritance Formulae

4. Presence and Absence Hypothesis

5. Summary of Mendelian Principles

a. The Principle of Unit Characters

b. The Principle of Dominance

c. The Principle of Segregation

II. Modifications and Extensions of Mendelian Principles

1. The Principle of Unit Characters and Inheritance Factors

Inheritance Factors and Germinal Units

2. Modifications of the Principle of Dominance

Sex and Sex Limited Inheritance Sex Linked Inheritance

3. The Principle of Segregation "Blending" Inheritance

Maternal Inheritance III. Mendelian Inheritance in Man

Chapter IV. Influence Of Environment A. Relative Importance Of Heredity And Environment

1. Former Emphasis on Environment

2. Present Emphasis on Heredity

Si Both Indispensable to Development xiv

B. EXPERIMENTAL MODIFICATIONS OF DEVELOPMENT

I. Developmental Stimuli

1. Physical Stimuli

2. Chemical Stimuli

II. Developmextal Responses

Dependent upon (a) Nature of Organism, (b) Nature of Stimulus, (c) Stage of Development

1. Modifications of Germ Cells before Fertilization

2. During Fertilization

3. After Fertilization

C. FUNCTIONAL ACTIVITY AS A FACTOR OF DEVELOPMENT

D. INHERITANCE OR NON-INHERITANCE OF ACQUIRED CHARACTERS

E. APPLICATIONS TO HUMAN DEVELOP- MENT: EUTHENICS

CHAPTER V. CONTROL OF HEREDITY: EU- GENICS A. DOMESTICATED ANIMALS AND CULTI- VATED PLANTS I. Influence of Environment in Producing New Races II. Artificial Selection

1. The Methods of Breeders

2. How has Selection acted?

III. Methods of Modern Genetics

1. Mendelian Association and Dissociation of Characters

2. Origin of Mutations

XT

B. CONTROL OF HUMAN HEREDITY I. Past Evolution of Man II. Can Human Evolution be Controlled?

1. Selective Breeding the only Method of Improving the Race.

2. No Improvement in Human Heredity within Historic Times

S. Why the Race has not Improved III. Eugenics

1. Possible and Impossible Ideals

2. Negative Eugenical Measures

3. Positive Eugenical Measures

4. Contributory Eugenical Measures

5. The Declining Birthrate CHAPTER VI. GENETICS AND ETHICS

I. The Voluntaristic Conception of Nature AND of Human Responsibility II. The Mechanistic Conception of Nature AND OF Personality

1. The Determinism of Heredity

2. The Determinism of Environment III. Determinism and Responsibility

1. Determinism not Fatalism

2. Control of Phenomena and of Self

3. Birth and Growth of Freedom

4. Responsibility and Will

5. Our Unused Talents

IV. The Individual and the Race

1. The Conflict between the Freedom of the Individual and the Good of Society

2. Perpetuation and Improvement of the

Race the Highest Ethical Obligation REFERENCES, GLOSSARY, INDEX

Chapter I Chapter I

FACTS AND FACTORS OF DEVELOPMENT INTRODUCTION

One of the greatest results of the doctrine of organic evolution has been the determination of man's place in nature. For many centuries it has been known that in bodily structure man is an animal; that he is born, nourished and developed, that he matures, reproduces and dies just as does the humblest animal or plant. For centuries it has been known that man belongs to that group of animals which have backbones, the vertebrates; to that class which have hair and suckle their young, the mammals, and to that order which have grasping hands, flat nails, and thoracic mammas, the primates, a group which includes also the monkeys and apes. But as long as it was supposed that every species was distinct in its origin from every other one, and that each arose by a i HEREDITY AND ENVIRONMENT

special divine fiat, it was possible to maintain that man was absolutely distinct from the rest of the animal world and that he had no kinship to the beasts, though undoubtedly he was made in their bodily image. But with the establishment of the doctrine of organic evolution this resemblance between man and the lower animals has come to have a new significance. The almost universal acceptance of this doctrine by scientific men, the many undoubted resemblances between man and the lower animals, and the discovery of the remains of lower types of man, real "missing links," have inevitably led to the conclusion that man also is a product of evolution, that he is a part of the great world of living things and not a being who stands apart in solitary grandeur in some isolated sphere.

But wholly aside from the doctrine of evolution, the fact that essential and fundamental resemblances exist among all kinds of organisms can not fail to impress thoughtful men. Life processes are everywhere the same in principle, though varying greatly in detail. All the general laws of life which apply to animals and plants apply also to man. This is no mere logical inference from the doctrine of evolution, but a fact which has been established by coimtless observations and experiments. The essential oneness of all life gives a direct human interest to all living things. If "the proper study of mankind is man," the proper study of man is the lower organisms in which life processes are reduced to their simplest terms, and where alone they may be subjected to conditions of rigid experimentation. Upon this fundamental likeness between the life processes of man and those of other animals are based the wonderful advances in experimental medicine, which may be counted among the greatest of all the achievements of science.

The experimental study of heredity, development and evolution in forms of life below man must certainly increase our knowledge of and our control over these processes in the human race. If human heredity, development and evolution may be controlled to even a slight extent we may expect that sooner or later the hvmian race will be changed for the better. At least no other scheme of social betterment and race improvement can compare for thoroughness, permanency of effect, and certainty of results, with that which attempts to change the natures of men by estabhshing in the blood the qualities which are desired. We hear much nowadays about man's control over nature, though in no single instance has he ever changed any law or principle of nature. What he can do is to put himself into such relations to natural phenomena that he may profit by them, and all that can be done toward the improvement of the human race is to apply consciously to man those great principles of development and evolution which have been at work, unknown to man, through all the ages.

A. PHENOMENA OF DEVELOPMENT One of the greatest and most far-reaching themes which have ever occupied the minds of men is the' problem of development. Whether it be the development of an animal from an egg, of a race or species from a pre-existing one, or of the body, mind and institutions of man, this problem is everywhere much the same in fundamental principles, and knowledge gained in one of these fields must be of value in each of the others. Ontogeny and phylogeny are not wholly distinct phenomena, but are only two aspects of the one general process of organic development. The evolution of races and of species is suificiently rare and xinfamiliar to attract much attention and serious thought; while the development of an individual is a phenomenon of such universal occvu'rence that it is taken as a matter of course by most people, something so evident that it seems to require no explanation; but familiarity with the fact of development does not remove the mystery which lies back of it, though it may make plain many of the processes concerned. The development of a human being, of a personality, from a germ cell is the climax of all wonders, greater even than that involved in the evolution of a species or in the making of a world.

The fact of development is everywhere apparent ; its principal steps or stages are known for thousands of animals and plants ; even the precise manner of develojiment and its factors or causes are being successfully explored. Let us briefly review some of the principal events in the development of animals, and particularly of man, and then consider some of the chief factors and processes of development. Most of our knowledge in this field is based upon a study of the development of animals below man, but enough is now known of human development to show that in all essential respects it resembles that of other animals, and that the problems of heredity and differentiation are fundamentally the same in man as in other animals.

I. Development of the Body

The entire individual — structure and fimctions, body and mind — develops as a single indivisible unity, but for the sake of clarity it is desirable to deal with one aspect of the individual at a time. For this reason we shall consider first the development of the body, and then the development of the mind.

1. The Germ Cells. — In practically all animals and plants individual development begins with the fertilization of a female sex cell, or egg, by a male sex cell, or spermatozoon. The epigram of Harvey, "Omne vivum ex ovo," has found abundant confirmation in all later studies. Both egg and spermatozoon are alive and manifest all the general properties of living things. How little this fact is appreciated by the public is shown by the repeated announcements by the newspapers that "Professor So-and-so has created life because he has made an egg develop without fertilization." An egg or a spermatozoon is as much alive as is any other cell; as characteristically alive as is the adult animal into which it develops.

It is difficult to define life, as it is also to define matter, energy, electricity, or any other fundamental phenomenon, but it is possible to describe in general terms what living things are and what they do. Every living thing whatever, from the smallest and simplest micro-organism to the largest and most complex animal, from the microscopic egg or spermatozoon to the adult man, manifests the following distinctive properties :

(a) It contains protoplasm, "the material basis of life," which is composed of the most complex substances known to chemistry. Protoplasm is not a homogeneous substance, but it always exists in the form of cells, which are minute masses of protoplasm composed of many distinct parts, the most important of these being the nucleus and the cytoplasm (Fig. 1). Protoplasm is therefore organized, that is composed of many parts all of which are integrated into a single system, the cell. Higher animals and plants are composed of multitudes of cells, differing more or less from one another, which are bound together and integrated into a single organism. Living cells and organisms are not static structures which are fixed and stable in character, but they are systems which are undergoing continual change. They are like the river, or the whirlpool, or the flame, which are never at two consecutive moments composed of the same particles but which nevertheless maintain a constant general appearance ; in short they are complex systems in dynamic equilibrium.

The principal physiological processes by which all living things maintain this equilibrium are:

Memh

B

Fig. 1. A nearly ripe human ovum in the living condition. The ovum is surrounded by a series of follicle cells {FC) inside of which is the clear membrane {Memb.) and within this is the ovum proper containing yolk granules (F) and a nucleus (iV) embedded in a clear mass of protoplasm. Magnified 500 diameters (x 500). (From O. Hertwig.) B, two humati spermatozoa drawn to about the same scale of magnification. (After G. Retzius.)

(b) Metabolism, or the transformation of matter and energy within the Uving thing, in the. course of which some substances are oxidized into waste products, with the hberation of energy, while other substances are built up into protoplasm, each part of every cell converting food substances into its own particular substance by the process of assimilation.

(c) Reproduction, or the capacity of organisms to give rise to new organisms, of cells to give rise to other cells, and of parts of cells to give rise to similar parts by the process of division.

(d) Irritability, or the capacity of receiving and responding to impinging energies, or stimuli, in a manner which is usually, but not invariably, adaptive or useful.

Both the egg and the sperm are living cells with typical cell structures and functions, but with none of the parts of the mature organism into which they later develop. But although they do not contain any of the differentiated structures and functions of the developed organism, they differ from other cells in that they are capable under suitable conditions of

FACTS AND FACTORS OF DEVELOPMENT IS

producing these structures and functions by the process of development or differentiation, in the course of which the general structures and functions of the germ cells are converted into the specific structures and functions of the mature animal or plant.

In both plants and animals the sex cells are alike in many respects, though they differ greatly in appearances. The female sex cells of animals are called ova, the male cells spermatozoa. The corresponding cells and adjacent parts of flowering plants are known as ovules and pollen. Collectively all kinds of sex cells are called gametes, and the individual formed by the union of a male and a female gamete is known as a zygote, while the cell formed by the union of egg and sperm is frequently called the oosperm.

The egg cell of animals is usually spherical in shape and contains more or less food substance in the form of yolk ; it varies greatly in size, depending chiefly upon the quantity of yolk, from the great egg of a bird, in which the yolk or egg proper may be hundrecfc of millimeters in diameter, to the miscroscopic eggs of oysters, worms, etc. which may be no more than a few thousandths of a millimeter in diameter. The human ovum (Fig. 1) is microscopic in size (about 0.2 mm. in diameter) but it is not smaller than is foimd in many other animals. It has all the characteristic parts of any egg cell, and can not be distinguished microscopically from the eggs of several other mammals, yet there is no doubt that the ova of each species differ from those of every other species, and later we shall see reasons for concluding that the ova produced by each individual are different from those produced by any other individual.

The sperm, or male gamete, is among the smallest of all cells and is usually many thou-

. T.. M B

B

Fig. 2. Two Human Spermatozoa. A, showing the side of tl|p flattened head; B, its edge; H, head; M, middle-piece; T, tail. (After G. Retzius.)

sands of times smaller than the egg. In most animals, and in all vertebrates, it is an elongated, thread-like cell with an enlarged head which contains the nucleus, a smaller middle-piece, and a very long and slender tail or flagellxmi, by the lashing of which the spermatozoon swims forward in the jerking fashion characteristic of many monads or flagellated protozoa. In different species of animals the spermatozoa differ more or less in size and appearance, and there is every reason to believe that the spermatozoa of each species are peculiar in certain respects even though we may not be able to distinguish any structural differences under the microscope. The himian spermatozoa (Fig. 2) closely resemble those of other primates but are still slightly different, and the conclusion is logically inevitable, as we shall see later, that the spermatozoa as well as the ova of each individual differ slightly from those of every other individual.

2. Fertilization. — If a spermatozoon in its swimming comes into contact with a ripe but unfertihzed egg, the head and middle-piece of the sperm sink into the egg while the tail is

Fig. 3. Enthance of the Speematozoon into the egg of THE Annelid Nereis. A, Entike Spermatozoon, showing perforatorium (P) ; head {H) ; middle-piece (M), and tail (T). B-O, Successive Stages in the Maturation and Fertilization showing the progress of the entrance cone (EC) and sperm nucleus ((?N) into the egg. D shows the first maturation spindle of the egg {1st Mat. Sp.) ; E the first polar body usually broken off and left outside (Figs. 3, 4) . The nucleus in the head of the sperm then begins to absorb material from the egg and to grow in size and at the same time a minute granule, the centrosome, appears, either from the middle-piece or from the head of the sperm, and radiating lines run out from the centrosome into the substance of the egg. The sperm nucleus and centrosome then approach the egg nucleus and ultimately the two nuclei come to he side by side (Figs. 3, 4) . Usually when one spermatozoon has entered an egg all others are barred from entering, probably by some change in the surface layer of the egg or in the chemical substances given out by the egg.

This union of a single spermatozoon with an egg is known as fertilization. Whereas egg cells are usually, but not invariably, incapable of development unless fertilized, there begins, immediately after fertilization, a long series of transformations and differentiations of the fertilized egg which leads to the develop-

(Ist PB) formed by this division; P the second maturation spindle (Sd Mat. Sp.) and the sperm nucleus and spindle (^N) ; O the division of the male and female nuclei in the first cleavage spindle (lat CI. 8p.). (After F. R. Lillie.)

merit of a complex animal — of a person. In the fusion of egg and sperm a new individual, the oosperm, comes into being. The oosperm, formed by the union of the two sex cells, is really a double cell, since parts of the egg and sperm never lose their identity, and the individual which develops from this oosperm is a double being; even in the adult man this double nature, caused by the union of egg and sperm, is never lost.

In by far the larger number of animal species the oosperm, either just before or shortly after fertilization, is set free to begin its own individual existence, and in such cases it is perfectly clear that the fertilization of the egg marks the beginning of the new individual. But in practically every class of animals there are some species in which the fertilized egg is retained within the body of the mother for a varying period during which development is proceeding. In such cases it is not quite so evident that the new individual comes into being with the fertilization of the egg; rather the moment of birth, or separation from the mother, is generally looked upon as the beginning of the individual's existence. And yet in all cases the new individual is always distinguishable from the body of the mother since there is no protoplasmic connection between the two. In mammals generally, including also the human species, not a strand of protoplasm, not a nerve fiber, not a blood vessel passes over from the mother to the embryo; the latter is from the moment of fertilization onward a distinct individual with particular individual characteristics, and this is just as true of viviparous animals in which the egg undergoes a part of its development within the body of the mother as it is of oviparous ones in which the eggs are laid before development begins.

The fertilized egg of a star-fish, or frog, or man is not a different individual from the adult form into which it develops, rather it is a star-fish, a frog, or a human being in the one-celled stage. This fertilized egg fuses with no other cells, it takes into itself no living substance, but manufactures its own protoplasm from food substances; it receives food and oxygen from without and it gives out

Fig. 4 Fig. 4. Fertilization of the Eggs op Staefish and SeauBCHiir. A-C, Successive stages in the entrance of a spermatozoon into the egg of IJhe starfish, Asterias glacialU. Only carbonic acid and other waste products^; it is sensitive to certain alterations in the environment such as thermal, chemical and electrical changes — it is, in short, a distinct living thing, an individuality. Under proper environmental conditions this fertilized egg. cell develops, step by step, without the addition of anything from the outside except food, water, oxygen, and such other raw materials as are necessary to the life of any adult animal, into the immensely complex body of a star-fish, a frog, or a man. At the same time, from the relatively simple reactions and activities of the fertilized egg there develop, step by step, without the addition of anything from without except raw materials and environmental stimuli, the multifarious activities, reactions,

one sperm has penetrated the jelly layer (jl) which surrounds the egg and the peripheral protoplasm (pp) of the egg protrudes as an entrance cone (ec) to meet it. (After Fol.) D, Mature spermatozoon of the sea-urchin, Toxopneustes, showing head {h) ; middle-piece (m) ; and tail t). E-H, Successive stages in the penetration of the sperm nucleus ((^N) and centposome (SO) into the egg of Toxopneustes. I-L, Stages in the approach of the sperm nucleus (c?iV) to the egg nucleus (JiV), and in the division of the sperm centrosome (c?C) and the formation of the first cleavage spindle. {D-L after Wilson.)

Fig. S. First Cleavage of the Egg of the Sea Urchin^, Echinus microtuberculatus. A, Nuclear division figure, or mitotic spindle vi'ith a centrosome at each pole and with the chromosomes from the egg and sperm nuclei at the equator.

instincts, habits, and intelligence of the mature animal.

Is not this miracle of development more wonderful than any possible miracle of creation? And yet as one watches this marvellous process by which the fertilized egg grows into the embryo, and this into the adult, each step appears relatively simple, each perceptible change is minute; but the changes are innvmierable and unceasing and in the end they accomplish this miracle of transforming the fertilized egg cell into the fish, or frog, or man — a thing which would be incredible were it not for the fact that it has been seen by hundreds of observers and can be verified at any time by those who will take the trouble to study the process for themselves.

3. Cleavage. — After its entrance into the egg the sperm nucleus moves toward the egg nucleus until the two meet when they divide

B and C, Later stages showing the separation of the daughter chromosomes from one another and their movement toward the two poles of the spindle. D and E, Still later stages showing the swelling of the chromosomes and their fusion to form nuclear vesicles. F, Complete division of egg into two cells, each containing one daughter nucleus and centrosome. (After Boveri.)

Fig. 6. Successive Stages in the Cleavage and Gasthtjla- TiON OF Amphioxus. A, one cell; B, two cells; C and D, four cells ; E, eight cells ; P, sixteen cells ; O, blastula stage of about ninety-six cells; H, section through the same showing the cleavage cavity; /, blastula seen from the left side showing three zones of cells, viz., an upper clear zone of ectoderm, a middle (faintly shaded) zone of mesoderm and a lower (deeply shaded) zone of endoderm cells; J, section through the same showing these three types of cells; K and L, successive stages in the infolding of the endoderm; cells indicated as in the preceding figure. In all figures except D the polar body is shown at the upper pole. Figs. A-H after Hatschek. a, anterior; p, posterior; v, ventral; d, dorsal; be, blastocoel; gc, gastrocoel. I and J should be reversed to correspond in position with