The breeding of animals

Front matter

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Preface

The problems of the animal-breeder may all be grouped under the three subjects, reproduction, inheritance, development. The mere multiplication of the species is now, and always has been, the major work of the breeder of domestic animals. But the real breeder is not only concerned with the production of mere numbers of animals of a given species but is primarily interested in securing animals possessing the largest number of desirable qualities and the least number of qualities undesirable to man.

How to maintain the good qualities that have already appeared in an individual, and how to cause other and better qualities to become dominant in future individuals of the same species, is the problem of inheritance that chiefly concerns the breeder of the domestic animals. The highest attainments in the breeder's art have come only to those who have had a good knowledge of the principles and laws of heredity. The development of animals from the fertilization of the egg to maturity and their proper maintenance throughout their productive lives is second in importance only to inheritance. The environment of the animal, including food, climate, and exercise of functions, determines the degree of development in the individual animal. The term Development, as used in this connection, has reference to the unfolding of capabilities that have come to the animal through inheritance.

vi PREFACE

The author has made no attempt to write a book on genetics or evolution ; but the principles of genetics as they apply to the practice of animal-breeding are discussed, in accordance with the conclusions of biologists. The problems of the animal-breeder are in many important particulars widely different from those of the plant-breeder ; and the emphasis has been placed on those principles and practices that belong peculiarly to the province of the animal-breeder, while not neglecting the lessons and illustrations to be drawn from the other field.

It has been the purpose to make a practical book which shall be directly useful to the student and to the breeder of animals, and the lessons and examples of which can be applied in the laboratory and on the farm.

F. B. MUMFORD.

Columbia, Mo.

November, 1916.

Chapter I

The cell theory, 1 ; The germ-cells, 2 ; The cell, 3 ; Is the cell the physiological unit ? 4 ; The structure of the cell, 5 ; Protoplasm, 6 ; The nucleus, 7 ; Growth by cell division, 8 ; How cells divide, 9 ; Prophase, 10 ; Metaphase, 11 ; Anaphase, 12 ; Telophase, 13; The germ-cells in detail, 14 ; The ovum, 15 ; The spermatozoon, 16.

Chapter II

Asexual reproduction, 17 ; Sexual reproduction, 18 ; The reproductive process, 19 ; Oviparous animals, 20 ; Primary and secondary sexual characters, 21 ; The reproductive organs of the male, 22 ; The testicles, 23 ; Castration, 24; The reproductive organs of the female, 25 ; The ovaries, 26 ; The Fallopian tubes, 27 ; The uterus, 28 ; The mammary glands, 29 ; Structure of mammary glands, 30 ; Pertilization of the ovum, 31 ; The nature of fertilization, 32 ; The process of fertilization, 33 ; The chromosomes, 34 ; Results of the union of egg and sperm, 35 ; Changes in the ovum, 36 ; Changes in the spermatozoon, 37 ; The significance of reduction, 38 ; The origin of the germ-cells, 39 ; Maturation and reduction in the female (oogenesis), 40 ; Reduction in the male (spermatogenesis), 41 ; The period of the oestrum or heat, 42 ; Artificial insemination, 43 ; Methods of artificial insemination, 44 ; Conditions influencing the vitality of the sperm-cells, 45 ; Effect of too

vii viii TABLE OF CONTENTS

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frequent breeding on the sperm-cells, 46 ; Vitality of spermatozoa within the female generative organs, 47 ; Effect of intoxication of the male parent on his offspring, 48 ; Effect of lead poisoning on the male germ-cells as indicated by the offspring, 49.

Chapter III

Changed conditions, 50 ; Phases of the breeding season, 51 ; Prooestrum, 62 ; Oestrum, 53 ; Metoestrum, 54 ; Dicestrum, 55 ; Puberty, 56 ; Conditions influencing puberty, 57 ; The oestrum and lactation, 58 ; Heat during pregnancy, 59 ; Superfoetation, 60 ; Examples of superfoetation, 61 ; Recurrence and duration of the oestrum, 62 ; Effect of ration on recurrence of oestrum, 63.

CHAPTER IV Gestation and Lactation ....... 66-84

Gestation : Indications of pregnancy, 64 ; Physical examination for pregnancy, 65 ; The period of gestation, 66 ; Causes of variation in length of gestation period, 67 ; Incubation, 68 ; Parturition, 69 ; Normal parturition of the domestic animals, 70 ; Mal-presentations, 71 ; Normal presentations, 72 ; Treatment for mal-presentation, 73.

Lactation : The mammary glands, 74 ; The duration of lactation, 75 ; The food supply, 76 ; Habit, 77 ; Heredity, 78 ; Exercise, 79 ; Climate, 80 ; Unusual lactation, 81.

CHAPTER V Fertility .......... 85-112

The number of young at a birth, 82 ; Period of gestation and fertility, 83 ; Fertility and the frequency of the recurrence of the oestrum, 84 ; Fertility and gestation, 85 ; Duration of the reproductive period, 86; Confinement and fertility, 87 ; The fertility of domesticated animals, 88 ; Age and fertility, 89 ; Relation of age to fertility in

TABLE OF CONTENTS IX swine, 90 ; Influence of age of sow on size of litter, 91 ; Relation of age to fertility in sheep, 92 ; Influence of age of ram on fertility of ewes, 93 ; The effect of the age of poultry parents on the offspring, 94 ; Age and fecundity, 95 ; Nutrition and fertility, 96 ; Excessive food supply and nutrition, 97 ; Other factors affecting fertility, 98 ; Relation of number of mammae in swine to fertility, 99 ; Twins, 100 ; Characters correlated with fertility, 101 ; In-breeding and fertility, 102 ; Cross-breeding and fertility, 103 ; Unusual fertility, 104 ; Unusual fertility among horses, 105 ; Unusual fertility among cattle, 106 ; Unusual fertility among sheep, 107 ; Unusual fertility among swine, 108 ; Unusual fertility among poultry, 109.

Chapter VI

The causes of sterility, 110 ; Causes of sterility in the male. 111 ; Sterility in the female, 112 ; Closure of the cervix, 113 ; Obstruction of Fallopian tubes resulting from excessive fatness, 114 ; Other causes of barrenness, 115 ; Sterility from fatty degeneration, 116 ; Sterility caused by abortion, 117 ; Contagious abortion and sterility, 118 ; Treatment for contagious abortion, 119 ; Diagnosis of contagious abortion, 120 ; The complement fixation test, 121 ; Sterility of free-martins, 122.

Chapter VII

Development, 123 ; Heredity defined, 124 ; Heredity and variation not antagonistic, 125; The kinds of heredity, 126 ; Blending inheritance, 127 ; Alternative inheritance, 128 ; Particulate or mosaic inheritance, 129 ; Mendelian inheritance, 130 ; The experiments of Mendel, 131 ; The law of dominance, 132 ; The law of segregation, 133; Unit characters, 134; Gametic purity, 135 ; Application of Mendel's law, 136 ; The complexity of animal characters, 137 ; The inheritance of polled and horned character in cattle, 138 ; Theory of pure lines,

X TABLE OF CONTENTS

139 ; Hallett's wheat breeding, 140 ; The presence and absence hypothesis, 141 ; The theory of mutations, 142 ; Two important classes of variation, 143 ; Kinds of mutations, 144 ; Importance of the mutation theory, 145 ; Mono-hybrids and di-hybrids, 146.

Chapter VIII

Inheritance of Acquired Characters .... 157-182 Belief in transmission of acquired characters, 147 ; Practical breeders believe in transmission of acquired characters, 148 ; Nature and nurture, 149 ; What are acquired characters ? 150 ; Somatoplasm and germ-plasm, 151 ; Examples of acquired characters, 152 ; Food supply, 153 ; Influence of the amount of food on body weight, 154 ; Food supply and body changes, 155 ; Influence of limited food supply from birth, 156 ; Telegony, 157 ; The Lord Morton mare, 158 ; The Penycuik experiments, 159 ; Telegony and mule hybrids, 160 ; Example of horse foals, 161 ; Possibility of influence from a previous impregnation, 162; Xenia in animals, 163 ; Xenia among poultry, 164.

Objections to the Theory that Acquired Characters are Transmitted : No mechanism for the inheritance of acquired characters, 165 ; The inheritance of disease, 166 ; Acquired diseases, 167 ; Congenital disease, 168 ; Predisposition to disease, 169; Immunity, 170.

Chapter IX

The significance of conjugation and fertilization, 171 ; Secondary sexual characters, 172 ; Secondary sexual characters and vigor, 173 ; Effects of castration and ovariotomy on the secondary sexual characters, 174 ; Effect of transplanting sexual glands, 175 ; Effect of internal secretion, 176 ; Sex-linked characters, 177 ; Color-blindness, 178 ; Controlling the sex of offspring, 179 ; Age or vigor of parents, 180 ; Comparative vigor ov

TABLE OF CONTENTS XI sexual superiority, 181 ; Nutrition and sex, 182 ; The maturity of the ovum, 183 ; Seasonal variations in proportion of sexes, 184 ; Sex cannot be controlled by external conditions, 185.

Chapter X

Importance of variability, 186 ; Morphological variations, 187 ; Physiological variations, 188 ; Meristic variation, 189 ; Functional variations, 190 ; Examples of functional variation, 191 ; Variation in fertility of animals, 192 ; Variation in the milking function, 193 ; Variations among different cows, 194 ; New characters originate in the germ-plasm, 195 ; Mutilations, 196 ; The Brown-Sequard experiments, 197 ; Causes of variation, 198 ; Cell division a cause of variation, 199 ; Influence of use and disuse in causing modifications, 200 ; Importance of causes of variation to the breeder of domestic animals, 201 ; Germinal variations, 202.

Chapter XI

Definitions, 203 ; Advantages claimed for in-breeding, 204 ; Bad results from in-breeding, 205 ; Decreased fertility and vigor from in-breeding, 206 ; Darwin's researches, 207 ; In-breeding cattle, 208 ; The Chillingham cattle, 209; Deer in parks, 210; In-breeding among pigs, • 211 ; In-breeding sheep, 212 ; In-breeding dogs, 213 ; Cornevin's experiments, 214 ; Weismann's and Von Guaita's experiments, 215 ; Researches of Ritzema Bos, 216 ; The Wistar Institute experiments, 217 ; In-breeding Berkshires by Mr. Gentry, 218 ; In-breeding corn, 219 ; How long is it safe to continue in-breeding ? 220 ; Selection important, 221 ; The truth about in-breeding, 222 ; Fixing characters by in-breeding, 223 ; In-breeding and prepotency, 224 ; Results of in-breeding vary with different species, 225.

xii TABLE OF CONTENTS

Chapter XII

Permanent and temporary results of cross-breeding, 226 ; Advantages from cross-breeding, 227 ; Grading, 228 ; Cross-breeding to increase fertility, 229 ; Cross-breeding to increase size and restore constitution, 230 ; Crossing and heredity, 231 ; First cross and improvement, 232 ; Cross-breeding as a cause of variation, 233 ; Crossing species, 234 ; Crossing bison and cattle, 235 ; The mule hybrid, 236 ; The hinny hybrid, 237 ; Crossing the horse and the zebra, 238 ; Crossing cattle and zebu, 239 ; Sheep-goat hybrid, 240.

CHAPTER XIII Development ......... 255-279

Growth, 241 ; The growth impulse, 242 ; Factors influencing growth, 243 ; Growth and food supply, 244 ; Capacity to grow, 246 ; Growth and the cell, 246 ; When the growth impulse is strongest, 247 ; Development of the foetus, 248 ; Heredity and foetal development, 249 ; Birth weight of lambs, 250 ; Effect of protein and ash in ration on foetal development, 251 ; High calcium rations for pregnant swine, 252 ; Size and vigor of foetus as influenced by corn and wheat rations, 253 ; The permanent effect of retarded growth, 254 ; Early stunting and the capacity to grow, 255 ; Climate, 256 ; The age factor in animal-breeding, 257 ; Premature breeding decreases size, 258 ; Decreased size due to early breeding not inherited, 259 ; Influence of early pregnancy on the mother, 260 ; Gestation and lactation in relation to growth, 261 ; The Missouri experiments, 262.

CHAPTER XIV The Practice of Breeding . . . . . . 280-303

Improvement in size, 263; Improvement in function, 264 ; The milking function, 265 ; Improvement in wool production, 266 ; Improvement in tendency to lay on fat.

TABLE OF CONTENTS xiii

267 ; Improvement in speed, 268 ; Selection, 269 ; Natural selection, 270 ; Methodical selection, 271 ; Importance of selection in animal-breeding, 272 ; Aids to selection, 273 ; The real results of selection in the improvement of the domestic animals, 274; Selection within pure lines, 275 ; Vilmorin's pure line wheat-breeding, 276 ; Selection most useful when genetic factors are not pure, 277 ; Pure line theory not opposed to improvement by selection, 278 ; Pedigree, 279 ; Registered breeding animals, 280 ; Registry associations, 281 ; Community breeding, 282 ; Importance of numbers, 283 ; Selecting the best, 284 ; Selecting chance variations, 286 ; The Burbank method, 286 ; The mendelian method, 287.

ILLUSTKATIONS

FIG.

1. Cell division. Prophases. Aftei* Wilson

2. Cell division. Later phases. After Wilson

3. The ovarian egg

4. Human spermatozoa ....

6. Genital organs of boar. After EUenberger

6. Sections through ovary of rat

7. Genital organs of mare. After EUenberger

8. Genital organs of cow. After EUenberger

9. Genital organs of sow. After EUenberger

10. Genital organs of bitch. After EUenberger

11. Normal presentation in mare

12. Posterior presentation .

13. Abnormal anterior presentation .

14. Abnormal posterior presentation .

15. Abnormal transverse presentation

16. Diagram illustrating mendelian inheritance

17. Diagram illustrating mendelian inheritance

PAGE

PLATE I.

VI. Plates

Genital organs of mare. After EUenberger Superfcetation in mare ....

Upper : A mare mule that secretes milk . Lower : A Free-Martin heifer .

Unusual fertility in a cow. Triplet calves Normal healthy uterus of sow .

Uterus of sterile sow

FACING PAGE

. 43

62^

84^,

84^

. 108

118 '

119 *^

XV

XVI

ILLUSTRATIONS

PLATE FA{

VII. Upper: Steer fed ration not restricted

Lower : Steer fed greatly restricted ration VIII. Upper : Steer fed ration not restricted Lower : Steer fed for normal growth . IX. Upper : Steer fed generously, at age 120 days . Lower : Same at age 27 months ....

X. Upper : Dam of seven mule foals followed by filly foal

Lower : Filly foal born after seven mule foals . XL Upper : Eleventh foal following ten mule foals . Lower : Dam of ten mule foals and their filly foal XII. Upper : Ninth foal following eight mule foals .

Lower : Foaled after eight mule foals in succession

XIII. Upper : Twelfth foal following eleven mule foals Lower : Mother of eleven mule foals and their horse

foal

XIV. Upper : Close in-breeding of fox terrier Lower : Eighth generation of intense in-breeding

XV. In-bred Berkshire ......

XVI. Cross-bred Hereford-Aberdeen Angus steer XVII. Upper: Half-blood buffalo (bison) heifer .

Lower : Cross-bred buffalo-cattle bulls XVIII. Upper : A five-year-old hinny ....

Lower : Sheep-goat hybrid .....

XIX. Effect of food supply on development. Side view

XX. Same. Front view

XXI. Effect of starvation on capacity to grow XXII. Recovery from starvation

XXIII. Upper : Cow fed corn products .... Lower : Calf from cow fed corn products .

XXIV. Upper : Cow fed wheat products Lower : Calf from cow fed wheat products

XXV. Permanent effect of retarded growth .

XXVI. Grace Briggs at age 18 years ....

XXVIII. Sophie 19th of Hood Farm

XXIX. Daughters of the same sire .....

XXX. Three generations showing impressive character of original dam ......

XXXI. Result of using a pure-bred sire ....

XXXII. Result of using a scrub sire ....

ACKNOWLEDGMENTS

The author is indebted to R. Pearl for Plate IV, to J. W. Connaway for Plates V and VI, to P. F. Trowbridge for Plates VII, VIII, IX, XXI, XXII, and XXV, to E. A. Trowbridge for Plate XVI, to M. Boyd for Plate XVII, to L. Monsees for Plate XVIII upper, to W. J. Spillman for Plate XVIII lower, to Hart et al, for Plates XXIII and XXIV, to C. H. Eckles for Plates XXVI, XXIX, and XXX, and to H. Hackedorn for Plates XXXI and XXXII.

Grateful acknowledgment is made to all these persons for their valuable assistance.

XVll

Chapter I The Cell

The greatest modern contribution to the science of animal-breeding was the formulation of the so-called cell theory. This fundamental biological generalization ranks with the evolution theory in importance in many respects ; it has given a definite physical basis for inheritance.

1. The cell theory. — All the higher forms of life are made up of cell units, and from these all parts of the body are constructed. Although various in form, all living cells are alike in having within a mass of protoplasm which Huxley called the '' physical basis of life." In the simple one-celled forms all functions are found in the one cell, but in the more complex higher forms a physiological division of labor results in the distribution of functions among the cells. ''It is to the cell," says Verworn,^ '' 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 contraction ; in the gland cell reside the causes of secretion ; in the epithelial cell, in the white blood cell, lies the problem of absorption of food, and the secrets of the

1 Wilson, " The Cell," p. 6, 1911.

B 1 mind are hidden in the ganglion cell." It is now clearlyapparent that the great questions of reproduction, inheritance and development, involving as they do the problems of embryology and evolution, are intricately bound up with the structure and functions of the cell.

2. The germ-cells. — The heritage of the species is contained within the germ-cell. The microscopic egg of the female carries within its minute structure the germ characters of all the maternal ancestors. The germ-cell of the male, the spermatozoon, holds within its exceedingly minute compass the sum total of all the heritable characteristics of the paternal ancestors. All cells are derived from other cells. Virchow's claim made in 1855 that every cell must have been formed by cell division from some previously formed cell, has now been definitely established. Not only does growth and development take place by cell division in the fertilized egg-cell, but the egg-cell itself is directly derived by cell division from an egg-cell of the immediately preceding generation, and so on indefinitely.

3. The cell. — The cell is a mass of protoplasm containing a nucleus, and both nucleus and protoplasm arise through division of the corresponding elements of a preexisting cell.^

The word cell is derived from the Greek and means a hollow chamber. The term came into common use before the form and structure of the cell were well understood. The cell-wall which is characteristic of most cells is not an essential part of its structure. It is also

1 Wilson, "The Cell in Development and Inheritance," p. 19, 1906.

true that living cells are never hollow chambers, but are filled in whole or in part by a colorless, viscid, semi-fluid substance, protoplasm. Many cells are simply masses of protoplasm lacking entirely any kind of an enclosing wall. Lying within the protoplasm is a minute body of spherical form which is the cell nucleus. These two, the protoplasm and the nucleus, are of universal occurrence and are the essential components of a living cell.

4. Is the cell the physiological unit ? — A study of the form and function of the cell leads to the inevitable conclusion that in a very real sense the cell is the morphological unit of the organism. In its physiological relations to the cells* of the body as a whole, however, it is not to be regarded as an independent unit but rather as a localized center of bodily activities. The individual cell in a multicellular body is influenced, sometimes in a marked degree, by the surrounding cells. The most fundamental problem in growth and development of animals is what and how much influence do the body-cells of one group have over the cells of another group. Is there a physiological connection between adjoining cells? Can a group of cells forming a so-called system like the reproductive system in the animal body be influenced by the soma or body-cells ? If influenced at all, can such influence have any effect upon the germ plasm in the nucleus of the germ-cell ? Is it probable that the germ characters may be changed by these influences so that the offspring of the parent bodies where these influences have been at work, will correspond in any way to the changes occurring in the parent body? In other words, are acquired habits transmitted ? It is to be regretted that the microscopic study of the cell has thrown little light upon this fundamental question. As indicated above, the cell in the animal body is in more or less close physiological relation with the other cells of the body, but these relationships and their bearings upon- the fundamental facts of biology have not yet been clearly determined.

5. The structure of the cell. — The cell contains a cell-body and the nucleus. The cell-body is all that portion of the protoplasm not contained in the nucleus. The cell in its simplest form is a rounded mass of protoplasm. This type is found generally in one-celled forms and is the characteristic form of the egg-cell of the higher animals. The fact that the form of cells in the higher plants and animals is not always* rounded spherical is due to unequal pressure and the movement of the cells comprising the body.

The nucleus is a definite, clearly-marked body existing within the protoplasmic contents of the living cell, and its relation to growth, reproduction and heredity have given it a commanding position in the study of modern biological problems. Other bodies are often found in the cell, such as food granules, products of excretion, fat globules and crystals. None of these plays an active part in the metabolism of the cell and may be regarded as accidental or at least subsidiary to the major role played by the protoplasm itself. Another body generally found in the cell is the centrosome which is concerned with the mechanism of cell division. The cell-wall is generally present in the higher forms of plant and animal life and consists of a membrane which is usually lifeless.

6. Protoplasm. — The protoplasm is universally present in every living cell. It is the most fundamentally important life substance. Huxley aptly designated protoplasm as " the pltysical basis of life." It is not to be regarded as being a definite chemical substance, as its composition changes. It is a viscid, colorless, semifluid material having a higher index of refraction than water, and hence appears brighter. It was called slime by Schleiden. The protoplasm of the cell has a definite structural arrangement appearing as a meshwork, or reticulum, and a ground substance, or cell-sap, filling the intervening spaces. In addition to these two definite substances there are present in the protoplasm minute granules or microsomes which are distributed regularly or irregularly along the lines of the meshwork. While other materials are often found in the protoplasm, the above materials are regarded as the essential elements of primary importance in the activities of the cell.

7. The nucleus. — The nucleus is the center of the constructive activities of the cell. When the nucleus is destroyed, those processes which result in the growth and development of the organism can no longer take place. Only destructive activities are possible in a cell devoid of a nucleus, and these can go forward for only a limited time. " The nucleus is generally regarded," says Wilson, '' as a controlling center of cell activity, and hence a primary factor in growth, development and the transmission of specific qualities from cell to cell, and from one generation to another." ^ Growth is the result of cell division, and the impetus for cell division appears to come from the nucleus. The essential fact in cell division is that a portion of the nuclear material of the parent cell shall pass into the new cell. The new cell in its turn becomes a parent cell, and so the process of growth continues. The nucleus is typically spherical and moves freely within the cell. It exhibits two distinct

1 Wilson, "The Cell in Development and Inheritance," p. 30.

phases which result from the varying degrees of activity present in the nuclear substance. One phase may be designated as the vegetative or quiescent stage and the other the active stage which is characteristic of that period in the development of the nucleus when the many complicated and significant changes occur which result in cell division and in reproduction.

The typical nucleus during the vegetative stage possesses certain distinct structural forms which are concerned in the many important nuclear activities: (1) The nuclear wall which encloses the nucleus and differentiates the nucleus from the cell-body; (2) The reticulum, which is the primary factor in nuclear activities, and appearing as an irregular network. The reticulum in turn comprises two structures, the linin and the chromatin. The latter is undoubtedly the most fundamentally important organic substance concerned with the growth, development and inheritance of plants and animals. It is apparently the only or chief material which is transmitted from the parent cell to the new or daughter cell by division, and from it all nuclear substance may be reformed. The word chromatin is given to this material because it becomes deeply stained upon the addition of certain well-known reagents. The chromatin may appear in the cell in scattered granules, varying in size and form, or in a single deeply staining mass, but more often the arrangement of the chromatin in the nucleus resembles a network which is closely associated with the clearly differentiated linin. (3) The nucleoli are generally but not always present, and their nature and functions are not well understood. By some authorities the nucleoli are the by-products of the activities going on in the nucleus. (4) The ground substance is a fluid filling the chromatin network and is not stained by ordinary reagents.

8. Growth by cell division. — How does growth take place in a living organism? What are the primary factors concerned in the increase in size? What changes in the organism result in the progressive development of the many useful qualities found in the improved types of the domestic animals? How does a microscopic egg-cell develop into a mature and highly organized animal possessing countless cells of many different forms and exercising various and important functions? A still more fundamental problem, if possible, is, how are the qualities of an individual transmitted from parent to offspring? What is the physical basis of heredity and what are the elements concerned in a study of inheritance ?

Many of these questions are answered by a study of the cell and specifically by a study of cell division. The entire tissue structure of the animal body arises by repeated division from the germ-cell. The germ-cell itself is the result of the division of a cell which formed a part of the body of the parent. Thus it is that the germ substance carrying the hereditary material is separated from the parent body by cell division. The fertilized egg by continued cell division passes on to every cell in the body a portion of its own substance. The process of cell division must therefore be regarded as a great fundamental fact in the growth and development of plants and animals as well as one of the most significant and primary facts in the transmission of qualities from parent to offspring.

Growth occurs as the result of continued cell division rather than by any material increase in the size of existing cells.

9. How cells divide. — All cells do not divide in the same manner, but the most typical process is known as indirect division or mitosis, and this will be here described. We have seen that the nucleus contains within its minute compass the active material which stimulates the cell to various activities and determines its physiological destiny. If further evidence was needed on this point, it would be found in the remarkable and interesting transformations which take place within the nucleus before and during the process of cell division.

The cell first passes through a vegetative or quiescent stage, and this is followed by a period of activity finally resulting in the formation of two cells from the original parent cell. Various clearly marked stages or phases are distinguishable in this process which have been accurately described by Wilson. The phases observed are for convenience named : (1) prophase, (2) metaphase, (3) anaphase, and (4) telophase.

10. Prophase (Fig. 1). — In the vegetative stage the chromatin of the nucleus exists in the form of a network. Generally during the prophase the chromatin loses its net-like arrangement and assumes the form of a skein like thread known as the spireme. During this stage the spireme thread stains intensely and is fine and closely convoluted. It gradually becomes thicker and the convolutions become more open, giving rise to the '' open spireme." Gradually the spireme breaks up into a number of definite straight or curved rods known as chromosomes. It is usual for the wall of the nucleus to disappear during this phase, and the chromosomes then lie naked in the protoplasm of the cell. It is a significant fact that every plant and animal possesses a characteristic number of chromosomes and that this number is always even.

Fig. 1. — Cell division. Diagram showing typical prophases in cell division. A, Vegetative or resting stage showing nucleus. B, The spireme thread. C, Preparation for division. D, Formation of chromosomes. E, Gradual fading of nuclear membrane. F, Chromosomes in equatorial plate ready for division.

In the ox and in man the number is sixteen. The fact that the number of chromosomes is even in all species is due to the fact that during the processes of fertilization one-half the chromosomes are derived from the female and one-half from the male parent.