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YALE UNIVERSITY
MRS. HEPSA ELY SILLIMAN MEMORIAL LECTURES

THE THEORY OF THE GENE

VOLUMES PUBLISHED BY YALE UNIVERSITY PRESS ON
THE SILLIMAN FOUNDATION

Electricity and Matter. By Joseph John Thomson, D.Sc, LL.D., Ph.D.,
F.R.S., Fellow of Trinity College and Cavendish Professor of Experimen-
tal Physics, Cambridge University. (Fourth printing.)

The Integrative Action of the Nervous System. By Charles S. Sher-
rington, D.Sc, M.D., Hon. LL.D. Tor., F.R.S., Holt Professor of Physi-
ology, University of Liverpool. (Seventh printing.)

Experimental and Theoretical Applications of Thermodynamics to
Chemistry. By Dr. Walter Nernst, Professor and Director of the In-
stitute of Physical Chemistry in the University of Berlin.

Radioactive Transformations. By Ernest Rutherford, D.Sc, LL.D.,
F.R.S., Macdonald Professor of Physics, McGill University. (Second
printing.)

Theories of Solutions. By Svante Arrhenius, Ph.D., Sc.D., M.D., Direc-
tor of the Physico-Chemical Department of the Nobel Institute, Stock-
holm, Sweden. (Fourth printing.)

Irritability. A Physiological Analysis of the General Effect of Stimuli in
Living Substances. By Max Verworn, M.D., Ph.D., Professor at Bonn
Physiological Institute. (Second printing.)

Stellar Motions. With Special Reference to Motions Determined by Means
of the Spectrograph. By William Wallace Campbell, Sc.D., LL.D.,
Director of the Lick Observatory, University of California. (Second print-
ing-)

Problems of Genetics. By William Bateson, M.A., F.R.S., Director of
the John Innes Horticultural Institution, Merton Park, Surrey, England.
(Second printing.)

The Problem of Volcanism. By Joseph Paxson Iddings, Ph.B., Sc.D.
(Second printing.)

Problems of American Geology. By William North Rice, Frank D.
Adams, Arthur P. Coleman, Charles D. Walcott, Waldemar Lind-
gren, Frederick Leslie Ransome, and William D. Matthew. (Second
printing.)

Organism and Environment as Illustrated by the Physiology of
Breathing. By J. S. Haldane, M.A., M.D., F.R.S., Hon. LL.D. Birm.
and Edin., Fellow of New College, Oxford; Honorary Professor, Birming-
ham University. (Second printing.)

A Century of Science in America. With Special Reference to the Ameri-
can Journal of Science 1818-1918. By Edward Salisbury Dana, Charles
Schuchert, Herbert E. Gregory, Joseph Barrell, George Otis Smith,
Richard Swann Lull, Louis V. Pirsson, William E. Ford, R. B. Sos-
man, Horace L. Wells, Harry W. Foote, Leigh Page, Wesley R. Coe,
and George L. Goodale.

A Treatise on the Transformation of the Intestinal Flora with Spe-
cial Reference to the Implantation of Bacillus Acidophilus. By
Leo F. Rettger, Professor of Bacteriology, Yale University, and Harry
A. Cheplin, Seessel Fellow in Bacteriology, Yale University.

The Evolution of Modern Medicine. By Sir William Osler, Bart., M.D.,
F.R.S. (Third printing.)

Respiration. By J. S. Haldane, M.A., M.D., F.R.S., Hon. LL.D. Birm.
and Edin., Fellow of New College, Oxford; Honorary Professor, Birming-
ham University.

After Life in Roman Paganism. By Franz Cumont. (Second printing.)

The Anatomy and Physiology of Capillaries. By August Krogh, Ph.D.,
LL.D., Professor of Zoo-physiology, Copenhagen University. (Second
printing.)

Lectures on Cauchy's Problem in Linear Partial Differential Equa-
tions. By Jacques Hadamard, LL.D., Member of the French Academy
of Sciences ; Foreign Honorary Member of the American Academy of
Arts and Sciences.

THE THEORY OF THE

GENE

BY
THOMAS HUNT MORGAN

Professor of Zoology in Columbia University.

NEW HAVEN
YALE UNIVERSITY PRESS

LONDON • HUMPHREY MILFORD • OXFORD UNIVERSITY PRESS

MDCCCCXXVI

THE SILLIMAN FOUNDATION

In the year 1883 a legacy of eighty thousand dollars was left to
the President and Fellows of Yale College in the city of New
Haven, to be held in trust, as a gift from her children, in memory
of their beloved and honored mother, Mrs. Hepsa Ely Silliman.

On this foundation Yale College was requested and directed
to establish an annual course of lectures designed to illustrate
the presence and providence, the wisdom and goodness of God,
as manifested in the natural and moral world. These were to be
designated as the Mrs. Hepsa Ely Silliman Memorial Lectures.
It was the belief of the testator that any orderly presentation of
the facts of nature or history contributed to the end of this
foundation more effectively than any attempt to emphasize the
elements of doctrine or of creed ; and he therefore provided that
lectures on dogmatic or polemical theology should be excluded
from the scope of this foundation, and that the subjects should
be selected rather from the domains of natural science and his-
tory, giving special prominence to astronomy, chemistry, geology
and anatomy.

It was further directed that each annual course should be made
the basis of a volume to form part of a series constituting a
memorial to Mrs. Silliman. The memorial fund came into the
possession of the Corporation of Yale University in the year
1901; and the present work constitutes the nineteenth volume
published on this foundation.

TABLE OF CONTENTS

List of Illustrations xi

I. The Fundamental Principles of Genetics 1

Mendel's Two Laws 1

Linkage 10

Crossing-Over 14
The Simultaneous Interchange of" Man v Genes in

Crossing- Over 20

The Linear Order of the Genes 22

The Theory of the Gene 25

II. Particulate Theories of Heredity 26

III. The Mechanism of Heredity 32

The Mechanism of Mendel's Two Laws 33
The Number of the Linkage Groups and the Number

of the Chromosome Pairs 36

The Integrity and Continuity of the Chromosomes 37

Mechanism of Crossing-Over 39

IV. Chromosomes and Genes 45

V. The Origin of Mutant Characters 59

VI. Are Mutant Recessive Genes Produced by

Losses of Genes? 72

Recessive Characters and Absences of Genes 74
The Bearing of Reverse Mutation (Atavism) on the

Interpretation of the Mutation Process 85

The Evidence from Multiple Allelomorphs 92

Conclusions 94

VII. The Location of Genes in Related Species 95

VIII. The Tetraploids, or Fourfold Type 105
Tetraploidy as a Means of Increasing the Number of

Genes in a Species 130

viii TABLE OF CONTENTS

IX. Triploids 131

X. Haploids 139

XI. Polyploid Series 150

The Polyploid Wheats 150

The Polyploid Roses 158

Other Polyploid Series 165

XII. Heteroploids 172

XIII. Species Crossing and Changes in Chromo-

some Nnmber 191

XIV. Sex and Genes 199

The Insect Type (XX-XY) 199

The Avian Type (WZ-ZZ) 206

Sex-Chromosomes in Dioecious Flowering Plants 212

Sex-Determination in Mosses 214

XV. Other Methods of Sex-Determination Involv-

ing the Sex-Chromosomes 219

The Attachment of the X-Chromosome to Auto-
somes 219
The Y-Chromosome 222
Degeneration of Male-Producing Sperm 228
The Elimination of one X-Chromosome from a Dip-
loid Egg to Produce a Male 228
Sex-Determination through the Accidental Loss of

a Chromosome in Spermatogenesis 231

Diploid Females and Haploid Males 233

XVI. Intersexes 240

Intersexes from Triploid Drosophila 240

Intersexes in the Gypsy Moth 243

The Free Martin 247

XVII. Sex Reversals 250

Environmental Changes 251

Changes of Sex Associated with Age 254

TABLE OF CONTENTS ix

Sex and Sex Reversal in Frogs 256
Transformation of Bidder's Organ of the Male Toad

into an Ovary 266

Sex Reversal in Miastor 269

Sex Reversal in Birds 271

The Effect of Ovariotomy in Birds 272

The Sex of Parabiotic Salamander Twins 275

Sex Reversal in Hemp 276

XVIII. Stability of the Gene 281

XIX. General Conclusions 300

The Effects Produced by a Change in Chromosome

Number and by a Change in a Gene 300

Is the Mutation Process Due to a Degradation of the

Gene ? 307

Are Genes of the Order of Organic Molecules? 309

Bibliography 311

Index 337

LIST OF ILLUSTRATIONS

FIG. PAGE

1. Inheritance of tall versus short peas 2

2. Hybrid pea back-crossed to recessive parent 3

3. Inheritance of brown versus blue eyes 4

4. Back-cross of hybrid to recessive blue-eyed individual . . 5

5. Inheritance of flower color of four-o'clock (Mirabilis) . . 6

6. Distribution of genes in cross shown in figure 5 . 7

7. Inheritance of yellow round and green wrinkled peas . . 8

8. Distribution of genes for three pairs of characters ... 9

9. Inheritance of two linked characters in sweet peas ... 11

10. Inheritance of four linked characters in Drosophila ... 13

11. Inheritance of two sex-linked characters in Drosophila . . 15

12. Inheritance of the same characters as in figure 11 in recipro-
cal combination 16

13. Inheritance of white eyes and miniature wings of Droso-
phila (back-cross) 18

14. 'Inheritance of white eyes and forked bristles of Drosophila 19

15. The location of nine sex-linked recessive genes .... 21

16. Crossing-over between garnet and vermilion in the series of
genes shown in figure 15 21

17. Crossing-over between echinus and cross-veinless in the series

of genes shown in figure 15 21

18. Double crossing-over 21

19. Chart of the linked genes of Drosophila 23

20. The order of the genes, yellow, white, bifid, of Drosophila . 24

21. Maturation of sperm-cell 33

22. Maturation of the egg 34

23. Independent assortment of X and a pair of autosomes . . 35

24. Haploid chromosome groups of edible pea, sweet pea, and
Indian corn 37

25. Nuclei of daughter cells of Ascaris 39

26. Conjugation of chromosomes of Batrachoseps 41

27. Twisted chromosomes of Batrachoseps 42

28. Conjugation of chromosomes of a planarian, Dendrocoelum 43

29. Normal and haplo-IV female of Drosophila 47

xii LIST OF ILLUSTRATIONS

FIG. PAGE

30. Three mutant types of chromosome-IV group of Drosophila 48

31. Cross between haplo-IV and diploid eyeless fly . . ... 49

32. Haplo-IV and triplo-IV female of Drosophila .... 50

33. Cross between triplo-IV and diploid eyeless Drosophila
(upper half of diagram). Continuation of last (lower half of
diagram), mating of heterozygous triplo-IV to diploid eye-
less 51

34. Primary non-disjunction. XX-egg fertilized by Y-sperm . . 53

35. Non-disjunction. White-eyed XXY female fertilized by red-
eyed XY male 54

36. Superfemale of Drosophila. (2n + 3X) 56

37. Fertilization of egg with attached X (double yellow female)

by sperm of wild-type male 57

38. Sex-linked inheritance of the mutant type, white eyes of
Drosophila 60

39. Sex-linked inheritance of a light-colored mutant type lacti-
color of Abraxas 61

40. Head of mutant type of Drosophila, Lobe2 (eye) .... 62

41. Mutant type of Drosophila, Curly wings 63

42. Oenothera Lamarckiana and 0. gigas 69

43. Combs of domestic races of fowls 73

44. Mutant type — Notch wings — of Drosophila 78

45. Inheritance of Notch wings 79

46. Notch- deficiency; duplication of not-vermilion; duplication

of not-yellow 80

47. Fertilization of egg-cell of flowering plant, and develop-
ment of endosperm 83

48. Triploid condition of (a) two floury genes versus one flinty
gene; (a') two flinty versus one floury gene 84

49. Types of bar-eye of Drosophila 87

50. Diagram of bar female heterozygous for forked and fused,

by forked bar fused male 88

51. Diagram of crossing-over of bar; of infra-bar; of bar-infra-
bar 90

52. Diagram of crossing-over of bar-infra-bar heterozygous for
forked and fused; of infra-bar-bar heterozygous for forked

and fused 91

53. Cross between two species of tobacco 96

54. Cross between two species of snapdragon 97

LIST OF ILLUSTRATIONS xiii

PIG. PAGE

55. Cross between a mutant type of one species of snapdragon

and a normal type of another species 97

56. Different recombination types resulting from cross of two
species of snapdragon (figure 55) 98

57. Varieties of two species of Helix, and hybrid between them . 99

58. Drosophila melanogaster and D. simulans 100

59. Homologous genes of Drosophila melanogaster and D. simu-
lans 101

60. Chart of chromosomes of Drosophila virilis 102

61. Chart of chromosomes of Drosophila obscura 103

62. Chromosomes of Metapodius with three m-chromosomes and
their reduction 106

63. First two divisions of egg of Ascaris 108

64. Polar, metaphase group of chromosomes of diploid and
tetraploid Artemia 109

65. Diploid and tetraploid chromosome group of Oenothera . . 110

66. Maturation of pollen mother cells of Oenothera .... Ill

67. Grafting of tomato and nightshade, and a resulting chimaera 113

68. Diploid and tetraploid nightshade 114

69. Seedlings, flowers, and cells of diploid and tetraploid night-
shade 115

70. Haploid and diploid cells of normal nightshade; diploid and
tetraploid cells of tetraploid 116

71. Same of tomato 117

72. Normal and tetraploid Datura 118

73. Haploid, diploid, triploid, and tetraploid Datura .... 119

74. Diploid and tetraploid chromosome groups (second matura-
tion division) of Datura 120

75. Methods of conjugation of chromosomes of diploid, triploid,

and tetraploid Datura 121

76. Conjugation of chromosomes of tetraploid Datura . . . 122

77. Chromosome groups of Euchlaena, annual and perennial, of
Indian corn, and of hybrid 123

78. Diagram of gametophyte and sporophyte generation of dioe-
cious moss 125

79. Diagram of formation of 2n gametophyte from regeneration
of 2n sporophyte, and 4n gametophyte by regeneration from

4n sporophyte 127

80. Triploid chromosome-group of hyacinth 132

81. Maturation division of diploid and triploid Datura . . . 133

xiv LIST OF ILLUSTRATIONS

FIG. PAGE

82. Diploid and triploid Drosophila 135

83. Constriction of egg of Triton isolating one half with a
single sperm 140

84. Haploid plant of Datura 143

85. Diagram of two maturation divisions of egg of bee, and fer-
tilization of egg by sperm 144

86. Two maturation stages of the germ-cell of the honey bee . 146

87. Parthenogenetic female, sexual female, and male, of Hyda-
tina 147

88. Chromosomes of diploid, tetraploid, and hexaploid wheats . 151

89. Normal maturation divisions of Einkorn, Emmer, and Vul-
gare wheats 153

90. Maturation divisions of hybrid between Emmer and Vulgare
wheats 154

91. Same as last, illustrating a somewhat different account of
process 155

92. Multiple chromosome groups of roses 159

93. Maturation division of pollen cells of a hybrid rose . . . 161

94. Maturation division of egg-cell of a hybrid rose .... 162

95. Five types of hybrid canina roses 164

96. Maturation of pollen of Hieracium 166

97. Chromosomes of chrysanthemums 167

98. Nuclear sizes of varieties of chrysanthemums 168

99. Chromosomes of chrysanthemums 168

100. Oenothera lata, heterosomic type 173

101. Mutant types of seed capsules of Datura 178

102. Normal and heterosomic (2n + 1 and 2n + 2) types of seed
capsules of Datura 179

103. Tetraploid and heterosomic tetraploid types of Datura . . 183

104. Union of chromosomes in Primary and Secondary types of
Datura 184

105. Diagram of hypothetical reversal of conjugating chromo-
somes 185

106. Conjugation of chromosomes of Primary and Secondary
heterosomic types of Datura 186

107. Imaginary relation of mutant types to specific chromosomes

in Oenothera 188

108. Diploid and haploid chromosome groups of Drosera . . . 191

109. Chromosome groups of Crepis setosa and C. biennis and that

of hybrid 195

LIST OF ILLUSTRATIONS xv

FIG. PAGE

110. Chromosome groups of perennial and annual teosinte . . 196

111. Chromosome groups of two species of poppies and that of
hybrid 197

112. Male and female chromosome groups of Protenor . . . 200

113. Male and female chromosome groups of Lygaeus .... 200

114. Diagram of XX-XY type of sex determination .... 201

115. Sex-linked inheritance of white eyes of Drosophila . . . 202

116. Chromosome groups in man 203

117. Separation of X- and Y-chromosomes in spermatogenesis in
man 204

118. Diagram of WZ-ZZ type of sex-determination .... 205

119. Male and female chromosome groups in fowl 206

120. Sex-linked inheritance in poultry 207

121. Sex-linked inheritance in a moth, Abraxas 208

122. Chromosomes of egg of moth, Fumea 209

123. Sex-determination in dioecious plants 213

124. Female and male prothallia of liverwort, with respective
haploid chromosome groups 215

125. Method of combining male- and female-determining groups

of mosses 217

126. Separation of X-chromosome from autosomes in Ascaris . 219

127. Diagram of sex determination when the X-chromosomes are
united to autosomes 220

128. Diagram of paternal inheritance in a fish, Lebistes, with
crossing-over between X and Y 223

129. Diagram of inheritance of white versus red body-color of fish 224

130. Diagram of second generation from last 225

131. Diagram of inheritance of white and red body-color of fish
with crossing-over between X and Y 226

132. Diagram of theoretical crossing-over between autosomes to
which the X-chromosomes are attached 227

133. Two maturation divisions of bearberry aphid 229

134. Polar spindle of male-producing egg, and polar spindle of
female-producing egg of Phylloxera 230

135. Two maturation divisions of sperm-cells of Rhabditis . . 231

136. Two maturation divisions (polar body formation) of egg

of Rhabditis 232

137. Parthenogenetic female, male-egg-producing female, sexual
egg-producing female, and male of Brachionus .... 234

xvi LIST OF ILLUSTRATIONS

FIG. PAGE

138. Sex formulae of diploid, triploid, tetraploid types of Droso-
phila 241

139. Superfemale and supermale of Drosophila 242

140. Normal male and female and two mosaics of gypsy moth . 244

141. Union of chorions of two foetal calves, one of which becomes

a free-martin 247

142. Normal male, normal female, and parasitized male of Inar-
chus 251

143. Larva of Perla and ovotestis of same 256

144. Chromosome groups of spermatogonia and oogonia, and
diploid male egg of Perla 257

145. Chromosome groups of frog 261

146. Hermaphroditic condition in frog 264

147. Bidder's organs of toad 266

148. Transformation of Bidder's organs, after castration, into
ovaries 267

149. Female and male hemp plants 277

150. Four pure lines and general population of Princess bean . 284

151. Types of hooded rat 286

152. Normal and mutant types of abnormal abdomen of Droso-
phila 291

153. Distribution of pigment cells in different color types of
pupae of cabbage butterfly 293

154. Diagram of percentages of color types of pupae of cabbage
butterfly after exposure to different colored lights . . . 295

155. Guinea pigs with alcoholic ancestry 296

156. Young mice with hemorrhagic areas after exposure of mother

to radium 298

CHAPTER I

THE FUNDAMENTAL PRINCIPLES OF GENETICS

THE modern theory of heredity is derived from
numerical data obtained by crossing two individ-
uals that differ in one or more characters. The
theory is primarily concerned with the distribution of
units between successive generations of individuals. In
the same sense in which the chemist postulates invisible
atoms and the physicist electrons, the student of heredity
appeals to invisible elements called genes. The essential
point in this comparison is that both the chemist and the
student of heredity — the sreneticist — have reached their
conclusion from numerical and quantitative data. The
theories justify themselves in so far as they permit
numerical and quantitative prediction of a specific kind.
In this essential respect the theory of the gene differs
from earlier biological theories that have also postulated
invisible units to which were arbitrarily assigned any
desired properties. The theory of the gene reverses this
order and derives the properties of the genes, so far as it
assigns properties to them, from the numerical data
alone.

Mendel's Two Laws.

We owe to Greffor Mendel the discovery of two of the
fundamental laws of heredity on which the modern theory
of heredity is based. Later work, clone by others during
the present century, has carried us further in the same
direction and made possible the elaboration of the theory
on a much broader basis. Mendel's discovery may be
illustrated by a few familiar examples.

2 THE THEORY OF THE GENE

He crossed a tall variety of edible pea to a short vari-
ety. The offspring, or hybrids, Fx, were all tall (Fig. 1).
These were allowed to self-fertilize. Their offspring were

TALL

SHORT

F1 TALL (SHORT)

Ft Sgg.

3 TALL

SHORT

Pollen

TALL

TALL
TALL

SHORT
TALL

SHORT

TALL
SHORT

SHORT
SHORT

1 TALL

TALL (SHORT) 1 SHORT

Fig. 1.

Tall peas crossed to short peas giving in the first generation (Fa),
tall peas that are "hybrid," viz., tall (short). The recombination
of the gametes (eggs and pollen grains) are shown in the square.
Three tall to one short peas result in the next or second (F2)
generation.

tall and short in the ratio of three tails to one short. If
the tall variety contains in its germ-cells something that
makes the plants tall, and if the short variety carries
something in its germ-cells that makes the plants short,
the hybrid contains both ; and since the hybrid is tall it is

PRINCIPLES OF GENETICS 3

evident that when both are brought together the tall
dominates the short, or, conversely, short is recessive to
tall.

Mendel pointed out that the 3 to 1 ratio that appears
in the second generation can be explained by means
of a very simple hypothesis. If the element for tall
and the one for short (that are both present in the hy-
brid) separate in the hybrid when the eggs and pollen

E^s short sliort

fj Poll en

tall

short
tall

sliort
tall

short

short
short

Fig. 2.

A "back-cross" of Fj hybrid, tall (short) peas to the recessive

type (short), giving equal numbers of tall and short offspring.

grains come to maturity, half the eggs will contain the
tall and half the short element (Fig. 1). Similarly for the
pollen grains. Chance fertilization of any egg by any
pollen grain will give on the average three tails to one
short ; for, when tall meets tall a tall plant is produced ;
when tall meets sliort a tall plant results; when short
meets tall, a tall plant is produced ; and when short meets
short, a short plant arises.

Mendel put this hypothesis to a simple test. The hybrid
was back-crossed to the recessive type. If the germ-cells
of the hybrid are of two kinds, tall and short, there should

4 THE THEORY OF THE GENE

be two kinds of offspring, tall and short in equal numbers
(Fig. 2). The results confirm the expectation.

The same relation shown by the tall and the short peas
can be illustrated by the inheritance of eye color in man.
Blue eyes mated to blue, give only blues; brown eyes

brbl

_yv_

Germ cells of Y

Fi£ and 6 bf> bl

Fig. 3.
Inheritance of brown eyes (brbr) versus blue (blbl) eyes in man.

bred to brown give only brown, provided the browns
have had only a brown ancestry. If a blue mates with a
pure brown, the children are brown (Fig. 3). If two indi-
viduals that have arisen from such parentage marry,
their children will be brown- and blue-eyed in the ratio
of 3 to 1.

PRINCIPLES OF GENETICS 5

If a hybrid brown-eyed individual (Fx brown-blue)
marries a blue-eyed individual, half the children will have
brown, and half will have blue eyes (Fig. 4).

There are other crosses that give, perhaps, a more
striking illustration of Mendel's first law. For instance,
when a red and a white-flowered four-o'clock are crossed,
the hybrid has pink flowers (Fig. 5). If these pink-

Sper m

brown

blue
brown

blue

blue
blue

Fig. 4.

A "back-cross" of a brown-eyed, Flf individual, heterozygous for
blue eyes, to the recessive type, blue eyes, giving equal numbers
brown-eyed and blue-eyed offspring.

flowered hybrid plants self-fertilize, some of their off-
spring (F2) are red like one grandparent, some of them
pink like the hybrid, and others white like the other
grandparent, in the ratio of 1:2:1. Here one original
parental color is restored when red germ-cell meets red,
the other color is restored when white meets white, and
the hybrid combinations appear as often as red meets
white, or white meets red. All the colored flowered plants
in the second generation taken together are to the white-
flowered plants as 3 : 1.

THE THEORY OF THE GENE

In passing it is important to note two facts. The red
and the white F, individuals are expected to breed true,
because they contain the elements for red, or for white,

Fig. 5.
A cross between a red-flowered four-o'clock (Mirabilis Jalapa)
and a white-flowered four-o'clock, giving pink in F1; and one red,
two pink, one white in F2.

twice present (Fig. 6), but the pink F2 individuals should
not breed true, since they are like the first hybrid genera-
tion, and contain one red and one white element (Fig. 6).
All this turns out to be true when these plants are tested.

PRINCIPLES OF GENETICS 7

So far the results tell us no more than that something
derived from one parent separates, in the germ-cells of
the hybrid, from something brought in by the other par-

Eggs

Pollen #><6

F«

o
o

Fig. 6.
Diagram to illustrate the history of the germ-cells in the cross
between red and white four-o'clock (Fig. 5). The small black
circles stand for red-producing genes and the small white circles
for white-producing genes.

ent. The results might be interpreted, on this evidence
alone, to mean that red-flowered and white-flowered
plants behave as wholes or entities in inheritance.

Another experiment, however, throws further light on
this question. Mendel crossed peas whose seeds were
yellow and round with peas whose seeds were green and
wrinkled. Other crosses had shown that yellow and green

8 THE THEORY OF THE GENE

constitute a pair of contrasted characters giving a 3 to 1
ratio in the second generation, and that round and wrin-
kled constitute another pair.

Yellow Round

green wrinkled

Yellow (green) Round(wrinkled)

Round
wrinkled

wrinkled
Round

Fig. 7.

Diagram to illustrate the inheritance of two pairs of Mendelian
characters, viz., yellow-round and green-wrinkled peas. In the
lower part of the diagram the four classes of F2 peas are shown,
viz., the two original classes, yellow-round and green-wrinkled,
and the two recombination classes, yellow-wrinkled and green-
round.

The offspring were yellow and round (Fig. 7). When
selfed, they produced four kinds of individuals, yellow
round, yellow wrinkled, green round, and green wrinkled
in the ratio of 9 : 3 : 3 : 1.

Mendel pointed out that the numerical results found
here can be explained, if the separation of the elements
for yellow and for green is independent of that for round
and wrinkled. This would give four kinds of germ-cells

PRINCIPLES OF GENETICS 9

in the hybrid, yellow round, yellow wrinkled, green round,
and green wrinkled (Fig. 8).

If the fertilization of the four kinds of ovules by the
four kinds of pollen grains is at random, there will be

_s^-

gW gW1

\jW

I GW J

V GW J

Fig. 8.
Diagram illustrating the sixteen F2 recombinations (from yellow-
round and green-wrinkled peas) that result when the four kinds of
eggs and the four kinds of pollen grains of the T?i hybrid come
together.

sixteen combinations possible. Eemembering that yellow
dominates green, and that round dominates wrinkled,
these sixteen combinations will fall into four classes, that
are in the ratios of 9 : 3 : 3 : 1.

The results of this experiment show that it can no
longer be assumed that the whole parental germ-materials

10 THE THEORY OF THE GENE

are separated in the hybrid; for yellow and round that
went in together have, in some cases, come out separated.
Similarly for green and wrinkled.

Mendel also showed that when three, and even four,
pairs of characters enter a cross their elements are inde-
pendently assorted in the germ-cells of the hybrid.

It might, then, have seemed justifiable to extend this
conclusion to as many pairs of characters as enter any
particular cross. This would mean that there are as many
independent pairs of elements in the germinal material
as there are possible characters. Subsequent work has
shown, however, that Mendel 's second law of independent
assortment has a more restricted application, since many
pairs of elements do not assort freely, but certain ele-
ments that enter together show a tendency to remain
together in succeeding generations. This is called linkage.

Linkage.

Mendel's paper was recovered in 1900. Four years
later Bateson and Punnett reported observations that
did not give the numerical results expected for two inde-
pendent pairs of characters. For instance, when a sweet
pea having purple flower-color and long pollen grains is
crossed to one with red flowers and round pollen grains,
the two types that go in together come out together more
frequently than expected for independent assortment of
purple-red and round -long (Fig. 9). They spoke of these
results as due to repulsion between the combinations
purple and long and red and round, that went^f rom oppo-
site parents. Today these relations are called linkage. By
linkage we mean that when certain characters enter a
cross together, they tend to remain together in later
generations, or, stated in a negative way, certain pairs
of characters do not assort at random.

It would seem, then, so far as linkage holds, that there

PRINCIPLES OF GENETICS

are limits to the subdivision of the germinal material.
For example in the vinegar fly, Drosophila melanogaster,
there are known about 400 new mutant types that fall
into only four linkage groups.

583

Fig. 9.

Cross between a sweet pea with purple flowers and long pollen
grains and one with white flowers and round pollen grains. In the
lower line the four classes of F2 individuals appeared in the pro-
portions given.

One of these groups of characters of Drosophila is said
to be sex-linked, because in inheritance the characters
show certain relations to sex. There are about 150 of these
sex-linked mutant characters. Several of them are modifi-
cations of the color of the eye, others relate to its shape

12 THE THEORY OF THE GENE

or its size, or to the regularity of the distribution of its
facets. Other characters involve the body color; others
the shape of the wings, or the distribution of its veins ;
others the spines and hairs that cover the body.

A second group of about 120 linked characters includes
changes in all parts of the body. None of the effects are
identical with those of the first group.

A third group of about 130 characters also involves all
parts of the body. None of these characters are the same
as those of the other two groups.

There is a small fourth group of only three characters :
one involves the size of the eyes, leading in extreme cases
to their total absence ; one involves the mode of carriage
of the wings; and the third relates to the reduction in
size of the hairs.

The method of inheritance of linked characters is given
in the following example. A male Drosophila with four
linked characters (belonging to the second group), black
body color, purple eyes, vestigial wings, and a speck at
the base of the wings (Fig. 10), is crossed to a wild type
female with the corresponding normal characters, that
may be called gray body color, red eyes, long wings, and
absence of speck. The offspring are wild type. If one of
the sons1 is now crossed to a stock female having the four
recessive characters (black, purple, vestigial, speck), the
offspring are of two kinds only, half are like one grand-
parent with the four recessive characters, and the other
half are wild type like the other grandparent.

Two sets of contrasted (or allelomorphic) linked
genes went into this cross. When the germ-cells in the
male hybrid matured, one of these sets of linked genes
went into half of the sperm-cells and the corresponding
allelomorphic set into the wild type half of the sperm-

i It is necessary to make this reservation as to the male Drosophila, be-
cause in the female these same characters are not completely linked.

Fig. 10.

The inheritance of four linked, recessive characters, vis., black
body color, purple eyes, vestigial wings, and speck, versus their
normal allelomorphs of the wild type fly. The Fx male is "back-
crossed" to a female of the multiple recessive stock, giving in
the second generation (shown below) only the two grand parental
combinations.

14 THE THEORY OF THE GENE

cells. This was revealed, as described above, by crossing
the hybrid (FJ male to a female pure for the four reces-
sive genes. All of her mature eggs contain one set of four
recessive genes. Anv egg fertilized by a sperm with one
set of the dominant wild type genes should give a wild
tvpe flv. Anv egg fertilized by a sperm with the four re-
cessive genes (which are the same as those in the female
here used) should give a black, purple, vestigial, speck
flv. These are two kinds of individuals obtained.

Crossing-Over.

The members of a linked group may not always be com-
pletely linked as in the case just given. In fact, in the Fx
female from the same cross, some of the recessive charac-
ters of one series may be interchanged for wild type
characters from the other series, but even then, since they
remain united more often than thev interchange, thev are
still said to be linked together. This interchange is called
crossing-over, which means that, between two corre-
sponding linked series, there may take place an orderly
interchange involving great numbers of genes. Since an
understanding of this process is essential to what fol-
lows, a few examples of crossing-over may be given.

When a male Drosophila with the two recessive mutant
characters, yellow wings and white eyes, is mated to a
female with the wild type characters, gray wings and red
eyes, the daughters and sons have gray wings and red
eyes (Fig. 11 ). If one of the daughters is mated to a male
with the two recessive characters, yellow wings and white
eyes, there are four kinds of offspring. Two kinds are
like the grandparents, that is, they have yellow wings and
white eyes, or gray wings and red eyes. Together they
constitute 90 per cent of the offspring. The characters
that went in together have come out together in a much
higher perc<-nta^e than expected from Mendel's second

PRINCIPLES OF GENETICS

law, vis., the law of free assortment. In addition to the
two classes, there are two other kinds of flies in the second
generation (Fig. 11). one with yellow wings and red
eves, and the other with gray wings and white eyes.
Together thev constitute 1 per cent of this generation.

ftfe&

99<*/°

1*1

Fig. 11.
The inheritance of two recessive sex-linked characters, vie., white
eyes and yellow wings and their ' ' normal ' ' allelomorphs, via.,

red eyes and gray wings.

THE THEORY OF THE GENE

They are the crossovers, and represent interchanges be-
tween the two linkage groups.

A similar experiment can be made in which the same
characters as before are differently combined. If a male
Drosophila with yellow wings and red eyes is mated to a

Fig. 12.

The inheritance of the same two sex-linked characters of Fig. 11,
but in reciprocal combinations, vis., red eyes and yellow wings, and
white eyes and gray wings.

PRINCIPLES OF GENETICS 17

female with gray wings and white eyes the daughters
have gray wings and red eyes (Fig. 12). If one of the
daughters is mated to a male with the two recessive
mutant characters, yellow wings and white eyes, there
are four kinds of flies produced. Two of these are like the
two grandparents, and constitute 99 per cent of the out-
put. Two are new combinations, or crossovers, one with
yellow wings and white eyes and the other with gray
wings and red eyes. Together they make up 1 per cent of
the second generation.

These results show that the same amount of crossing-
over takes place irrespective of the way in which the
combinations of the same characters enter the cross. If
the two recessives enter together, they tend to hold to-
gether. This relation was called coupling by Bateson and
Punnett. If one of the recessives enters from one parent
and the other recessive from the other parent, they tend
to come out separately (each in combination with the
dominant that went in with it). This relation was called
repulsion. It is clear, however, from the two crosses that
have just been given, that these relations are not two
phenomena, but expressions of the same one, namely, that
the two linked characters that enter a cross, quite irre-
spective of their dominance or recessiveness, tend to hold
together.

Other characters give different percentages of cross-
ing-over. For example, when a male Drosophila with the
two mutant characters, white eyes and miniature wings
(Fig. 13), is mated to a wild type fly with red eyes and
long wings the offspring have long wings and red eyes.
If one of the daughters is mated to a male with white
eyes and miniature wings the offspring are of four kinds.
The two grandparental types constitute 67 per cent and
the two cross-over types 33 per cent of this generation.

A still higher percentage of crossing-over is given in

THE THEORY OF THE GENE

the following experiment. A male with white eyes and
forked bristles is mated to a wild type female (Fig. 14).
The offspring have red eyes and straight bristles. If one
of the daughters is mated to a male with white eyes and
forked bristles, there are four kinds of individuals pro-

67 o/o 33 <jo

Fig. 13.
The inheritance of two sex-linked characters, white eyes and
miniature wings, and red eyes and long wings.

PRINCIPLES OF GENETICS

duced. The grandparental types constitute 60 per cent
and the crossovers 40 per cent of this second generation.
A study of crossing-over has shown that all possible
percentages of crossing-over occur, up to nearly 50 per
cent. If exactly 50 per cent of crossing-over took place,

^\_

-yr

60*jo 40«jo

Fig. 14.

The inheritance of two sex-linked characters, white eyes and forked
bristles, and red eyes and normal bristles.

20 THE THEORY OF THE GENE

the numerical result would be the same as when free
assortment occurs. That is, no linkage would be observed
even though the characters involved are in the same link-
age group. Their relation as members of the same group
could, nevertheless, be shown by their common linkage
to some third member of the series. If more than 50 per
cent crossing-over should be found, a sort of inverted
linkage would appear, since the cross-over combinations
would then be more frequent than the grandparental
types.

The fact that crossing-over in the female of Drosophila
is always less than 50 per cent, is due to another corre-
lated phenomenon called double crossing-over. By double
crossing-over is meant that interchange takes place twice
between two pairs of genes involved in the cross. The
result is to lower the observed cases of crossing-over,
since a second crossing-over undoes the effect of a single
crossing-over. This will be explained later.

The Simultaneous Interchange of Many Genes
in Crossing-Over.

In the examples of crossing-over just given, two pairs
of characters were studied. The evidence involved only
those cases of crossing-over that took place once between
the two pairs of genes involved in the cross. In order to
obtain information as to how frequently crossing-over
takes place elsewhere, i.e., in the rest of the linkage
group, it is necessary to include pairs of characters that
cover the entire group. For example, if a female with the
following nine characters of Group I, scute, echinus,
cross-veinless, cut, tan, vermilion, garnet, forked and
bobbed, is crossed to a wild type male, and if the Fx
female (Fig. 15) is back-crossed to the same multiple re-
cessive type, the offspring produced will give a record of
every crossing-over. If crossing-over had taken place

I 1 1 1 1 1 1— 1

I— I 1 1 I 1 I 1

Fig. 15.
Diagram of two allelomorphic series of linked genes. In the upper
line the approximate location of nine, sex-linked recessive genes
is indicated. In the lower line the normal allelomorphic genes are
indicated.

Fig. 16.

Diagram to show crossing-over between garnet and vermilion, i.e.,

near the middle of the series shown in Fig. 15.

Fig. 17.

Diagram to show crossing-over between echinus and cross -veinless

near the left end of the series. See Fig. 15.

Fig. 18.
Double crossing-over between the two series of genes indicated in
Fig. 15. One crossing-over is between cut and tan and the other
between garnet and forked.

22 THE THEORY OF THE GENE

near the middle of the series (between vermilion and
garnet), the result would be that shown in Fig. 16. Two
complete halves have interchanged.

In other cases, crossing-over may take place near one
end (for example, between echinus and cross-veinless).
The result is like that shown in Fig. 17. Only the short
ends of the two series have interchanged. The same
kind of process occurs whenever an interchange takes
place. Whole series of genes are interchanged, although
as a rule the interchange is noticed only between the
genes on each side of the crossing-over.

When simultaneous crossing-over occurs at two levels
at the same time (Fig. 18) very many genes are also in-
volved. For example, in the series just given one cross-
ing-over is supposed to take place between cut and tan,
and another crossing-over between garnet and forked.
All the genes in the middle of the two series have been
interchanged. This would pass unobserved were there no
mutant genes in the region to indicate the fact that two
crossings-over had taken place, since the two ends of both
series remain the same as before.

The Linear Order of the Genes.

It is self-evident that if two pairs of genes should be
near together, the chance that crossing-over occurs be-
tween them is smaller than if they are further apart. If
two other genes are still further apart the chance of cross-
ing-over is correspondingly increased. We may utilize
these relations to obtain information as to the ' ' distance ' '
at which any two pairs of elements lie with respect to
each other. With this information we can construct charts
of the series of elements in each of the linkage groups.
This has been done for all the linkage groups of Droso-
phila. Such a chart (Fig. 19) gives the result as far as
carried out.

I p

- -004.S

~~13.7 crosay'less!

it — 16.± club
^-17.+ deltex

0.3
0.6
1.0
1.5
3.0

5
5.5
6.9
7.5

yellow !

Hairy-wing ♦
scuta !
lethal -7
broad +
prune +
white J
facet
Notch
Abnormal
echinus !
bifid !
ruby !

■20.0
■21.0

cut !

singed +

v^- 27.5 tan +

^27.7 lozenge ♦

^33.0 Terrailion !

\~~36.1 miniature ♦

0"36.2 dusky +

^38.± furrowed

.^"-43.0 sable ♦

~^44.4 garnet J

^, 54.2 small-wing

54,5 rudimentary

:— 56.5 forked [

■^ 57.0 Bar [

:S>58.5 small-eye

.\59.0 fused t-

\ 59.6 Beadex ♦

\\62.± Uinute-n ♦

^65.0 cleft

^70.0 bobbed I

0.0

2.0 Star •

3.± aristaless

6.± expanded

12.1 Gull

13.0 Truncate !

14. ± dacha ous +

16.0 Streak +

-I 0.0 roughoidl

31.0 dacha +

35.0 Ski-II

41.0 Jammed !

46.± Uinute-e

-48.5 black i

-48.7 jaunty

0.0 bent
0.5 ± shaven
0.9 eyeless

20.0 divergent ♦

26.0 sepia !
26.5 hairy !

^/4

- - 54.5 purple \ 54,6 Hairy-wing sup. •

57.5 cinnabar +
•60.+ safranln

-J- 64. i pink-wing ♦

67.0 vestigial ■

68. t telescope

72.0 Lobe \

74. ± gap

75.5 curved '.

83.5 fringed

90.0 humpy

J 99.5 are ♦

L00. 5 plexus !

L02.+ lethal-IIa

105.0 brown [

.'l\105.± blistered

/,106.i purploid

./l07.± morula +

Ml07.0 speck S

M07.5 balloon

.35.0
/,36.5

/,40.1
40.2
40.4
/42.2
-V44.0
/,46.±
^46.5
I>47.5

: 48.0

^-49.7

\|50.t

150.0

rose +
cream-III +
Uinute-h
tilt

Dichaete !
thread !
scarlet !
warped
ski-III
Deformed
pink !
maroon ♦
dwarf
curled |

58.2
58.5
58.7
59.5
62.0
63.1
66.2

Stubble 1
spineless \
bi thorax +
bithorax-b
stripe •
glass ♦
Delta '.

69.5 Hairless •

70.7 ebony !

72.0 band

75.7 cardinal ♦

76.2 white-ocelli ♦

91.1 rough !

93.0 crumpled
93.8 Beaded

94.1 Pointed +

100.7 claret I
101.0 Uinute

-1- 106.2 Uinute-g !

Fig. 19.
Map or chart of the four series, I, II, III, IV, of linked genes of
Drosophila melanogaster. The "map distance" is given in the
numerals to the left of each character.

24- THE THEORY OF THE GENE

In the preceding illustrations of linkage and crossing-
over, that have been given, the genes are represented as
lying in a line — like beads on a string. The numerical data
from crossing-over show, in fact, that this arrangement is
the only one that is consistent with the results obtained,
as the following example will serve to illustrate.

Fig. 20.

Diagram illustrating the linear order of three sex-linked genes,

viz., yellow wings, white eyes, bifid wings.

Suppose that crossing-over between yellow wings and
white eyes occurs in 1.2 per cent of cases. If we then test
white with a third member of the same series, such as
bifid wings, we find 3.5 per cent of crossing-over (Fig.
20). If bifid is in line and on one side of white it is ex-
pected to give with yellow 4.7 per cent crossing-over, if on
the other side of white it is expected to give 2.3 per cent
of crossing-over with yellow. In fact, it gives one of these
values, namely, 4.7. We place it, therefore, below white in
the diagram. This sort of result is obtained whenever a
new character is compared with two other members of
the same linkage group. The crossing-over of a new
character is found to give, in relation to two other known
factors, either the sum or the difference of their respec-
tive cross-over values. This is the known relation of
points on a line, and is the proof of the linear order of the
genes ; for no other spatial relation has yet been found
that fulfills these conditions.

PRINCIPLES OF GENETICS 25

The Theory of the Gene.

We are now in a position to formulate the theory of the
gene. The theory states that the characters of the indi-
vidual are referable to paired elements {genes) in the
germinal material that are held together in a definite
number of linkage groups; it states that the members of
each pair of genes separate when the germ-cells mature
in accordance ivith Mendel's first laiv, and in consequence
each germ-cell comes to contain one set only; it states
that the members belonging to different linkage groups
assort independently in accordance with Mendel's second
law; it states that an orderly interchange — crossing-over
— also takes place, at times, between the elements in
corresponding linkage groups; and it states that the fre-
quency of crossing-over furnishes evidence of the linear
order of the elements in each linkage group and of the
relative position of the elements with respect to each
other.

These principles, which, taken together, I have ventured
to call the theory of the gene, enable us to handle prob-
lems of genetics on a strictly numerical basis, and allow
us to predict, with a great deal of precision, what will
occur in any given situation. In these respects the theory
fulfills the requirements of a scientific theory in the full-
est sense.

CHAPTER II
PARTICULATE THEORIES OF HEREDITY

THE evidence given in the last chapter led to the
conclusion that there are hereditary units in the
germinal material that are, to a greater or less
extent, independently sorted out between successive gen-
erations of individuals. Stated more accurately, the inde-
pendent reappearance in later generations of the charac-
ters of two individuals combined in a cross can be
explained by the theory of independent units in the
germinal material.

Between the characters, that furnish the data for the
theory, and the postulated genes, to which the characters
are referred, lies the whole field of embryonic develop-
ment. The theory of the gene, as here formulated, states
nothing with respect to the way in which the genes are
connected with the end-product or character. The absence
of information relating to this interval does not mean
that the process of embryonic development is not of in-
terest for genetics. A knowledge of the way in which the
genes produce their effects on the developing individual
would, no doubt, greatly broaden our ideas relating to
heredity, and probably make clearer many phenomena
that are obscure at present, but the fact remains that the
sorting out of the characters in successive generations
can be explained at present without reference to the way
in which the gene affects the developmental process.

There is, nevertheless, a fundamental assumption im-
plied in the statement just made, viz., that the develop-
mental process follows strictly causal laws. A change in a

PARTICULATE THEORIES OF HEREDITY 27

gene produces definite effects on the developmental proc-
esses. It affects one or more of the characters that ap-
pear at some later stage in the individual. In this sense,
the theory of the gene is justified without attempting to
explain the nature of the causal processes that connect
the gene and the characters. Some needless criticism of
the theory has arisen from failure to clearly understand
this relation.

It has been said, for example, that the assumption of
invisible units in the germ-materials really explains noth-
ing, since to these are ascribed the very properties that
the theory sets out to explain. In fact, however, the only
properties ascribed to the gene are those given in the
numerical data supplied by the individuals. This criti-
cism, like others of its kind, arises from confusing the
problems of genetics with those of development.

Again, the theory has been unfairly criticised on the
grounds that the organism is a physico-chemical mechan-
ism, while the genetic theory fails to account for the
mechanism that is involved. But the only assumptions
made by the theory, the relative constancy of the gene,
its property of multiplying itself, the union of the genes
and their separation when the germ-cells mature, involve
no assumptions inconsistent with physical principles, and
while it is true the physical and chemical processes in-
volved in these events cannot be explicitly stated, they
relate at least to phenomena that we are familiar with in
living things.

A part of the criticism of Mendel's theory arises from
a failure to appreciate the evidence on which the theory
rests, and also from a failure to realize that its proce-
dure is different from that which, in the past, has led to
the formulation of other particulate theories of heredity
and of development. There have been a good many of
these theories, and biologists have become, through ex-

28 THE THEORY OF THE GENE

perience, somewhat incredulous in respect to any and all
theories that postulate invisible units. A brief examina-
tion of a few of the earlier speculations may serve to
make the difference between the old and the new proce-
dure more apparent.1

Herbert Spencer's theory of physiological units, pro-
posed in 1863, assumes that each species of animal or
plant is composed of fundamental units that are all alike
for each species. The elements concerned are supposed
to be larger than protein molecules and more complex in
structure. One of the reasons that led Spencer to this
view is that any part of the organism may in certain
cases reproduce the whole again. The egg and the sperm
are such fragments of the whole. The diversity of struc-
ture in each individual is vaguely ascribed to a "polar-
ity" or some sort of crystal-like arrangement of the ele-
ments in different regions of the body.

Spencer's theory is purely speculative. It rests on the
evidence that a part may produce a new whole like itself,
and infers from this that all parts of the organism con-
tain material out of which a new whole may develop, but,
while this is, in part, true, it does not follow that the
whole must be made up of a single kind of unit. Our
modern interpretation of the ability of a part to develop
into a new whole must also assume that each such part
contains the elements for the construction of a new whole,
but these elements may be different from each other, and
to this difference the differentiation of the body is re-
ferred. So long as a complete set of units is present, the
power to produce a new whole is potentially given.

Darwin's theory of pangenesis, proposed in 1868, ap-
pealed to a host of different invisible particles. The
theory states that minute representative elements, called

i A full discussion of earlier theories is given by Delage in Eeredite and
by Weismann in the Germ-Plasm.

PARTICULATE THEORIES OF HEREDITY 29

genimules, are being continually thrown off from every
part of the body. Those that reach the germ-cells become
incorporated there with the hereditary units of the same
general kind already present.

The theory was proposed primarily to explain how
acquired characters are transmitted. If specific changes
in the body of the parent are transmitted to the offspring,
some such theory is required. If the changes in the body
are not transmitted, there is no need of such a theory.

Weismann in 1883 challenged the entire transport
theory, and convinced many, but not all, biologists that
the evidence for the transmission of acquired characters
was inadequate. This led him to develop his theory of
the isolation of the germ-plasm. The egg produces not
onlv a new individual, but other eggs like itself, carried
by the new individual. The egg produces the individual,
but the individual has no subsequent influence on the
germ-plasm of the eggs contained in it, except to protect
and to nourish them.

From this beginning Weismann developed a theory of
particulate inheritance of representative elements. He
appealed to evidence derived from variation, and he ex-
tended his theory to include a purely formal explanation
of embryonic development.

We are concerned, in the first place, with Weismann 's
views as to the nature of the hereditary elements or ids
as he calls them. The ids he identified in his later writings
as small chromosomes when many small chromosomes
are present, but when only a few chromosomes are pres-
ent he supposed that each is made up of several or many
ids. Each id contains all the elements that are essential
to the development of a single individual. Each is a mi-
crocosm. The ids differ from each other in that they are
the representatives of ancestral individuals or germ-

30 THE THEORY OF THE GENE

plasms, each different from the others in one or another
way.

The individual variations shown by animals are due
to the different recombinations of ids. This is brought
about by the union of eggs and sperms. The number of
ids would become indefinitely large were it not that, at
the ripening of the germ-cells, the number of ids is re-
duced to half.

Weismann also formulated an elaborate theory of
embryonic development based on the idea that the ids are
separated into their smaller elements as the egg divides,
until, finally, each kind of cell in the body comes to con-
tain one of the ultimate components of the ids, i.e., deter-
minants. In the cells destined to become germ-cells the
disintegration of the ids does not take place. Hence the
continuity of the germ-plasm, or of the colony of ids. The
application of his theory to embryonic development lies
outside the modern theory of heredity that either ignores
the developmental process, or else postulates a view
exactly the opposite of that of Weismann, namely, that
in every cell of the body the entire heredity complex is
present.

It will be seen without further elaboration that Weis-
mann 's ingenious speculation invokes, in order to explain
variation, processes that are akin to those we adopt
today. Variation, he believed, is due to the recombination
of units from the parents. These are reduced to half in
the process of maturation of the egg and sperm. The
units are wholes and each represents an ancestral stage.

W^e owe to Weismann largely the idea of the isolation
and continuity of the germinal material. His challenge of
the Lamarckian theory was of immense service to clear
thinking. The theory of the inheritance of acquired
characters had obscured for a long time all problems
dealing with heredity. Weismann 's writings were also

PARTICULATE THEORIES OF HEREDITY 31

unquestionably important in keeping in the foreground
the intimate relation between heredity and cytology. It is
difficult for us to estimate to what extent his fascinating
speculations have influenced our later attempts to inter-
pret heredity in terms of chromosome constitution and
behavior.

These and other earlier speculations have today mainly
an historical interest. They do not represent the main
path along which the modern theory of the gene has de-
veloped, which rests its claims to recognition on the
method by which it is derived and on its ability to predict
exact numerical results of a specific kind.

I venture to think that, however similar to the older
theories the modern theory may appear, it stands apart
from them, in that it has arisen step by step from experi-
mentally determined genetic evidence that has been care-
fully controlled at every point. The theory need not and
does not, of course, pretend to be final. It will, no doubt,
undergo many changes and improvements in new direc-
tions, but most of the facts concerning heredity, known
to us at present, can be accounted for by the theory as
it stands.

CHAPTER III

THE MECHANISM OF HEREDITY

THE statement of the theory of the gene at the end
of the first chapter is derived from purely numeri-
cal data without respect to any known or assumed
changes in the animal or plant that bring about, in the
way postulated, the distribution of the genes. However
satisfactory the theory may be in this respect, biologists
will seek to discover in the organism how the orderly
redistribution of the genes takes place.

During the last quarter of the last century, and con-
tinuously through the first quarter of the present century,
the study of the changes that take place during the final
stages in the maturation of the egg and sperm-cell have
revealed a remarkable series of events that go far toward
furnishing a mechanism of heredity.

It was discovered that there is a double set of chromo-
somes in each cell of the body and in the early stages of
the germ-cells. The evidence of this duality came from
observations on differences in the sizes of the chromo-
somes. Whenever recognizable differences exist there are
two chromosomes of each kind in the somatic cells and
one of each in the germ cells after maturation. One mem-
ber of each kind has been shown to come from the father
and the other from the mother. At the present time the
duality of the chromosome complex is one of the best
established facts of cytology. The only striking exception
to the rule is sometimes found in the sex-chromosomes,
but even here the duality holds for one sex, and often for
both.

THE MECHANISM OF HEREDITY 33

The Mechanism of Mendel's Two Laws.

Toward the end of the ripening period of the germ-
cells, chromosomes of the same size come together in
pairs. This is followed by a division of the cell, when the
members of each pair go into opposite cells. Each mature
germ-cell comes to contain only one set of chromosomes,
(Figs. 21 and 22).

Fig. 21.
Diagram of the two maturation divisions of sperm-cells. Three
pairs of chromosomes are represented; those from the father in
black, those from the mother in white (except in a, b, c). The
first maturation division, here the reduction division, is shown in
d, e, f. The second, or equational division, in which each chromo-
some splits lengthwise into two daughter chromosomes, is shown
in g, h.

34 THE THEORY OF THE GEXE

This behavior of the chromosomes in the maturation
stages parallels Mendel's first law. A chromosome de-
rived from the father separates from a chromosome de-
rived from the mother for each pair of chromosomes. The

■ -

■ -.- b

«.

i)(2l_JP)

• ' :•

e ■ • f

Fig. 22.
Diagram of two maturation divisions of the egg. The first polar
spindle is shown in a. The separation of the paternal and maternal
chromosomes (reduction) is shown in b. The first polar body has
been given off in c. The second polar spindle is formed in d; each
chromosome has split lengthwise into daughter halves (equational
division). The second polar body is being given off in e. The egg-
nucleus is left in f with the half (haploid) number of chromo-
somes.

orerm-cells that result contain one chromosome of each
kind. Taking the chromosomes in pairs we may say, half
of the germ-cells, when mature, contain one member of
each pair, the other half the mates of those chromosomes,
pair for pair. If one substitutes Mendelian units for chro-
mosomes, the statement is the same.

THE MECHANISM OF HEREDITY 35

One member of each pair of chromosomes comes from
the father, its mate from the mother. If, when the con-
jugants come to lie on the spindle, all the paternally de-
rived chromosomes were to go to one pole, and all the
maternally derived to the other pole, the two resulting

\/ \/

f *l

■ op i

I I

/ \ ' / \

Fig. 23.
Diagram to illustrate the random assortment of a pair of chromo-
somes with respect to the X-chromosome. (After Carothers.)

germ-cells would be like those of the father and of the
mother. There is no a priori reason for supposing that
the conjugants would behave in this way, but it has been
extremely difficult to prove that they do not do so, because
from the very nature of the case, the conjugating chro-
mosomes being alike in shape and size, it is not as a rule
possible to tell from observation which member is pater-
nal, which maternal.

In recent years, however, a few cases have been found
in grasshoppers where slight differences are sometimes

36 THE THEORY OF THE GENE

present between the members of certain pairs — differ-
ences in shape, or in the attachment to the spindle fibers
(Fig. 23). When the germ-cells mature these chromo-
somes conjugate and then separate. Since they retain
their individual differences, they can be traced to the
poles.

Now in these grasshoppers there is, in the male, an un-
paired chromosome that is connected with sex determina-
tion (Fig. 23). It passes at the maturation division to one
pole or to the other. It serves as a land-mark for the
other pairs of chromosomes. Miss Carothers, who first
made these observations, found that a marked pair (one
bent, one straight) separates at random with respect to
the sex chromosome.

Carrying the matter further, other chromosome pairs
were found to show at times constant differences in some
individuals. A study of these chromosome pairs at reduc-
tion has shown, again, a random distribution of the mem-
bers of the pairs with respect to one another. Here then
we have objective evidence of the independent assort-
ment of the pairs of chromosomes. This evidence paral-
lels Mendel 's second law, which calls for independent dis-
tribution of the members of different linkage groups.

The Number of the Linkage Groups and the
Number of the Chromosome Pairs.

Genetics has shown that the hereditary elements are
linked in groups, and in one case with certainty, and in
several other cases with some probability, there is a defi-
nite and fixed number of these linkage groups. In Droso-
phila there are only four such groups, and there are four
pairs of chromosomes. In the sweet pea there are seven
chromosome pairs (Fig. 24), and probably seven inde-
pendent pairs of Mendelian characters have been found
by Punnett. In the edible pea there are also seven pairs

THE MECHANISM OF HEREDITY 37

of chromosomes (Fig. 24) and seven independent pairs
of Mendelian characters, according* to White. In Indian
corn there are ten to twelve (?) pairs of chromosomes,
and several groups of linked genes have been detected. In
the snapdragon, with sixteen pairs of chromosomes, the
number of independent genes approaches the number of
the chromosomes. In other animals and plants, also,
linked genes have been reported, but as yet this number
is small in comparison with the chromosome numbers.

Edible Pea Sweet Pea

Indian Corn

Fig. 24.

The reduced number of chromosomes in the edible pea (n— 7),
sweet pea (n— 7), and Indian corn (n— 10 or 12?).

The further fact that, to date, no case is known where
there are more independently assorting pairs than there
are pairs of chromosomes is further evidence, as far as it
goes, in favor of the view that the linkage groups and
the chromosomes correspond in number.

The Integrity and Continuity of the Chromosomes.

The integrity of the chromosomes, or their continuity
from one cell generation to the next, is also essential for

38 THE THEORY OF THE GENE

the chromosome theory. There is general agreement
amongst cytologists that when the chromosomes are set
free in the protoplasm they remain intact through the
entire period of cell division, but when they take up fluid
and combine to form the resting nucleus, it is no longer
possible to trace their history. By indirect means, how-
ever, it has been possible to get some evidence as to the
conditions of the chromosomes in the resting stages.

After each division the individual chromosomes be-
come vacuolated as they come together to form a new
resting nucleus. They can be followed for some time,
forming separate compartments of the single nucleus that
re-forms. They then lose their staining quality and can
no longer be identified. "When the chromosomes are again
about to appear, sac-like bodies are seen. This suggests,
if it does not prove, that the chromosomes have remained
in place during the resting stage.

Boveri showed that when egg-cells of Ascaris divide,
the daughter chromosomes of each pair are pulled apart
in the same way, and often show characteristic shapes
(Fig. 25). At the next division of such cells, when the
chromosomes of daughter cells are about to reappear,
they show similar arrangements of their threads. The
inference is clear. The threads retain in the resting
nucleus the shapes that they had when they entered the
nucleus. This evidence is favorable to the view that the
chromosomes have not passed into solution, and later
reformed, but have retained their integrity.

Finally, there are cases where the chromosome num-
bers have been increased, either by becoming doubled, or
by crossing species with different numbers of chromo-
somes. There may be, then, three or four chromosomes of
each kind. The same number is retained as a rule through
all successive divisions.

On the whole, then, while the cytological evidence does

THE MECHANISM OF HEREDITY

not demonstrate completely that the chromosomes remain
intact throughout their history, the evidence, as far as it
goes, is favorable to this view.

There is, however, a very important limitation that
must be placed on this statement. The genetic evidence
clearly proves that between the members of the same pair
of chromosomes there is at times an orderly interchange
of parts. Does the cytological evidence show any indica-
tion of sucli an interchange 1 Here we enter on more ques-
tionable ground.

a b c d

Fro. 25.
The nuclei of four pairs of sister cells (above and below) show-
ing the position of the daughter chromosomes as they come out of
the resting nuclei. (After Boveri.)

Mechanism of Crossing-Over.

If, as other evidence clearly shows, the chromosomes
are the bearers of genes, and if the genes may inter-
change between members of the same pair, it follows that
sooner or later we may expect to find some kind of mecha-
nism by which such interchange takes place.

Several years before the genetic discovery of crossing-
over, the process of conjugation of the chromosomes, and
their reduction in number in the mature germ cells had
been fully established. It was demonstrated that at the

40 THE THEORY OF THE GENE

time of conjugation the members of the same pair of
chromosomes are those that combine. In other words,
conjugation is not at random, as one might possibly have
inferred from the earlier accounts of the process, but
conjugation is always between a paternally derived and
a maternally derived specific chromosome.

We may now add to this information the following
fact, namely, that conjugation takes place because the
members of a pair are alike, not because they have come
from a male and a female respectively. This has been
shown in two ways. In hermaphroditic types the same
union occurs, although, after self-fertilization, both mem-
bers of each pair have come from the same individual.
Secondly, in exceptional cases, the two members of a
pair have come from the same egg, yet presumably they
conjugate since crossing-over takes place.

The cytological evidence of the conjugation of like
chromosomes supplies the first steps for a mechanical
explanation as to how an interchange might take place,
for, obviously, if the two members of each pair come to
lie side by side throughout their length, gene to gene as
it were, the chromosomes are brought into a position
where equivalent blocks might be interchanged in an
orderly way. Of course, it does not follow that in con-
sequence of their side to side apposition an inter-
change would necessarily follow; in fact, a study of the
crossing-over in a linkage group, such as the sex-linked
group of genes of Drosophila (where a sufficient number
of genes is present to furnish complete evidence of what
takes place in the linkage series), shows that there is no
interchange at all in about 43.5 per cent of the eggs for
that pair of chromosomes. The same evidence shows that
one interchange takes place in about 43 per cent of the
eggs; that two interchanges take place in about 13 per
cent (double crossing-over) and three interchanges in 0.5

THE MECHANISM OF HEREDITY

per cent. In the Drosophila male, no interchange at all
takes place.

In 1909 Janssens published a detailed account of a
process that he called Chiasmatypie. Without entering
here into the details of Janssens' work it may be stated
that he brought forward evidence which he believed to
show that there is an interchange of blocks or segments
between the members of the conjugating pairs of chromo-
somes which is traceable to an earlier twisting of the
two conjugating chromosomes around each other (Fig.
26).

Fig. 26.
The conjugation of the chromosomes in Batrachoseps. The twisting
of the two thin threads around each other is suggested in one of
the two chromosomes in the middle figure. (After Janssens.)

Unfortunately there is scarcely any stage in the matu-
ration divisions that is as much in dispute as this one
involving the twisting of the chromosomes. From the
nature of the case it is practically impossible to demon-
strate, even when twisting of the chromosomes is ad-
mitted, that it actually leads to an interchange of the
kind demanded by the genetic evidence.

42 THE THEORY OF THE GENE

There are many published figures of the chromosomes
twisted about each other. But in some respects this evi-
dence proves too much. For instance, the most familiar
and best ascertained stage, where twisting is obviously
present, is found at the time when the conjugant pairs are
shortening preparatory to entrance into the equator of
the spindle (Fig. 27). The usual interpretation of the

Fig. 27.
The late twisting of the thick threads (chromosomes) just before
they enter the spindle of the first maturation division of Batracho-
seps. (After Janssens.)

twisting at this stage is that it is in some way connected
with the shortening of the two conjugants. There is noth-
ing in these figures to show that this leads up to inter-
change. While it is possible that some of the cases of this
kind may be due to an earlier twisting of the threads
about each other, yet the persistence of the spiral would
rather indicate that crossing-over had not taken place,
for this would undo the twist.

If we turn next to the published drawings of the earlier
stages we find a number of cases in which the thin threads
(leptotene stage) are represented as though twisted

THE MECHANISM OF HEREDITY 43

about each other (Fig. 28b), but this interpretation is
often open to suspicion. It is extremely difficult, in fact, to
determine when threads as delicate as these come into
contact with each other whether they pass above or below,
i.e., above at one node and below at the next, etc. The
difficulty is enormously enhanced by the coagulated con-
dition of the threads, and it is only in this condition that
they are stained for microscopic study.

b -'.a

Fig. 28.
Conjugation of a pair of chromosomes of a planarian. In a, the
two thin threads are coming together; in b, there are indications,
at two levels, of crossing-over of the two united strands. (After
Gelei.)

The preparations that most nearly approach a demon-
stration of the twisting of the leptotene threads are those
in which the conjugation begins at one end (or at both
ends of bent chromosomes) and progresses toward the
other end (or toward the middle of the bend). The sperm-
cells of Batrachoseps presents perhaps the most seduc-
tive preparations of this sort (Fig. 26), but the figures of
Tomopteris are almost or quite as good. The drawings

44 THE THEORY OF THE GENE

of the eggs of Planaria (Fig. 28) are also quite convinc-
ing. Some at least of these figures give the impression
that as the threads are coming together they overlap one
or more times, but this impression is not sufficient to
show that they do more than lie across each other as seen
from certain levels. It does not follow moreover that they
will interchange where they overlap. While it must be
admitted, then, that the cytological evidence of crossing-
over has not been demonstrated, and from the nature of
the conditions it will be extremely difficult to actually
prove; nevertheless, it has been shown in a number of
cases that the chromosomes are brought into a position
where such an interchange might readily be supposed to
take place.

The cytologist, then, has given us an account of the
chromosomes that fulfills to a degree the requirements of
genetics. When we recall the fact that much of the evi-
dence was obtained prior to the rediscovery of Mendel's
paper, and that none of the work has been done with a
genetic bias, but quite independently of what the students
of heredity were doing, it does not seem probable that
these relations are mere coincidences, but rather that
students of the cell have discovered many of the essential
parts of the mechanism by which the hereditary elements
are sorted out according to Mendel's two laws and are
interchanged in an orderly way between members of the
same pair of chromosomes.

CHAPTER IV
CHROMOSOMES AND GENES

NOT only do the chromosomes pass through a
series of manoeuvres that go far toward supply-
ing a mechanism for the theory of heredity, but
from other sources evidence has accumulated supporting
the view that the chromosomes are the bearers of the
hereditary elements or genes, and this evidence has
steadily grown stronger each year. The evidence comes
from several sources. The earliest indication came from
the discovery that the male transmits equally with the
female. In animals, the male contributes, as a rule, only
the head of the spermatozoon, which contains almost ex-
clusivelv the nucleus composed of the condensed chromo-
somes. Although the egg contributes all the visible proto-
plasm of the future embryo, it has no preponderating
influence on development, except so far as the beginning
stages of development are determined by the egg proto-
plasm that has been under the influence of the maternal
chromosomes. Despite this initial influence, which can be
entirely ascribed to the previous influence of its own
chromosomes, the later stages of development and the
adult show no preponderance of maternal influence.

This evidence from the mutual influence of the two
parents is not, however, in itself convincing, for, dealing
with elements that are ultramicroscopical, it might be
claimed that the sperm contributes something more than
its chromosomes to the future embrvo. In fact, in recent
years it has been shown that visible protoplasm elements,
the centrosomes, may possibly be brought into the egg by

46 THE THEORY OF THE GENE

the sperm. It has not been established, however, that the
centrosomes have any specific effects on the develop-
mental process.

From another quarter the significance of the chromo-
somes was shown. "When two (or more) sperms enter the
egg, the three sets of chromosomes that result may be dis-
tributed irregularly at the first division of the egg. Four
instead of two cells, as in normal development, are
formed. It has been shown by a detailed study of such
eggs, combined with a study of the development of each
of the isolated quarters, that normal development does
not take place unless at least one full set of chromosomes
is present. At least this is the most reasonable interpreta-
tion of the results. Since in these cases the chromosomes
are not marked, the evidence does not do more than create
a presumption that at least one full set of chromosomes
must be present.

More recently still evidence in favor of such an inter-
pretation has come from other sources. It has been shown,
for example, that one set of chromosomes alone (haploid)
is capable of producing an individual which, to a large
extent, is a replica of the normal form, but this evidence
also indicates that these haploid individuals are not as
vigorous as the normal diploid type of the species. While
this difference may depend on factors other than the
chromosomes, the presumption remains that two sets of
chromosomes are better than one, as things stand. On
the other hand, in mosses, where there is a haploid stage
in the life cycle, the artificial transformation of the hap-
loid stage into a diploid stage does not appear to give an
advantage. Furthermore, it remains to be shown that
twice the number of chromosomes present in artificial
tetraploids confers any advantage over the normal dip-
loid set. It is evident, then, that we must be cautious as
to the merits of one, two, three, or four sets of chromo-

CHROMOSOMES AND GENES

somes, especially when suddenly an artificial situation is
created by increasing or decreasing the normal comple-
ment of chromosomes to which the machinery of develop-
ment is already adjusted.

>**f

Norma

Hapio- r?

Fig. 29.
Normal and haplo-IV flies of Drosophila melanogaster. Their
respective chromosome groups are shown above and to the right
of each.

Probably the most complete and convincing evidence
concerning the significance of the chromosomes in hered-
ity has come from the recent genetic results that have to
do with the specific effects of changes in the number of
the chromosomes where each one carries genetic factors
that enable us to identify its presence.

Eecent evidence of this kind comes from the loss or
from the addition of one of the small fourth chromosomes
of Drosophila (chromosome-IV). It has been shown both
by genetic and cytological methods that chromosome-IV
is sometimes lost from one of the germ-cells — egg or
sperm. If an egg lacking this chromosome is fertilized by

THE THEORY OF THE GENE

a normal sperm, the fertilized egg contains only one of
the fourth chromosomes. It develops into a fly ("haplo-
IV") that shows in many parts of its body slight differ-
ences from the normal fly (Fig. 29).

Fig. 30.
Characters in the fourth linkage group of D. melanogaster. To the
left, bent wings; to the right (above), four heads showing "eye-
less," one in dorsal, three in side view; below, and to the right,
shaven.

The result shows that specific effects are produced
when one of these chromosomes is absent, even in the
presence of the other fourth chromosome.

There are three mutant elements or genes in this chro-
mosome, namely, eyeless, bent, shaven (Fig. 30). All three
are recessives. If a haplo-IV female is mated to a diploid
eyeless male with two fourth chromosomes (each ripe
sperm with one) some of the offspring that hatch are
eyeless, and if the pupae that do not hatch are removed

CHROMOSOMES AND GENES

from their pupa-cases and examined, more eyeless flies
are detected. The eyeless fly has come from an egg that
did not carry chromosome-IV and was fertilized by a
sperm with chromosome-IV carrying the eyeless gene.

Haplo-lV

Eyeless
Diplo-lV

Germ-cells

Normal Diploid Haplo-lV Eyeless

Fig. 31.
Diagram of a cross between a normal-eyed, haplo-IV fly, and an
eyeless fly with two fourth chromosomes, each carrying a gene for
eyeless. The fourth chromosome carrying the eyeless gene is here
represented by an open circle, that for normal eyes by a black dot.

As shown in the diagram (Fig. 31), half of the flies should
be eyeless, but most of these do not pass beyond the pupal
stage, which means that the eyeless gene itself has a
weakening effect on the individual, and that when to this
is added the effects due to the absence of one of the fourth

50 THE THEORY OF THE GENE

chromosomes only a few such flies survive. The occur-
rence, however, of such recessive eyeless flies in the first
generation corroborates the interpretation that the eye-
less gene is carried by chromosome-IV.

The same results are obtained when the two other
mutant genes, bent and shaven, are used in a similar

Haplo-Ef

Fig. 32.

Haplo-IV and triplo-IV of D. melanogaster. The chromosome
groups are represented, respectively, above to the left, and to the
right of the figures.

experiment, but the proportion of recessive flies that
hatch in Fj is still smaller, indicating that these genes
have an even greater weakening effect than the eyeless
gene.

Occasionallv flies arise in which three chromosome-
IV 's are present. These are triplo-IV 's (Fig. 32). They
also differ from the wild type in several, or many, per-
haps in all their characters. The eyes are smaller, the
body color is darker, and the wings are narrower. If a

CHROMOSOMES AND GENES

triplo-TV is bred to an eyeless fly two kinds of offspring

result (Fig. 33). Half are triplo-IV 's, and half have the

normal number of chromosomes, as shown in the diagram.

If, now, one of these triplo-IV flies is back-crossed to

Triplo-H

Gametes

f",

sless

F, Triplo-D?

FiEgg*

Eijele&s Sperm

-Q-.

• O •

o o

IP •

o o

5 Wild Type

P 8

1 Eyeless

Fig. 33.
Diagram of a cross between a triplo-IV fly with normal eyes and
a normal diploid fly, pure for eyeless. In the lower half of the
diagram an Fj triplo-IV fly (whose gametes are represented in
"Fj eggs") is crossed to a diploid eyeless fly (whose "eyeless
sperm" is represented by the open circle), giving five kinds of
flies in the ratio of five wild type eyes to one eyeless.

52 THE THEORY OF THE GENE

an eyeless fly (from stock) the expectation is that there
will be five wild-type flies to one eyeless (Fig. 33, lower
half) instead of equality as in the ordinary case when a
heterozygous individual is back-crossed to its recessive.
The diagram (Fig. 33) shows the recombinations of
germ-cells that are expected to give rise to the 5 to 1
ratio. The actual number of eyeless obtained approxi-
mates expectation.

These and other experiments of the same kind show
that the genetic results check up at every point with the
known history of chromosome-IV. No one familiar with
the evidence can doubt for a moment that there is some-
thing in this chromosome that is responsible for the
observed results.

There is also evidence that the sex-chromosomes are
the bearers of certain genes. In Drosophila there are as
many as 200 characters whose inheritance is said to be
sex-linked. This term means only that they are carried
by the sex-chromosomes. It does not mean that the
characters are confined to one or the other sex. Owing to
the differential pair of sex-chromosomes in the male, the
X and the Y, the inheritance of characters whose gene
lies in the X-chromosomes is somewhat different from
that of anv of the other characters. There is evidence
that the Y-chromosome does not contain in Drosophila
any genes that conceal the recessives in the X. It may,
therefore, be ignored except in so far as it acts as the
mate of the X in the male at the reduction division of
the sperm-cells. The mode of inheritance of linked charac-
ters of Drosophila has already been given in Chapter I
(Figs. 11, 12, 13, 14). The mode of transmission of the
sex-chromosome is given in Fig. 38. An examination of
the latter shows that these characters follow the known
distribution of the chromosome.

Occasionally the sex-chromosomes "go wrong," and

CHROMOSOMES AND GENES

this slip furnishes an opportunity to study the changes
that take place in sex-linked inheritance. The most com-
mon disturbance is due to the failure of the two X's in
the female to disjoin at one of the maturation divisions.
The process is called non-disjunction. If an egg that has

disjunction

Non-disjunc+ion

Fig. 34.

Diagram to show the fertilization of an '"* allached X^^-er XX-egg
by a Y-sperm, producing a non-disjunctional XXY female.

retained its two X-chromosomes (and one of each of the
other chromosomes, Fig. 34) is fertilized by a Y-sperm,
an individual is produced — a female — that has two X's
and a Y. When the eggs of the XXY female mature, that
is, when the reduction of the chromosomes takes place,
some irregularity is introduced in the distribution of the

POLAR
BODY

EGGS

SPERM

POLAR
BODY

ECCS

SPERM

WHITE d
5

Fig. 35.

Diagram illustrating the fertilization of an XXY-egg, whose X-
chromosomes carry each the gene for white eyes, by a red-eyed
male. In the upper half of the diagram the fertilization of the
four possible kinds of eggs by the red-eyed producing X-chromo-
some of the male is shown. In the lower part of the diagram the
fertilization of the same four kinds of eggs by the Y-chromosome
of a male is shown.

CHROMOSOMES AND GENES 55

two X's and the Y, because the X's may conjugate, leav-
ing the Y free to move to either pole, or one X and the Y
may mate, leaving a free X. Possibly all three may come
together, and then separate so that two go to one pole of
the maturation spindle and one to the opposite pole. The
results are practically the same in either case. Four kinds
of eggs are expected, as shown in the diagram (Fig. 35).

In order to follow the genetic changes it is necessary
that the X-chromosomes of the female or of the male carry
one or more recessive genes. For instance, if the two X's
in the female carry each the gene for white eyes, and the
X in the male carries the allelomorphic gene for red eyes,
and if the former are indicated by open (white) X's and
the latter by a black X (Fig. 35), the combinations that
result are those indicated in the diagram (Fig. 35). Eight
kinds of individuals are expected, one of which (YY), not
containing even one X-chromosome, is expected to die. In
fact, this individual does not appear. Two of these indi-
viduals, viz., No. 4 and No. 7, never appear when an ordi-
nary white-eyed (XX) female is fertilized by a red-eyed
male. Their presence here, however, is in accord with the
expectation from an XXY white-eyed female. They have
been tested by genetic evidence and found to correspond
to the formula here given them. Furthermore, the white-
eyed XXY female has been also shown, by cytological
examination, to have two X's and a Y in her cells.

There is one additional kind of female expected that
has three X-chromosomes. The diagram indicates that
she dies, and this happens in the great majority of cases ;
but rarely one comes through. She has certain peculiari-
ties by which she can be easily identified. She is sluggish,
her wings are short and often irregular (Fig. 36) and she
is sterile. A microscopic examination of her cells has
shown that she contains three X-chromosomes.

This evidence points to the correctness of the theory

56 THE THEORY OF THE GENE

that the sex-linked genes are carried by the X-chromo-
somes.

There is another aberrant condition in the X-chromo-
somes that also supports this conclusion. A type of
female arose whose genetic behavior could be explained

Fig. 36.

A three-X female, a, having three X-ehromosomes and two of each

of the other kinds (autosomes), as shown in b and c.

only on the assumption that her two X-chromosomes had
become attached to each other. During the maturation
division of her eggs both X's go together, i.e., they both
stay in the egg, or both go out together (Fig. 37). A
microscopic examination shows in fact that the two X's
of these females are stuck together end to end, and it
shows also that these females contain a Y-chromo-
some that acts, presumably, as a mate of the two attached

CHROMOSOMES AND GENES

chromosomes. The diagram gives the expected results
when such a female is fertilized. By good fortune the
X-chromosomes that became attached carried each the
recessive gene for yellow wings. The presence of the two

Double-X yellow by

rtl9

Y +

Wild

l%6

Fig. 37.
Diagram illustrating the fertilization of the two kinds of eggs of
an attached, XX, yellow female (whose double X-chromosome is
represented here in solid black) by wild type male. There is a
Y-chromosome in the double-X female. It is represented here by
cross-hatching. The Y-chromosome in the male is indicated in the
same way. After reduction two kinds of eggs are present (see
above to left). These fertilized by the two kinds of spermatozoa
of the normal (wild type) male (see above to right) give the four
classes at the bottom of the diagram.

58 THE THEORY OF THE GENE

genes for yellow enables us to follow the genetic history
of the attached X's when such a female is bred to a nor-
mal wild type male with gray wings. For example: the
diagram (Fig. 37) shows that two kinds of eggs are ex-
pected after the maturation division: one egg retains the
double yellow X, the other egg retains the Y-chromosome.
If these eggs are fertilized by any kind of male, prefer-
ably by one whose X-chromosome contains recessive
genes, four kinds of offspring should be produced, two of
which die. The two that survive are a double XXY female
that is yellow, like her mother, and an XY male that is
like his father with respect to his sex-linked characters
because he gets his X from his father.

This result is exactly the reverse of what happens
when a normal female with recessive genes is fertilized
by a different kind of male, and the apparent contradic-
tion is understandable, at once, on the assumption of
attached X-chromosomes. A cytological examination of
these double X females never fails to show two X's
attached to each other.

CHAPTER V
THE ORIGIN OF MUTANT CHARACTERS

THE modern study of heredity has been intimately
bound up with the origin of new characters. In
fact, the study of Mendelian inheritance is possible
only when there are pairs of contrasted characters that
can be followed. Mendel found such contrasted characters
in the commercial stocks that he used, tall and short, yel-
low and green, round and wrinkled peas. Later work has
also extensively used such material, but some of the best
material is supplied by new types whose origin, in pedi-
gree cultures, is better known.

These new characters arise for the most part suddenly,
fully equipped, and maintain their constancy in the same
way as do the characters in the original type from which
they arose. For example, the white-eyed mutant of Droso-
phila appeared in a culture as a single male. When mated
to a common red-eyed female, all the offspring had red
eyes (Fig. 38). These were inbred and produced in the
next generation red-eyed and white-eyed individuals. All
the white-eved individuals were males.

These white-eved males were then mated to different
red-eved females of the same generation. Some of the
pairs produced equal numbers of white-eyed and red-
eyed offspring, both males and females. When the white-
eyed individuals were bred together they gave rise to
pure white-eyed stock.

We explain these results in accordance with Mendel's
first law, which postulates a red-producing and a white-
producing element (or gene) in the germinal material.

Fig. 38.
Sex-linked inheritance of white eyes in D. melanogaster. A white-
eyed male is bred to a red-eyed female. The X-chromosome carry-
ing the gene for red eye is represented by the black rod; the X-
chromosome carrying the gene for white eyes is represented by
the open rod, and the white recessive gene carried in the chromo-
some, by small w. The Y-chromosome is stippled.

ORIGIN OF MUTANT CHARACTERS 61

They behave as a pair of contrasted elements, that are
separated in the hybrid at the maturation of the eggs and
sperm.

LACTICOLOR 9 01

GR0SSULAR1ATA d LL

O © GERM CELLS ©

CROSSULAR^TA 9 OL

GR0SSULAR1ATA d 1 L

O ©'

LACTICOLOR 9 01

O ©

GR0SSULARIATA90L

CROSSULARlATAdLl

CR05SULARIATA d LL

Fig. 39.
Diagram showing a cross between the light-colored type (lacti-
color) of Abraxas and the common or dark type (grossulariata).
The sex-chromosome carrying the gene for dark color is here indi-
cated by the circle enclosing L, that for light color by the circle
with 1. The open circle (without an included letter) stands for
the W-chromosome that is confined to the females.

It is important to observe that the theory does not
state that the white-eyed gene alone produces white eyes.
It states, only, that a change took place in some part of
the original material, and in consequence of this single
change, the material, taken as a whole, now gives rise to

62 THE THEORY OF THE GENE

a different end-product. In fact, the change not only af-
fects the eyes, but other parts of the body as well. The
sheath of the testes is colorless, while it is greenish in
red-eyed flies. The white-eyed flies are more sluggish
than their red-eved fellows, and have a shorter life. It
is probable that many parts of the body are affected by
the change that took place in some part of the germinal
material.

At rare intervals, lighter colored, or pale individuals,
of the currant moth, Abraxas, appear in nature. They
are females as a rule. A pale, mutant female bred to a
dark, wild type male (Fig. 39) gives offspring that are

Fig. 40.
The mutant character Lobe* of D. melanogaster. The eyes are

small and protruding.

like the dark type. These, inbred, give the old and the new
types as 3 to 1. Pale F2 individuals are all females. If
they are bred to males of the same generation, some
pairs give pale males and females, as well as dark types
in equal numbers. From the former a pale stock can be
reared.

The two preceding mutant characters act as recessives
toward the corresponding character in the wild type, but
other mutants act as dominants. For example : lobe2 is
characterized by the peculiar shape and size of the eye
(Fig. 40). It arose as a single individual. Half of its off-

ORIGIN OF MUTANT CHARACTERS 63

spring showed the same character. A change in a gene in
one of the second chromosomes must have taken place,
either in the mother or father of the mutant. The germ-
cell containing this gene met a cell containing a normal
o-ene at the time of fertilization, and the first mutant
arose. The first individual was, therefore, hybrid or

Fig. 41.

The mutant character Curly of D. melanogaster. The wings curl

up at the ends and are held somewhat apart.

heterozygous, and, as stated above, produced, when mated
to a normal individual, both lobe2 and normal offspring
in equal numbers. From these heterozygous forms some
pure lobe2 flies were produced by mating two lobe2 indi-
viduals. The pure type (homozygous for lobe2) resembles
the heterozygous type, but the eyes are often smaller, and
one or both may be absent.

It is a curious fact that many dominant mutants are
lethal in homozygous conditions. Thus curly wing (Fig.
41), a dominant character, nearly always dies when homo-

M THE THEORY OF THE OEM

ivgous. Rarely, however, an individual survives. The
mutant, yellow coat color in mice, is lethal as double

dominant, as is also the mutant gene for black-eyed white
in mice. In all types of this sort, pure breeding stock can-
not be produeed (except by "balancing" the dominant
with another lethal). They produce, in each generation,
individuals like themselves and some other type (the
normal allelomorph) in equal numbers.

The short -tinkered or braehvdaetvl type in man is a
striking dominant character whose inheritance is well
known. It will hardly be questioned that it arose as a
dominant mutant that established itself in certain fami-
lies.

All the stoeks of Drosophila have arisen as mutants.
In the eases that T have given the mutant first appeared
as a sino-lo individual. In several other eases, however,
the new mutant type first appeared in several individuals.
In such eases the mutation must have appeared early in
the germ-track, so that several eggs or sperm-eells came
to carry the mutated element.

At other times a quarter of all the offspring from a
pair are mutants. These mutants are reeessives. and the
evidence shows, ill such eases, that the mutation had
occurred in an ancestor, and, being a recessive, it did not
appear on the surface until two individuals each having
the mutated gene met. A quarter of their offspring are
then expected to show the recessive character.

Closely inbred stoeks are expected to give this sort of
result more often than outbred stoeks. If outbred, the
recessive gene may be distributed to a Large number of
individuals before two such individuals meet by chance.

It is probable that there are many concealed recessive
genes in the human germ-material, since some defective
characters recur oftener than expected by independent
mutation. When their pedigrees are traced they often

ORIGIN OF MUTANT CHARACTERS 65

show relatives or ancestors with the same mutant charac-
ter. Human albinos furnish, perhaps, the best example of
this sort. In many cases they come from stocks both of
which carry the recessive gene, but it is always possible
thai a new gene for albinos may have been produced by
mutation. Even then it cannot come to expression until
it meets another like gene.

Most of our domesticated animals and plants show
characters that behave in inheritance in the same way as
do the mutants whose origin is established. There can be
no reasonable doubt that many of the characters have
arisen by sudden mutations, especially in cases where the
domesticated types have come from inbred pedigree
stocks.

It is not to be inferred from the preceding examples
that the production of mutants is peculiar to domesticated
races; for this is not the ease. There is abundant evi-
dence that the same kinds of mutations occur also in
nature. Since most of the mutants are weaker or less well-
adapted type- than the wild type, they disappear before
they are recognized. In cultivation, on the other hand,
the individual is protected, and tin; weaker types have a
chance to survive. Moreover, domestic forms, especially
those reared for genetic purposes, are carefully scruti-
nized, and our familiarity with them accounts for the
detection of many new types.

A study of the occurrence of mutations in the stocks of
Drosophila has brought to light a curious and unexpected
fact. The mutational change takes place in one member
only of a pair of genes — not in both at the same time.
It is difficult to imagine what kind of an environmental
effect could cause one gene in one cell to change, and not
the other identical gene. Hence it may seem that the cause
of the change is internal rather than external. This ques-
tion will be further discussed later.

THE THEORY OF THE GENE

TABLE I
Becurrent Mutations and Allelomorphic Series

Distinct

Total

Mutant

Total

Mutant

Locus

Occurrences

Types

Locus

Occurrences

Types

apterous

lethal-a

ascute

4±

lethal-b

Bar

lethal-c

bent

lethal-e

bifid

Lobe

bithorax
black

3
3 +

lozenge
maroon

bobbed

miniature

brown

Notches

25-4-

broad
cinnabar

4
3

pink
purple

11 +
6

club

reduced

cross-reinless

rough

curved
cut

dachs
dachsoid

16+

roughoid

ruby

rudimentary

sable

2
6

14 +
3

2
2

5+
2

Delta

scarlet

deltex

scute

Diehaete

dusky

ebony

eyeless

fat

forked

fringed

furrowed

6+
10
2
2
9
2
2

3
3

5
2
2
4

1
2

sepia

singed

Star

tan

tetraploidy

triploidy

Truncate

vermilion

4
5
2
3
3

15±

8±

12±

1
3
1
2
1
1
5
2

fused
garnet
Haplo-IV
inflated

35=h

2
3
1

vestigial

white

yellow

6
25±
15±

4
11

There is also another fact that a study of the mutation
process has brought to our attention. The same mutation
may recur again and again. A list of these recurrent mu-
tations of Drosophila is given above. The reappearance
of the same mutant indicates that we are dealing with
a specific and orderly process. Its recurrence recalls
Galton's famous analogy of a polygon. Each change
corresponds to a new stable position (here perhaps in a

ORIGIN OF MUTANT CHARACTERS 67

chemical sense) of the gene. Tempting as is this com-
parison, we must remember that, as yet, we have almost
no evidence as to the real nature of the mutation process.

The mutant types that are most often referred to, or
used for genetic material, are as a rule rather extreme
modifications or aberrations. This has sometimes given
the impression that a mutant change involves a great
departure from the original type. Darwin spoke of salta-
tions, which are only extreme mutations, and he rejected
them as materials for evolution, because, he said, such
great alterations in one part of the body would be likely
to throw the organism out of harmony with its environ-
ment, to which it is nicely adapted. Today, while we real-
ize fully the truth of this statement, when applied to
extreme changes producing malformations or aberra-
tions, we have come, nevertheless, to a realization that
minute changes are as characteristic of mutation as are
the grosser changes. In fact, it has been shown many
times that small changes that make a part a little larger
or a little smaller may also be due to genes in the germ-
material. Since only the differences that are due to genes
are inherited, it seems to follow that evolution must have
taken place through changes in the genes. It does not
follow, however, that these evolutionary changes are
identical with those that we see arising as mutations. It is
possible that the genes of wild types have had a different
origin. In fact, this view is often implied and sometimes
vigorously asserted. It is important, therefore, to find
out whether there is any evidence in support of such a
view. De Vries' earlier formulation of his famous muta-
tion theory might at first sight seem to suggest the crea-
tion of new genes.

The opening sentence of the mutation theory states
"that the properties of the organism are made up of
units, sharply distinguishable from one another. These

68 THE THEORY OF THE GENE

units are bound up in groups, and, in related species, the
same units and groups of units recur. Transitions, such
as seen in the outer forms of animals and plants, no more
exist between the units than between the molecules of the
chemist.

''Species are not continuously connected, but arise
through sudden changes or steps. Each new unit added
to those already present forms a step, and separates the
new type as an independent species from the species from
which it arises. The new species is 'presto change,' there.
It arises without visible preparation and without transi-
tions."

It may appear from this statement that a mutation
that produces a new elementary species is due to the
sudden appearance or creation of a new element — a new
gene. Put in another way, we witness at mutation the
birth of a new gene or at least its activation. The number
of active genes in the world has been increased by one.

De Vries has further elaborated his views on mutation
in the concluding chapters of The Mutation Theory and
in his later lectures on ' ' Species and Varieties. ' ' He recog-
nizes two processes, one the addition of a new element
that gives rise to a new species ; and the other, the inacti-
vation of a gene already present. It is the second view
that interests us at present, because, except for the man-
ner of expression, it is essentially the view that is today
sometimes said to be the way in which the new types in
our cultures arise — through the loss of a gene. De Vries
himself, in fact, places in this category all the commonly
observed cases of loss mutations without respect to their
dominance or recessiveness, implying, however, that they
are recessive because their gene has become inactive.
Mendelian results, he thinks, belong solely to this second
category, because of the existence of contrasting pairs of
genes — the active one and its inactive mate. These segre-

ORIGIN OF MUTANT CHARACTERS

gate, giving the two kinds of gametes peculiar to Men-
delian inheritance.

De Vries says that such a process represents a step
backward in evolution. It is not progressive but degres-
sive and produces a " retrograde variety." This inter-
pretation is, as I have said, closely akin to a current
interpretation of mutational changes as due to a loss of a
gene — in principle the two ideas are the same.

Fig. 42.

Several plants of Oenothera Lamarekiana (to the left), and O.

gigas (to the right). (After de Vries.)

It is not without interest, therefore, to examine the
evidence that led de Vries to develop his mutation hy-
pothesis.

De Vries found near Amsterdam, in a waste field, a
colony of evening primroses, Oenothera Lamarekiana
(Fig. 42). Amongst them were a few individuals that dif-
fered somewhat from the common forms. He brought
some of these into his garden and found that they bred
true for the most part. He also bred the parent form, or
Lamarekiana. It produced, in each generation, a small
number of the same new types. In all, about nine such

70 THE THEORY OF THE GENE

types were recognized at that time. These were the new
mutants.

Now it has turned out that one of these types is due to
doubling the number of the chromosomes. It is called
gigas (Fig. 42). One is a triploid, semigigas. Several of
the types are due to the presence of an extra chromosome.
These are called lata and semilata forms. One at least,
brevistylis, is a point-mutant, like the recessive mutants
of Drosophila. It is, then, to 0. brevistylis, and to the
residue of recessive mutants, that de Vries must appeal.1
It appears, now, highly probable that this residue (the
recessive mutants) conform to the Drosophila mutant
types, but their manner of reappearance in nearly every
generation gives a picture entirely different from that of
mutation in Drosophila and other animals and plants. A
possible interpretation may be found in the presence of
lethal genes closely linked with these recessive mutant
genes. Only when the recessive gene is released from its
near-by lethal through crossing-over is there an opportu-
nity for it to come to expression. It has been possible
in Drosophila to make up balanced lethal stocks carrying
recessive genes that simulate closely Oenothera. Only
when crossing-over occurs does the recessive reappear.
The frequency of its appearance is dependent on the
closeness of the lethal to the recessive gene.

It has been found that other species of wild Oenotheras
behave in the same way as Lamarck's evening primrose,
whose peculiarities in inheritance are, therefore, not due
to a hybrid origin (as has been sometimes surmised),
but due, in the main, to the presence of recessive genes
linked to lethal factors. The appearance of the mutant
types does not represent the mutation process that pro-

i De Vries and Stomps both thought that some of the peculiarities of
O. gigas are due to other factors than chromosome number.

ORIGIN OF MUTANT CHARACTERS 71

duced the mutant gene but rather its release from its
lethal linkage.2

It seems, then, that the mutation process in Lamarck's
primrose is probably not essentially different from famil-
iar processes that occur in other plants and animals. In
other words, there are no longer grounds for interpreting
the mutation process that it shows as differing essentially
from what takes place in other animals and plants, except
that some of its recessive mutant genes are concealed,
owing to the presence of lethal genes.

These considerations remove, I think, any necessity for
assuming that a new gene is added, even when a new or
progressive type of Oenothera appears. It may be that
such progressive types as de Vries had in mind arise
through the accidental addition of a whole chromosome to
the normal set. This question will be considered in Chap-
ter XII, but it may be said here that there is very little
evidence that new species can often be produced in this
way.

2 Shull has interpreted the appearance of a number of the recessive types
of O. Lamar cTciane, on the lethal-linkage hypothesis. S. H. Emerson has re-
cently pointed out that Shull's evidence, so far published, is not entirely
cogent, but it may, nevertheless, be valid. De Vries himself, in recent pub-
lications, seems not averse to accepting the lethal interpretation for certain
of the oft-repeated recessive mutants that he places in the "central chromo-
some. ' '

CHAPTER VI

ARE MUTANT RECESSIVE GENES PRODUCED
BY LOSSES OF GENES?

MENDEL did not consider the question of the
origin or the nature of the genes. He repre-
sented in his formula the dominant gene by a
capital letter and the recessive gene by a small letter. The
pure dominant was AA and the recessive was aa and the
hybrid, or Fj, was Aa. The question as to origin did not
arise, because the characters yellow and green, tall and
short, round and wrinkled, were already present in the
peas selected for the experiment. Only later, when the
relation of the mutants to the wild species from which
thev were supposed to have come was considered, did
their origin arouse interest. A specific case, that of rose
comb and pea comb in domestic fowls, seems to have had
something to do with the reasoning that led to an inter-
pretation of recessive genes as losses or absences.

Certain breeds of domestic poultry have a comb called
rose (Fig. 43). They breed true to this type of comb.
Other races have a comb called pea (Fig. 43). They also
breed true to their type. If these breeds are crossed, the
Fx has a new form of comb, called walnut (Fig. 43). If
two Ft fowls with walnut combs are mated, the offspring
show 9 walnut, 3 rose, 3 pea, to 1 single. The numerical
result shows that two pairs of genes are involved, rose
and not-rose, pea and not-pea. The single comb is not-
rose, not-pea, which was then interpreted to mean the
absence of pea and of rose genes. But the not-presence
of pea and not-presence of rose genes does not prove

ARE RECESSIVES LOSSES OF GENES?

necessarily that the allelomorphs of these genes are ab-
sences. The allelomorphs may be only other genes that do
not give rise either to pea or to rose comb.

Fig. 43.

Combs of domesticated races of fowls, a, single comb; b, pea
comb; c, rose comb; d, walnut comb (the hybrid or Fj type when
pea and rose are crossed).

The result may be stated in another way that may make
the situation more obvious. If we assume that the wild
jungle fowl, from which our domesticated races have
come, had a single comb, and that at some time a domi-
nant mutation occurred that gave a pea comb, and at

74 THE THEORY OF THE GENE

another time, in another bird, a dominant mutation oc-
curred that gave a rose comb, it follows that in the cross
described above, the F2 single comb is due to the presence
of the original wild type genes. Thus, a race with pea
comb (PP) will contain the wild type genes (rr), from
which the rose comb arose by mutation. Similarly the
race with rose comb (RR) will contain the wild type
genes (pp), from which the pea comb arose by mutation.
The formula for the pea comb race is then PPrr and
for the rose comb race RRpp. The germ cells of these two
races will be Pr and Rp respectively, and the F1 will be
PpRr. The two dominants produce a new type, the wal-
nut comb. Since two pairs of genes are present in ¥1
there will be 16 combinations in F2, and of these one will
be pprr or single comb. The single comb is due then to
the recombination of the wild type recessive genes that
entered the cross.

Recessive Characters and Absences of Genes.

In the background of the presence and absence theory
there lurks, beyond doubt, the idea that many recessive
characters are actual losses of some character that was
once present in the original type, hence by implication the
gene of that character is also absent. This idea is a hang-
over of Weismann's theory of the relation of determinant
to character.

It is instructive to look a little closer into the evidence
that may have seemed at first to support such an inter-
pretation.

An albino rabbit or rat or guinea pig may be inter-
preted to have lost the pigment characteristic of the origi-
nal type. In a sense no one will deny that the relation of
the two types may be expressed in this way, but, in pass-
ing, it may be noted that many albino guinea pigs have a
few colored hairs on the feet or toes. If the pigment-pro-

ARE RECESSIVES LOSSES OF GENES? 75

ducing gene is absent and if color depends on the pres-
ence of this gene it is difficult to explain the presence
of these colored hairs.

A mutant race of Drosophila is called vestigial (Fig.
10) because only vestiges of the wings are present, but if
the larvae are reared at a temperature of about 31 °C. the
rudiments are quite long and in extreme cases may be
almost as long as the wings of the wild type. If the gene
for producing long wings is absent, how can a high tem-
perature bring it back again?

There is another highly selected race of Drosophila in
which the eyes are absent in most individuals, but small
eyes may be present in other individuals (Fig. 30). As
the culture °;ets older more and more of the flies have
eyes and the average size of the eyes is larger. It is not
probable that the gene changes as the culture gets older
and if it were absent in the eyeless flies that first hatch, it
is not likely that the age of the culture could bring back
the missing gene. Moreover, if this were the case, flies
from the older culture should produce offspring more of
which had eyes or larger eyes than the average of the
race, but this does not happen.

In still other recessive mutant types the loss of the
character itself is by no means obvious. A black rabbit
is recessive to the gray wild type. The black rabbit has
actually more pigment than has the gray rabbit.

There are dominant genes that produce pure white
individuals. The white leghorn race of fowls is due to
such a factor. Here the argument is reversed, and it is
said that there is present in the wild type jungle fowl a
gene that suppresses white plumage. When this suppress-
ing gene is lost the bird is then able to develop white
plumage. Logical as this argument may appear, the as-
sumption of the presence of factors of this sort in the
wild bird seems far fetched, and in the light of the occur-

76 THE THEORY OF THE GENE

rence of other dominant characters, the argument is not
one that makes a favorable appeal but seems rather a
forced attempt to save the theory at all costs.

It must be remembered, too, that the distinction be-
tween recessive and dominant genes is largely arbitrary.
Experience has shown that characters are by no means
always recessive or dominant. On the contrary, in a large
majority of cases, a character is not completely dominant
or completely recessive. In other words, the hybrid type,
containing a dominant and a recessive gene, lies some-
where between the parent types — both genes have some
effect on the character produced. When this relation is
realized, the theory that a recessive gene is an absence
does not appear in so favorable a light. It is true that it
might be claimed in such cases, with some grounds for
justification, perhaps, that the hybrid is intermediate
because one dominant gene produces less effect than two
dominant genes, but this introduces a new feature into
the situation. It does not necessarilv mean that the effect
is really due to one absence. It can be brought into line
with this assumption perhaps, but is not a necessary in-
ference.

If the preceding arguments are admitted as cogent we
might dismiss this interpretation of the meaning of the
recessive gene taken in a literal sense. But in recent years
another interpretation of the relation between the effect
of all the genes and the character has appeared that
makes the refutation of the presence and absence view
much more difficult. For example, suppose a gene were
actually lost from a chromosome and that when two such
chromosomes are brought together, some character of the
individual is modified or even absent. The modification or
absence might be said to be the effect produced by all
the rest of the genes. It is not the absence, as such, that
determines the result, but the effect produced, when two

ARE RECESSIVES LOSSES OF GENES? 77

genes are absent, by the rest of the genes. Such an inter-
pretation avoids the rather naive assumption that each
gene in itself represents a character of the individual.

Before discussing this view it should be pointed out
that in certain aspects this interpretation is similar to,
and in fact derived from, another more familiar interpre-
tation of the relation between gene and character. For
instance, if the mutation process is interpreted to mean
a change in the constitution of a gene, the result that
follows, when two recessive mutant genes are present,
is not that the new character is due to the new gene alone,
but that the new character is the end product of the
activity of all the genes, including the new ones, in the
same sense that the original character was also due to
the original gene (that mutated) and to the rest of the
genes.

These last two interpretations may be briefly stated
as follows : The first one states that in the absence of a
pair of genes all the rest of the genes are responsible for
the mutant character. The second states that when a gene
changes its constitution, the end-result, produced by the
new genes, and the rest of the genes, is the mutant
character.

There is a certain amount of recently obtained evidence
that has some bearing on the question here at issue
although it cannot be said to furnish a decisive answer
in favor of either view. The evidence is, nevertheless,
worth considering on its own merits, since it brings out
certain possibilities relating to mutation that have not,
so far, been discussed.

There are several mutant stocks called collectively
notch that can be identified by one or more incisions at
the end of the wings and by the thickening of the third
vein of the wing (Fig. 44). Only females having these
characteristics appear. Any male carrying the gene for

THE THEORY OF THE GENE

notch dies. The factor is carried in the X-chromosome.
The notch female has one X carrying the factor for notch
and another X with its normal allelomorph (Fig. 45).
Half of her ripe eggs retain one X, half the other. If she

Fig. 44.

Notch-win g, a dominant sex-linked, recessive lethal of Drosophila

melanogaster.

is fertilized by a normal male, an X-sperm uniting with
an egg carrying the normal X produces a normal daugh-
ter ; an X-sperm uniting with an egg carrying the notch-
bearing X produces a notch daughter. A Y-sperm uniting
with an egg carrying a normal X produces a normal son;
a Y-sperm uniting with an egg carrying a notch-bearing
X forms a combination that dies. The output is two
daughters to one son.

ARE RECESSIVES LOSSES OF GENES?

As far as this evidence goes, notch might be interpreted
as a recessive lethal gene that acts as a dominant wing
modifier in the hybrid. It was, however, later shown by
Bridges and by Mohr that a much greater length of the
X-chromosome is involved in the notch mutation than in

Notch O

N -n

XX t4

normal cf

X^Y

Gametes

JVotch

Fig. 45.
Diagram to illustrate a cross of a Notch female, XNXn by a normal
male, XnY. The X-chromosome with Notch is XN; the other X,
carries the normal allelomorph, Xn.

an ordinary ' ' point mutation ' ' ; for when recessive genes
are present in the region of notch in one X-chromosome,
and notch in the other X-chromosome, the recessive
characters appear in such an individual as though a cer-
tain region of the notch chromosome were absent or at
any rate inactive (Fig. 46a). The result is practically the
same as though an absence had actually arisen. In some
of the notch mutants the "lost" region extends over
about 3.8 units (from the left of white to the right of

THE THEORY OF THE GENE

abnormal) (see chart, Fig. 19) ; but in other notches the
lost region extends over fewer units. In each case the test
seems to mean that a small piece (more or less) of the
chromosome has, in some sense, dropped out.

white
facet——

aDnor-m.

\-not ver

ver .... v ev

1-14 Mel

/-not v/el

Fig. 46.
Diagram a, showing the location of the genes in the Notch-bearing
chromosome. In the right-hand rod the break in the chromosome
stands for Notch. In the left-hand rod the location of three reces-
sive genes (white, facet, abnormal), that stand opposite Notch, are
indicated. In b the translocation of a piece of an X-chromosome to
another X is shown. There are two vermilion-bearing X-chromo-
somes to one of which the piece is attached carrying the normal
allelomorph of vermilion, i.e., not-vermilion. In c there are two
yellow-bearing X-chromosomes to one of which is attached a piece
carrying the normal allelomorph of yellow, i.e., not-yellow.

As has been stated, recessive genes, when opposite to
notch, produce their recessive characters. This is con-
sistent either with the view that these recessives are
absences and the effect is produced by all the rest of the
genes, or with the view that the recessive genes are pres-
ent and produce their effect in combination with all the
rest of the genes. The result does not permit a decision
between the two views.

ARE RECESSIVES LOSSES OF GENES? 81

There is, nevertheless, a slight difference between the
character produced by two recessive genes in this region
and one recessive and the notch "absence." This differ-
ence might seem to be due to one real absence (notch) and
one recessive not being equivalent to two recessive genes,
but further consideration shows that the two situations
are not quite comparable owing to the absence of other
genes in the lost notch piece. These genes are present in
the double recessive type, and the slight differences in the
result in the two cases may be referable to the presence
or absence of these other genes.

In the preceding case it has not been possible to show
by cytological evidence that a piece of the X-chromosome
is absent in the notch mutant — its absence is deduced
from the genetic evidence alone. In the next case, how-
ever, an actual absence has been demonstrated.

Occasionally one of the small fourth chromosomes is
lost (haplo-IV, Fig. 29). This chromosome carries, in
certain mutant stocks, recessive genes. It is possible to
make up an individual that has a recessive gene — eyeless
for example — in its single IV-chromosome. Such individ-
uals show the characteristics of the eveless stock but are,
as a class, more extreme than when two eyeless genes are
present. This difference may be due to the absence of the
other genes in the absent chromosome.

A different relation arises in the case of translocation,
so called by Bridges, which means (from genetic evidence)
that a piece of a chromosome has become detached and re-
attached to some other chromosome. It perpetuates itself,
and, owing to the genes that it carries, introduces a com-
plication into the genetic results. For example, a piece
of the normal X-chromosome in the region of the ver-
milion locus became attached to another X-chromosome
(Fig. 46b). A female with vermilion genes in each of its
X-chromosomes and the transposed piece attached to one

82 THE THEORY OF THE GENE

of them (Fig. 46b) is vermilion despite the fact that one
normal allelomorph of vermilion is present in the piece.
At first sight it may seem, if the vermilion genes are
interpreted as absences, that two absences cannot pos-
sibly dominate one presence. On second thought, how-
ever, another explanation is possible, for, if the vermilion
eye color is due to the action of all the other genes when
vermilion is absent, the same result might happen even
though one dominant normal allelomorph is present. The
situation is not identical with one in which a vermilion
gene is present in one chromosome and its normal alle-
lomorph in the other.

The relation shown here between two recessive genes
and a dominant gene in the translocated piece does not
always lead to the development of the recessive charac-
ter. For example, there is another case of translocation
reported by L. V. Morgan. A piece of an X-chromosome
of the region of the mutant genes yellow and scute be-
came stuck to the right end of an X-chromosome. A
female that has the recessive genes for yellow or for
scute in each of her X-chromosomes (Fig. 46c) and a
piece attached to one of these X's shows the wild type
character. Here the effects of the recessive genes are coun-
terbalanced by the dominant allelomorphs of the attached
piece. This is interpreted to mean that all the other genes,
plus those in the attached piece, combine to turn the scale
toward the dominant type and this is expected on either
of the contrasted interpretations as to the nature of the
gene.

The relation of two recessive genes to one dominant
has also been studied in the triploid endosperm of corn
and in one triploid animal. The nuclei of the endosperm
cells of the seed of corn arise from the union of one pollen
grain (haploid in chromosome number) and two nuclei of
the embryo sac (each haploid). A triploid or threefold

ARE RECESSIVES LOSSES OF GENES?

nucleus results (Fig. 47), which by division gives rise to
the triploid nuclei of the endosperm cells. In floury corn
the endosperm is composed of soft starch, while flint corn
has a large amount of corneous starch in the endosperm.
If a floury corn be used as the female parent (ovule) and

polar
nucleus

synergidae

--ovum

ego; nucleus
sperm nucleus

ertili'zed

endosperm
nude «

Fig. 47.

Three stages in the fertilization of the egg-nucleus in the embryo
sac of plants. In b the two maternal haploid nuclei and the single
paternal haploid sperm nucleus are shown. By their union the trip-
loid endosperm is produced as shown in c. (After Strasburger and
Guinard from Wilson.)

flinty corn as the male parent (pollen) the seeds pro-
duced by the Fa plant have floury endosperm. The result
shows that two floury genes are dominant over one flinty
gene (Fig. 48a). If the reciprocal combination is made,
flinty female parent and floury pollen, the Fx seeds are
flinty (Fig. 48b). Here two flinty genes dominate one
floury. It is a matter of choice which gene is chosen to
represent the absence of the other. If the absence is floury,
then two absences would be said to dominate one presence

THE THEORY OF THE GENE

in the first case, and two presences to dominate one ab-
sence in the second case.

The interpretation of two absences dominating a pres-
ence would have no meaning if taken literally, but as has
been pointed out it is possible to explain such a statement,
if, in the absence of a gene, the floury character is deter-
mined by the rest of the genes, and, of course, the same

(a)

(a'j

fl-

f I O UT Y

ft-

-ft

-fl

flint

Fig. 48.

Diagrams of triploid condition of endosperm of corn when, as in a,
two floury genes and one flinty are present giving floury endo-
sperm; and when, as in a', two flinty genes and one floury gene
are present giving flinty endosperm.

explanation applies if there is present a gene for floury
(a mutated gene from flinty) whose effect is produced by
itself plus the rest of the genes. This evidence from trip-
loid endosperm is no more decisive than when a trans-
located piece of a chromosome introduces a third element
into the situation.

There are several other cases in corn where two reces-
sive elements do not dominate a single dominant, but
these have no further bearing on the present question.

If a triploid female Drosophila has a vermilion gene in
each of two of its X-chromosomes and a red gene in the
third, the resulting eye color is red. One dominant gene
here dominates two recessives. This result is the opposite

ARE RECESSIVES LOSSES OF GENES? 85

of that where the wild type (dominant) gene present in
the duplicated piece was opposed to two vermilion genes.
The two situations are, however, not identical in all re-
spects, for the triploid differs from duplication by the
occurrence of nearly an entire X-chromosome instead of
only a short piece of this chromosome. The excess of
genes in the extra X-chromosome may account for the
difference in the two cases and this holds equally whether
the recessive genes be interpreted as real absences or
as mutated genes.

The Bearing of Reverse Mutation (Atavism) on the
Interpretation of the Mutation Process.

If recessive genes arise by losses, then there is little
expectation that a pure recessive stock would ever pro-
duce again the original gene, since this would mean ap-
parently the production of something highly specific from
nothing. On the other hand, if mutation is due to a change
in the constitution of the gene, it seems less difficult to
imagine that the mutated gene might sometimes return
to the original condition. It may be that we know too little
about the gene to give much weight to such arguments;
nevertheless, the occurrence of return mutants would ap-
pear more plausibly explained on the latter view. Unfor-
tunately the evidence bearing on the question is not en-
tirelv satisfactory. There are, it is true, a number of
instances in Drosophila where a mutant recessive stock
has given rise to an individual with the original or wild
type character; but an occurrence of this sort, unless
controlled, cannot be accepted as sufficient evidence, since
the chance of contamination of the stock by a wild type
individual is not to be ignored. If, however, a mutant
stock is marked by several mutant characters, one only
of which reverts, the occurrence furnishes the desired
evidence, provided no other combinations of these mu-

86 THE THEORY OF THE GENE

tants are present in the vicinity at the time. There are a
few recorded cases in our stocks that fulfill these condi-
tions, and the evidence, as far as it goes, shows that
reversal may take place. There is also another possibility
that has to be guarded against. Some of the mutant
stocks have, after a time, seemed to lose more or less the
characteristics of that stock, yet when outbred it has been
found that the mutant character can be recovered in its
original strength. The fourth chromosome character, bent
wings (Fig. 30), that is itself variable, and subject to
environmental influence, has shown, when not selected, a
tendency to return to the wild type in appearance. If a
fly of this kind is outcrossed to wild stock and the F^s
inbred, the bent character reappears in many of the indi-
viduals of the expected bent class. A similar result has
been found in another mutant stock called scute, charac-
terized by the absence of certain bristles on the thorax.
Individuals appeared in certain pure cultures in which
the " missing' ' bristles were present. Apparently the
mutant had reverted to wild type. But that this had not
occurred was shown by breeding such flies to wild type
stock. In the second generation, scute flies reappeared.
A study of this case has shown that the return to normal
was due to the appearance of a recessive factor which,
when present in homozygous condition in scute stock,
brings about the development there of the missing bris-
tles. Aside from the bearing of this result on the question
under discussion, the fact of a new recessive mutation
occurring, that brings back the mutant character to the
original type, is in itself an interesting and important
fact.

Finally, there is the peculiar reversion to normal of the
dominant or semi-dominant character, bar-eye (Fig. 49,
1 and 2). For some years it has been known from the
observations of May and Zeleny that bar-eye reverts

Fig. 49.

Different types of bar eyes; 1, homozygous bar female; 2, bar
male; 3, bar-over-round eye female; 4, female homozygous for
round that was obtained by reversion; 5, male that carries the
gene for round eye obtained by reversion; 6, double-bar male;
7, homozygous infra-bar female; 8, infra-bar male; 9, infra -bar-
over-round female; 10, double-infra-bar male.

» 88 THE THEORY OF THE GENE

to normal eye, and this instance has been cited as evi-
dence that reverse mutation may take place. The fre-
quency of the return mutation varies in different stocks.
It has been estimated to occur about once in 1600 times.
It was later discovered by Sturtevant and Morgan that
when the reversion occurs, crossing-over takes place in
the vicinity of the bar gene, and Sturtevant has obtained
crucial evidence in regard to the nature of the changes
that there take place.

V 1 — i 1 l j \r f B fu

?x — ■ ■ ■ — h dx — ' '

y + B +

Fig. 50.

Diagram of a cross between a female bar-eyed fly, heterozygous in

forked and fused, and a forked bar fused male.

The demonstration that crossing-over takes place
whenever reversion occurs, was as follows: To the left
and very close to bar (% unit) there is a gene called
forked : to the right and very near bar (2U2 units) a gene
called fused. A female is made up with bar lying between
these two genes in one X-chromosome, and bar with the
wild type allelomorphs of forked and fused in the other
(Fig-. 50^. Such a female is bred to a forked bar fused
male. The ordinary sons will be either forked bar fused

or bar, since each has received either the forked bar
fused or the not-forked bar not-fused X-chromosome of
the mother. When, as happens rarely, a reversion takes
place, i.e., a male appears that has round eyes, it is ob-
served that crossing-over has taken place between forked
and fused. For example, the reverted male is either fused
or else forked ; it is never forked and fused, nor is it ever
both not-forked and not-fused. Crossing-over must have

ARE RECESSIVES LOSSES OF GENES? 89

taken place in the mother of the male between forked and
fused. The total cross-overs between forked and fused
are less than 3 per cent and yet they include all the rever-
sions to full eye.

Only the reverted sons have been spoken of above in
order to simplify the situation, but of course the reverted
chromosome might have passed into an egg that de-
velops into a female. The experiment can be so planned
that evidence of crossing-over will also be detected in the
reverted daughter. The ordinary daughters will be homo-
zygous bar (see Fig. 49, 1). The reverted daughters will
be heterozygous for bar eyes, and either forked or fused.
None of them are both forked and fused and none of them
are not-forked not-fused.

The crossing-over, that brings about reversion to
round eye, must not only have left one X-chromosome
without a bar gene, but must have put the other bar gene
into the bar chromosome (Fig. 51a). In appearance a
male with two bar genes (double bar) is similar to a
male with one bar gene, but its eyes are smaller and
the number of its facets is reduced. It has been named
double bar (Fig. 49,6). The presence of two allelomorphic
genes in the same linear series is an exceptional occur-
rence that has as yet not been observed in any other
mutation. It can be pictured only by supposing that the
bar genes lying opposite to each other before crossing-
over are shifted a little when the crossing-over occurs.
The result is that the double bar chromosome is length-
ened by one bar gene at least, and conversely that the
other chromosome has been shortened by the ''loss" of
the bar gene.

Sturtevant has put the theory of reversion of bar to a
number of critical tests. There is an allelomorph of bar
(that arose as a mutation of bar) called infra-bar (Fig.
49, 7 and 8), whose eyes are somewhat different in size

90 THE THEORY OF THE GENE

and in the number of facets. In infra-bar stock, reversion
also takes place (Fig. 51b), producing a full round eye
a^A closely similar to wild type, whreh-4s a new type called
double-infra-bar (Fig. 49, 10).

(a)

(b)

B Bar BB Double Bar

"g Bar Normal

_B InfraBar BB Double Infra Bar

d' Infra Bar " Normal

B Bay
~ Jhfra Bar

r B B Bar Infra Bar
Normal

B B Infra Bar Bar
N ormal

Fig. 51.
Diagram of mutation in bar and infra-bar and bar-infra-bar.

A female with bar in one chromosome and infra-bar
in the other (Fig. 51c) produces, when reversion takes
place, a full round eye (wild type) and bar-infra-bar or
infra-bar-bar types (Fig. 51c).

Sturtevant has also utilized these two types, the bar-
infra-bar type and the infra-bar-bar, in order to prove
that when crossing-over takes place in bar-infra-bar over
normal (Fig. 52a), the result gives either forked bar or
else infra-bar fused, and when crossing-over takes place
in infra-bar-bar over normal (Fig. 52b) the result is
either forked infra-bar or bar fused, provided the mutant
genes all lie in the same chromosome, as shown in the
diagram (Fig. 52a, b).

ARE RECESSIVES LOSSES OF GENES? 91

It follows that in both types the genes not only keep
their identity but also their sequence. From the way in
which the two types, fBB'fu and fB'Bfu, were made up,
the sequence of the genes is known, and in all cases the
breaking apart of B and B' agrees with the sequence pre-
viously determined.

(a)

j B B'-fa BarlnfraBar f B Bar

Normal g7 ju InfraBar

(b)

f B'B tu. InfraBar Bar f B' InfraBar

Normal B fu Bar

Fig. 52.

Diagram, in a, of a mutation of forked bar, infra-bar fused and
in b, a mutation of forked infra-bar, bar fused.

These results furnish crucial evidence in favor of the
correctness of the theory that reversion in bar is due to
crossing-over. This is, at present, a unique case. There
would seem to be some peculiarity in the X-chromosome
at the bar locus that allows crossing-over between allelo-
morphic factors to occur. Sturtevant speaks of this as
unequal crossing-over.1

This result raises the question as to whether all muta-
tions may not be due to crossing-over. There is explicit
evidence in Drosophila that this is not the general expla-

i Several curious problems concerning the bar locus are involved in these
relations. For instance, when bar crosses over what is left in the bar locus?
Is it an absence of bar? Did the original bar arise by mutation in a wild
type gene, or was a new gene created? These questions are still under
investigation.

92 THE THEORY OF THE GENE

nation of mutations, because, for one reason, it is well
known that mutation may occur in the male of Drosophila
as well as in the female. In the male of Drosophila there
is no crossing-over.

The Evidence from Multiple Allelomorphs.

It has been shown in Drosophila, as well as in a few
other types (in corn, for example), that more than a
single mutation may occur at the same locus. The series
of allelomorphs at the locus for white eye in Drosophila
is the clearest case of the sort. No less than eleven eye
colors, in addition to the red eye of the wild fly, have been
recorded. They form a graded series from white to red
as follows : white, ecru, tinged, buff, ivory, eosin, apricot,
cherry, blood, coral, wine. They have not, however, ap-
peared in this sequence, although white was the first
mutation observed at this locus. That they have not
arisen by the mutation of a series of adjacent genes is
clearly shown by their origin and their relation to each
other. For example, if the white were due to a mutation
from the wild type at one locus and cherry by mutation
at an adjacent locus, then when white is crossed to cherry
the female offspring should have red eyes, because white
would, on this assumption, carry the wild type allelo-
morph of cherry and cherry would carry that of white.
When white and cherry are crossed they do not give this
result, but the daughters have an intermediate eye color.
The Ft female gives again white and cherry sons in equal
numbers. The same relation holds for all the other alle-
lomorphs, any two of which can exist simultaneously in
any one female.

If the presence and absence theory is taken literally
there cannot be more than one absence for each gene. The
theory in this form is disproven in all cases where the
occurrence of multiple allelomorphs is known to have

ARE RECESSIVES LOSSES OF GENES? 93

taken place independently from the wild type ;2 but it is
possible to interpret absence in such a way that it is not
in contradiction with the occurrence of multiple allelo-
morphs. Suppose, for instance, that different quantities
of materials are lost at the locus in question for each
mutant type. The loss of one quantity might stand for
white, another quantity for cherry, and so on. The result
might then not appear to be inconsistent with the facts,
although it should be noted that the assumption calls for
a somewhat different interpretation of the gene as a unit.
The ' ' compound ' ' formed by the presence of two of these
allelomorphs might then not be expected to give the wild
type but something else. To admit this, however, changes
the idea of presence and absence in such a way as to make
it essentiallv the same as the view that is here main-
tained, namely, that mutation is due to a change of some
sort in a gene. There is no advantage, that I can see, in
urging that the change must be a loss of part of the gene
(gene meaning a quantity of something at a given locus).
This is a gratuitous assumption in regard to the nature
of the change — one that is not necessary to explain the
results. It may be, of course, that a gene may be lost or a
part of a gene may be lost and a new mutant result, but
it is theoretically possible that the constitution of the
gene may change in some other way. So long as we do not
know anything definite concerning the kind of change
that takes place there is nothing to be gained by limiting
it to only one kind of process.

2 If the multiple allelomorphs had arisen seriatim, i.e., one from another,
then of course it might be possible that each one carried the preceding
mutant gene. If so the two when crossed would not give the wild type. But
when, as in Drosophila, each has arisen independently from the wild type
the situation is different, as explained in the text.

94 THE THEORY OF THE GENE

Conclusions.

An analysis of the evidence at hand does not justify
the view that the actual loss of some character present in
the original type must be interpreted to mean that a cor-
responding loss has taken place in the germinal material.

Even by extending the literal interpretation of the pres-
ence and absence idea so that the postulated connection
between the loss of the character and the loss of the gene
means the effect produced by other genes, the assumption
of a loss still has no advantage over the alternative
view that a mutation is due to a change of some sort in
the gene. Furthermore, the occurrence of mutation in the
reverse direction ( omitting the special case of bar rever-
sion), while not sufficiently established as yet, is in better
accord with the view that genes may mutate by a change
in their constitution without that change being neces-
sarily a loss of the whole gene. And finally, the evidence
from multiple allelomorphs seems more consistent with
the view that each is due to a change in the same gene.

The theory of the gene as here formulated, regards the
wild type genes as specific elements in the chromosomes,
that are relatively stable over long periods. There is at
present no evidence that new genes arise except by
changes in the constitution of the old genes. The total
number of the genes remains on the whole constant over
long periods. Their number may be changed, however,
by a process of doubling the full set of chromosomes and
perhaps in other similar ways. The effect of changes of
this sort will be considered in later chapters.

CHAPTER VII

THE LOCATION OF GENES IN RELATED

SPECIES

DE VRIES' mutation theory quite apart from its
| special interpretation discussed in the la3t-chap-
ter postulates that "elementary" species are
made up of a large number of identical genes ; and that
their differences are due to different recombinations of
these genes. The more recent work on hybridizing re-
lated species has furnished evidence bearing on this
theory.

The most obvious way to study the problem would be
to cross species and determine in this way, if possible,
whether they are made up of the same number of ho-
mologous genes, but several difficulties stand in the way.
Many species cannot be crossed, and some of those that
can be crossed produce sterile hybrids. Nevertheless, a
few species are fertile inter se, and some of them also
give fertile hybrids. Even then, another difficulty arises,
namely, the identification in the two species of the char-
acters that behave as Mendelian pairs; for the differ-
ences that serve to distinguish one species from another
species are dependent on a multitude of factors in each
case. In other words, it is rare to find two well-marked
species in which any single difference is due to one dif-
ferential factor. Mutant differences of recent origin in
one or in both species must be resorted to for the neces-
sary evidence.

There are several cases in plants and two at least in
animals where species having mutant types have been

THE THEORY OF THE GENE

crossed with other species, and produced fertile off-
spring. These, when inbred, or back-crossed, have fur-
nished the only crucial evidence concerning the allelo-
morphic relation of genes in different species.

Fig. 53.
Cross between two species of tobaeco, Nicotiana Langsdorffii and
N. alata. In a and c the two original types of flowers are shown,
and in b the hybrid type. In d and e, two of the recovered types
in F2 are shown. (After East.)

East crossed two species of tobacco, Nicotiana Langs-
dorffii and N. alata (Fig. 53). One plant with white
flowers was a mutant type. Despite the wide variability
of many characters in the second generation, the white
flowers appeared in one-fourth of the individuals of this
generation. The mutant gene of one species behaved
toward a gene of the other species in the same way as it
behaves with its own normal partner.

Correns crossed Mirabilis Jalapa with M. longiflora.
A recessive mutant of Jalapa (chlorina) was used. This

majus

Amolle Hybrid

Fig. 54.

Two species of snapdragon, Antirrhinum molle and A. majus with

the hybrid between them. (After Baur.)

A.molJ

A. majusf (peloric)

Fig. 55.
A bilateral type of flower of Antirrhinum molle by a peloric type
of A. majus, which, when crossed, gives the hybrid "wild" type
seen below. (After Baur.)

THE THEORY OF THE GENE

character reappeared in almost one-quarter of the indi-
viduals in the second generation.

Baur crossed two species of snapdragon, Antirrhinum
majus and A. molle (Fig. 54). He used at least five

Fig. 56.

Types of F2 flowers from the cross shown in Fig. 55. (After

Baur.)

mutant types of A. majus and recovered them in the sec-
ond generation in the expected number of individuals
(Fig. 55 and 56).

Detlefsen crossed two species of guinea pigs, Cavia
porcellus and C. rufescens. The hybrid females (the hy-
brid males are sterile) were mated to C. porcellus males

GENES IN RELATED SPECIES 99

with mutant characters, seven in all. The mutant charac-
ters were inherited in the same way as in C. porcellus.
This result again shows that the two species carry some
identical loci. The results do not show, however, that
identical mutants exist in the two species, for no mutant
races with characters similar to those of porcellus have
been studied.

c d

Ftg. 57.
a, Helix nemoralis, 00000, yellow, Zurich type; b, ditto 00345, red-
dish (Aarburger type); c, typical H. hortensis, 12345; d, ditto;
e, hybrid 00000. (After LaDg.)

One of the clearest cases where the characters of one
species behave toward the characters of the other species
in the dominance-recessive relation as do the same
character-pairs within the species is described by Lang
in his experiments with two wild species of snail, Helix
hortensis and H. nemoralis (Fig. 57).

There are two wild species of Drosophila that are so
much alike externally that they were put into the same
species. One is now called D. melanogaster, the other D.

100 THE THEORY OF THE GENE

simulans (Fig-. 58). Careful scrutiny shows them to be
different in many ways. They cross with difficulty and the
hybrids produced are completely sterile.

Forty-two mutant types are now known in D. simulans.
These fall into three linkage groups.

Fig. 58.
Drosophila melanogaster to the left, and D. simulans to the right;

both males.

Twenty-three of these recessive mutant genes in simu-
lans are recessive in the hybrid, and 65 recessive mutant
genes of melanogaster have also been shown to be reces-
sive in the hybrid. This result means that each species
carries the standard or wild type gene of each of the
recessive genes of the other species.

Sixteen dominant genes have also been tested. All but
one produced nearly the same effect in the hybrid that
they produce within their own species. This means that

GENES IN RELATED SPECIES

101

sixteen normal genes are recessive to the dominant mu-
tant genes of the other species.

Mutants of simulans have been mated to mutants of
melanogaster. In twenty cases tested, the mutant charac-
ter proved to be the same.

20 30

90 100 110 120 130 140

H — Jr

t^v *$ ty 3p <ty

KW — 1 1 ii \ +■

tfi

I'll

i !■■--.

-*-mel

u lib
i — f— siml

— H

FTT&r-

-m —

-p*—- .

^mel

-*sim.

Mg_

mel

sim.

Fig. 59.
Chart showing, above, the corresponding loci of identical mutant
genes of the first or X-chromosome in Drosophila melanogaster and
in D. simulans, similarly, in the middle, of the second chromo-
some; and, at the bottom, of the third chromosome. (After Sturte-
vant.)

This last result establishes the identity of the mutants
in the two types, and enables one to discover whether they
lie in the same linkage series, and in the same relative
position in each series. The chart (Fig. 59) shows by the
connecting dotted lines the relative position of the loci
of identical mutants so far worked out by Sturtevant. In
chromosome-I there is a remarkable agreement. In chro-
mosome-II only two identical loci have been determined.
In chromosome-III the agreement is not complete. It can

102 THE THEORY OF THE GENE

probably be explained on the assumption that a large
section of this chromosome has been reversed, and the
corresponding loci are in inverse order.

These results of Sturtevant's are not only important
in themselves, but help to make probable the view that
similar mutants in different species that occupy the same
relative position in the linkage series, are identical mu-
tants, but unless their identity can be tested by crossing,

I
0.0-f se

0.6- -ij

27.0
30.0

450--SI

57.0

66.0
70.0

78.0 --

,C
v

m
f

s
tr

"T.g.W

102.04- dr

n m

0.0+ cf 0.0-t-Sc

• d

42.0 4- Cc

41.0

62.0 --t

.-hch

D. virilis

o.o-t-Fu.ap I

-- 1

21.0 --Br

Fig. 60.

Chart of the location of the mutant genes in the six chromosomes
of Drosophila virilis. (After Metz and Weinstein.)

GENES IN RELATED SPECIES

103

as in the case of D. melanogaster, and D. simulans, there
may always remain some doubt as to their identity, be-
cause similar mutant types that are not identical are

62.0 -j-P
58.0 -|- gz

42.0 --b

o.oN.

6 0'

14.5
160

240 -I- r

36 0
420

fu
CO

n
s

ro
bx

St
Tm
Sb
gP

Ac
Ms

D. obscura

86 5 4- as
115 4

10"] .0 4- S

Fig. 61.

Chart of the location of the mutant genes in the chromosomes of
Drosophila obscura. The loci corresponding with those of D.
melanogaster are sc=scute, y=yellow, No2=Notch, wh=:white.
(After Lancefield.)

104 THE THEORY OF THE GENE

known, and sometimes these lie near together in the same
linkage group.1

In two other species of Drosophila the work has prog-
ressed to a point where the comparisons are at least very
interesting. In Drosophila virilis, Metz and Weinstein
have determined the location of several mutant genes,
and Metz has compared the order of the series with that
of D. melanogaster. The chart (Fig. 60) shows that there
are five apparently similar mutants in the sex-chromo-
some that stand in the same order as those of melano-
gaster, viz., yellow (y), cross-veinless (c), -singed (si),
miniature (m), forked (f).

Another species, Drosophila obscura, has a genetic sex-
chromosome twice as long as that of melanogaster (Fig.
61). It is probably significant that the four characteris-
tic mutant types, yellow, white eyes, scute, and notch
wings, that lie in the middle of this long sex-chromosome,
are identical with the same mutant characters of D.
melanogaster that lie at the end of the shorter sex-chro-
mosome of melanogaster and simulans. The interpreta-
tion of this relation is still being carefully studied by
Lancefield.

These and other results should make us extremely cau-
tious in drawing phylogenetic conclusions from inspec-
tion alone of the chromosome groups ; for, it follows from
the Drosophila evidence that very closely related species
may have their genes arranged in a different order in the
same chromosomes. Similar groups of chromosomes may
at times contain different assortments of genes. Since it
is the genes, and not the chromosomes as such, that are
important, the final analysis of the hereditary construc-
tion must be determined by genetics rather than by
cytology.

i By taking into account more than a single effect of each gene the
identification may be made more probable.

CHAPTER VIII

THE TETRAPLOIDS, OR FOURFOLD TYPE-

THE chromosomes have been counted in more than
a thousand species of animals and probably in as
many or more species of plants. In two or three
species only one pair of chromosomes is present. At the
other extreme there are species with over one hundred
chromosomes. No matter how many or how few the chro-
mosomes, the number is found to be constant for each
species.

It is true that irregularities in the distribution of the
chromosomes occasionally take place. Most of these ir-
regularities are, as a rule, automatically straightened out
in one or another way. It is also true that, in one or two
cases, a slightly variable number of chromosomes may
occur, as in Metapodius where one or more small, extra
chromosomes, sometimes the Y-chromosomes, sometimes
another chromosome called the M-chromosome, mav or
may not be present (Fig. 62). As Wilson has shown, these
chromosomes may, perhaps, be looked upon as indifferent
bodies that have lost their importance, since there are no
corresponding variations in the characters of the indi-
viduals.

It is known, furthermore, that chromosomes may join
together, decreasing the number by one or more, but the
totality of the genes is still preserved, and this also holds
for cases where a chromosome may break, increasing for
a time at least the number by one.1 Finally, there are

1 The occasional breaking apart of chromosomes in Oenothera has been
described by Hance. In the moth PhragmO(tobia, and in other moths also,
Seiler has described several cases where certain chromosomes that are united

106

THE THEORY OF THE GENE

fcin <t:v

•v

♦ •

• • d * w e

Fig. 62.

Chromosomes of Metapodius. a, spermatogonial group with three
small m-chromosomes ; b and c, side view of spermatocytes, con-
jugation of three m% two pass to one pole, one to the other, as
seen in d and e (anaphase plates of c). (After Wilson.)

species where the female has one or more chromosomes
than the male, and there are other species where the

in the eggs and sperms, are separate in the embryonic cells. In the bee each
chromosome is supposed to break into two parts in all of the somatic cells.
In some of the tissue cells of flies and other animals the chromosomes may
divide without the cell dividing and in this way become doubled or quad-
rupled.

TETRAPLOIDS 107

reverse may be true. All these situations have been ex-
tensively studied, and are familiar to every student of the
cell. The occurrence of such cases does not invalidate the
general statement, that the number of the chromosomes
is constant and characteristic of each species.2

In recent years an ever increasing number of cases has
been reported in which individuals have suddenly ap-
peared that have double the number of chromosomes
characteristic of the species. These are the fourfold types,
or tetraploids. Other multiple types have also been found,
some arising spontaneously, others derived from the
tetraploids. We speak of these collectively as polyploids.
Of these polyploids, the fourfold group is in many ways
the most interesting.

In animals there are only three cases of tetraploidy
that are certainly known. The parasitic threadworm of
the horse, Ascaris, occurs in two types, one with two and
one with four chromosomes, respectively. These two
varieties are like each other, even as to the size of their
cells. The chromosomes of Ascaris are regarded as com-
pound and as formed by the union of a number of smaller
chromosomes sometimes called chromomeres. In the cells
of the embryo that will become body-cells, each chromo-
some breaks up into its constituent elements (Fig. 63, a,
b, d). These are constant in number or approximately so,

2 Delia Valle and Hovasse have in recent years denied that the number
of chromosomes is constant in different tissue cells, but -since their conclu-
sions are based on an examination of the somatic cells of amphibia that
have a large number of chromosomes difficult to identify with accuracy,
their results do not suffice to overthrow the overwhelming number of obser-
vations on other forms (and even on some Amphibia) where the number
of the chromosomes can be accurately determined.

It is also known that in certain tissues the number of the chromosomes
may be doubled or quadrupled, either by failure of cells to divide when the
chromosomes divide, or else by the chromosomes breaking up into a con-
stant number of parts. These are special cases that do not affect the general
situation.

108

THE THEORY OF THE GENE

and there are in all about twice as many elements in
bivalens as in univalens. This supports the view that
there are twice as many chromosomes in one type as

Fig. 63.

First and second cleavages of the egg of Ascaris univalens with
two chromosomes. In a and b the fragmentation of the two chro-
mosomes in one of the cells is shown. In d, three cells show frag-
mented chromosomes, while in the fourth cell the chromosomes are
intact. The latter gives rise to the germ-cells. (After Boveri.)

in the other, rather than that bivalens has arisen through
the halving of the univalens chromosomes.

One form of the brine shrimp, Artemia salina, is, ac-
cording to Artom, a tetraploid. There are two races, one
with 42 chromosomes, the other with 84 chromosomes
(Fig. 64). The latter propagates by parthenogenesis.

TETRAPLOIDS

109

Under these circumstances it is not difficult to imagine
that the tetraploid originated in a variety that was
already parthenogenetic, for, should an egg-cell double
the number of its chromosomes by the retention of one of
its polar bodies, or become double through the chromo-
somes failing to separate after the first division of a
nucleus, the double condition might continue to perpetu-
ate itself.

HaploJcf 21 ^Diploid 4>z)

Fig. 64.

The chromosomes, in reduced number, of the diploid and tetraploid

races of Artemia salina. (After Artom.)

One of the first tetraploids in plants was discovered by
de Vries, and named Oenothera gigas (Fig. 42). It was
not known, at first, that this giant was a fourfold chromo-
some type, but de Vries saw that it was stouter than
plants of the parent species (Lamarck's evening prim-
rose) and different in manv other minor characteristic
details. Its chromosome number was later made out.

Lamarck's evening primrose (Oenothera Lamarcki-
ana) has 14 chromosomes (haploid 7). The giant form 0.
gigas has 28 chromosomes (haploid 14). The two chromo-
some groups are drawn in Fig. 65.

Gates has made measurements of the cells of differ-
ent tissues. The epidermal cells of the anthers of gigas

110 THE THEORY OF THE GENE

have almost four times the volume of the normal type;
those of the stigma three times the volume ; those of the
petals twice the volume and the pollen mother cells are
about one and a half times larger. The nuclei of the latter
have, in gigas, twice the volume of the parent type. The
cells in the two types also differ sometimes markedly in
their superficial dimensions. Most species of evening
primroses have 3-lobed discoidal pollen grains, some of
those of gigas are 4-lobed.

'it*

a b

Fig. 65.

a, The fourteen diploid chromosomes of Oenothera Lamarckiana;

6, the twenty-eight diploid chromosomes of O. gigas.

The maturation of the pollen mother cells has been
studied by Gates, Davis, Cleland, and Boedjm. Gates
reports that in 0. Lamarckiana there are, as a rule, 14
pairs of bivalent chromosomes (gemini) in the giant. At
the first maturation division, half of each bivalent goes to
each daughter cell. At the second division each chromo-
some splits lengthwise and gives 14 chromosomes to each
pollen grain. A similar process presumably occurs in
the ripening of the ovules. Davis describes the chromo-
somes of 0. Lamarckiana that emerge from the synaptic
tangle as stuck together somewhat irregularly and not
strictly in side to side union. Later they move toward one

TETRAPLOIDS

111

or the other pole bringing about reduction. Cleland has
recently described an end-to-end union of the chromo-
somes of another diploid species, Oenothera franciscana,
as they enter the maturation spindle (Fig. 66). Some of

Fig. 66.
The maturation of the pollen cells in Oenothera franciscana.

(After Cleland.)

the earlier figures of Davis had also to some extent indi-
cated an end-to-end union.

In other monoecious flowering plants tetraploids have
also been found in recent years. It is obvious why these
occurrences should be more frequent in monoecious spe-
cies than in species with separate sexes; for, in the

112 THE THEORY OF THE GENE

former, eggs and pollen are produced on the same plant.
Hence if a plant lias started as a tetraploid, it will pro-
duce both egg-cells and pollen-cells with a diploid number
of chromosomes. Self-fertilization will give tetraploids
again. On the other hand, in animals or plants with sepa-
rate sexes the eggs of one individual must be fertilized by
sperm from another individual. Now, if a tetraploid fe-
male should arise, her ripe eggs, with the diploid number
of chromosomes, will ordinarily be fertilized by the hap-
loid sperm from a normal male, with the result that a
threefold type, or triploid is formed. From a triploid
the chance of recovering a tetraploid again is very small.

The tetraploids that have arisen in pedigreed cultures
furnish more accurate information as to their origin
than do tetraploids found accidentally. There are, in fact,
other records where tetraploids have arisen under con-
trolled conditions. In Primula sinensis, Gregory has
found two giant types, one of which appeared in a cross
between two diploid plants. Since the parent plants con-
tained known genetic factors, Gregory was enabled to
study the inheritance of the characters in the fourfold
type. His results left him undecided as to whether they
indicated that a given member of each of the four like
chromosomes unites with a specific mate or equally with
any member of its group. Muller's analysis of the same
data indicates the latter as the more probable conclusion.

Winkler has obtained a giant nightshade (Solanum
nigrum) and a giant tomato (Solanum lycopersicum)
through the intermediate process of grafting, which has
in itself, so far as known, no direct relation to the produc-
tion of the double forms.

The tetraploid nightshade was obtained in the follow-
ing way. A piece of a young tomato plant was grafted
into a young nightshade plant from which the axial buds
were then removed. A cross cut was made, ten days later,

TETRAPLOIDS

113

at the graft level (Fig. 67). Adventitious buds grew up
from the callus tissue of the exposed surface. One of
these plants was a chimaera, i.e., a plant part of whose
tissue was nightshade and part tomato. It was removed
and propagated. Some of the axial buds of the new plant

Fig. 67.

To the left a, fc, c, methods of grafting of tomato and nightshade.
To the right, a periclinal chimaera, S. lycopersicum. (After
Winkler.)

had a tomato epidermis and a nightshade core. These
branches were then isolated and planted. The plantlets
differed from other chimaeras known to be diploid, which
created a suspicion that the new type might have a tetra-
ploid core, which was confirmed by examination. The tops
of these chimaeras were cut off, and the axial buds of the
basal half removed. From the adventitious buds of the

114

THE THEORY OF THE GENE

callus, young plants were obtained that were tetraploid
throughout. One of these gigas nightshade plants is
shown in Fig. 68, to the right, and a normal (diploid) or
parent type, to the left ; a flower of gigas is shown above
to the right in Fig. 69 and the parent type to the left. A
seedling gigas is shown and a seedling of the parent type
above left, Fig. 69.

Fig. 68.
Normal diploid parent plant of solanum to the left, and tetra-
ploid to the right. (After Winkler.)

The differences in the cells of some of the tissues are
shown in Fig. 69. The palisade cells of the leaf of the
gigas type and the corresponding cells of the parent type
are shown below to the left ; the guard cells of the gigas
stomata and those of the parent type are shown below
to the right ; the hairs of the gigas form and those of the
parent are shown at the bottom to the right ; the pith cells
of the giant are correspondingly larger than those of the
normal plant. The pollen grains of the giant are repre-

TETRAPLOIDS

115

sentecl in the middle to the right and those of the parent
type to the left.

A tetraploid tomato plant, also, was obtained as follows.
A piece of a young tomato plant was grafted on to a stock
of nightshade in the usual way (Fig. 67). After union

Nopmal(n) Gi§as(g) Nor>mal(n) Gigas(g)

Fig. 69.
Diploid and tetraploid seedling and flowers of the nightshade are
shown above, and tissue cells below. Above to left, seedlings;
above to right, flowers; below to left, palisade cells; in middle,
pollen grains; to right stomata, above, and hairs below. (After
Winkler. )

had been perfected, a cut was made across the union of
the two plants and the axial buds removed from the stock.
From the cut surface, young buds developed in the callus
tissue. These were removed and planted. One of these
had an epidermis of nightshade cells and a core of tomato
cells. It was found on further examination that the epi-
dermal cells were diploid and the cells of the core were

116

THE THEORY OF THE GENE

tetraploid. In order to obtain, from this composite plant,
a tetraploid in all of its parts, the stem of a young plant
was cut across and the axial buds below the cut were
removed. New adventitious buds appeared on the cut
surface which were, for the most part, made up of the

Giant Haploid 24 Giant Diploid 48

c d

Fig. 70.
a, Haploid; b, diploid cell and chromosomes of nightshade; c, hap-
loid, and d, diploid cell and chromosomes of tetraploid night-
shade. (After Winkler.)

tomato tissues both within and without. The giant tomato
plant differs from the parent plant in the same way as
does the giant nightshade from its parent.

The diploid nightshade has 24 chromosomes, its hapr
loid number is 12; the tetraploid has 48 chromosomes,
and its haploid number is 24 chromosomes. The diploid
tomato has 72 chromosomes (haploid 36). The tetraploid

TETRAPLOIDS

117

tomato has 144 chromosomes (haploid 72 chromosomes).
These chromosomes are shown in Figs. 70 and 71.

Giant haploid Giant diploid

12 144

C d

Fig. 71.

a, Haploid ; &, diploid cell and chromosomes of tomato ; c, haploid,
and d, diploid cell and chromosomes of tetraploid tomato. (After
Winkler.)

As has been said, there is no obvious relation in these
cases, as far as known, between grafting and the forma-
tion of tetraploid cells in the callus. How these cells arise
is uncertain. It is possible that two cells of the callus fuse

118

THE THEORY OF THE GENE

together, as Winkler at one time thought probable, but
it seems more likely that the tetraploids arise by the sup-
pression of the cytoplasmic division of a dividing cell,
which would thereby double the number of its chromo-
somes. Such a tetraploid cell might form the whole or
only the core, or any other part of a young plant.

A tetraploid of the common Jimson weed (Datura
stramonium) (Fig. 72 below) was found by Blakeslee,

sl«2r^ k~- A- *

■Fa 9L^-/W^ ' .^i^iHA ■"■

^- '^ySL* *8JJUjX' Jfr> ^Vf^^y "V f v^/L1^ *i**fS^ "3^ '$£*&*•*''

Fig. 72.
Diploid plant of Datura stramonium, above, and tetraploid, below.

(After Blakeslee.)

TETRAPLOIDS

119

Belling, and Farnham. In appearance it is described as
differing in several respects from the diploid type. The
differences in the capsule, flower, and stamens in the
diploid (second column) and tetraploid (fourth column)
are shown in Fig. 73.

]fl

Haploid

/ I .

_\ /_

/ \

fcu "

Diploid

„

% 1

Tnploid

t\ 1/

■JJM.

Tetraploid

Fig. 73.

Capsules, flowers, and stamens of haploid, diploid, triploid, and
tetraploid D. stramonium. (After Blakeslee.)

120 THE THEORY OF THE GENE

The diploid plant has 12 pairs of chromosomes (24
chromosomes) which according to Belling and Blakeslee
can be arranged in six sizes (Fig. 74), namely, large (L
and 1), medium (M and m), and small (S and s), or
o (L-j-41+3M+2m+S+s). The formula for the haploid

* «

»s«t i * tm

•i « *i

♦ '

Fig. 74.
o, Second metaphase chromosome group of diploid Datura stramo-
nium with 12 chromosomes (each constricted) ; and b, correspond-
ing group of tetraploid with 24 chromosomes. (After Belling and
Blakeslee.)

group is L+4.1+3M+2ni+S+s. These chromosomes,
when about to enter the first maturation division (pro-
phase), form pairs of rings or else are united by one end
(Fig. 75, second column). One conjugant of each pair then
moves to one pole and its mate to the opposite pole. Pre-
paratory to the second maturation division, each chromo-
some constricts, producing the appearance shown in Fig.
74b. One constricted half passes to one pole of the spindle,

TETRAPLOIDS

121

the other half to the other pole. Each daughter cell gets
12 chromosomes.

The tetraploid has 24 pairs or 48 chromosomes. Prior
to their entrance into the first maturation spindle they
come together in fours (Fig. 76 and Fig. 75). The differ-

Ot i 2^

Haji&tcCU)ifi£olci

Tnift£axcL j Tethciji£crvct

Fig. 75.

Methods of conjugation of the chromosomes in diploid, triploid,
and tetraploid types of Datura stramonium. (After Belling and
Blakeslee.)

ent ways in which these chromosomes are combined in
these quadrivalent groups is shown in these figures. They
enter the first maturation spindle in approximately this
condition. At the first maturation division two members
of each quadrivalent pass to one pole and two to the op-
posite pole (Fig. 75). Each pollen grain has 24 chromo-
somes. Occasionally, however, three chromosomes may
pass to one pole and one to the other.

122

THE THEORY OF THE GENE

The 24 chromosomes of the tetraploid at the second
maturation division are shown in Fig. 74. They re-
semble those of the diploid at the same stage. Half of
each passes to one pole, half to the opposite pole. Belling
records that in 68 per cent the distribution is regular, i.e.,
24 to each pole (24+24). In 30 per cent of cases the dis-

**r*> /-

-«//#/

^ <?

^i-

Fig. 76.
Conjugation of the chromosomes of the tetraploid of Datura stra-
monium. Four like chromosomes unite to make up each group.
(After Belling and Blakeslee.)

tribution gives 23 at one pole and 25 at the other (23+
25). In 2 per cent there were 22 at one pole and 26 at the
other. In one case the distribution was 21-27. The result
shows that irregularities of distribution are not uncom-
mon in the tetraploid Datura. A further test of this was
made by self-fertilizing a tetraploid. The progeny was
grown to maturity and the chromosomes in their germ-
cells counted. The number of chromosomes in 55 of the
plants was 48 ; in five plants it was 49 ; in one plant it was
47 ; in another it was 48 ( ?) . If the distribution in the egg-
cells is like that in the pollen cells, it follows that the

TETRAPLOIDS 123

germ-cells with 24 chromosomes are those most likely to
survive and function. Some of these plants with more
than 48 chromosomes might give new types with still
greater irregularities of distribution of the chromosomes,
owing to the additional extra chromosomes.

v*WW*t

0*«S

• c

Fig. 77.

a, Euchlaena perennis, first maturation division, prophase; with
19 bivalents and two single chromosomes, a1, Metaphase of last.
a2, Anaphase of same, b, Zea mays, first maturation prophase with
ten bivalents. c, Euchlaena mexicana, first maturation division,
prophase, with ten chromosomes, ab, Hybrid (Fx) between E.
perennis and Zea mays, prophase of first maturation division with
3 trivalent, 8 bivalent, and 5 single chromosomes, ab1 Same as
last, late anaphase of first maturation division. (After Longley.)

124 THE THEORY OF THE GENE

A tetraploid Narcissus has been reported by de Mol.
The diploid species has 14 chromosomes (7 pairs) while
two cultivated varieties were found to have 28 chromo-
somes. De Mol points out that until 1885 the small diploid
varieties were chiefly cultivated. Then the larger triploid
types appeared and finally about 1899 the first tetraploid
was obtained.

The perennial teosinte of Mexico has twice as many
chromosomes as the annual teosinte, according to Long-
ley. The perennial Fig. 77a, has 40 chromosomes (n=20)
and the annual 20 chromosomes (n=10) Fig. 77c. Longley
crossed both of these with corn (maize), that has 20
chromosomes (n=10) Fig. 77b. The hybrid between the
annual teosinte and corn has 20 chromosomes. At the
maturation stages of the pollen mother cells there are 10
bivalents, and these divide and pass to the poles without
any lagging chromosomes. This means that the 10 chro-
mosomes that have come from the teosinte conjugate with
the 10 that have come from the corn. "When the perennial
teosinte is crossed to corn the hybrid has 30 chromo-
somes. At the ripening of the pollen mother cells of the
hybrid the chromosomes are found to be united, some in
threes, others in twos; the rest have no partners (Fig.
77ab). This leads to irregularities in the division that
follows (Fig. 77aV).

In hermaphroditic or monoecious plants, where the
question of sex determination is not involved with differ-
ential sex-chromosomes, the tetraploid may be said to be
both balanced and stable. By balanced is meant that the
numerical relations of the genes is the same as that in the
diploid or normal type. By stable is meant that the
mechanism of maturation is such that the type, once
established, perpetuates itself.3

Tetraploids in mosses were produced as early as 1907

3 Blakeslee used the terms differently.

TETRAPLOIDS

125

by Elie and fimile Marchal by artificial means. Each
moss plant has two generations, a haploid protonema
stage (gametophyte) that produces eggs and sperm-cells
and a diploid stage (sporophyte) that produces asexually

X(Tl)

x(m

Xy(2Tl) Sporophyte

X(TD Gametophyte

lerna

@ mm ®xm)

Fig. 78.
Normal life cvele of dioecious moss.

126 THE THEORY OF THE GENE

the spores (Fig. 78). Pieces of the sporophyte if kept
under moist conditions give rise to threads whose cells
are diploid. These become a true protonema that gives
rise in time to diploid eggs and diploid sperm-cells. By
the union of these germ-cells tetraploid sporophyte
plants are formed (Fig. 79). Here the normal haploids
have been duplicated by a diploid protonema and moss
plant, and the diploid sporophyte has been duplicated by
a tetraploid sporophyte.

The Marchals have made comparative measurements
of the size of the cells of the normal plants and of those
of the tetraploids. In three species the volume of the
normal perianth cells to that of the doubles was found to
be as 1 to 2.3 ; 1 to 1.8 ; and 1 to 2. The volumes of the cells
of the normal antheridia in the two types were as 1 to 1.8
and those of the nuclei were about as 1 to 2. The egg-cells
were as 1 to 1.9. Measurements of the antheridial organs
(that carry the sperm-cells) and of the archegonial
organs (that carry the egg) showed in all cases that
the double types are longer and broader than are the
normal types. It is evident that the increase in size of
the double types is due to larger cells and these in turn
have larger nuclei, which, other evidence has shown,
have in the double types twice as many chromosomes as
in the normal type. This was, of course, to be expected
from their origin by regeneration from the normal sporo-
phyte.

In the sporophyte generation the mother cells of the 2n
spores were to those of the 4n spores about as 1 to 2.

The two maturation divisions in mosses, i.e., the divi-
sions following conjugation of the chromosomes, take
place in the sporophyte at the time when the spores are
formed — four from each spore mother cell. If, in mosses,
the chromosomes carrv the ofenes, the doubling: of the
chromosomes (tetraploid) in double types is expected to

>A)3

xycin)

@®

Xlj(AU)

7@® @@

xy(An)

XljilU)

XljiATi)

XljiZTl)

©xycn)

Fig. 79.

Formation of a diploid protonema (2n) by regeneration from a
2n sporophyte of a normal, monoecious moss. By self-fertilization
a 2n gametophyte gives rise to a tetraploid or 4n sporophyte. By
regeneration from the latter a tetraploid gametophyte is produced.

128 THE THEORY OF THE GENE

give ratios different from those in the normal plants. As
yet little has been done in this direction, although Wett-
stein has found clear evidence of genetic inheritance in a
few species crosses of mosses, and Allen, in the related
group of liverworts, has genetic evidence for two charac-
ters of the gametophyte.

In those mosses with separate sexes and in certain
liverworts it has been shown by the Marchals, by Allen,
by Schmidt, and by Wettstein that the sex-determining
elements are sorted out at the time of spore formation.
An account of these observations and experiments will
be given in the chapter on sex.

There are many important questions for embryology
rather than for genetics relating to the size of the cells
of tetraploids. In general it may be said that the cells
are larger, and frequently twice as large, but there is a
good deal of variation in the different tissues in these
respects.

The size of the whole plant as well as some of the other
peculiarities of the tetraploid are due apparently to the
increase in size of its cells. If this is correct, it means
that these characteristics are developmental rather than
genetic. The way in which tetraploids arise has to some
extent been already considered. The methods that have
been suggested, as to how the increase in the amount of
cytoplasm in the cells of the tetraploids takes place, call
for further examination.

If two cells in the germ-track should fuse, and their
nuclei then or later unite, a tetraploid cell might result.
If the double cell continued to maintain a double volume
in the growth period, an egg of twice the normal size
would be expected to result. The number of cells of the
larger embryo would also be expected to be the same as
that characteristic of the normal embryo.

There is, however, another possibility, namely, that the

TETRAPLOIDS 129

double germ-cell might not be able to increase to double
size in the germ-track of its diploid mother. The egg
might not then be any larger than the normal egg, but
have twice as many chromosomes. The embryo develop-
ing from this egg might not be able to get enough nour-
ishment to increase the size of its cells until the post-
embryonic or larval stages were reached, when food is
obtainable from the outside. Whether at this late period
the presence of a double set of chromosomes in each cell
would bring about an enlargement of the cytoplasm of
each cell is uncertain. In the next generation, however,
the eggs would develop from the beginning with a four-
fold set of chromosomes in a tetraploid body, and under
these circumstances it is conceivable that the egg might
grow to double size before dividing.

It is even less to be expected, perhaps, that an imme-
diate increase in amount of the cytoplasm could take
place if the doubling of the chromosomes occurred in a
mature egg after it is fertilized. The embryos of animals
pass through a rather definite number of cell-divisions
before organ formation begins. If an embryo should start
as an egg of normal size but with double the number of
chromosomes, and if, in consequence of the double num-
ber present, cleavages should cease sooner than in the
normal egg when organ formation sets in, such a tetra-
ploid embryo would then have cells twice the size of the
normal embryo but only half as many cells.

In the flowering plants where ample space and food
supply is present in the embryo sac, the development of
an egg with a larger amount of cytoplasm may have a
more favorable chance to take place.

130 THE THEORY OF THE GENE

Tetraploidy as a Means of Increasing the Number
of Genes in a Species.

One of the most interesting considerations connected
with tetraploids from an evolutionary standpoint is the
opportunity they may seem to furnish for increasing the
number of new genes. If new and stable types arise
through doubling the number of the chromosomes, and
if, after doubling, the four like chromosomes should be-
come different in the course of time, so that two become
more like each other, and the other two also become more
like each other, the tetraploid would then resemble geneti-
cally a diploid, except in so far as many of the genes
remained unchanged. Many like genes would then be
present in four chromosomes of each set, and the expec-
tation for the F2, when an individual is heterozygous for
only one pair of genes, would be a Mendelian ratio of 15
to 1 instead of 3 to 1. Such ratios have in fact been found
(wheat, shepherd's purse) but whether tetraploidy ac-
counts for the result or whether doubling has occurred in
some other way remains to be determined.

On the whole, it seems that until we know something
more as to the way in which new genes arise — if they do
now arise — it is rather hazardous to take advantage of
tetraploidy as a general explanation to account for a
change in number of the genes. It is true that in monoe-
cious plants new types may arise in this way, yet it is
improbable that, in animals with separate sexes, tetra-
ploidy could become established (except in parthenoge-
netic species), because, as has been pointed out above, the
tetraploid is lost by crossing to an ordinary or diploid
individual and not easily recovered afterwards.

CHAPTER IX

TRIPLOIDS

TN recent work a number of threefold, or triploid,
types have also been recorded. Some of these trip-
loids have arisen from known diploid types ; others
have been found in cultivated plants, while still others
have been found in the wild state.

Stomps and Anne Lutz described triploid plants of
Oenothera (scmi-gigas), with 21 chromosomes. Triploids
of Oenothera have since been described by de Vries, van
Overeem, and others. They are supposed to be produced
by the union of a diploid with a haploid germ-cell.

The distribution of the chromosomes of triploids dur-
ing: maturation has been studied bv Gates and Geerts and
van Overeem. They find that while, in some cases, the
chromosomes are rather regularly distributed at reduc-
tion, in other cases some of the chromosomes are lost and
degenerate. Miss Lutz found in fact great variation in the
kind of offspring produced by triploids. Gates records
that, in one 21-chromosome plant, the two cells resulting
from the first maturation division contained "almost in-
variably" 10 and 11 chromosomes respectively and only
occasionally 9 and 12. Geerts found more numerous ir-
regularities. He describes 7 of the chromosomes going
regularly to each pole, while the remaining 7 that were
unpaired were irregularly distributed to the poles. This
account fits well with the view that 7 conjugate with 7,
leaving the remaining 7 without partners. Van Overeem
states that in Oenothera, when the triploid serves as the
mother plant, the results show that most of the ovules are

132

THE THEORY OF THE GENE

functional, regardless of the distribution of the unpaired
chromosomes, or, in other words, all or most of the pos-
sible different groups of egg-cells survive and may be
fertilized. The outcome is a varied assortment of forms
with many different combinations of chromosomes. On
the other hand, when the pollen of a triploid Oenothera

Fig. 80.

Triploid chromosome-group of the pollen mother cell of the Hya-
cinth. (After Belling.)

is used, the results show that only those carrying 7 or 14
chromosomes are functional. The pollen grains with
intermediate numbers are, for the most part, not func-
tional.

Triploid hyacinths have been found under cultivation
by de Mol. He states that they are replacing the older
types as a result of selection for commerce. Some of their
derivatives, with chromosome numbers varying around
the triploid, constitute a considerable part of modern cul-

TRIPLOIDS 133

tivated types. Since hyacinths are usually reproduced by
bulbs, any particular form can be perpetuated. De Mol
has studied the maturation of the germ-cells, both of the
normal and the triploid hyacinths (Fig. 80). The normal
diploid type has 8 long, 4 medium, and 4 short chromo-
somes. The haploid germ-cell contains 4 long, 2 medium,
and 2 short chromosomes. Both de Mol and Belling: have
pointed out that the "normal" may be already a tetra-
ploid, since in the reduced group there are two chromo-
somes of each size. If so, the so-called triploid may pos-
sibly be a double triploid, since it has 12 long, 6 medium,
and 6 small chromosomes.

ft r

S***

fl"

p^ m

i, M

yV,

vv?

s M

a b

Fig. 83.
a, Eedueed chromosome group of diploid Datura; b, reduced chro-
mosome group of triploid Datura. (After Belling and Blakeslee.)

Belling has also studied the maturation divisions of a
triploid variety of Canna. The chromosomes of each type
conjugate in threes. When the chromosomes separate two
of each triplet pass as a rule to one pole and one to the
other pole, but since the distribution for different triplets
is at random only rarely will a diploid and a haploid
sister cell result.

A triploid Datura has been reported by Blakeslee,
Belling, and Farnham. It arose from a tetraploid fertil-
ized by a normal. The normal diploid type has 24 chromo-

134 THE THEORY OF THE GENE

somes (n=12) (Fig. 81a). The triploid has 36 chromo-
somes (Fig. 81b). The haploid group is composed of 1
extra large (L), 4 large (1), 3 large medium (M), 2 small
medium (m), 1 small (S), and 1 extra small (s) chromo-
somes. The diploid group is therefore 2 (L+41+3M-J-
2m+lS+ls) and the triploid has three of each kind.

The maturation divisions have been studied by Belling
and Blakeslee. The reduced groups consist of 12 sets
of three each, united as in Fig. 81b. These trivalents have
the same size relations as have the bivalents in the dip-
loid group, i.e., they are formed by the union of like
chromosomes only, which are united in various ways as
seen in the figures. Two may be united at both ends and
the third joined on at one end only, etc.

At the first division two of each triplo-set pass*to one
pole and one to the other pole of the spindle (Fig. 75,
third column), and since the assortment takes place at
random in the different triplets several combinations of
chromosomes are realized. The numbers found in one
count of 84 pollen mother cells are recorded below in
Table I. The results are in close agreement with the ex-
pectation for random assortment.

table I

Assortment of Chromosomes in 84 Pollen Mother Cells of Triploid

Datura, 19729(1)

Metaphase of Second Division.

Assortment of Chromosomes

Nos. of double groups 1

Calculated on random orienta-
tion of trivalents 0.04 0.5 2.7 9.0 20.3 32.5 19.0

Rarely the first division of the triploid may be omitted.
This is favored by transient cold. At the second division

TRIPLOIDS

135

an equatorial division of the chromosomes takes place,
giving two giant cells with 36 chromosomes each.

As a rule very few functional pollen grains are formed
in the triploid, but apparently the egg-cells are more
often functional. For instance, when a triploid is polli-
nated by a normal plant, the number of normal offspring
(2n) produced is much beyond expectation on the as-
sumption that the chromosomes of the egg are freely
assorted.

Fig. 82.

a, Normal or diploid female, and b, triploid of Drosophila melano-

gaster.

Triploid Drosophilas have been found by Bridges
(Fig. 82). They are females because they have three X-
chromosomes balanced against three of each kind of auto-
some. This is the same balance that produces the normal
female. Since genetic factors in all the chromosomes are
known, it has been possible to study the behavior of the

136 THE THEORY OF THE GENE

chromosomes at maturation by means of the character-
distribution in the progeny. It has also been possible to
study the crossing-over, and to determine that the chro-
mosomes mate in threes.

In true triploid Drosophilas there are three sets of
ordinary chromosomes and three X-chromosomes also.
If, on the other hand, there are only two X-chromosomes
present the individual is an intersex. If only one X is
present the individual is a supermale. These relations
are as follows :

3a+3X=triploid female
3a+2X=intersex
3a+lX= supermale

In bisexual animals another triploid is known in an
embryonic stage. Females of the bivalens variety of the
threadworm Ascaris have been reported whose ripe eggs
with two chromosomes have been fertilized each by a
spermatozoon of a univalens variety with one chromo-
some. These eggs produce embryos with three chromo-
somes in each cell. Since the embryos escape before their
own germ-cells mature, the most significant feature of
their chromosome behavior, namely, union during con-
jugation, has not been observed, for as yet no adult trip-
loids of Ascaris have been reported.

Triploids have been produced by crossing diploid spe-
cies and back-crossing the hybrid (that has diploid germ-
cells owing to the failure of conjugation and reduction)
to one of the parental stocks. The experiment was carried
out by Federley with three species of moths with the fol-
lowing chromosome numbers.

Diploid Eaploid

Pygaera anachoreta 60 30

Pygaera curtula 58 29

Pygaera pigra 46 23

TRIPLOIDS 137

The hybrid between the first two species has 59 chromo-
somes (30+29). When the germ-cells of the hybrid
reaches the maturation stages no union takes place be-
tween the chromosomes. At the first maturation division,
each of the 59 chromosomes splits into daughter halves.
Each daughter cell receives this number. At the second
maturation division many irregularities occur. The chro-
mosomes split again, but the halves often fail to separate.
Nevertheless, the male is partially fertile and, as the
result shows, some of his germ-cells contain the full num-
ber (59) chromosomes. The Fx female is sterile.

If the F2 male is back-crossed to a female of one of the
parent species, to anachoreta, for example, whose ripe
eggs contain 30 chromosomes, the second hybrid has 89
chromosomes (59+30), and is therefore a hybrid trip-
loid. These F2 hybrids resemble closely the Fx hybrids.
They have two sets of anachoreta chromosomes and one
set of curtula chromosomes. They are, in a sense, per-
manent hybrids, although in each generation only half of
their chromosomes conjugate. For instance, in the ripen-
ing of the germ-cells of these 89 chromosome hybrids the
double set of anachoreta chromosomes (30+30) conju-
gates, the 29 curtula chromosomes remain single. The
former separate at the first division, the latter divide,
giving 59 to each cell. At the second division all 59 chro-
mosomes divide. The germ-cells contain, therefore, 59
chromosomes and are diploid. As long as back-crossing
continues it should be possible to produce triploid indi-
viduals. While under controlled conditions it might be
possible to maintain a triploid line in this way, it is not
probable, owing to the sterility of the offspring resulting
from irregularities in the spermatogenesis of the hybrid,
that under natural conditions a permanent triploid race
could be established.1

i The account in the text has been intentionally somewhat simplified. In

138 THE THEORY OF THE GENE

The embryonic development of triploid individuals is
expected to be normal because of the balanced condition
of the genes. The only inharmonious factor that may
enter into the situation is the relation between three sets
of chromosomes and the inherited quantity of cytoplasm.
How far auto-regulation takes place is not definitely
known, but it may be surmised that in plants at least the
cells of the triploid are larger than those of the normal
type.

Other triploid types that have arisen or have been pro-
duced by crossing wild species, one of which has twice as
manv chromosomes as the other, will be described in a
later chapter.

the Fx hybrid one or more of the chromosomes appear to conjugate at
times. Probably reduction follows for this pair, which would change by one
or more the actual number of chromosomes in the germ-cells of the F2
individuals.

CHAPTER X

HAPLOIDS

THE genetic evidence indicates that one complete
set of chromosomes at least is required for normal
development. A cell with one set of chromosomes
is said to be haploid, and an individual made up of such
cells is sometimes called a haplont or frequently, by ex-
tension, a haploid. The embryological evidence also indi-
cates that one set of chromosomes is necessary for de-
velopment. It does not follow, however, that the diploid
set can be replaced directly by a haploid set without seri-
ous consequences, so far as the developmental conditions
are involved.

Eggs that have been incited to develop by artificial
agents may develop into embryos whose cells have only
one set of chromosomes. Not infrequently, however, the
eggs double the number of the chromosomes (by sup-
pressing a protoplasmic division) before they begin to
develop, and these fare better than the haploids.

By cutting off a fragment from a sea urchin egg, and
fertilizing it with a single sperm, an embryo can be ob-
tained with only one set of chromosomes, the paternal
set. By constricting the egg of triton immediately after
fertilization, Spemann and later Baltzer have sometimes
been able to separate a piece of the egg that contains only
a single sperm-nucleus (Fig. 83), and one such embryo
was carried through by Baltzer to the time of metamor-
phosis.

If frogs' eggs are exposed to X-rays, or to radium for
a sufficient time to injure or to destroy the chromosomes,

140

THE THEORY OF THE GENE

and if, as Oscar and Grunther Hertwig have shown, these
eggs are then fertilized, they may produce embryos whose
cells have the half number of chromosomes. Conversely,
if the spermatozoa of the frog are radiated they may
enter the eggs, but may fail to take further part in the
development. Under these circumstances the egg may
develop, for a time, with a haploid set derived from the
egg nucleus. In some of these eggs, on the other hand,

Fig. 83.

Egg of Triton constricted in two, immediately after fertilization.

In the right half the polar body is shown. (After Spemann.)

the chromosomes of the egg may first divide without the
protoplasm dividing, and in this way the full number of.
chromosomes is restored before development begins.
These eggs produce embryos that develop into normal
tadpoles.

Most of the artificial haploid forms obtained in these
various ways are weak. They die, in most cases, long
before the adult stages are reached. It is not evident why
this should be true, but there are several possibilities
that may be taken into account. If a whole egg with a
haploid nucleus is incited by artificial means to partheno-
genetic development, and if, before differentiation sets

HAPLOIDS 141

in, it divides the same number of times as does the normal
egg, each of its cells will be in proportion to its chromo-
some number twice as large as the normal cells in propor-
tion to their chromosome number. In so far as the de-
velopment of the cell is dependent on its genes there may
be an insufficiency of gene material to produce a normal
effect on a cytoplasm of double volume.

On the other hand, if such an egg should pass through
one more division than does the normal egg before dif-
ferentiation (organ formation) begins, the number of
chromosomes (the nuclear size) would then be propor-
tionate to the cell size — there would be twice as manv
cells, and twice as many nuclei in the whole embryo as
in the normal. The embryo as a whole would then contain
the same total number of chromosomes as does the nor-
mal embrvo. How far the smaller size of the cells in such
a case might affect the developmental process we do not
know at present. Observation of the cell-size of haplonts
seems to show that the cells have the normal size and that
the nuclei are only half as large as the normal ones. It
appears, then, that the embryo does not rectify its nu-
clear cytoplasmic relation as just indicated.

It might be possible in another way to determine
whether the weakness of the artificial haplonts is due to
an insufficiency of genes for cells as large as normal ones.
Half of an egg, containing a single sperm nucleus, would,
if it passed through the number of divisions characteris-
tic of the normal egg, be made up of cells and nuclei hav-
ing the normal size-ratio to each other. Sea urchin em-
bryos of this kind have, in fact, long been known. They
become plutei that appear to be normal, but none have
been carried beyond the pluteus stage because, for one
reason, it is difficult to carry even normal embryos fur-
ther than this stage under artificial conditions. It is not
certain, therefore, whether these haplonts are as viable

142 THE THEORY OF THE GENE

as normal embryos. Boveri and others have studied ex-
tensively fragments of sea urchins' eggs, most of which
were probably smaller than half an egg. Boveri concluded
that these haplonts die, for the most part, before the
gastrulation stages or soon thereafter. It is possible that
these "fragments" never entirelv recover from the
operation, or that they do not contain all the essential
constituents of the cytoplasm.

A comparison of these embryos with those obtained by
isolating blastomeres of normal diploid eggs has certain
points of interest. It is possible by means of calcium-free
sea water to isolate the first two, or the first four, or the
first eight blastomeres of the segmenting egg of the sea
urchin. Here there is no operative injury, and each cell
has the double number of chromosomes. Nevertheless,
many of the y2 blastomeres develop abnormally, fewer
still of the 14 blastomeres produce plutei, and probably
none of the ys blastomeres pass beyond the gastrula
stage. This evidence shows that, aside from the number
of chromosomes and from the nucleo-plasma ratio, small
size in itself has a deleterious influence. What this may
mean is not known, but the surface relations to the vol-
ume vary with the size and may possibly enter into the
result.

These experiments do not hold out much promise of
obtaining normal vigorous haplonts by diminishing arti-
ficially the amount of the protoplasm of the eggs in spe-
cies already adjusted to the diploid condition. Neverthe-
less, under natural conditions there are several cases
known where haplonts exist, and there is one case re-
corded where a haplont of a diploid species has reached
maturity.

Blakeslee discovered a plant, in his cultures of Datura,
that was haploid, Fig. 84. With care it was kept alive and
by grafting upon diploid plants it has been maintained

HAPLOIDS

143

for several years. This plant resembles, in all essential
respects, the normal plant, except that it produces a very
small number of haploid pollen grains. These pollen

L i^^. ~->aC!y ^KJJ^E JSP *^^ ~~^ 0&

^u^fftB^

™ 4 '

K ' ' A

l)tZi3l3(Bfin){1

Fig. 84.
A haploid plant of Datura. (After Blakeslee.)

grains are the ones that have received one set of chro-
mosomes after a rather devastating attempt to pass
through the maturation stages.

Two haploid tobacco plants have been reported by
Clausen and Mann (1924) that appeared in a cross be-
tween Nicotiana Tabacum and A7, sylvestris. Each had 24

144-

THE THEORY OF THE GENE

chromosomes, which is the haploid number of the -tsba-
cum species. One of these haplonts was "a reduced
replica" of the "variety" of the Tabacum parent, but the
expression of the characters was somewhat exaggerated.

First polar body

Egg nucleus qz

Division of
first pol&r tody

8 Second polar body

Sperm (82)

Male82(l6)or Female 82(l6) + 82(16) =32

Fig. 85.
Diagram illustrating the two maturation divisions of the egg of
the honey bee. The fertilization of the egg by the sperm is indi-
cated in the lower part of the diagram with a subsequent doubling
of the chromosomes by breaking into two parts.

It was about three-fourths the height of the parent type ;
the leaves were smaller, the branches more slender, and
the flowers distinctly smaller. It was less vigorous than
the parent type; it bloomed profusely but produced no
seeds. Its pollen was completely defective. The other hap-
lont showed similar relations to the variety of Tabacum

HAPLOIDS 145

from which it was derived. The first maturation of the
pollen mother cells of these haplonts was irregular, few
or many of the chromosomes passing to the poles, the rest
remaining at the equator of the spindle. The second matu-
ration division was somewhat more regular, but lagging
chromosomes failed to reach either pole.

Nature seems to have been successful in producing a
few haplonts in species in which one sex is diploid. Male
bees, wasps, and ants are haplonts. The eggs of the queen
bee contain 16 chromosomes, which become 8 bivalents
after conjugation (Fig. 85). Two maturation divisions
take place, reducing the number to 8 chromosomes. If
an egg is fertilized it produces a female (queen or
worker) with the diploid number of chromosomes, but if
an egg is not fertilized it develops parthenogenetically
with the half number of chromosomes.

An examination of the nuclear and cell-size of the dif-
ferent tissues of the female and male bees (Boveri, Mela-
ling, Nachtsheim) has shown that, in general, there is no
constant difference between the cliplont and the hap-
lont. There is, however, a peculiar condition in the early
embryonic stages both of the female and male bee that
has somewhat complicated the situation. In the cells of
the embryo of the female, the chromosomes become twice
as numerous as at first, apparently by each chromosome
separating into two parts. In the cells of the embryo of
the male, the same process occurs, and is there repeated
even a second time, so that there appear to be 32 chromo-
somes present. The evidence seems to indicate that the
chromosomes do not actually increase in number but
''fragment." If this is the correct interpretation there is
no increase in the number of the genes. The female has
still twice the number of those in the male. What relation,
if any, this fragmentation may have to nuclear size is not
clear at present.

146

THE THEORY OF THE GENE

In the germ-track of the male and female the fragmen-
tation does not seem to take place, or if it does the pieces
rejoin before the maturation stage.

The best evidence that the male bee is a haplont, or at
least that its germ-cells are haploid, is found in the be-

d e f g

Fig. 86.
The two maturation divisions of the germ-cells of the male of the
honey bee. (After Meves.)

havior of the cells at the maturation divisions. The first
division is abortive (Fig. 86, a, b). An imperfect spindle
forms in connection with 8 chromosomes. A piece of the
protoplasm constricts off without chromatin. A second
spindle develops and the chromosomes divide (Fig. 86,
d-g), presumably by splitting lengthwise, and the daugh-

HAPLOIDS

147

ter halves pass to the poles. A small cell cuts off from
a large one. The latter becomes the functional sperm.
It has the haploid number of chromosomes.

The male of the rotifer, Hydatina senta, is a haplont
(Fig. 87c), and the females are diplonts. Under unfavor-

Fig. 87.
A, parthenogenetic female of Hydatina senta; B, young female
of same; C, male of same; D, parthenogenetic egg; E, male-pro-
ducing egg; F, winter egg. (After Whitney.)

148 THE THEORY OF THE GENE

able conditions of food, or when fed on the protozoon
Polytoma, only female rotifers occur. Each female is
diploid, and her eggs are at first diploid. Each egg gives
off only one polar body — each chromosome splitting into
like halves. The full number of chromosomes is retained
in the egg that develops by parthenogenesis into a female.
When fed on other food (Euglena, for example), a new-
type of female appears. If she is fertilized by a male at
the moment she emerges from the egg, she produces
sexual eggs only, which give off two polar bodies and
retain the haploid number of chromosomes. The sperm
nucleus, already within the egg, unites with the egg nu-
cleus to form a diploid female that starts once more a
parthenogenetic line. If, however, the special type of
female, just described, is not fertilized, she produces
smaller eggs. These eggs also give off two polar bodies
and retain the half number of chromosomes. They de-
velop by parthenogenesis into male haplonts. The male
is sexually mature a few hours after birth; he never
grows any larger and dies after a few days.

The males of the white "fly," Trialeurodes vaporari-
orum, have been shown by Schrader to be haplonts. It
had been discovered by A. W. Morrill that, in America,
virgin females of this fly give rise to male offspring only,
and later Back found this holds for another member of
the same family. On the other hand, in England, virgin
females of the same white fly give rise to females only,
according to Hargreaves and later to Williams. Schrader
has studied the chromosomes in the American form.
There are 22 chromosomes in the female and 11 in the
male. The mature eggs have 11 bivalent chromosomes.
Two polar bodies are given off, leaving 11 single chromo-
somes in the egg. If the egg is fertilized 11 chromosomes
are added by the sperm nucleus. If the egg is not fertil-
ized it develops by parthenogenesis with 11 chromosomes

HAPLOIDS 149

present in all cells of the embryo. In the maturation
stages of the germ-cells of the male, there is no evidence
of a reduction division (not even a rudimentary process
as in the bee) and the equational division, if it is present,
does not differ from the earlier or oogonial divisions.

There is some evidence that the unfertilized eggs of
lice develop into males, as suggested by the breeding
experiments of Hindle. In one of the mites, Tetranychus
bimaculatus, the unfertilized eggs produce males, the
fertilized, females (Perkins, H. A. Morgan, Bank, Ewing,
Parker). It has been shown by Schrader that the males
are haplonts with only three chromosomes, the females
are diplonts with six chromosomes. The early ovarian
eggs have six chromosomes that conjugate to give three
bivalents. Two polar bodies are given off, leaving three
chromosomes in the egg. If the egg is fertilized three
chromosomes are added, giving six in the female, if the
egg is not fertilized it develops directly into a male with
three chromosomes in each cell.

Virgin females of one species of thrips, Anthothrips
verbasci, examined by A. F. Shull, produce only males
from unfertilized eggs. These males are probably hap-
lonts.

In mosses and liverworts the protonema and moss
plant stage (gametophyte) are haplonts. Wettstein has
by artificial means brought about the doubling of the
number of chromosomes in cells of the protonema and
from these has obtained diploid protonema and moss
plants. This result proves that the difference between
this stage and the sporophyte stage is not due to the num-
ber of chromosomes that each contains but is a develop-
mental phenomenon in the sense that in order to reach
the sporophyte stage the spore must pass through the
gametophyte condition.

CHAPTER XI

POLYPLOID SERIES

IN recent years an ever increasing number of closely
related wild and of cultivated types have been re-
ported whose chromosome numbers are multiples of
a basal haploid number. The polyploid series run in
groups which suggest that members of the series with the
higher numbers have come from the lower members by a
continuous process of additions. Whether taxonomists
will decide to give such forms as are stable specific rank
is for them to decide.

It is probably significant that the polyploid series have
been found in several groups that were known as poly-
morphic groups that had bewildered taxonomists owing
to their variability and to their close resemblance to each
other, to their failure in many cases to breed true from
seeds, etc. x\ll this accords with the cytological findings.
In so far as the chromosome groups are balanced, the
genetic expectation is that these plants would be very
similar, except in so far as the increase in the size of the
cells may introduce physical factors that affect the struc-
ture of the plant, and except in so far as the increased
number of the genes may introduce chemical effects in
the cytoplasm.

The Polyploid Wheats.

In the small grains, wheat, oats, rye, and barley, multi-
ple chromosome groups have been found. The wheat
series has been most extensively studied and the hybrid
types produced by crossing them have been examined in a

POLYPLOID SERIES

151

number of cases. Of these, T. monococcuin, has the fewest
chromosomes, viz., 14 (n=7). It belongs to the Einkorn
group and can be traced back, according to Percival

'Aiv.

Fig. 88.

Eeduced number of chromosomes of diploid, tetraploid, and hexa-
ploid wheats. (After Kihara.)

152 THE THEORY OF THE GENE

(1921), to the Neolithic period in Europe. Another type,
the Emmer group, with 28 chromosomes, was grown in
Europe in prehistoric times, and in Egypt as early as
5400 b.c. It was later supplanted in the Graeco Roman
period by wheat with 28 chromosomes, and by one with
42 chromosomes of the Vulgare group (Fig. 88). The
number of varieties is greatest in the Emmer group, but
there are more different "forms" in the Vulgare group.
The chromosomes have been studied by several investi-
gators. The most recent work is that of Sakamura (1900)
and Kihara (1918, 1924) and Sax (1922). The follow-
ing account is taken largely from Kihara 's monograph
and to some extent also from Sax's papers. The next
table gives the observed diploid number of chromosomes
and the observed or estimated haploid number.

Haploid Diploid

Einkorn group, Triticum monococcum 7 14

Einkorn group, Triticum dicoccum 14 28

Einkorn group, Triticum polonicum 14 28

Emmer group, Triticum durum 14 28

Emmer group, Triticum turgidum 14 28

Vulgare group, Triticum Spelta 21 42

Vulgare group, Triticum compactum 21 42

Vulgare group, Triticum vulgare 21 42

The haploid groups are represented in Fig. 88a (mo-
nococcum), Fig. 88e (durum), and Fig. 88h (vulgare).

The normal maturation of a member of each of these
groups is shown in Fig. 89 from Sax. In the Einkorn
wheat the seven gemini (conjugated chromosomes) divide
at the first division, seven going to each pole. There are
no lagging chromosomes. At the second division of each
daughter cell the seven chromosomes split into daughter
halves. Seven go to each pole. In the Emmer type the 14
gemini divide at the first maturation stage. Fourteen
chromosomes go to each pole. At the second division each

POLYPLOID SERIES

153

chromosome splits, and 14 daughter chromosomes move
to each pole. In the Vulgare type the 21 gemini divide
at the first maturation division. Twenty-one go to each
pole. At the second division the daughter halves split and
21 move to each pole.

Hqploid

Em Korn

mmer

Vul

are

"&*

,•*

»%'/..
W

<&<

— ■

Fig. 89.

The first, or reduction, division of diploid, tetraploid, and hexa-

ploid wheats. (After Sax.)

This series of types may be interpreted as diploid,
tetraploid, and hexaploid. Each is balanced and each is
stable.

Crosses have been made between several of these types
with different chromosome numbers. Some of the com-
binations produce slightly fertile hybrids, others com-
pletely sterile ones. The behavior of the chromosomes in
several of the combinations, where different parental

154

THE THEORY OF THE GENE

numbers are involved, brings out some interesting rela-
tions. A few examples will serve as illustrations.

Kihara examined the hybrid produced by crosses be-
tween an Eminer with 28 chromosomes (n=14) and a
Vulgare type with 42 chromosomes (n=21). The hybrid

Fig. 90.
Reduction division of hybrid wheats. (After Kihara.)

has 35 chromosomes. It is therefore a pentaploid hybrid.
In the maturation stages (Fig. 90a-d) there are 14
gemini and 7 single chromosomes. The former divide, 14
going to each pole; the latter, the single chromosomes,
are irregularly scattered on the spindle, where they lag
for some time after the "reduced" chromosomes have
reached the poles (Fig. 90d). Later these single chromo-

POLYPLOID SERIES

155

somes split lengthwise, and the daughter chromosomes
move to the poles, not, however, with complete regu-
larity. When the distribution is equal there will be 21
chromosomes at each pole.

EMMER BY VULGARE

m3r

*^4

. s

•7 s

Fig. 91.

Eeduction division of the hybrid between Emmer and Vulgare

wheat. (After Sax.)

In passing it should be recorded that according to
Sax's results in a similar cross, the 7 single chromo-
somes do not divide at this time, but are distributed un-
equally to the poles, the more common distribution being
3 and 4 (Fig. 91).

At the second division, according to Kihara, 14 chro-
mosomes that are split lengthwise appear and 7 chromo-
somes that are not split. The former divide, 14 going to

156 THE THEORY OF THE GENE

each pole, while the 7 singles are distributed at random —
more often 3 going to one pole and 4 to the other. Accord-
ing to Sax, the 7 single as well as the 14 reduced chro-
mosomes split at the second division.

Whichever interpretation holds for the single chromo-
somes (and there are in other forms precedents for either
interpretation), one important fact is evident, viz., that
conjugation takes place only between 14 chromosomes.
Whether this union is between the 14 chromosomes de-
rived from the Emmer and 14 chromosomes derived from
the Vulgare, or whether the 14 chromosomes of the Emmer
unite to make 7 conjugants and 14 of the Vulgare unite to
make 7 conjugants, leaving one set of 7 over, is not clear
from the cytological evidence. A genetic study of these or
similar combinations (this one gives a sterile hybrid)
may furnish decisive evidence, but this is lacking at
present.

Kihara also crossed Einkorn, having 14 chromosomes
(n=7), with Emmer wheat, having 28 chromosomes (n=
14). The hybrid, having 21 chromosomes, is a triploid.
In the maturation of the germ-cells of the hybrid (pollen
mother cells) there is much more irregularity than in the
last case (Fig. 90e-k). The number of the conjugating
chromosomes is variable and their union, when it occurs,
is less complete. The number of the gemini varies as
shown in the next table.

Somatic nun

iber

Gemini

Singles

7 (Fig. 90e)

9 (Fig. 90b)

11 (Fig. 90g)

13 (Fig. 90h)

At the first division the components of the gemini sepa-
rate and pass to the poles. The splitting of the single
chromosomes does not alwavs take place before thev have

POLYPLOID SERIES 157

moved to one or the other pole ; some reach the poles un-
divided, others split and the halves move to the poles.
Not infrequently 7 single chromosomes are left in the
middle plane between the two polar groups (Fig. 90i).
Three counts are given in the following table :

Upper pole

Between the poles

Lower pole

7 (Fig. 90i)

8 (Fig. 90 j)

9 (Fig. 90k)

At the second division 11 or 12 chromosomes are, as a
rule, present; some are doubles (split lengthwise), others
singles. The former divide normally, the daughter chro-
mosomes going to one or the other pole; the singles are
distributed without division to one or the other pole.

From this evidence it is not possible to determine
which chromosomes conjugate in the hybrids. Since the
number of gemini does not exceed 7, these may be inter-
preted as the result of union of the 14 chromosomes of
the Emmer parent, or as the result of the union of 7 of
the Einkorn with 7 of the Emmer chromosomes.

In a few crosses between Emmer and Vulgare, fertile
hybrids have been obtained. Kihara has studied the chro-
mosomes in the maturation division of some of the F3, F4,
and later generations. The chromosome numbers in the
plants vary and there are irregularities in the distribu-
tion of some of them during maturation, leading to fur-
ther irregularities, or to the reestablishment of a stable
type like one of the original types, etc. These results,
important for the genetic study of the hybrids, are too
complex for our present purpose.

Kihara studied hybrids (one combination) between a
Vulgare wheat and a race of rye, the former having 42
chromosomes (n=21), the latter 14 chromosomes (n=
7). The hybrid (with 28 chromosomes) may be called a

158 THE THEORY OF THE GENE

tetraploid. This hybrid between these two widely differ-
ent species is, according to earlier observations, sterile,
but fertile according to other observers.

In the maturation stages of the germ-cells, few or even
no conjugating chromosomes were observed, as shown in
the next table :

Gemini

Singles

The distribution of the chromosomes to the poles is
very irregular; few if any of the singles divide before
reaching the poles ; some of them are left scattered in the
cell. In the second division many of the chromosomes
split, but those that divided in the first division lag and
pass slowly to the pole ; the number that lag is, however,
much less than in the first division.

The almost complete absence of conjugating chromo-
somes in the cross between wheat and rye is the most
interesting feature of the cross. The resulting irregu-
larity in the distribution of the chromosomes will prob-
ably account for the generally observed sterility of the
hybrid. There is a possibility that all the chromosomes
(or most of them) belonging to one species might, as a
rare event, pass to one pole. This might lead to the for-
mation of a functional pollen grain.

The Polyploid Roses.

Since the time of Linnaeus the classification of many
of the roses has baffled the skill of taxonomists. The re-
cent discoveries of a Swedish botanist, Tackholm, and of
three English botanists, Harrison and Blackburn in
collaboration, and Hurst, a rose expert and geneticist,

POLYPLOID SERIES 159

have shown that certain groups of roses, especially those
belonging to the family of canina rose, are polyploid
types. Their differences are not only due to polyploidy,
but combined with this there is evidence of extensive
hybridization.

*7V* C9 t(r>

diploid triploid teLraploid

jy&/t

hexaploid

pentaploid octoploid

Fig. 92.
Polyploid series of roses. (After Tackholm.)

Tackholm has recentlv made an elaborate studv of
these roses. His account may first be followed. The spe-
cies with 14 chromosomes (n=7) have the smallest num-
ber, and may be taken as the basal type. There are trip-
loids (3 times 7), tetraploids with 28 chromosomes (4
times 7), pentaploids (5 times 7), hexaploids with 42 (6
times 7), and octoploids with 56 (8 times 7). See Fig. 92.
In the maturation division of some of these polyploids

160 THE THEORY OF THE GENE

that are balanced, all the chromosomes are united in pairs
(gemini), while in those polyploids with odd numbers and
even in some of those with even numbers (taken to be hy-
brids) only 7 (or 14) gemini are present, the rest of the
chromosomes being single in the first maturation division.
In other words, when there are four, six, or eight chromo-
somes of each of seven kinds they conjugate in twos, as
though these types were diploid. Whatever their origin
may have been, the chromosomes never conjugate in
fours, sixes, or eights. In these polyploids, the conjugants
separate at the first maturation division, half going to
each pole. At the second division each chromosome di-
vides, and half of each goes to one or the other pole. The
germ-cells, whether pollen or ovules, thus come to con-
tain half the original number of chromosomes. Hence, if
they propagate sexually, the characteristic number is
maintained.

Another group of roses is regarded as hybrid by Tack-
holm, because the changes that take place in their germ-
cells show them to be unstable forms. Some of these have
21 chromosomes, hence are triploids. In the early matu-
ration stages of the pollen mother cells there are 7 biva-
lents (gemini) and 7 single chromosomes. At the first
division the 7 bivalents divide and 7 go to each pole;
the 7 single chromosomes do not divide and are distrib-
uted at random to the poles. Hence several combinations
are possible. The type is unstable in this respect. At the
second maturation division, all the single chromosomes
divide, whether they come from the earlier bivalents or
from single chromosomes. Many of the resulting cells
degenerate.

In other hybrids there are 28 chromosomes (4 times 7),
but these are not classified as true tetraploids by Tack-
holm, because the behavior of the chromosomes at the
time of conjugation indicates that there are not four of

POLYPLOID SERIES

161

each kind. Only 7 bivalents appear and 14 single chromo-
somes. At the first division the 7 bivalents split, the 14
singles do not divide and are distributed irregularly.

In other hybrids there are 35 chromosomes (7 times 5).
At maturation there are 7 bivalents and 21 single chro-
mosomes (Fig. 93). Both behave as in the last case.

a ~ c

Fig. 93.

First maturation division of a thirty-five chromosome heterotypic
rose. (After Tackholm.)

In a fourth type of hybrid there are 42 chromosomes
(7 times 6). At maturation there are again only 7 biva-
lents, and, here, 28 single chromosomes. The behavior of
the chromosomes at maturation is the same as before.

These four types of "hybrid roses" are classified
below in tabular form in regard to their pollen formation.

7 bivalent and 7 single chromosomes. Whole number 21
7 bivalent and 14 single chromosomes. Whole number 28
7 bivalent and 21 single chromosomes. Whole number 35
7 bivalent and 28 single chromosomes. Whole number 42

The unique behavior of these hybrids consists in the
conjugation of only 14 chromosomes to give the 7 biva-
lents. These chromosomes, we must suppose, are identi-

162

THE THEORY OF THE GENE

cal, or so nearly alike that they conjugate. It is not ob-
vious why the other sets do not conjugate, unless, as
Tackholm suggests, each set of 7 has come from a differ-
ent wild species by crossing. The additional chromosomes
arising in this way are sufficiently different from the
original set and from each other to interfere with conju-
gation.

Fig. 94.

Maturation division of egg-cell of rose. All the single chromo-
somes move to one pole where they are joined by half of the con-
jugants. (After Tackholm.)

Two other hybrid forms may be mentioned; in both
there are 14 bivalents and 7 single chromosomes. In these
there are twice as many conjugating chromosomes as in
the former hybrids.

In only a few hybrids of the canina group is the history
of the chromosomes in the embryo mother sac (where the
egg develops) described (Fig. 94). There are 7 bivalents

POLYPLOID SERIES 163

lying in the equator of the spindle, while all the single
chromosomes are collected at one pole. The bivalents
separate, half of each going to one pole, half to the other.
One of the resulting daughter nuclei contains 7 chromo-
somes (derived from the bivalents) and all of the 21
single chromosomes, while its sister cell contains only 7
chromosomes. The egg-cell is derived from the former
group. If the egg develops, as appears to be the case,
from the (7+21) chromosome cell, and is fertilized by a
sperm with 7 chromosomes (the other pollen grains as-
sumed to be non-functional), the fertilized egg will con-
tain 35 chromosomes, the original number of such a type.

The reproductive processes in these polyploid hybrid
roses has not been fully worked out. In so far as they
reproduce by stolons they will maintain whatever num-
ber of chromosomes may result from fertilization. Those
that form seeds by parthenogenesis may also maintain a
definite somatic number. It seems probable that, as a
result of the irregularities in the formation of the pollen
and egg-cells many different combinations may be estab-
lished. Without a knowledge of the chromosome interre-
lations of these types the hereditary processes would
have been very baffling. Even with this advance in our
knowledge there still remains a great deal to make clear
the composition of these hybrid roses.

Hurst, who has studied species of Rosa, both wild and
cultivated, thinks that the wild diploid species consist of
five primary groups that may be designated AA, BB, CC,
DD, EE, Fig. 95, a-d, e-h, i-1, m-p, q-t. Many combinations
of these five fundamental types are recognizable. Thus,
one tetraploid is designated BB, CC ; another, BB, DD ;
one hexaploid is AA, DD, EE ; another hexaploid is AA,
BB, EE ; an octoploid is BB, CC, DD, EE.

Hurst states that each member of the five primary
series has at least 50 diagnostic characters. These can be

Fig. 95.

The five types of canina roses, viz., a-d, e-h, i-1, rn-p, q-t. The char-
acteristics of each type are indicated in the same horizontal line
including flower, seed capsule, method of branching, spines, and
leaf insertion. (After Hurst.)

POLYPLOID SERIES 165

recognized in combinations in the hybrids. The environ-
mental conditions may alternately favor the expression
of one or the other set of characters. Hurst believes that
a classification of the species of the genus is possible on
the basis of these interrelations.

Other Polyploid Series.

In addition to the types that have just been described,
there are a number of other groups in which multiple
chromosome varieties and species have been reported.

The genus Hieracium is known to contain some species
that reproduce by sexual methods, and other species that
reproduce by parthenogenesis, even although stamens are
sometimes present in them that may contain some normal
pollen grains. Rosenberg has studied the development of
the pollen of several species that produce pollen. He has
also examined hybrids between different species. In the
latter he has studied the maturation divisions of the
pollen cells of the hybrid between H. auricula with 18
chromosomes (n=9) and H. aurantiacum with 36 (n=
18). In the hybrid there are 9 gemini and 9 single chro-
mosomes in the first maturation division but some excep-
tional cases are found, due perhaps to aberrant numbers
of chromosomes in the pollen of one of the parents, viz.,
H. aurantiacum. At the first division the gemini separate,
and most of the single chromosomes divide.

Rosenberg has also studied the maturation division of
Fx hybrids between two tetraploid or 36 chromosome
types, viz., H. pilosella and H. aurantiacum. The somatic
cells of the hybrid have 38 to 40 chromosomes. In two
cases 18 gemini were present and 4 single chromosomes.
In another cross between H. excellens, with 36 or 42
chromosomes (n=21), and H. aurantiacum, with 36 (n=
18), there were in one case 18 gemini. It is probable that
the H. excellens parent had 36 chromosomes. In another

166

THE THEORY OF THE GENE

similar cross, in which the pollen in Fa was largely abor-
tive, there were large numbers of gemini present and
many single chromosomes. Results similar to these were

# r

\W%

Fig. 96.

Maturation stages of several types of apogamous species of Hiera-

cium. (After Eosenberg.)

found in two other tetraploid crosses. In general, the
result with tetraploids shows that like chromosomes are
present in these different species that conjugate with
each other, or at least it seems more probable that the

POLYPLOID SERIES

167

S'emini are formed in this way rather than that thev are
formed bv the union of the like chromosomes within each
species group.

Rosenberg has also studied the maturation of the pol-
len in species of Archieracium, in which species both
sexual and parthenogenetic methods of reproduction
occur, the latter being the more common method. There
is no reduction division in the parthenogenetic types in

Fig. 97.
Types of chromosomes of eight varieties of chrysanthemums, each
having the reduced number of nine chromosomes. (After Tahara.)

the embryo sac, but the diploid number of chromosomes
is retained. The pollen development is much altered and
good pollen is seldom present. The reduction divisions in
the pollen mother cells are very irregular. Rosenberg has
described the maturation stages of several apogamous
species of Hieracium in which the pollen is scarcely ever
functional (Fig. 96). He interprets the changes as, in
part, due to their tetraploid origin (bivalent and single
chromosomes appear in most types) and in part due to a
progressive loss of all conjugation between the chromo-
somes, accompanied by a suppression of one of the matu-

Fig. 98.
Multiple chromosome groups of different varieties of chrysanthe-
mums; a, with 9; 6, with 9; c, with 18; d, with 21; e, with 36;
/, with 45 chromosomes. (After Tahara.)

Fig. 99.
Nuclei in the diakinetic stage of several varieties of chrysanthe-
mums, a and b with 18 chromosomes; c with 27; d with 36; e with
45; f with 45 chromosomes. (After Tahara.)

POLYPLOID SERIES 169

ration divisions. It is suggested that a comparable series
of changes may exist in the egg mother cells and lead to
the retention of all the chromosomes in the parthenoge-
netic egg-cells.

In the cultivated varieties of chrysanthemum, Tahara
has found a polyploid series. In ten varieties (Fig. 97)
nine haploid chromosomes are present, but the chromo-
somes themselves have different sizes, and, more impor-
tant still, the relative size of the chromosomes may be
different in different species (Fig. 98). This point will be
considered later. It is also significant that the nuclear
size may be different in some of these cases where the
total number of chromosomes is the same. Other species
of chrysanthemum have multiples of nine (Fig. 99) ; two
species have 18, two have 27, one has 36, two have 45. The
following table gives the relation between chromosome
number and nuclear size.

Chromosome

Nuclear

Name

number

diameter

Radius3

Ch. lavanduloefolium

5.1

17.6

Ch. roseum

5.4

19.7

Ch. japonicum

6.0

29.0

Ch. nipponicum

6.0

27.0

Ch. coronarium

7.0

43.1

Ch. carinatum

7.0

43.1

Ch. Leucanthemum

7.3

50.7

Ch. morifolium

7.8

57.3

Ch. Decaisneanum

8.8

85.4

Ch. arcticum

9.9

125.0

Triploid varieties of the mulberry (Morus) have been
reported by Osawa. Of the 85 varieties studied, 40 are
triploids. The diploid number of chromosomes is 28 (n=
14) and the triploid 42 (3X14). The diploid plants are
fertile, while the maturation divisions of the triploid
show irregularities (univalent chromosomes) and have
abortive pollen grains and embryo sacs. In the first matu-

170 THE THEORY OF THE GENE

ration division of the triploid, both in the pollen and in
the megaspore mother cell, there are 28 bivalents and 14
univalents. The latter pass to the poles at random. They
all divide at the second division.

In the maples (Acer) there appears to be a possible
polyploid species. Taylor reported two species with 26
(n=13), two with 52 (n=26), and others with approxi-
mately 144 (n=72), or 108 (n=54), or 72 (n=36). Other
species with different numbers were also found.

In the sugar cane (Saccharum) Tischler found races
with the haploid numbers 8, 16, and 24 (bivalent) chro-
mosomes. Bremer reports about 40 haploid chromosomes
in another variety and 56 in a third. Other numbers have
also been reported. Some of the combinations may be due
to hybridization, but little is known at present to what
extent the observed differences in number have arisen in
this wav. Bremer has also studied maturation divisions
of a few hybrids.

In the genus Carex, Heilborn states that the chromo-
some numbers are quite different and that no apparent
polyploid series exists in this genus. "It is of importance
now to define somewhat more clearly the meaning of the
word polyploid. It appears from the list of chromosome
numbers in Chap. II that there are several numbers that
constitute, apparently, a series of multiples with 3 as the
fundamental number (9, 15, 24, 27, 33, 36, and 42), others,
again, that form a series with 4 as fundamental number
(16, 24, 28, 32, 36, 40, and 56), others with 7 (28, 35, 42,
and 56) and so on, but, according to the author's opinion,
these merely arithmetical relations cannot be regarded
as cases of polyploidy. The chromosome group of a poly-
ploid species must necessarily contain a certain number
of complete haploid chromosome sets and it must have
arisen through addition of such sets. We know, however,
that, for instance, C. pilulifera does not contain 3 sets of

POLYPLOID SERIES 171

3 chromosomes, but 3 long, 4 medium, and 2 short chro-
mosomes; that C. ericetorum does not contain 5 such
sets, but 1 medium and 14 short chromosomes, and that,
consequently, the chromosome groups in these two spe-
cies have not arisen through an addition of such sets but
in some other way." More problematical polyploid series
are reported in Rumex, Papaver, Callitriche, Viola, Cam-
panula, Lactuca. Two numbers, one of which is double or
triple the other, have been found in Plantago (6, 12),
Atriplex (9, 18), Drosera (10, 20), Platanthera (21, 63).
It has also been recently reported by Longley that haw-
thorns and raspberries, known to be complex polymor-
phic species, show extensive polyploidy.

CHAPTER XII
HETEROPLOIDS

IRREGULARITIES in the division or the separa-
tion of the chromosomes occasionally cause a single
chromosome to be added to the group. Conversely,
one may be lost from the group. In so far as the addition
of one or more chromosomes to, or loss from, a given
group produces a new number, the word heteroploid has
been used. Another word, trisomic, has also been used
for cases where three of one kind are present (in contrast
to triploid, where there are three of each kind present)
and the word triplo combined with the name of the par-
ticular chromosome in triplicate has also been used, as
triplo-IV in Drosophila. Still earlier, an extra chromo-
some was called a supernumerary or m-chromosome, etc.
The loss of one member of a pair is designated by the
term haplo- combined with the name of the particular
chromosome, as in the haplo-IV type in Drosophila.

Certain mutant types of Oenothera have been found to
be associated with the addition of a fifteenth chromosome.

Normally Lamarck's evening primrose has 14 chromo-
somes. Certain mutant types, known as lata and semi-
lata, have 15 chromosomes, i.e., one additional chromo-
some (Fig. 100). The lata plants differ from Lamarckiana
in many small details, although most of the differences
are so slight that only an expert would notice them. Ac-
cording to Gates, one of the lata mutants is almost com-
pletely male-sterile, and its production of seed is also
greatly reduced. In one of the semi-lata types some good
pollen is produced.

HETEROPLOIDS

173

The frequency of occurrence of lata types varies in dif-
ferent progenies from 0.1 to 1.8 per cent, according to
Gates.

At the maturation of the pollen of the 15 chromosome
types, 8 chromosomes are present. Seven are in pairs

Fig. 100.
Oenothera lata. (After Anne Lutz.)

and 1 is unpaired. The conjugants separate and pass to
opposite poles at the first maturation division. The un-
paired chromosome does not divide at this time, but
passes intact to one or to the other pole. Other irregu-
larities in the maturation divisions occur in some cases,
but whether or not they are caused by the extra chromo-
some is unknown, although Gates states that these irregu-

174 THE THEORY OF THE GENE

larities are much more frequent in triplo-typic individ-
uals than in normals.

From the 15 chromosome types two kinds of germ-cells
are expected, one with 8, one with 7 chromosomes. It has
been shown that these two kinds are produced. From a
genetic standpoint the lata type, crossed to a normal
type, should produce equal numbers of lata (8+7) and
normal (7+7) offspring. This is approximately what
happens.

The most interesting question concerning these triplo-
types relates to the particular chromosome that becomes
the supernumerary. Since there are seven kinds of chro-
mosomes, we may anticipate that any one may appear in
triplicate. De Vries has recently suggested that there are
seven trisomic types in Oenothera, corresponding to the
seven possible supernumeraries.

It is also important to bear in mind that types with two
supernumeraries (either like or unlike), the tetrasomic
types, may not be as viable as trisomic types. It is known
that such types occur. For instance, amongst the off-
spring of a triplo-type there seems to be a good chance
for the formation of an individual with two like super-
numeraries when an 8-chromosome pollen grain fertilizes
an 8-chromosome egg. This would give a tetra-type or
tetrasomic group for one particular chromosome. It
would be a stable type to the extent that 8 paired chro-
mosomes are present in each germ-cell, but it might be
even more unbalanced than a triplo-type with only one
extra chromosome. Sixteen-chromosome types have been
recorded, some of which are probably multiples of the
same chromosome when they are derived from a 15 triplo-
type, but their relative viability is not recorded.

It seems, a priori, possible that duplication of any pair
of chromosomes may be brought about through a triplo-
type giving rise to a tetratypic individual. But even if

HETEROPLOIDS 175

stability should be attained, the more important factor
of gene balancing may make it improbable that a per-
manent increase in the chromosome pairs could be estab-
lished in this way. When a large chromosome number is
present the initial stages of unbalancing might be slight
as compared with forms having fewer chromosomes, be-
cause in the former the ratio would be less disturbed.

In Drosophila Bridges found a triplo-type for the
small IV-chromosome, and since three genetic factors
are present in this small chromosome it has also been
possible to study not only the characters that are affected
by the presence of an additional IV-chromosome, but the
bearing of this condition on genetic questions in general.
On the other hand, it has been found that an individual
with three X-chromosomes usually dies, and that indi-
viduals with either chromosome-II or -III in triplicate do
not live.

The triplo-IV Drosophila is not strikingly different
from the normal, and the two can be distinguished only
with difficulty. The general color of the body is a little
darker and the trident marking on the thorax is absent
(Fig. 32) ; the eyes are somewhat smaller and have a
smooth surface; the wings are narrower and more
pointed than those of the wild type. That these slight
effects are due to the presence of an additional small
chromosome was shown both by a cytological demonstra-
tion of its presence (Fig. 32) and by genetic tests. When
a triplo-IV is crossed to eyeless (eyeless is a IV-chromo-
some recessive mutant type) some of the offspring (Fx)
can be distinguished by the characters given above as
triplo-IV flies. If these are back-crossed to eyeless (Fig.
33), flies with full eyes and flies with " eyeless eyes" are
produced approximately in the ratio of 5 to 1. As shown
in Fig. 12 this result agrees with expectation provided
that one normal gene is dominant to two eyeless genes.

176 THE THEORY OF THE GENE

When two triplo-IV flies (obtained in the way de-
scribed above) that have two ordinary IV-chromosomes
and another IV-chromosome carrying eyeless, are mated,
they give approximately 26 full-eyed flies to one eyeless.

From this cross some flies might be expected that con-
tained four chromosome-IVs, since half of the eggs and
half of the sperm-cells carry two of these chromosomes.
If such tetra-typic flies developed, the expected ratio
would be 35 full-eyed to one eyeless. The ratio found (26
to 1) instead of the expected ratio (on the assumption
that the tetra-typic flies come through) is due to the death
of the tetra-types. In fact, no flies of this composition
have been detected, which means that, despite the small-
ness of these chromosomes, the presence of four of them
upsets the balance of the genes to such an extent that
such an individual does not develop into an adult.

In contrast to these triplo-types of Drosophila there is
another heteroploid type, the haplo-IV type (Fig. 29), in
which one of the small chromosomes is absent. This type
has appeared very often, which is interpreted to mean
that one of these small chromosomes is sometimes lost in
the germ track — possibly as a result of two passing to
one pole at the reduction division. The haplo-IV has a
paler body color but a more marked trident on the thorax,
rather large eyes with a rough surface, slender bristles,
and somewhat shortened wings, and the aristae are re-
duced or even absent. In all these respects its characters
are the opposite of those of the triplo-type. This is not at
all surprising if the IV-chromosome contains genes that
affect many parts of the body in conjunction with other
genes. These effects are increased by the presence of an
additional chromosome and diminished when one is ab-
sent. The haplo-IV 's emerge four or five days later than
the normals; they are often sterile and generally poor
producers ; their mortality is very high. There is abun-

HETEROPLOIDS 177

dant cytological and genetic evidence that these flies owe
their peculiarities to the absence of one chromosome.

Flies lacking both IV-chromosomes have not been
found and the ratio obtained when two haplo-IV's are
bred together (giving 130 haplo-IV's to 100 normals)
shows that the nullo-IV's die.

If a diploid fly that is eyeless is mated to a haplo-IV
fly carrying wild type genes in its single chromosome-IV,
some of the 1\ offspring will be eyeless and these will be
haplo-IV. Theoretically, half of the offspring should be
eyeless, but the presence of the eyeless gene in the single
fourth chromosome lowers the viability of the haploid
98 per cent of expectation, and this relation holds when
the other recessive mutant types (bent and shaven) are
present in the single IV-chromosome. According to
Bridges, bent lowers survival by 95 per cent and shaven,
100 per cent, i.e., haplo-shaven does not develop.

The Jimson weed, Datura stramonium, has 24 chromo-
somes. A number of types under cultivation have been
detected by Blakeslee and Belling with 25 chromosomes
(2n+l). It is probable that there are 12 such types, each
of which has a different extra chromosome. The slight
but constant differences shown by these 12 triplo-types
(2n+l) involve all parts of the plant. These differences
are well shown in the capsules (Fig. 101). In two of these,
at least (triplo-globe and triplo-poinsettia), in which Men-
delian factors are present in the extra-chromosome
group, it has been shown by Blakeslee, Avery, Farnham,
and Belling, that the twenty-fifth chromosome involved is
a different one in the two cases. In one of these in particu-
lar, namely, the trisomic type poinsettia, involving a chro-
mosome that carries the gene- for purple stem pigment
and white flower color, the effects on the inheritance due
to one extra chromosome have given the clearest results.
These show that those germ-cells carrying the extra chro-

Norma/

(''air Poin>,/f,v Cocklebvr J /r ,

ff/i,/,us f?o//pJ Reduced BarM/uy

6&>*sy HkroearpA C/ongafe Sa,na<h

Fig. 101.

The original type or seed-capsule of Datura stramonium, and the

twelve probable trisomic types. (After Blakeslee.)

HETEROPLOIDS

179

mosome are less viable than the normal, hence deficiencies
in certain expected classes occur; in fact, these germ-
cells (n+1) are not transmitted at all through the pollen

Diploid

(2-.)

Ftg. 102.

Normal or diploid type of capsule of Datura (2u) as contrasted
(below) with 2n-j-l and 2n-f2 types of capsule. (After Blakeslee.)

(or only to a slight extent), and through only about 30
per cent of the eggs. When these relations are allowed
for, the genetic results agree with expectation.

180

THE THEORY OF THE GENE

Iii their study of the trisomic types of Datura, Blakes-
lee aud Belling have found about 12 distinct types be-
lono-ino: to the 2n+l or trisomic series. Since there are
just 12 pairs of chromosomes, only 12 simple trisomic
types are expected, and, in fact, evidence has been found
that there are only 12 such primary types. The rest,
called secondaries, appear to belong to one or another of
the 12 primary types (Fig. 102). The evidence for this
comes from several sources, from similarities in external
appearance, from internal structures (as shown by Sin-
nott), from their similar mode of inheritance (giving the
same trisomic inheritance for marked chromosomes),
from the reciprocal throwing of one member of the group
by the other, and from the sizes of the extra chromosomes
(Belling).

In the following table a list of the primaries and their
secondaries is given. These have been derived from trip-
loids.

Primary and
(Primaries

Secondary (2n+l) Types in Offspring from Triploids
are printed in capitals, secondaries in lower case type.)

3n x

3n X

sn x

3n X

SELF

TOTAL

SELF

TOTAL

GLOBE

8. BUCKLING
Strawberrv

•■>

POIN-
SETTIA

Maple

Wiry

9. GLOSSY

COCKLE-

10. MICRO-

BUR

CARPIC

Wedge

11. ELONGATE

ILEX

Undulate

ECHINUS
Mutilated
Nubbin (?)

(2?)

IS
(t)

12. SPINACH(?)

Totals (2n + 1)

381

424

ROLLED

Sugarloaf

(2u + 1 + 1)

11
30

101

215

112

248

Polycarpic

• • •

REDUCED

Grand Totals

697

7S4

HETEROPLOIDS

181

The spontaneous occurrence of primaries and second-
aries is given in the next table. The primaries arise in
this way more frequently than the secondaries. Breed-
ing experiments have shown that whereas primaries may
occasionally throw secondaries, the secondaries regularly
throw their primaries more frequently than they throw
new mutants belonging to the other groups. Thus of
31,000 offspring from poinsettias about 28 per cent were
poinsettia and about 0.25 per cent were the secondary
wiry. Conversely, when wirys were the parents about
0.75 per cent of the offspring were the primary poin-
settia.

Spontaneous Occurrence of Primary and Secondary (2n-)-l) Mutants
(Primaries are printed in capitals, secondaries in lower-case type.)

FROM

PARENTS

O "J B 5

TOTALS

FROM

PARENTS

FROM UN-
RELATED
(211+ 1)
PARENTS

1. GLOBE

2. POIN-

SETTIA
Wiry

107

148

75
1

8. BUCKLING
Strawberry
Maple

9. GLOSSY

98
2
2

3. COCKLE-

BUR

Wedge

4. ILEX

7
19

17
27

24
46

10. MICRO-

CARPIC

11. ELONGATE
Undulate

100

164

2
1

5. ECHINUS
Mutilated

10
2

6
1

12. SPINACH(?)

Nubbin(?)

Totals (2n -f 1)

269

506

775

6. ROLLED

Sugarloaf
Polycarpic

24
3
3

47
9

Related (2n -f 1)
types
2n

Grand totals

32J523

22,123

70,281

22,123
102,804

7. REDUCED

32,792

92,910

125,027

The breeding experiments of "Wedge — a Secondary of
the Cocklebur group — furnishes the following evidence
as to the relation of secondaries to primaries. "Both
Poinsettia and its Secondary Wiry give trisomic ratios
for the color factors P, p, but give disomic ratios for

182 THE THEORY OF THE GENE

spine factors A, a, indicating that both Poinsettia and
Wiry have their extra chromosomes in the set carrying
the factors P, p, but not in the set with the factors A, a.
Similarly, the ratios for Cocklebur indicate that this Pri-
mary has its extra chromosome in the set carrying the
factors A, a, but not in the set with factors P, p. Its
Secondary Wedge, however, fails to give trisomic ratios
for A, a. The ratios actually found resemble those in
disomic rather than in trisomic inheritance and seem to
indicate a deficiency in the extra chromosome of Wedge
for the locus A, a, since the evidence strongly indicates
that it is a Secondary of Cocklebur. If A' indicates the
modified chromosome and A and a go to opposite poles
at reduction division in a Wedge plant with the formula
AA'a, the gametes would be A-f-a+AA'+aA'. Such be-
havior would account for the ratios [in table 5]. If A' is
deficient for the factor A, the gamete aA' would carry no
factor for A ; hence the disomic ratios between armed and
inermis Wedges found but not shown in the table. If A
and a, occasionally should go to the same pole, the
gametes would be A' (which would probably die) and Aa,
which would go to form a Primary Cocklebur occasion-
ally thrown by Wedges.

"The hypothesis of a deficiency in the extra chromo-
some of Secondaries has been strengthened by Dr. Bell-
ing's cytological findings. His hypothesis of reversed
crossing-over, however, completes the picture by indicat-
ing a doubling of a part of the chromosome along with a
deficiency of the remaining portion."

Tetraploids of Datura with an additional chromosome
have also been reported (Fig. 103). In one of these shown
in the figure there are five like chromosomes in one group,
and in the other there are six like chromosomes.

Belling and Blakeslee have studied the modes of union
of the three chromosomes in the primary and in the sec-

HETEROPLOIDS

183

ondary trisomic types of Datura, and have found certain
differences that offer a suggestion as to the relation of
these two types. In the upper row of Fig. 104 the differ-

Fig. 103.
Tetraploid capsule above, and below 4n-f-l, 4n-(-2, and 4n-f-3 cap-
sules. (After Blakeslee.)

ent ways in which the three chromosomes of the primary
type are united are shown. The numbers below each
show the frequency of the type. Of these types the triva-

184 THE THEORY OF THE GENE

lent V is the most common form of union (48) ; next in
frequency is the ring-and-rod type (33) ; then the Y (17) ;
the straight chain (9) ; the ring (1) ; the double ring (1) ;
the ring of two with the third member left over (9-f-).

From ten of the primary 25-chromosome forms

O 8 6 I

48 33 17 9 1 1 9+

From ei§ht of the secondary 25-chrom. forms

O0o

26 13 1 3 2 51 Z0+

Fig. 104.

Methods of union of the three chromosomes of a trisomic type of

Datura. (After Belling and Blakeslee.)

Since chromosomes are supposed to conjugate by like
ends coming together it is reasonable to assume that, in
these types, like ends (a and a, Z and Z) are still in con-
tact (see Fig. 104, upper row).

In the lower row of Fig. 104 the different ways in which
the three chromosomes of the secondary types are united
are shown. In general the types are the same as those of

HETEROPLOIDS

185

the primaries, but the frequencies are different. The
most noticeable features are seen in the last two types (to
the right). One of these is an elongated ring of three
chromosomes, the other is a ring of two chromosomes and
a small single-ring chromosome. These two types suggest

A Z

Z A

Fig. 105.

Diagram illustrating possible conjugation of two chromosomes,
turned in opposite directions.

that, in some way, the end of one chromosome has been
changed. Belling and Blakeslee offer the following provi-
sional suggestions as to how such a change may have been
brought about at a preceding stage in the triploid parent
or in a trivalent of a primary type. Suppose, for example,
two chromosomes should come to lie side by side in re-
versed position as shown in Fig. 105, and suppose they
should cross over in the middle, which is the only level

186

THE THEORY OF THE GENE

at which like genes come together. The result will give
two chromosomes each having its two ends alike, i.e., one
has A and A at its ends, the other Z and Z. If now such a
chromosome becomes in the next generation a member

Primary 2n+l plants

Secondary 2n + l plants

A A

Fig. 106.

Diagram illustrating possible types of conjugation of three chro-
mosomes of trisomic types. (After Belling and Blakeslee.)

of a trivalent group, it is possible to construct such modes
of union as indicated in Fig. 106, where in a Z-Z chromo-
some, combined with two normal partners, like ends meet
each other.

If these rings, peculiar to the secondaries, can be ac-
counted for in the way suggested, it follows that one of

HETEROPLOIDS 187

the trivalent chromosomes differs from the other two by
a duplicated half. Hence the secondary has a different
gene combination from the primary.

Kuwada reports 20 chromosomes (n=10) for corn
(Zea mays), but certain sugar corns were found to have
21, 22, aud even 23 or 24 chromosomes. Kuwada suggests
that corn is a hybrid, one of whose parents was the Mexi-
can teosinte (Euchlaena). One of the corn chromosomes
that is longer than its mate was derived from teosinte,
he thinks, and its mate from some unknown species. The
longer one sometimes breaks into two pieces, which ac-
counts for the additional chromosomes found in sugar
corns. If this interpretation is verified (it has recently
been questioned), these 21, 22, and 23 chromosome types
are not strictly trisomic.

De Vries' conclusions relating to the extra chromosome
types of Oenothera Lamarckiana had an important bear-
ing on his interpretation of the origin of progressive
mutation, hence on his interpretation of the relation of
mutation to evolution. The numerous small changes in the
characters of the individual frequently observed in triso-
mic types fulfill de Vries' early definition as to what
constitutes an elementary species, causing at a stroke,
as it were, the appearance of two elementary species.

It should be observed that when a»mutational effect is
produced by the addition of a whole chromosome the re-
sult involves, so far as the germ material is concerned,
an enormous alteration in the actual number of the he-
reditary units. This change is scarcely compatible with
the comparison to a change in a single chemical molecule.
Only by treating the chromosomes as a unit could such a
comparison have any weight. The constitution of the
chromosomes, from the viewpoint of their genes, is hardly
consistent with such a comparison.

The chief interest in these heteroploids, as I interpret

188 THE THEORY OF THE GENE

them, lies in the explanation they offer of a peculiar and
interesting genetic situation arising from the occasional
erratic behavior of the mechanism that is involved in the
processes of cell division and maturation. Unstable forms
are produced, that, in so far as they maintain themselves,
do so by remaining unstable, i.e., with an extra chromo-
some. In this respect they differ obviously from normal

central

lata / m V \ scintillans

>allescens \ m m cana

liquida \ ^ 0 / spathulata

Fig. 107.
Diagram illustrating de Vries' idea of the relation between the
seven chromosomes of O. Lamarckiana and types of trisomic mu-
tants.

types and species. Furthermore, most of the evidence
indicates that these heteroploids are not so viable as the
balanced types from which they arise, hence would rarely
be able to replace them or act as substitutes in a different
environment.

Nevertheless, the occurrence of heteroploidy must be
regarded as a significant genetic event whose explanation
promises to clear up many situations that would be very
puzzling without the information which a study of their
chromosomes has revealed.

HETEROPLOIDS 189

De Vries identifies six trisomic mutant types, and a
seventh one, also, that differs genetically more strikingly
from the other six than they do from each other. These
seven trisomic types may, he suggests, correspond to the
seven chromosomes of the evening primrose. A list of six
of them is given below. A diagram of the corresponding
chromosome groups is given in Fig. 107.

15-chromosome mutants.

1. Lata group.

a. Semi-lata.

b. Sesquiplex mutants: albida, flava, delata.

c. Subovata, sublinearis.

2. Scintillans group.

a. Sesquiplex mutants: oblonga, aurita, auricula,

nitens, distans.

b. Diluta, militaris, venusta.

3. Cana group : candicans.

■4. Pallescens group : lactuca.

5. Liquida.

6. Spathulata.

This list of six 15-chromosome primary mutants in-
cludes some secondary mutant types arranged under
their primaries. Their interrelations are shown not only
by similarities in characters, but also by the frequency
with which one throws the other. Two of them, albida and
oblonga, have two kinds of eggs but only one kind of pol-
len, and are called one-and-one-half or sesquiplex mu-
tants. Another secondary, candicans, is also a sesquiplex
type. The central or largest ' ' chromosome ' ' of the group
(Fig 107) carries the "factors" for velutina or for those
of laeta. De Vries assigns to them also, from evidence
found by Skull, the new mutants funif&lia and perrivens.
It may seem probable, therefore, following Shull, that

190 THE THEORY OF THE GENE

the factors for five other mutant types1 of Lamarckiana
belong in this group, as well as the lethal factors that
put these factors in a balanced lethal condition. Accord-
ing to Shull the appearance of these recessive characters
is due to crossing-over between the members of a pair of
chromosomes here identified provisionally as the large
central chromosome.2

i Kubricalyx buds, and its allelomorph red stem (intensifier), nanella
(dwarf), pink-coned buds, sulfur colored flowers, revolute leaves.

2 Emerson has recently pointed out that the evidence so far published by
Shull does not necessarily prove his interpretation of the balanced lethal
relation.

CHAPTER XIII

SPECIES CROSSING AND CHANGES IN
CHROMOSOME NUMBER

SOME interesting relations have come to light as a
result of crossing species having different chromo-
some numbers. One species may have exactly twice
or three times as many chromosomes as the other; in
other cases, the larger chromosome group may not be a
multiple of the other.

»* * a b

Fig. 108.

Diploid and haploid groups of the sundew, Drosera rotundifolia.

(After Rosenberg.)

The classic case is that of the cross between two species
of sundew by Eosenberg in 1903-1904.

One species of sundew, Drosera longifolia, has 40 chro-
mosomes (n=20), another species, rotundifolia, has 20
chromosomes (n=10) (Fig. 108). The hybrid has 30
chromosomes (20+10). In the maturation of the germ-
cells of the hybrid, there are 10 conjugating chromo-
somes, often called gemini or bivalents, and 10 singles
(univalents). Rosenberg interpreted this condition to
mean that 10 of the longifolia unite with 10 of the rotun-
difolia leaving 10 of the former without a mate. At the
first maturation division of the germ-cell, the conjugants

192 THE THEORY OF THE GENE

separate, the members going to opposite poles; the 10
single chromosomes are distributed irregularly, without
division, to the daughter cells. Unfortunately the hybrid
is sterile, and cannot be used for further genetic work.

The cross between two species of tobacco, Nicotiana
Tabacum and N. sylvestris, has been extensively studied
by Goodspeed and Clausen. Only recently, however, has
the chromosome number been determined; Tabacum has
24 (n=12) and sylvestris 48 (n=24) chromosomes. This
difference in chromosome number has not as yet been
correlated with the genetic results ; and the behavior of
the chromosomes in the maturation divisions has not
been reported.

The hybrid from crossing these two species resembles
in every particular the Tabacum parent, even when that
parent is pure for factors that behave as recessives
toward the normal factors of the type Tabacum {i.e., in
crosses with varieties or races of Tabacum). Goodspeed
and Clausen interpret this result to mean that the Taba-
cum genes dominate as a group the sylvestris genes. They
have expressed this by saying that the ' ' reaction system ' '
of Tabacum dominates the embryological processes of the
hybrid; or "the elements of the two systems must be
largely mutually incompatable. "

The hybrids are highly sterile, but a few functional
ovules are formed. As the breeding results show, these
functional ovules are exclusively (or predominately)
either pure Tabacum or pure sylvestris. It may seem,
therefore, that in the hybrid only those (or largely only
those) ovules that contain a complete set (or nearly com-
plete set) of one or the other group of chromosomes are
functional. This view is based on the following experi-
ments.

When the hybrid is fertilized with the pollen of sylves-
tris, a variety of forms is produced, among which there

SPECIES CROSSING 193

is a considerable proportion of plants that are pure syl-
vestris in all their characters. These plants are fertile
and breed true to sylvestris. They must be supposed to
have come from ovules with a sylvestris chromosome
group, fertilized by sylvestris pollen. There are also
plants that resemble sylvestris, but contain other ele-
ments, probably derived from the Tabacum group of
chromosomes. They are sterile.

Back-crossing to Tabacum was unsuccessful, but a few
hybrids have appeared in the field from open pollination
that are like Tabacum and have undoubtedly come from
Tabacum pollen. Some of them are fertile. Their descend-
ants never show sylvestris characters. They exhibit segre-
gation for whatever Tabacum genes were present. There
are also sterile forms in the series, and these resemble the
F1 hybrids between Tabacum and sylvestris.

These remarkable results are important in another re-
spect. The Fx hybrid may be obtained both ways; i.e.,
either species may be the ovule parent. It follows that
even with a sylvestris protoplasm the Tabacum group of
genes completely determines the character of the indi-
vidual. This is strong evidence in favor of the influence
of the scenes in the determination of the character of the
individual, since this result is obtained when the proto-
plasm belongs to a widely different species.

The idea of a reaction system, proposed by Clausen
and Goodspeed, while novel, contains nothing in principle
that is opposed to the general interpretation of the gene.
It means only that the haploid set of genes of sylvestris,
when placed in opposition to the haploid set of genes of
Tabacum, is totally eclipsed and ineffectual. The sylves-
tris chromosomes, nevertheless, retain their identity.
They are not eliminated or injured, since from the hybrid
a set of functional sylvestris chromosomes may be re-
gained in back-crosses to a sylvestris parent.

194 THE THEORY OF THE GENE

An extensive series of crosses between species of
Crepis have been carried out by Babcock and Collins.
The chromosomes of these hybrids have also been studied
by Miss Mann.

Crosses between Crepis setosa with 8 chromosomes
(n=4), and C. capillaris with 6 chromosomes (n=3) have
been made by Collins and Mann. The hybrid has 7 chro-
mosomes. At maturation some of the chromosomes con-
jugate and other chromosomes, without dividing, are scat-
tered in the pollen mother cells, forming nuclei with
from two to six chromosomes. At the second division all
the chromosomes divide, at least, those in the larger
groups, and pass to opposite poles. The cytoplasm usu-
ally divides into four cells, but sometimes into 2, 3, 5, or 6
microspores.

These 7-chromosome hybrids do not give functional
pollen, but some of the ovules are functional. When the
hybrid was used as pistil parent and fertilized by pollen
from one of the parents, five plants were obtained with 8
and 7 chromosomes. The maturation stage of one with 8
chromosomes was examined. It had 4 bivalents, which
divided normally. The plant resembles C. setosa in its
characters and has the same type of chromosomes. One
of the parental types has been recovered.

Another cross was made between Crepis biennis with
40 chromosomes (n=20) and C. setosa with 8 chromo-
somes (n=4) (Fig. 109). The hybrid has 24 chromosomes
(20+4). In the maturation of the hybrid, at least 10
bivalents are present, and a few univalents. It follows
that some of the biennis chromosomes must conjugate
with each other, since setosa contributes only 4 chromo-
somes. At the ensuing division 2 to 4 chromosomes lag
behind the rest, but finally pass, in most cases, to one or
the other nucleus.

The hybrids are fertile. They produce (F2) plants hav-

SPECIES CROSSING

195

ing 24 or 25 chromosomes. There seems to be a chance
here of producing new stable types with a new chromo-
some number that may contain one or more pairs of chro-
mosomes derived from the species contributing the

«&•

setose

Crepis

biennis

Fig. 109.
Chromosome groups of Crepis setosa and C. biennis. (After Collins

and Mann.)

smaller number. The presence of 10 conjugants in the
hybrid suggests that Crepis biennis is a polyploid, pos-
sibly an octoploid. In the hybrid the like chromosomes
unite in pairs. This Fx hybrid, with half the full number
of biennis chromosomes, is an annual, while biennis itself
is biennial. The reduction in the number of its chromo-
somes has caused a change in its habits. It reaches matu-
rity in half the time necessary for biennis.

196 THE THEORY OF THE GENE

Two types of Mexican teosinte have been described by
Longley, one, mexicana, an annual type with 20 chro-
mosomes (n=10), the other, perennis, a perennial with
40 chromosomes (n=20). Both plants have normal reduc-
tion divisions. When the diploid teosinte (n=10) is
crossed to Indian corn (n=10), the hybrid has 20 chro-
mosomes. At maturation, there are 10 bivalents in the
hybrid 's germ-cells. This would ordinarily be interpreted
to mean that 10 chromosomes of teosinte have united with
10 of Indian corn.

*•*

/tfy\

* >

6
Fig. 110.

Keduced chromosome

group, a, of perennial

teosinte; b, of hybrid

with maize; c, reduction division of last. (After Longley.)

When the perennial teosinte (n=20) is crossed to In-
dian corn (u=10) the hybrid has 30 chromosomes. At the
first maturation division of the pollen mother cells there
were found some trivalent groups loosely held together,
some bivalents, and some single chromosomes in varying
numbers, thus as 4 : 6 : 6 ; or as 1 : 9 : 9 ; or as 2 : 10 : 4, etc. ;
see Fip\ 110b. At the first division the bivalents divide
and the partners move to opposite poles; the trivalents
divide, two going to one pole, one to the other ; the singles
lag and are distributed (without division) irregularly to
the two poles (Fig. 110c). A very unequal distribution
results.

Quite recentlv a case has been described in which a

••222*

Fig. 111.

Cross between two species of poppies, one, a, Papaver nudicaule,
having 14 chromosomes (n=7) and the other, c, P. striatocarpum
having 70 chromosomes (n— 35). The hybrid, b, has 42 (n=21).
d-e, embryo mother cell of hybrid. /, first maturation division of
hybrid, anaphase. (After Ljungdahl.)

198 THE THEORY OF THE GENE

new stable hybrid that is fertile has been produced by
crossing two species with widely different chromosome
numbers. Ljungdahl (1924) crossed Papaver nudicaule,
having 14 chromosomes (n=7), with P. striatocarpum,
with 70 chromosomes (n=35) (Fig. 111). The hybrid has
42 chromosomes. At maturation of the hybrid germ-cells
there are 21 bivalents (Fig. Ill, b, c-e). These divide, 21
going to each pole. No single chromosomes are present,
and none lag on the spindle. The result must be inter-
preted to mean that the 7 chromosomes of nudicaule have
mated with 7 chromosomes of striatocarpum, and that the
remaining 28 chromosomes of striatocarpum have con-
jugated in twos to give 14 bivalents. This gives a total of
21 bivalents, the number observed. It seems natural to
assume that the form striatocarpum, with 70 chromo-
somes (n=35), is probably a decaploid type, i.e., a type
with ten times each kind of chromosome.

The new type (Fx) produces germ-cells with 21 chro-
mosomes. It is balanced and stable. It is also fertile and
may be expected to produce a new stable type. From it
still other stable types are theoretically possible. If back-
crossed to nudicaule it should give rise to a tetraploid
type (21+7=28). Back-crossed with striatocarpum it
should produce an octoploid type (21+35=46). Here,
through hybridization of a diploid and a decaploid type,
there may be produced in subsequent generations tetra-
ploids, hexaploids, and octoploid types that are stable.

Federley's experiment (Chapter IX) with species of
moths of the genus Pygaera illustrate a very different
relation. Owing to the failure of the chromosomes to con-
jugate in the germ-cells of the hybrid the double number
is retained. By back-crossing the double number may be
continued, but as the hybrids are very sterile nothing
permanent could result from these combinations under
natural conditions.

CHAPTER XIV

SEX AND GENES

OUR present understanding of the mechanism of
I sex-determination has come from two sources.
Students of the cell have discovered the role
played by certain chromosomes and students of genetics
have gone further and have discovered important facts
as to the role of the genes.

Two principal types of mechanism for sex-determina-
tion are known. They both involve the same principle,
although they may seem, at first, to be the converse of
each other.

The first type may be called the insect type, because in
insects we have the best cytological and genetic evidence
for this kind of sex-determining mechanism. The second
type may be called the avian type, because in birds we
now have both cytological and genetic evidence for this
alternative mechanism. It is also present in moths.

The Insect Type (XX-XY).

In the insect type the female has two sex-chromosomes
that are called X-chromosomes (Fig. 109). When the eggs
of the female ripen (that is, after each has given off its
two polar bodies), the number of the chromosomes is re-
duced to one-half. Each ripe egg, then, contains one X
and, in addition, one set of ordinary chromosomes. The
male has one X-chromosome only (Fig. 112). In some
species this X has no mate ; but in other species it has a
mate that is called the Y-chromosome (Fig. 113). At one
of the maturation divisions the X and the Y pass to oppo-

Iff

t t •

J{lt#«<

f>n Mm

t(f • • • 4

Fig. 112.
Chromosome group of male and of female Protenor, the former
having one X-chromosome and no Y -chromosome; the latter having
two X-chromosomes. (After Wilson.)

•••

«r

•••itt#
tin •*•

Fig. 113.

The male and female type of chromosome groups of Lygaeus, the
former with X and Y; the latter with two X-chromosomes. (After
Wilson.)

SEX AND GENES

201

site poles (Fig. 113). One daughter cell gets the X, the
other the Y. At the other maturation division each splits
into daughter chromosomes. The outcome is four cells
that later become spermatozoa; two contain an X-chro-
mosome, two contain a Y-chromosome.

Any egg fertilized by an X-sperm (Fig. 114) gives rise
to a female that has two X's. Any egg that is fertilized

Diploid Nuclei XX

Gametes
Fertilization

Zugotes

Fig. 114.

Diagram illustrating the XX-XY type of mechanism in sex deter-
mination.

by a Y-sperm gives rise to a male. Since the chances are
equal that an egg will be fertilized by one or the other
kind of sperm, the expectation is that half the offspring
will be female and half will be male.

Given such a mechanism, certain kinds of inheritance
are explicable, some of them including ratios that do not
appear, at first sight, to conform to Mendel 's 3 to 1 ratio.
On closer scrutiny, however, the apparently exceptional
ratios are found to furnish confirmation of Mendel 's first
law. For instance, if a white-eyed female of Drosophila
is bred to a red-eyed male, the female offspring are
red-eyed and the sons are white-eyed (Fig. 115). The

r\ r\

Fig. 115.
Inheritance of the white-eyed character in Drosophila. The gene
for white eye is carried by the X-chromosome represented here
by an open rod (w). The normal allelomorph to the "white-eyed
gene," namely, the "red-eyed gene" is carried here by the black
rod. The Y-chromosome is stippled.

SEX AND GENES

203

explanation is obvious, if the X-chromosomes carry the
differential genes involved, namely, the red- and white-
producing genes. The son gets his single X from his
white-eyed mother ; and the daughter gets also such an X,
but also one from her red-eyed father. The latter being
dominant, the daughter has red eyes.

x<?&

U>i

Fig. 116.
a, The reduced group of chromosomes in man, according to de
Winiwarter; b, chromosome group in man, according to Painter;
c and d, side view of first maturation division showing the separa-
tion of the X- and Y-chromosomes, according to Painter.

If these two individuals are inbred there will appear
in the next generation white-eyed and red-eyed offspring
in the ratio of 1 : 1 : 1 : 1. This ratio is due to the distribu-
tion of the X-chromosomes, as shown in the middle of the
diagram (Fig. 115).

In passing, it is not without interest to note that the
cytological evidence and the genetic evidence, especially

204

THE THEORY OF THE GENE

the latter, show that man belongs to the XX-XO or to the
XX-XY type. The number of chromosomes in the human
race has only recently been determined with any degree
of accuracy. The earlier observations giving fewer have

Fig. 117.

Maturation division of the germ-cells of man, illustrating the
separation of the X- and the Y-ehromosome. (After Painter.)

been shown to be faulty, owing to the tendency of the
chromosomes to stick together in groups when the cells
are preserved. De Winiwarter gives the number as 48
in the female (n=24) and 47 in the male (Fig. 116a),
and this count is practically confirmed by Painter, who
recently has shown that there is also present in the
male a small chromosome that acts as the mate of a much

SEX AND GENES

205

larger X (Fig. 117). He interprets these two as an XY
pair. If so, there are 48 chromosomes in each sex, but in
the male those of one pair, the sex-chromosomes, are
unequal in size.

More recently still Oguma, who confirms de Winiwar-
ter's numbers, finds no Y-chromosome in the male.

The genetic evidence for man is quite clear. The in-
heritance of haemophilia (or bleeding), of color blind-

Diploid Nuclei VZ

Gametes W

Fertilization

Zygotes WZ

Fig. 118.
Diagram illustrating the WZ-ZZ type of mechanism in sex deter-
mination.

ness, and of two or three other human characters, follow
in their inheritance the same method of transmission seen
in the white-eyed flies.

The following groups of animals belong to the XX-XY
type or to a modification of this type, vie., the XX-XO
type, in which 0 means the absence of Y or no X. Several
mammals in addition to man have been reported to have
this mechanism — the horse and the opossum, and pos-
sibly the guinea pig. It is probable that the Amphibia
also belong here, as well as teleostean fish. Most of the

206 THE THEORY OF THE GENE

insects belong to this group, with the exception of the
Lepidoptera (moths and butterflies). In the Hymenop-
tera, however, another mechanism determines sex (see
below). The roundworms (Nematodes) and sea urchins
belong also to the XX-XO type.

The Avian Type {WZ-ZZ).

The other type of sex mechanism, the avian-moth type,
is shown in the diagram (Fig. 118). The male has two
like sex-chromosomes that may be called ZZ. These sepa-

M&le Female

Fig. 119.
Male and female chromosome groups of the fowl. (After Shiwago.)

rate at one of the two maturation divisions and each ripe
sperm-cell comes to contain one Z. The female has one
Z-chromosome and a W-chromosome. When the eggs ma-
ture, each egg is left with one or the other of these chro-
mosomes. Half the eggs contain a Z- and half contain a
W-chromosome. Any W-egg fertilized by a Z-sperm pro-
duces a female (WZ). Any Z-egg fertilized by a Z-sperm
produces a male (ZZ).

Here again we find a mechanism that automatically
produces two kinds of individuals, females and males,
in equal numbers. As before, a 1 to 1 sex-ratio results
from the combination of chromosomes that takes place

SEX AND GENES 207

at fertilization. The evidence for this mechanism in birds
comes both from cytology and from genetics, although
the former is as yet not entirely satisfactory.

Fig. 120.
Diagram of cross between black and barred poultry, showing

sex-linked inheritance.

208

THE THEORY OF THE GENE

According to Stevens, in the chick the male appears to
have two large chromosomes equal in size (Fig. 119),
presumably X's; the female has only one of these. Shi-
wasfo and Hance confirm these relations.

CR0S5ULARIATA 9 .01

LACT1C0L0R d 11

O © CffiM CELLS ®

LACT1C0L0R 9 01

\ GROSSULARIATA d LI

O ©CERM CELLS ..© ©

O ©'" O ©" *© ® "0 ^©

GR0SSULAR1ATA 9 OL LACT1C0L0R 9 01 CR05SULAR1ATA d 1 L LACTIC0L0R 6

Fig. 121.
Sex-linked inheritance in the currant moth, Abraxas.

The genetic evidence for birds is beyond dispute. It
comes from sex-linked inheritance. If a Black Langshan
male is mated to a Barred Plymouth Rock female, the
sons are barred, the daughters are black (Fig. 120). This
is expected if the differential genes are carried by the
Z-chromosomes, because the daughter gets her single Z-
chromosome from her father. If the Ft offspring are bred

SEX AND GENES 209

together, they produce barred and black males and
females as 1:1:1:1.

A similar mechanism is found in moths, where the cyto-
logical evidence is more certain. When a female of the
darker wild type variety of the currant moth (Abraxas)
is mated to a lighter mutant type, the daughters have a

• »

Fig. 122.

c, Eeduced group of chromosomes of the egg of Fumea casta; b
and &' outer and inner pole of the first maturation division of the
egg; the single X -chromosome is present only at one pole. (After
Seiler.)

210 THE THEORY OF THE GENE

lighter color, like the father; the sons a darker color, like
the mother (Fig. 121). The daughter gets a single Z from
her father ; the son also gets this Z from his father, but
another from his mother. This maternal Z carries the
gene for darker color that is dominant, hence the darker
color of her sons.

In the silkworm moth, Tanaka has found a sex-linked
character, translucent skin of the larva, that is inherited
as though carried in the Z-chromosomes.

In the moth Fumea casta there are 61 chromosomes in
the female and 62 in the male. After conjugation of the
chromosomes in the egg there are 31 chromosomes pres-
ent (Fig. 122a). At the first polar division, when the first
polar body is given off, 30 of the chromosomes (bivalents)
divide and pass to opposite poles ; the thirty-first sin-
gle chromosome passes undivided to one or the other
pole (Fig. 119b and b'). Half of the eggs will come to
contain 31, half 30 chromosomes. At the next polar divi-
sion all the chromosomes present divide, leaving each
egg with the number it had before this division (i.e., 31
or 30). In the, ripening of the sperm of this moth, 31
bivalent chromosomes are present after conjugation of
the chromosomes. At the first division the members of
each pair separate and at the second each divides. Each
spermatozoon carries 31 chromosomes. Fertilization of
the eggs gives the following combinations :

Females = 61<^

30s
x31v

Males = 62<

In another moth, Talaeporia tubulosa, Seiler finds 59
chromosomes in the female and 60 in the male. In Soleno-

SEX AND GENES 211

bia pineti an unpaired chromosome is not visible in the
female or in the male, nor is an unpaired chromosome
visible in several other moths. On the other hand, in
Phragmatobia fuliginosa there is a compound chromo-
some containing the sex-chromosome. In the male there
are two of these present; in the female only one is com-
pound like those of the male. It seems not improbable
that this relation may also exist in other moths where the
W-element and the Z-elements do not appear as separate
chromosomes.

Another demonstration of sex-linked inheritance in
moths has been given by Federley in a cross between two
species of moths (Pygaera anachoreta and P. curtula).
This case is interesting because within each species the
male and female caterpillars are alike. They show specific
differences, however, when the caterpillars in the two
species are compared. This specific difference, that is not
dimorphic within the species, becomes the basis for a
sexual dimorphism in the Fx caterpillars (when the cross
is made "one way"), because, as the results show, the
main genetic difference between the caterpillars in the
two races lies in the Z-chromosomes. When anachoreta is
the mother and curtula the father, the hybrid caterpillars,
after the first molt, are markedly different. The hybrid
male caterpillars are closely similar to those of the mater-
nal race (anachoreta), while the hybrid female caterpil-
lars resemble those of the paternal race (curtula).

The reciprocal cross gives offspring that are all alike.
These results are explicable on the assumption that the
anachoreta Z-chromosome carries a gene (or genes)
dominant to the gene (or genes) in the Z-chromosome of
curtula. The special interest in this case is due to the
genes in one species acting as a dominant to allelomor-
phic genes in the same chromosome of the other species.
The analysis of the result can be carried over consistentlv

212 THE THEORY OF THE GENE

to the next generation, produced by back-crossing the Fx
male to either parent stock, provided, however, the trip-
loid nature of the offspring be taken into account. (See
Chap. IX.)

There are no grounds for supposing that the chromo-
somes involved in the XX-XY and in the WZ-ZZ types
are the same. On the contrary, it is difficult to imagine
how one type could change over directly into the other.
There is no theoretical difficulty, however, in supposing
that the change in balance that gives the two sexes may
have arisen independently in the two types, even although
the actual genes involved are the same or nearly the same
in both.

Sex-Chromosomes in Dioecious Flowering Plants.

One of the surprises of the year 1923 was the simul-
taneous announcement by four independent workers that
in some of the flowering plants with separate sexes a
mechanism is present that follows the XX-XY type.

Santos found in the male of Elodea that 48 somatic
chromosomes are present (Fig. 123), consisting of 23
pairs of autosomes and an XY unequal pair. At matura-
tion the X and Y separate. Two kinds of pollen grains
result, one with X, the other with Y.

Two other cytologists, Kihara and Ono, found in male
plants of Rumex 15 somatic chromosomes consisting of
6 pairs of autosomes and 3 heterochromosomes (mx, m2,
and M). These three come together at maturation of the
germ-cells to form a group (Fig. 123). The M goes to one
pole, the two small m's to the opposite. Two kinds of pol-
len grains result, 6a-f M and 6a+m+m. The latter are
male-determining.

Winge found an XY pair of chromosomes in two spe-
cies of hops (Humulus lupulens and H. japonica). Nine

*•*••

Elodea

r_Y

■x

Humulus

M M
•mm mm

Rumex

V^llisnerm

Vadlisnerm

Mel&ndrium

Me tan dpi urn

Pollen Ovule

Fig. 123.
Maturation groups of several dioecious plants. (After Belar.)

214 THE THEORY OF THE GENE

autosomes and an XY pair are present in the male. He
also found in Vallisneria spirales that in the male there is
an unpaired X-chromosome. The formula is 8a+X.

In Melandrium, Correns has concluded from breeding
work that the male is heterogametic. Winge reported that
the male formula is 22a+X-f Y, which confirms Correns'
deduction.

Miss Blackburn also reported an unequal pair of chro-
mosomes in the male of Melandrium. She adds one all-
important link to the chain of evidence. The female has
two equal sex-chromosomes, one of them corresponding
to one of the sex-chromosomes of the male (Fig. 123). At
maturation they conjugate and reduce.

From this evidence we may, I think, safely conclude
that some at least of the dioecious flowering plants make
use of the same kind of mechanism for sex-determination
that is present in many animals.

Sex-Determination in Mosses.

Several years before these observations on flowering
plants had been made, it had been shown by the Marchals
that when the spores are formed in dioecious mosses —
mosses that have separate male and female gametophytes1
(or sexual prothallia) — two of the spores derived from

i In mosses, ferns, and liverworts the haploid or gametophyte generation
is spoken of as consisting of two sexes, male and female, and the diploid
generation (sporophyte) as non-sexual or neutral. In flowering plants, the
plant itself corresponds to the sporophyte of the mosses. It carries, as
it were, the gametophyte generation within its pistil and stamens. A paradox
arises from the use of the same terms male and female in mosses for one
generation, that is, the haploid one, and for the alternative generation in
flowering plants, that is, the diploid. The paradox is not so much a question
of diploid and haploid (this contrast is encountered even within the same
generation in some animals — bee, rotifers, etc.), but in using the same terms
for contrasted generations, one sexual, the other non-sexual. With this under-
standing, however, no serious difficulty arises by following conventional
usage.

SEX AND GENES

215

the same sporophyte mother cell produce female game-
tophytes and the other two male gametophytes.

Somewhat later Allen discovered in the nearly related
group of liverworts (Fig. 124) that in the haploid female

X....

saSU

Y ^^>

Fig. 124.
a, Female and b, male prothallia of liverwort. The female with
one large X-chromosome, a'; the male with one small Y-chromo-
some, b'. (After Allen.)

prothallium (gametophyte), with eight chromosomes,
there is one (X) that is much larger than the other seven
chromosomes ; and in the haploid male prothallium (game-
tophyte), with eight chromosomes, one (Y) is much
smaller than the other seven (Fig. 121b'). Each egg will

216 THE THEORY OF THE GENE

contain an X-, and each sperm a Y-chromosome. After
fertilization the sporophyte will have 16 chromosomes
(including one X and one Y). When the spores are
formed, reduction takes place, the X and the Y separat-
ing. Half of the haploid spores so formed will contain an
X and give rise to a female prothallium, and half will con-
tain a Y and give rise to a male prothallium.

More recently still, Wettstein has made some critical
experiments with dioecious mosses, experiments that
carry the analysis further. By utilizing a discovery of the
Marchals, he produced gainetophytes that contained both
the male and the female groups of chromosomes (Fig. 125
to the left). For example, following the Marchals' method,
he cut off pieces of the spore-bearing stalk (whose cells
are diploid). From the fragment a gametophyte devel-
oped, also diploid. In this way he obtained FM game-
tophytes.

Then in another way he made diploid male and female
moss plants that were double females (FF) and double
males (MM). This was accomplished as follows:

By treating the protonema threads with chloral hy-
drate and other drugs and reagents, he brought about the
suppression of a cell division in an individual cell after
the chromosomes had already divided. In this way he
could produce in these dioecious species, diploid giant
cells that were doubled in their female or else in their
male elements, chromosomes, for example. From such a
diploid cell a protonema or moss plant was produced.
By artificial means Wettstein then brought about several
new combinations, some triploids, others tetraploids.
Some of the most interesting of these combinations are
shown in the diagram (Fig. 125, to the right).

A diploid cell from a female thread gives a diploid
moss plant, FF, that produces diploid egg-cells. Similarly
an MM plant is produced from a diploid male thread.

SEX AND GENES

217

When an FF egg and an MM sperm are brought together
a tetraploid sporophyte (FFMM) is produced.

When the FF ovule is fertilized by a normal male

\\//

-5

W// *S

/ \

??<?

^
*

^
^
^

^<f_ '

^
^

Fig. 125.

Diagram illustrating different combinations of diploids and

triploid mosses. (After Wettstein.)

sperm cell M, a triploid sporophyte (FFM) is produced.
Thus:

FFM

FF
/ \

FFMM

From each sporophyte, FFM and FFMM, a gametophyte
can be regenerated. Each of these plants develops both

218 THE THEORY OF THE GENE

male and female elements, and both produce eggs and
sperm-cells; but the number of female organs (arche-
gonia) and of male organs (antheridia) and their time of
appearance show characteristic differences.

The Marchals had obtained, as has been said, the dip-
loid FM gametophyte in the same species used by Wett-
stein, and had shown that it produces both female and
male organs. Wettstein confirms this and reports that the
male organs develop before the female.

A comparison of the three types, FM, FFM, FFMM,
is interesting. The FM plant is strongly protandric. At
first there is a great excess of antheridia or male organs
compared with archegonia. The archegonia develop later.

The FFMM plant is, as Wettstein says, twice as
strongly protandric as the FM plant. At first only anthe-
ridia appear. Very late in the year, when the old anthe-
ridia have gone, a few young archegonia appear — some
plants never develop them. Still later a vigorous develop-
ment of female organs may set in.

The triploid plants are protogynic. At least, at the time
when the FFMM tetraploid plants have only male organs
(in July), the triploids have only female organs. Later
(in September) both organs are present.

These experiments are interesting in showing how arti-
ficial hermaphroditic individuals may be made from
plants that normally have separate sexes by combining
the two sets of elements. The results also show that the
sequence in which the sexual organs develop is deter-
mined by the age of the plant. More important is the
actual reversal of this time relation by changing the
genetic composition in the opposite direction.

CHAPTER XV

OTHER METHODS OF SEX-DETERMINATION
INVOLVING THE SEX-CHROMOSOMES

THE determination of sex through the redistribu-
tion of the sex-chromosomes in the germ-cells is
regulated in some animals in other ways than
those described in the preceding chapter.

«*

«•

Fig. 126.

Separation of the two small X-chromosomes from the autosomes in

Ascaris eggs. (After Geinitz.)

The Attachment of the X-Chromo somes to Autosomes.

The attachment of the sex-chromosomes to other chro-
mosomes, that is known to occur in a few forms, tends to
conceal the differential character of X- and Y-chromo-

220

THE THEORY OF THE GENE

somes. Their presence has been detected, in such cases, by
their occasional separation, as in Ascaris (Fig. 126),
from their attachment, or by the differential staining
properties of the X-chromosome in the male, or, as in
certain moths studied by Seiler, by the regular separation
of the compound chromosome into its components in the
somatic cells of the embryo.

o^ 4,.

Ooqon ia

)erma

onia

Fig. 127.

Diagram illustrating the distribution of the attached X-chromo-
somes in the male and female of Ascaris. (After Boveri.)

The attachment of the sex-chromosomes to ordinary
chromosomes, or autosomes, as they are called, involves
the mechanism of sex-linked inheritance, more particu-
larly should crossing-over take place in the male between
the autosome attached to the X and its mate lacking the
attached X. An example will illustrate the point at issue.
In Fig. 127 the X-chromosomes of Ascaris are indicated
by the black ends of those chromosomes to which they are
attached. In the female there are two X-chromosomes,

OTHER METHODS OF SEX-DETERMINATION 221

each attached to a member of the same pair of autosomes.
In the mature egg one such compound chromosome (there-
fore one X) is left in each egg. In the male, one X is pres-
ent, attached to the corresponding autosome, but the
other autosome has no attached X. After maturation half
of the sperm-cells will contain an X, half will be without
an X. The mechanism for sex-determination is obviously
here the same as in the XX-XO type.

In the female, crossing-over might take place both be-
tween the two X-chromosomes and between the two at-
tached autosomes. But in the XO male the situation
would be different ; for in the male the X part of the com-
pound chromosome has no opposite, hence no crossing-
over is expected in that part. This would insure the
coherence of the sex-differentiating genes and of the sex-
mechanism ; but between the autosomal parts of the com-
pound chromosome an interchange might then take place
without affecting the sex-mechanism. The characters
whose genes lie in the X-component will show sex-linked
inheritance, i.e., the recessive character, will appear in
the sons. The recessive characters whose genes are in the
autosomal part will not appear in the sons. However, the
character whose genes are in the autosomal part will
show partial linkage to sex and to the characters whose
genes lie in the X-component.1

In the imaginary example just given, the autosome
without an attached X, that is, the mate of the compound
chromosome with an X in the male, will appear to corre-
spond to the Y-chromosome of the ordinary XX-XY type

i According to McChmg the X-chromosome in the male of Hesperotettix
is not constantly attached to the same autosome, although in a given indi-
vidual its attachment is constant. In other individuals it may be free. Were
sex-linked characters known in such a type, their inheritance might be
complicated by this inconstant relation of the X-chromosome to the auto-
somes.

222 THE THEORY OF THE GENE

(because it is confined to the male line), except, as just
pointed out, that it carries genes that are like those in the
corresponding part of the compound X-chromosomes.
Cases of inheritance have, in fact, been recently recorded
where certain genes appear to be carried by the Y-chro-
mosome, and such cases have been interpreted to mean
that the Y-chromosome itself may sometimes carry genes.
There is no objection to such a statement if interpreted
as above, but there is an obvious objection to this state-
ment if it is intended to mean more than this; for the
chromosomal sex-mechanism would break down if the
X and the Y of the male interchanged throughout. If this
happened, the two chromosomes would after a time be-
come identical, and the difference in balance that gives
males and females would be lost.

The Y -Chromosome.

There are two groups in which the genetic evidence has
been interpreted to mean that Mendelian factors may be
carried in the Y-chromosome. In fish, belonging to two
different families, it has been shown by Schmidt, Aida,
and Winge that the Y carries genes. In the gypsy moth
Goldschmidt has interpreted the result of species-crosses
in the same way (here the W-chromosome). The latter
results will be considered in the chapter on sex inter-
grades ; the former may now be taken up.

In the small aquarium fish, Lebistes reticulatus, a na-
tive of the West Indies and northern South America, the
males are highly colored and strikingly different from the
females (Fig. 128). The females in different races are
closely similar to each other, while the males show char-
acteristic differences in color. Schmidt has found that
when a male of one race is bred to a female of another
race, the sons are like the father. If these hybrids (Fx)
are inbred, their sons (F2) are again all, like the father,

OTHER METHODS OF SEX-DETERMINATION 223

and none of them show any characters of males of the
maternal grandmother's race. The F3 and F4 males are
again all like those of the paternal forefather. There
seems to be here no Mendelian splitting for any charac-
ters that might have been expected to have come through
the maternal grandparent.

crossover!

Fig. 128.
Diagram illustrating the inheritance of a sex-linked character in
fish, carried both by the X- and the " Y-chromosome. " After
Winge.)

The same results are obtained when the reciprocal
cross is made; the sons and grandsons are all like the
paternal parent, etc.

In another fish, Aplocheilus latipes, inhabiting small
streams and paddy fields of Japan, several types differ-
ing in color are found. Other types have also appeared in
cultures. In these fish both males and females of each
type occur. Aida has shown that several of these differ-
ences are transmitted through the sex-chromosomes (both
X and Y). The genetic transmission of these characters
can be explained on the hypothesis that the genes are
carried sometimes in the Y- and sometimes in the X-

221

THE THEORY OF THE GENE

chromosomes, and that crossing-over between these chro-
mosomes may take place.

For example, the character, white body color, is sex-
linked in inheritance. Its allelomorph is red body color.
When a pure white female was mated to a pnre red male,
the sons and daughters (F±) were all red. When these
were inbred they gave,

Red 2 Red $ White ? White $
41 76 43 0

Assuming that the genes for white are carried in the two
X-chromosomes of the female and designating such a
chromosome by Xw (Fig. 129), and assuming that the

white Q

x^x"

dc^

X Y

germ cells

Fig. 129.

Diagram showing the inheritance of a character whose gene is
carried by the ' ' Y-chromosome, ' ' as well as by the X-chromosome.

gene for red is carried both by the X and the Y of the
male, and designating these by Xr and Yr, the cross given
above works out on the XX-XY formula as shown in Fig.
129. If red (r) dominates white (w), both sons and daugh-

OTHER METHODS OF SEX-DETERMINATION 225

ters (Fx) will be red. If these are inbred the results are
shown in the next diagram (Fig. 130). White and red
daughters in equal numbers are expected and red sons
only, that are equal in number to the sum of the two
female classes.

^ red c?

Fig. 130.
Diagram illustrating the inheritance of red and white color from
two Ft heterozygous male and female fish. The Y-chromosome as
well as the X may carry the gene for red (r).

Thus from a red male and a white female no white
grandsons are expected on this formulation unless in an
FjXwYr red male, crossing-over between X and Y occurs
to give a Yw chromosome (Fig. 131). When such a chro-
mosome meets an egg-carrying Xw, a white male, XWYW,
will be produced. A white male appeared, in fact, in one
experiment in which an ¥1 heterozygous red male, XwYr
(obtained in the above experiment), was back-crossed to
a pure white female. The results obtained were :

Eed $
2

White 2
197

Red S
251

White $
1

226 THE THEORY OF THE GENE

The occurrence here of two red 9 's and one white male
may be accounted for, if, in the Fx$ (XwYr), an inter-
change occurred about once out of 451 times, as shown in
Fig. 131. Similar results were obtained when white and
brown males were crossed, but no cross-overs were re-
corded. When variegated red females and white males
were crossed, the same kind of results were obtained with
11 cross-overs out of 172 individuals in the back-cross.

red f^ d

X'Yr

crossover gametes
,erm cells X" X*- Y - X*"- Y*

F2 xY x"Y. x"x' Yt

white p redd* redo white d

crossover zygotes

Fig. 131.

Diagram illustrating crossing-over between genes for red and for
white carried by the X- and ' ' Y-chromosome ' ' of an F1 male fish.
These genes are interpreted as allelomorphs.

Winge (1922-1923) extended Schmidt's experiments
with Lebistes, and independently reached the same con-
clusions concerning the Y-chromosomes as had Aida. The
results of a cross between a female of one race, X0X0, and
a male of another race, XeYm, are shown in Fig. 128. The
ripe germ-cells of this heterozygous male are represented
by two non-cross-over classes, Xe and Ym, and two cross-
over classes, X0 and Yem. Correspondingly there were two

OTHER METHODS OF SEX-DETERMINATION 227

kinds of males, X0Ym and X0Yera. The latter are rare, one
ont of 73 sons.2 Whether crossing-over also occurs in the
female cannot be determined from Winge 's data, since he
gives no cases of XeXM females. Moreover, he represents
one type of female as X0 and implies that the X0-chromo-
some is lacking in certain genes. Two pairs of genes are
necessary to show crossing-over when two X's are pres-
ent. In fact, Winge represents an Xe that has crossed over
to a Ym as X0 without representing the reciprocal allelo-

_m . — x_m

m II \/ m

non cross over crossover

Fig. 132.
Diagram illustrating the possible relations of an attached X-chro-
mosome to crossing-over between the autosomal portion of this
compound chromosome and the autosome (the male of the latter)
here called Y.

morphic change. The full formula should represent one
of the X's containing the genes M and e, and the Y, in
this case, as containing the genes m and E. After cross-
ing-over the X would then contain E and M and the Y
would contain e and m, as shown in Fig. 132. The X-chro-
mosome after crossing-over is not X0, but XME, and the
Y-chromosome Yme. If m and e are dominant over M and
E, the results would be as recorded, except that another
cross-over is expected, namely, XME. If the part of the X
to the left of the M contains the sex-determining genes
(the heavier part of the X in the figure) the absence of
this cross-over in the experiment might be explained as
due to the proximity of M to the X-component.

Aside from these questions of interpretation, the re-

2 In another experiment 4 cross-overs out of 68 sons are recorded.

228 THE THEORY OF THE GENE

suits show that certain characters follow the Y-chromo-
some, so-called, in inheritance. The results are not incon-
sistent with those recorded in other cases of sex-linked
inheritance, provided the X-component of the compound
chromosome is absent in the Y. Whether crossing-over
occurs in the female of the two species of fish that have
been studied is not evident from the published results,
partly because the crosses have not been made in a way
to bring out this possibility, and partly because the nota-
tion used is such as to obscure this possibility.

Degeneration of Male-Producing Sperm.

In two closely related families of bugs, the Phylloxe-
rans and Aphids, belonging to the XX-XO type, the male-
producing class of sperms (no X) degenerate (Fig. 133).
This leaves only the female-producing sperms (X). The
sexual egg (XX), after extrusion of two polar bodies, is
left with one X-chromosome. Fertilized by the X-sperm,
these eggs produce only females (XX). These females are
called stem mothers. They are parthenogenetic and be-
come the starting point of a succession of other partheno-
genetic females. After a time, some of these females may
produce male offspring, others producing sexual females.
The latter are diploid, like their mothers, but in them the
chromosomes conjugate and their number becomes re-
duced to half. The former individuals that produce males
do so by a process that will be described in the next
section.

The Elimination of One X-Chromosome from a
Diploid Egg to Produce a Male.

In the Phylloxerans, as stated above, a certain kind of
female appears near the end of the parthenogenetic cycle
whose eggs are a little smaller than those of the earlier
females. Just before maturation of the smaller eggs the

OTHER METHODS OF SEX-DETERMINATION 229

X-chromosomes come together (there are four X's pres-
ent). Two of them pass out of the egg into the single
polar body that is given off (Fig. 134). The autosomes
at this time divide, and half of each is eliminated. The

d e f ' g

Fig. 133.
First maturation division of the bearberry aphid. At the first divi-
sion, a-c, the large X-chromosome passes into one cell. At the
second division, e, f, g, this cell divides again producing two func-
tional female-determining sperms. The rudimentary cell does not
divide again.

egg is left with a diploid set of autosomes and half of
the X-chromosomes. It develops, by parthenogenesis, into
a male.

In the Aphids a similar process takes place. The actual
extrusion of one of the X's from the egg (there are only
two X's present) has not been observed, but since, after
the single polar body is given off, there is one less chro-
mosome present in the egg, there can be no doubt but that
one is lost, as in the Phylloxerans.

In these two groups the male sex is determined by a

230 THE THEORY OF THE GENE

different process from that which takes place in other
insects, but the same mechanism is utilized in a different
way to bring about the same end-result.

There is one further fact of unusual interest in this
case. In the Phylloxerans the female that gives rise to the
male eggs — she is called the male-egg producer — forms
smaller eggs than did her parthenogenetic forbears. The

b
Fig. 134.

a, The first polar spindle of a" male egg ' ' of Phylloxera in which
two chromosomes lag on the spindle and are ultimately thrown out
of the egg, leaving five chromosomes in the egg nucleus, b, The
first polar spindle of a female egg, in which all six chromosomes
divide leaving six chromosomes in the egg nucleus.

fate of the eggs is, therefore, indicated before the X-
chromosomes are eliminated from them. It might appear
that, here, sex is determined by the size of the egg, which
might mean the amount of cytoplasm contained in it;
but the conclusion is an illegitimate inference from the
evidence, since the egg becomes a male only after half of
its X-chromosomes are eliminated. What would happen
if they were retained we do not know — probably the egg
would develop into a female. At any rate, we have here an
instance of a change that has taken place in the mother
that leads to the formation of the smaller egg, which, in
turn, reduces the number of its X-chromosomes to pro-

OTHER METHODS OF SEX-DETERMINATION 231

duce a male. The nature of the change in the mother is
unknown at present.3

Sex-Determination through the Accidental Loss
of a Chromosome in Spermatogenesis.

In hermaphroditic animals no sex-determining mecha-
nism has been found, and none is expected, since all the
individuals are alike, each with an ovary and a testis. In

Fig. 135.

First and second maturation division of the sperm cell of Angio-

stomum nigrovenosum. At the second division (lower line) one of

the X-chromosomes gets caught in the division-plane. (After
Schleip.)

one species of nematode worms, Angiostomum nigro-
venosum, there is an hermaphroditic generation that alter-
nates with a generation consisting of males and females.
Boveri and Schleip have shown that when the sperm-cells
mature in the parthenogenetic generation (Fig. 135) one

3 In one of the worms, Dinophilus apatris, eggs of two sizes are produced
by each female. Both kinds give off two polar bodies, resulting in a haploid
pro-nucleus. Both kinds of eggs are fertilized; the larger egg produces
females, the smaller one males (Naehtsheim). At present the cause of the
production of two kinds of eggs in the ovary is entirelv unknown.

232

THE THEORY OF THE GENE

of the X-chroniosomes frequently gets lost (being caught
in the division plane) and this leads to the production of
two classes of sperm, with five and six chromosomes.
In the maturation of the eggs of the same female the

■■-■■' * > . t

0 0 °0

)

}

Co* f

o O

:> 0

O-

<~\

^JO-V

-rX - %v. •

-!'-Z.1\" V

Lt>

Fig. 136.
The two maturation divisions of the egg of Angiostomum nigro-
venosum. Six chromosomes are left in the egg nucleus. (After
Schleip.)

twelve chromosomes conjugate, giving six gemini (Fig.
136). At the first maturation six go into the first polar
body and six remain in the egg. These split and six daugh-
ter chromosomes go into the second polar body, leaving
six chromosomes in the egg, each with one X-chromo-
some. An egg fertilized by a sperm with six chromosomes

OTHER METHODS OF SEX-DETERMINATION 233

produces a female ; an egg fertilized by a sperm with five
chromosomes produces a male. Here an accident in cell-
division becomes the mechanism of sex-determination.

Diploid Females and Haploid Males.

In the rotifers there is, first, a long series of genera-
tions of parthenogenetic females with the diploid number
of chromosomes. No reduction takes place in the eggs and
one polar body is given off. The series may apparently
continue indefinitely under certain conditions of nourish-
ment. The series can, however, as shown by Whitney, be
brought to an end by a change in diet — such as feeding
the females on a green flagellate. A female feeding on
such a diet now produces daughters (by parthenogenesis)
with dual possibilities. If one of these daughters is fertil-
ized by a male (that may have then appeared), each egg,
before maturation, is entered by a single sperm. The egg
enlarges in the ovary and a thicker shell is laid over it
(Fig. 137). It gives off two polar bodies, and then the
sperm nucleus (haploid) unites with the haploid nucleus
of the egg, restoring the full number of chromosomes.
This egg is a resting or winter egg. It contains the dip-
loid set of chromosomes, and after a time develops into
the stem mother of a new line of parthenogenetic females,
etc.

On the other hand, if the female in question is not fer-
tilized, she produces eggs that are smaller than the ordi-
nary parthenogenetic eggs. The chromosomes conjugate,
and two polar bodies are given off. The egg is left with a
haploid set of chromosomes. It segments, without dou-
bling the number of its chromosomes, and produces a male.
In the development of the sperm-cells in this male, only
one maturation division takes place. The functional
sperm with the haploid number of chromosomes fertilizes
the resting egg of the female.

Fig. 137.
The rotifer, Brachionus bakeri. A, female with attached partheno-
genetic female-producing eggs. B, female with attached partheno-
genetic male-producing eggs. C, female with attached sexual eggs.
D, male. (After Whitney.)

OTHER METHODS OF SEX-DETERMINATION 235

The evidence, taken at its face value, appears to mean
that the haploid number of chromosomes produces a
male, the diploid a female. The presence of sex-chromo-
somes is nowhere apparent, hence the presence of specific
sex genes cannot be assumed. Even if the absence of such
genes be granted, it is not apparent why the half number
of chromosomes should produce a male and the diploid
number a female, unless the differential factor here in-
volved be the relation between the amount of cytoplasm
in the two kinds of eggs and the number of chromosomes
present. Even then, however, the result is difficult to
bring into accord with the case of the bee (described
below), where the diploid egg, that produces a female,
and the haploid egg, that produces a male, have the same
size. The outstanding fact in both cases is that the hap-
loid number of chromosomes determines the male sex,
even although something else determines which eggs be-
come haploid.

It might be possible to invent an explanation involving
sex-chromosomes if two kinds of X-chromosomes were
postulated and if, at the reduction division, one passes
out into the polar body of the male egg and the other one
from the sexual egg (both being retained in the partheno-
genetic egg) ; but it must be confessed that at present
there is no excuse or need perhaps for advancing such a
speculation.

Sex-determination in bees, and in their near relatives,
the wasps and ants, is also connected with the diploid and
haploid condition of the nuclei. The facts seem well estab-
lished, but the interpretation here is also obscure. The
queen bee deposits eggs in the queen-cells, in the worker-
cells, and in the drone-cells. These eggs are, before being
laid, all alike. The eggs in the worker-cells and the queen-
cells are fertilized at the time of deposition ; in the drone
cells the eggs are not fertilized. All eggs give off two

236 THE THEORY OF THE GENE

polar bodies. The egg nucleus is left with the haploid
number of chromosomes. In the fertilized eggs the sperm
brings in a haploid set of chromosomes, which, uniting
with the egg nucleus, gives the diploid number. From
these eggs females develop (queens or workers). The
queens owe their more complete development to the food
supplied to the larvae in the queen-cells. This food is
different from that given to the larvae in the worker-cells.
The males (drones) are, as has been said, haploid.4

In this case, the determination of sex cannot be sup-
posed to be due to any effect preceding maturation. There
is no evidence that the presence of the sperm-nucleus in
the egg affects the way in which the maturation division
of the chromosomes takes place. Furthermore, there is
no evidence that the environment (drone-cell or worker-
cell) has any effect on the course of development. There
is, in fact, no evidence here that any particular set of
chromosomes has been set apart as sex-chromosomes.
The only known difference between the two kinds of indi-
viduals, females and males, is the number of chromo-
somes present. We can, at present, only fall back on this
relation as the one that is in some unknown way corre-
lated with sex-determination. At present it cannot be
satisfactorily brought into line with other cases in in-
sects, where sex is related to a balance between genes in
the chromosomes, but it may still be due to a balance be-
tween the chromosomes (genes) and the cytoplasm.

There is one further fact that involves sex-determina-

* It is known that, as the cleavage of the unfertilized egg of the male
proceeds, each chromosome breaks into two parts (except possibly in the
nuclei that pass into the germ-track). This process does not appear to be a
' ' division ' ' of each chromosome, but rather its breaking or separating into
two pieces. If this interpretation is correct there is no actual increase in
the number of the genes and the occurrence of this process (also known in
some of the nematodes) does not throw any light on the question of sex-
determination.

OTHER METHODS OF SEX-DETERMINATION 237

tion in bees. When the maturation of the germ-cells in the
male takes place, the first division is abortive. A small
cell is pinched off without chromosomes (Fig. 86). At the
second division the chromosomes divide. Half pass into
one cell, that is very small and later degenerates; half
remain in the larger cell, that becomes the functional
spermatozoon and contains the haploid number of chro-
mosomes. This number it brings into the egg, which, as
stated, then develops into a female.

There are a few cases on record (Newell) where two
races of bees have been crossed and the progeny of the
hybrid recorded. The males are said to show the charac-
ters of one or the other original race. This is expected, in
so far as the two races differ in genes in one and the same
pair of chromosomes, because these would be separated
at reduction, and one or the other would be retained in
the haploid egg that produces a male. But if the racial
differences depend on genes lying in different pairs of
chromosomes, no such sharp distinction into two classes
of grandsons is to be expected.

The worker bees (and ants) occasionally lay eggs.
These become males, as a rule, which is expected, since
the workers cannot be inseminated by the drones. There
are records in ants of the rare appearance of sexual fe-
males from workers' eggs. It may be supposed that this
is due to the retention of a double set of chromosomes. In
the "Cape bees" the production of females (queens)
from workers' eggs is said to be a common occurrence.
Provisionally we may apply the same explanation as that
given above for the females of worker ants that rarely
produce eggs some of which, under special conditions,
develop into females.

The direct transmission of the characters of the mother
to her haploid sons has been more completely demon-
strated in Whitings ' work on the parasitic wasp, Habro-

238 THE THEORY OF THE GENE

bracon. The common type has black eyes. A mutant male
with orange eyes appeared in the cultures. Crossed to
black-eyed females, there were produced by parthenogen-
esis 415 black-eyed sons, and from fertilized eggs 383
black-eyed daughters.

Four of these (Fj) daughters, when isolated, produced
parthenogenetically 268 black-eyed males and 326 orange-
eyed males and no females.

Eight other Fx daughters (from the original orange
male) were mated with their F, brothers. There were
produced 257 black-eyed sons, 239 orange-eyed sons, and
425 black-eyed daughters.

The original mutant orange-eyed male, when bred to
his Fj daughter, gave 221 black males, 243 orange males,
44 black females, and 59 orange females.

These results are expected on the hypothesis that the
male is haploid and comes from an unfertilized egg. The
gene for orange eyes and that for black eyes separate in
the germ-cells of the hybrid mother when her germ-cells
mature, half of the gametes then have one kind of gene,
half the other kind. Any pair of genes in any pair of chro-
mosomes will give the same result.

The reciprocal cross was also made, namely, an orange
female was crossed to a black male. Eleven such matings
gave 183 black daughters and 445 orange males, as ex-
pected ; but twenty-two matings gave, in addition to 816
black females and 889 orange males, 57 black males. The
occurrence of these black males calls for a different ex-
planation. They have obviously come from eggs fertilized
by a black-producing sperm. A possible explanation
would seem to be that the haploid sperm-nucleus has de-
veloped in the egg, and has given rise to those parts from
which the eyes at least have come. The rest of the egg
might then get its nuclei from the haploid egg-nucleus.
There is, in fact, some evidence that this is the correct

OTHER METHODS OF SEX-DETERMINATION 239

explanation, since Whiting has shown that some of these
exceptional black males may breed as though all their
sperm carried only the orange gene of the mother. But
there are other facts indicating that in these cases the
explanation is not so simple as this, for most of the black
males are sterile, as well as the few daughters arising
from those males that are fertile (the mosaic males).5
Whatever the final solution may be for these exceptional
cases, the main results of the crosses confirm the theory
that the males are haploid.

s According to Anna K. Whiting (1925), "the black-eyed patroclinous
males show a higher percentage of morphological abnormalities than do
males and females normally produced. The majority of patroclinous males
tested have been sterile, some have bred as blacks and been partially fertile,
while a few mosaics have produced orange-eyed daughters and have been
fully fertile. The orange-eyed daughters of patroclinous males are normal
in morphology and fertility. The black-eyed daughters of patroclinous males
are few in number and show a large percentage of abnormalities and are
almost completely sterile." The exceptional males in Hadrobracon may ex-
plain some of the anomalous cases that have been recorded in honey bees.

CHAPTER XVI

INTERSEXES

IN recent years some curious individuals have been
found in species with separate sexes, that combine
to varying- degrees the characters of males and fe-
males. At present most of these intersexes, or sex inter-
grades, may be referred to four sources: (a) to changes
in the ratio of the sex-chromosomes to the rest of the
chromosomes ; (b) to changes in the genes not visibly
connected with changes in chromosome number; (c) to
changes that result from crossing wild races, and (d) to
changes in the environment.

Intersexes from Triploid Drosophila.

To the first class of intersexes belong some of the off-
spring of triploid females of Drosophila. When the eggs
of a triploid female mature, the chromosomes are irregu-
larly distributed, and, after the polar bodies have been
given off, the eggs are left with different numbers of
chromosomes. If such a female is mated to a normal male
whose sperm carries one set of chromosomes, the off-
spring that come through are of several kinds (Fig. 138).
There is reason to believe that many eggs do not develop
at all, because they lack the right combination to produce
a new individual; but amongst the survivors there are
some triploids, more diploids (normals), and a few inter-
sexes. These intersexes (Fig. 139) have three sets of
autosomes and two X-chromosomes (Fig. 138). The for-
mula is 3a+2X (or 3a+2X+Y). Thus, although the
intersex has the same number of X-chromosomes as has

INTERSEXES 241

an ordinary female, it has one set more of the ordinary
chromosomes. It is clear from this that sex is determined
not by the actual number of the X-chromosomes present,
but by the ratio of these to the other chromosomes.

Diploid Triploid Tetraploid

2a-f2X=$ 3a+3X=$ 4a+4X=$

Za-r X+Y=d Sa+X4Y= Super d 4a+2X+Y=cf

3a4-2X= Inters ex
2>a+2X+Y=

3a(-IV)+2X=

> >

■>■>

2>a(-IV)+2X=
+Y

Fig. 138.

Diagram giving the formulae of normal, triploid, tetraploid, and

intersexes of Drosophila melanogaster. (After Bridges.)

From these exceptional relations amongst the chromo-
somes, described by Bridges, he concluded that sex is
determined by a balance between the X's and the other
chromosomes. We may think of the X-chromosomes as
containing more of the genes that go to produce a female,
and the rest of the chromosomes as containing more of
the genes that go to produce a male. In the normal fe-
male, 2a+2X, the two X's turn the scale toward female-
ness. In the normal male there is only one X, and the
balance turns the other way. The triploid, 3a+3X, and
the tetraploid, 4a+4X, have the same balance as the
normal female and are practically identical with her. The

242

THE THEORY OF THE GENE

expectation for the tetraploid male, 4a+2X-f-Y (that has
not yet been obtained), is that he will be like the normal
male, since the balance is the same in both.

This evidence from triploids gives no specific informa-
tion as to the occurrence of genes for sex-determination.
If we think of the chromosomes only in terms of genes, it
follows that genes are involved, but the evidence does not

SuperfemaJ

Fig. 139.

Supermale

Supersedes of Drosophila. The superfemale has three- sets of auto-
somes and three X-chromosomes. The supermale has three sets of
autosomes and an X- and Y-chromosome. (After Bridges.)

Twc —

show what they are like. Even if genes are involved, we
cannot state whether there is one gene in the X that
stands for femaleness, or hundreds of such genes. Simi-
larly for the ordinary chromosomes — the evidence does
not tell us whether the genes for maleness, if there be
such, are in all the chromosomes or in only one pair.

There are, however, two ways in which we may hope,
some day, to discover something about the genes that
influence sex. The X-chromosome may become frag-
mented in such a way as to reveal the location of the spe-
cial genes relating to sex, if there are such. The other
hope rests on the occurrence of a gene mutation. If other

INTERSEXES 243

genes mutate why not sex-genes, if there are such specific
genes f

There is, in fact, one certain case of the occurrence of
an intersex that arose by a mutant change in the second
chromosome of Drosophila. Sturtevant, who has studied
this case, found that it is due to a change in genes in the
second chromosome. The female is turned into an inter-
sex. Unfortunately, the evidence does not show whether
or not a single gene only was affected.

It is apparent, from what has been said, that while we
can interpret the sex-determining formulae in terms of
genes, we have no direct evidence, at present, that there
are any specific genes for maleness and femaleness.
There may be such genes, or it may be that sex is deter-
mined by a quantitative balance between all the genes.
But since we have much evidence that the genes differ
amongst themselves very greatly as to the kind of effects
that they produce, it seems probable, I think, that certain
genes may be more influential as sex differentials than
are other genes.

Intersexes in the Gypsy Moth.

Goldschmidt has carried out an extensive series of very
interesting and important experiments in the production
of intersexes in racial crosses of the gypsy moth.

When the female of the common European gypsy moth
(Fig. 140a, b) is crossed to a Japanese male, equal num-
bers of male and female offspring are produced. When
the cross is made the other way the sons are normal, but
the daughters are intersexes or male-like females (Fig.
140c, d).

Later Goldschmidt carried out an elaborate series of
crosses between the European species and several Japa-
nese species and also between different races of Japanese
varieties or species. The results may be arranged in two

244

THE THEORY OF THE GENE

series. In one series the females are finally all changed
over into males ; in the other series the males are changed
over into females. The former change is spoken of as
female intersexuality ; the latter, as male intersexuality.
Without attempting to review the long series of experi-
ments from which the evidence has come, Goldschmidt 's
theoretical deductions may be stated as briefly as possible.

Fig. 140.

a, Male and b, female of Lymantria dispar; c and d two inter-
sexes. (After Goldschmidt.)

The formula he uses for the male is MM and for the
female Mm ; in other words, the WZ-ZZ formula. In addi-
tion, however, Goldschmidt adds another set of sex-de-
termining factors that at first he called FF, which stand,
in a way, for femaleness. The male factors are supposed
to segregate, as do Mendelian factors in general, but the
FF factors do not segregate and are transmitted only
through the egg. They were supposed to reside in the
cytoplasm, although Goldschmidt has later shown an in-
clination to locate them in the W-chromosome.

INTERSEXES 245

By assigning numerical values to the big M's (none to
the m) and to FF he has built up a scheme to show how,
in the cross first mentioned, equal numbers of males and
females result when the cross is made in one direction,
and intersexes when it is made in the opposite direction.

In like manner, by assigning arbitrary values to the
letters in each of the other crosses a more or less con-
sistent account can be given of the results.

The unique feature of these formulas of Goldschmidt
is not, in my opinion, the numerical values attached to
the factors, for these are arbitrary, but the statements
that the results can be explained only by the assumption
that the factors for femaleness are in the cytoplasm, or
else in the W-chromosome. In this respect his view runs
counter to the conclusions to which we have come from a
study of the triploids in Drosophila, where the opposing
influences are in the X-chromosomes and in the auto-
somes.

Goldschmidt has recently (1923) reported a few ex-
ceptional cases in which the evidence indicates, he be-
lieves, that the female-producing factors lie in the W-
chromosome. One such case relates to certain racial
crosses, where, through non-disjunction, a female re-
ceives a W-chromosome (Y in his formula) from the
father and the Z from the mother. This is the reverse of
the ordinary transmission of these chromosomes. The
results indicate that the female factors follow the W.
Logically, the evidence appears satisfactory, but on the
other hand both Doncaster and Seiler have reported a
few exceptional female moths in which the W-chromo-
some is at times absent. These moths were normal females
in every respect and bred as such.1 They could not be

i There are 56 chromosomes present in the female and in the male of
Abraxas. That one of those in the female is a W-chromosome is very prob-
able, from Doncaster 's discovery of a strain in which the females have only

246 THE THEORY OF THE GENE

females, on Goldschmidt 's view, if the female factors are
in the W-chromosome.

Before leaving Goldschmidt 's theories a very interest-
ing suggestion that he has made to account for the mosaic
character of the intersexes must be mentioned. The inter-
sex consists of parts that are male and parts that are
female — patches of each. Now Goldschmidt suggests that
this is brought about by a difference of time at which the
male and the female parts are determined in the embryo.
Expressed in a different way, one may say that in certain
combinations of the sex factors of the racial hybrid-
intersexes, the individual starts as a male. The organs of
the embryo that are the first to be laid down are therefore
male-like. In later stages, the female factors overtake
and surpass the male-producing ones, so that the later
stages of the embryo are like the female. Hence the
mosaic characters for this one class of intersexes.

Conversely, in the reciprocal type the embryo starts
under the influence of the female factors, and the first
parts of the embryo to be laid down are female-like. In
later stages the male-producing factors overtake and sur-
pass the female tendencies, and male organs develop.

This is his theory in broad outline. When examined in
detail doubts arise, since it is bound up with assumptions
concerning enzymes that are philosophical rather than
chemical. Moreover, the male- and the female-producing
factors are identified as the genes themselves. Such an
interpretation of the process is at present purely specu-
lative. Furthermore, his basal assumption, namely, that
whichever enzyme starts first, it is overtaken later by the

55 chromosomes. The absence of one chromosome, presumably the W, pro-
duces no visible changes in the character of the female. That the missing
chromosome is really a sex-chromosome and not an autosome is highly
probable from the fact that individuals lacking it are always females.

INTERSEXES 247

other competing enzyme, really begs the entire question,
since this is not a recognized feature of enzyme behavior.

The Free Martin.

It has long been known that when twins in cattle are
born, one of which is a normal male, the other a "fe-
male," the latter is usually sterile. It is known as a free
martin. The external genitalia of the free martin are

Fig. 141.

Two embrvo calves, one of which will become a free martin, whose

placentas are united. (After Lillie.)

generally female, or much more female-like than male,
but it has been demonstrated that the gonads may resem-
ble testes. It was shown by Tandler and Keller (1911)
that the twins (one of which is a free martin) come from
two eggs, and Lillie (1917) has fully confirmed this fact.
It was also shown by Tandler and Keller that there is
present a vascular connection between the two embryos
in utero by means of intra-chorionic connections (Fig.
141). Magnussen (1918) described a considerable number
of free martins of various ages, and has shown by his-

248 THE THEORY OF THE GENE

tological examination that well-developed testicular-like
organs are present in older free martins, i.e., that the
characteristic tubular structure of the testes, including
rete tubules, sexual cords, and epididymis, is present.
Chapin (1917) and Willier (1921) have confirmed these
observations, and the latter especially has given a de-
tailed account of the transformation of the "indifferent
stage" of the ovary into a testis-like structure.

Magnussen (who erroneously believed the free martin
to be a male) found no spermatozoa in the "testes."
Their absence he believed to be due to the retention of
the testes within the body cavity (cryptorchidism). It is
known that in those mammals in which the testes nor-
mally descend into the scrotal sacs, sperm-cells are absent
when the testes are retained, but in the early embryo
germ-cells appear while the testes are still within the
body cavity. In the free martin there are, according to
Willier, no primordial germ-cells present in the so-called
testis.

Lillie's conclusion that the free martin is a female
whose gonads have been transformed into a testis-like
organ is so strongly supported by this evidence that it
can scarcely be questioned, but whether the effect is to be
referred to the composition of the blood of the male, or,
as he thinks, to an ovarian hormone in the blood is open
to question, since there is at present no evidence of any
specific substance produced by the gonad of the male
embryo that produces such an effect on the development
of the young ovary. Since all the tissues of the male
embryo have the male chromosomal composition, the
blood may likewise have a different chemical constitution
from that of the female, and affect, in consequence, the
development of the gonad. It is generally recognized that
the young gonads have rudiments of both ovary and testes
present, or, as Willier puts it, "the primordium of each

INTERSEXES 249

male structure developed in the free martin gonad is
present in the ovary at the time of sex differentiation. '
The most significant fact in these observations is the
absence of male germ-cells in the free martin. The influ-
ence of the blood of the male co-twin does not bring about
the transformation of the primordial egg cells into
sperm-producing cells.

Individuals with both male and female sexual organs,
even including ovaries and testes, have been frequently
recorded in mammals, including man. These were for-
merly called hermaphrodites, but now are sometimes
called intersexes or sex intergrades. The conditions that
give rise to them are unknown. Crew reports twenty-five
cases in goats, seven in pigs.2 These, Crew believes, are
modified males, since testes were present in all of them.
Baker has recently reported that the sex intergrading
pigs are surprisingly common on some of the islands
[New Hebrides] ; "one finds them in nearly every little
village. ' ' This tendency to sexual abnormality is inherited
through the male in some cases reported by him. Baker
regards them as probably transformed females.3

2 Pick and others had earlier described such individuals, two in horses, one
in sheep, one in cattle.

3 Prange has described four hermaphroditic goats with external female
genitalia, but with undeveloped mammae. In sex behavior and in coat they
were male-like. Internally both male and female ducts were present, but
the gonads were testes (cryptorchid).

Miss Harman has described a " gynandromorphous " cat that had a
testis on the left side and an ovotestis on the right side. The reproductive
system of the left side is like that of a normal male, while that of the right
side is like that of the female, except for the si2e, etc., of the uterine tube.

CHAPTER XVII

SEX REVERSALS

IN the older literature dealing with sex-determina-
tion the idea is often expressed that the sex of the
embryo is determined by the environmental condi-
tion under which the embryo develops. In other words,
the young embryo has no sex, or is indifferent, and its
fate is determined by its environment. It is unnecessary
to go over again the evidence from which this idea origi-
nated, since practically all of it has been shown to be
defective in one way or another.

In recent years there has been some discussion con-
cerning the reversal of sex, which means, by implication,
that a male, already determined as such, can become
changed into a female, and vice versa. It has even been
suggested that, if this can be shown to occur, the genetic
interpretation of sex is discredited or even overthrown.
It is scarcely necessary to point out that there is nothing
in the theory of sex as determined by sex-chromosomes
or genes contradictory to the idea that other influences
may so affect the development of the individual as to
change or even reverse the balance normally determined
by the genes. To fail to appreciate this is to fail entirely
in grasping the ideas that underlie the theory of the gene ;
for this theory postulates no more than that in a given
environment such and such effects are expected as a
result of the genes present.

It is no more surprising that a genetic male might, in
an abnormal environment, turn into a female, or vice
versa, than that an individual might at one stage of its

SEX REVERSALS

251

development function as a male and at a later stage as
a female. It remains, then, entirely a question of fact
whether evidence can be produced proving that an indi-
vidual having the genetic make-up of a male may, under a

Fig. 142.
Spider crab, a, normal male; a', abdomen of normal male from
below; b, normal female; &', abdomen of normal female from
below; c, parasitized male; c' , abdomen of parasitized male from
below. (After Geoffrey Smith.)

different set of conditions, become a functional female,
or the reverse. Several such cases have been reported in
recent years which call for a careful and unprejudiced
scrutiny.

Environmental Changes.

It was shown by Giard in 1886 that when the males of
crabs are parasitized by other crustaceans, such as Pelto-

252 THE THEORY OF THE GENE

gaster or Sacculina, they then develop external charac-
ters like those of a female. In Fig. 142a, an adult male
crab is shown, with its large claws, and in a' the under
side of its abdomen with the copulatory appendages, and
in b an adult female is shown, with her small claws, and
in b' the under side of her abdomen with the setose bifur-
cated egg-carrying appendages. In c is shown a male
that has been infected at an early stage; the claws are
small, resembling those of the female, and the abdomen is
broad and female-like ; in c' the under side of the abdo-
men of the infected male is shown. It has small bifurcated
appendages like those of the female.

The parasite sends long root-like processes into the
body of the crab, on which the parasite lives by absorbing
the juices, and may, in turn, set up physiological proc-
esses in the crab itself. The testes of the crab may not
at first be affected, but later may degenerate. In one case,
at least, where the parasite had fallen off, Geoffrey Smith
found large germ-cells developing in the regenerating
testis, which he interpreted as eggs.

Giard left open the possibility as to whether the change
in the crab was due to the absorbtion of the testis, or
whether the action was more direct on the host. Geoffrey
Smith has brought forward some evidence relating to
fat in the blood, and certain arguments in favor of the
view that the change in the crab is due to the physiologi-
cal effects on the host. There is no evidence in Crustacea
that the destruction of the gonads affects the secondary
sexual characters.

In insects, where there is evidence from castration, it
has been shown that the removal of the testes or ovary
does not alter the secondary sexual characters. It is all
the more significant, therefore, that in one case described
by Kornhauser, in one of the bugs (Thelia) that is para-
sitized by a hymenopter (Aphelopus), the male shows

SEX REVERSALS 253

the secondary characters of the female or at least fails
to develop those of the male.

While most of the decapod Crustacea have male and
female sexes there are a few cases where both ovaries
and testes are present in one or in both sexes, and there
are a few cases where the young males may have large
egg-like cells in the testes. Several crayfish have also been
described that are sex intergrades, but no complete re-
versals are known.1

In Daphnians, and related forms, intersexual individ-
uals have been described by several observers (Kuttner,
Agar, Banta, etc.), but no complete reversals are known.
Sexton and Huxley have recently described some indi-
viduals of Gammarus that are called female intersexes,
which, ''on reaching maturity, more or less closely resem-
ble females but gradually come to resemble males more
and more nearly."

Most of the barnacles are hermaphroditic. In some
genera there are, in addition to the large sessile hermaph-
rodites, minute complemental males, and there are a few
other species with sessile female individuals and comple-
mental males. The sessile individuals are generally sup-
posed to be true females, but Geoffrey Smith has sug-
gested that if a free-swimming larva becomes fixed it
grows to full size, passing through the male stage to be-
come a female, but if a free-swimming larva attaches
itself to a female it develops no further than the male
stage. This seems to mean no more than that the environ-
ment determines whether a potential individual develops
into a female or being arrested in its development be-
comes a male.

The last case is similar to another in the gephyrean
worm, Bonellia, described by Baltzer. If a free-swimming

i See Faxon, Hay, Ortman, Andrews, Turner.

254 THE THEORY OF THE GENE

larva attaches itself to the proboscis of a female it remains
extremely small and develops testes, but if it settles down
by itself it becomes a large female individual. The evi-
dence does not positively rule out the possibility that
there are two kinds of individuals that behave in one or
the other way, but Baltzer's interpretation seems very
probable.

If the correct interpretation for the barnacles and for
Bonellia is that suggested above, it means that sex is
determined in these forms by environmental conditions,
which means, in terms of genes, that all the individuals
are alike.2

Changes of Sex Associated with Age.

Biologists are familiar with several cases both in ani-
mals and in plants where an individual may first function
as a male and later as a female, or vice versa. But the
special cases in which sex reversal takes place are those
whose sex is known to be determined in the first place
by their chromosomal make-up, yet which are said, in
rare cases, to turn into the opposite sex without changing
their chromosome complex.

The hagfish, Myxine, according to Nansen and Cun-
ningham, is male when young, and later becomes female ;
but the subsequent observations of the Schreiners indi-
cate that while the young Myxine is hermaphroditic — the
anterior end of the gonad being a testis, the posterior
an ovary — it is not so functionally. Later each individual
becomes definitively male or female.

Breeders of the aquarium fish, Xiphophorus helleri,
have reported at various times that females change into
males, but, unfortunately, as yet there is no account of

2 According to Gould, if a young individual of Crepidula plana settles
down near a female it becomes at first a male and remains so permanently;
but if it settles down away from large individuals it fails to develop testes
and passes later into a female.

SEX REVERSALS 255

the sex of the offspring produced by these transformed
females, although ripe sperm has been found in one case
at least. Recently Essenberg has studied the development
of the gonads in the young of this fish. At birth the fish
measures 8 mm. and the gonads are in an "indifferent
stage," containing two kinds of cells of peritoneal origin.
At 10 mm. the sexes are distinct; in the females the pri-
mordial germ-cells gradually change into young eggs ; in
the male the definitive germ-cells (sperm-cells) come
from the peritoneal cells. In the immature condition, be-
tween 10 and 26 mm. in length, Essenberg records 74
females and 36 males, counting amongst the females the
retrogressive types, i.e., those in process of transforma-
tion from "females" to "males." The sex ratio of adult
fish, taken from Bellamy's records, is given as 75 5 to
25 9 . The change does not appear to be due to differential
viability, but to "sex inversion." This occurs most com-
monly in fishes from 16 to 27 mm., but may occur in later
stages also. The data indicate, then, that approximately
half of the "females" become males. This statement does
not mean, however, that functional females have changed
into males, but that half of the young "females" are
identified as such by the presence of an ovary, which
later changes into a testis. Whether functional females
may later become functional males, as breeders believe,
is as vet not so well established.

A change of functional females into individuals with
the secondary male characters in another fish, Glary-
dichthys janarius, has been reported by Philippi, and
similar changes in two other species by Herzenstein.

A curious case has recently been described by Junker
in the stonefly, Perla marginata. The young males (Fig.
143) pass through a stage in which an ovary is present
that contains rudimentary eggs (Fig. 143). The male has
an X- and a Y-chromosome and the female two X's (Fig.

256

THE THEORY OF THE GENE

144). The ovary in the male disappears when the insect
becomes adult, and the testes produce normal sperma-
tozoa. In this instance, then, we must infer that, in the
young stages of the male, the absence of one X does not

Pe-pla marginata

ovary

Fig. 143.

Perla marginata to the left. The ovotestis of a young male to the

right. (After Junker.)

suffice to suppress the development of an ovary, but when
the individual becomes adult its chromosome composition
asserts itself.

Sex and Sex Reversal in Frogs.
It has been known ever since the work of Pfliiger in
1881-1882 that sex ratios in young frogs are peculiar, and
that, at the time of metamorphosis of the tadpole into

SEX REVERSALS 257

the frog, the gonads often appear to be intermediate. The
classification of individuals of this sort as male or female
has led to much dispute. In recent years it has been
shown that these intermediate forms often become males,
and it has even been claimed that in many races all males
pass through this stage.

y
Spermatagonia DiPloid Male ^

'<•

«»*^

Oogonia

Is Spermatocyte

Fig. 144.

Chromosome groups, spermatogonia and oogonia, and diploid male
egg of Perla. (After Junker.)

The experiments of Richard Hertwig have shown that
by delaying the fertilization of the frog's egg, the pro-
portion of males is greatly increased, and, in extreme
cases, all individuals become males. The attempts to cor-
relate these cases of retarded fertilization with chromo-
somal alterations have not been successful.

258 THE THEORY OF THE GENE

Further study lias shown that the earlier results were
obscured by failure to realize that different races of frogs
show remarkable differences in the development of the
testes and ovaries. Witschi has shown that in general
there are two kinds or races of the European grass frog,
Rana temporaria. In one of these the testes and ovaries
differentiate directly from the early gonad. Such races
are found in the mountains and in the far North. In the
other races, living in the valleys and in the middle of
Europe, the gonad in those individuals that will become
males passes through an intermediate stage in which
large cells are present in its interior which he regards
as immature eggs. These are replaced later by a new
set of germ-cells that become the definitive sperm. These
races are called undifferentiated races.

Swingle also finds in the American bullfrog two types
or races, speaking broadly, in one of which the testes and
the ovaries differentiate early from the progonad. In the
other races the differentiation is delayed. In the female
of these races the larger cells of the progonad become
later the definitive eggs, but in the male the progonad
persists for some time after the female has differentiated.
Its large cells may differentiate into spermatozoa. These
are, however, later absorbed for the most part, but some
of those that remain undifferentiated become the defini-
tive sperm-cells. Swingle does not interpret the large
cells in the male progonad as eggs, but as male sperma-
tocytes. He shows that these cells pass through an abor-
tive maturation division and then, for the most part,
break down. In other words, the male does not pass
through a female stage, but makes, as it were, an abortive
attempt to form sperm before its second and later differ-
entiation takes place.

Whatever interpretation is placed on these large cells
in the progonad, the important point for present con-

SEX REVERSALS 259

sideration is whether external or internal conditions may
affect the progonad of the prospective female in such a
way that it produces later functional sperm-cells. "Wit-
schi's evidence is in favor of such a transformation in
those races that are indifferent.

In the following table (Table III) Witschi has brought
together the sex ratios reported by different observers
from different parts of Germany and Switzerland. In the
right-hand column the per cent of females is given; 50
per cent means a 1 to 1 ratio. It will be seen that in the
first two groups (Group I and II) the sex ratio is ap-
proximately 1 to 1, while in the last three groups (III,
IV, V) the proportion of females is higher, culminating
in those regions where all the individuals from a pair
may be females (100 per cent). These belong to the indif-
ferent races.

The most important facts discovered by Witschi relate
to the inheritance of these differences shown by the dif-
ferentiated and undifferentiated races. He made the fol-
lowing crosses between females and males of the differ-
ent races.

(1) 9 undif.by $ differ.=69 undif. 9 + 545
(2)9 dif. by S undif. =34 9 + 52 S

In (1) the daughters were all undifferentiated; in (2) the
daughters differentiated early. He draws the conclusion
that the eggs of a differentiated race are more strongly
female-determining than the eggs of an undifferentiated
race.

In another experiment he crossed undifferentiated
races whose " female determining power" (Kraft) was
greater or less, and concluded that weak eggs by strong
determining sperm gave the same result as strong eggs
by weak sperm. ' l Eggs and female determining sperm of
the same type have the same genetic constitution. ' '

260

THE THEORY OF THE GENE

TABLE III

Sex Ratios in Different Local Races of the Grass Frog Shortly
after Metamorphosis (at most two months)

Those with asterisk were caught in the open.

CrTOU})

Locality

Author

X timber

of
Animals

Examined

Percent of
Females

Ursprungtal

(Bayr. Alpen) . .
Sertigtal, Davos

(Ratisehe Alpen) .
Spitalboden (Grim-

sel, Berneralpen) .

Konigsberg .

Witschi (1914 b) .

Witschi (1923 b) .

Witschi . . .
Witschi ....

Pfliiger (1882) . j

490
814

46*

272
370
500*

44.5
51.5
53

Elsass (Mm) . . .

Witschi ....
Witschi ....
Witschi ....
v. Griesheim und J
Pfliiger (1881-82) \
v. Griesheim (1881)

424

471

290

806

668*

245*

405

62.5

Rostock ....

Witschi ....

Pfliiger (1882) . .

Lochhausen

(Munchen) . . .
Dorfen (Munchen) .

Utrecht ....

Witschi (1914 b) .
Schmitt (1908) . .

Pfliiger (1882) . j

221
925*

780
459*

83
85
87

Freiburg (in Baden)
Breslau ....
Breslau ....

Elsass (r) . . . .

Irschenhausen ( Isar-

tal siidl. Munchen)

Witschi (1923 a) .
Born (1881) . . .
Witschi ....
Witschi ....

Witschi (1914) . .

276
1,272
213
237
241

100

Total ....

10,483

The chromosome composition of frogs has been in dis-
pute for several years, not only as to the number of chro-
mosomes present, but as to whether the male or the female
is digametic. The most probable number of chromosomes
for several species seems to be 26 (n=13). Other num-

SEX REVERSALS 261

bers (24, 25, 28) have, however, been reported. According
to the most recent account, that of Witschi, Rana tem-
poraria has 26 chromosomes, including a slightly un-

%> 1* ffo tf w*

Fig. 145.
Chromosome groups of the frog Eana temporaria. a, Diploid male
group, b and b' anaphase plates of first spermatocyte division each
showing thirteen chromosomes, c and c' ditto, d division of XY-
chromosome of first spermatocyte, e, separation of X and Y at
second spermatocyte division. (After Witschi.)

equal XY pair in the male (Fig. 145). If this is confirmed,
the female is XX (homogametic) and the male XY (het-
erogametic).

Pniiger (1882), Richard Hertwig (1905), and later
Kuschakewitsch (1910) have shown that overripe eggs
give an increased percentage of males. In so far as these

262 THE THEORY OF THE GENE

experiments were not made with the same male for the
same sets of eggs, the results are doubtful. Hertwig him-
self points out there are many resemblances between the
effect of cold and that of overripening. Many embryos
are deformed. Witschi has confirmed Hertwig 's results
(with the Irschenhausen race). Eggs estimated to be 80
to 100 hours overripe gave 74 $ , 21 2 , 20 indifferent tad-
poles.3

Oscar Hertwig compared the sex ratio of normal and
delayed eggs (67 hours' interval) with the following re-
sults. Larvae 49 days old (just before metamorphosis)
that came from normal fertilization gave 46 indiffer-
ent 5 ; those from 'delayed fertilization, 38 indifferent 2
and 39 $ . The normal frogs about 150 days old were
differentiated females, indifferent females as to gonads,
and males (numbers not given), and from the delayed
eggs 45 indifferent 2 and 313 $ . Yearling frogs gave
6 2 and 1 $ (normal fertilization) and 1 2 and 7 $ (de-
layed fertilization). The overripeness here would seem
to hasten the male differentiation and in the second place
transform the indifferent individuals (here ranked undif-
ferentiated females) into males.

The interpretation of the results produced by over-
ripening the eggs is still very obscure. Taken at their
face value they seem to show that individuals that would
normally become females may become males. As yet no
genetic tests have been made of the sex-determining
properties of the spermatozoa of individuals obtained in
this way. Theoretically, these should be homogametic. It
seems improbable that such individuals could live and
function under natural conditions, for, although over-
ripeness must not infrequently occur, normal males giv-
ing 100 per cent females are practically unknown. Wit-

3 There was 20 per cent mortality in the tadpoles and 35 per cent in the
young frogs.

SEX REVERSALS 263

schi has pointed out that the overripe eggs undergo an
unusual type of cleavage, and that a few embryos that he
examined show internal defects, but the relation of these
defects to the transformation of females into males is not
apparent.

The possibility of transforming individuals having an
undifferentiated or juvenile hermaphroditic gonad (or
progonad) into females by external agencies is furnished
by the following evidence from Witschi's experiments
(1914-1915).

Tadpoles of the Ursprungtal race, that is, probably a
differentiated race, gave, at 10° C, 23 males and 44 fe-
males; at 15° C, 1315 and 140 5 ; and at 21° C, 115 $
and 104 9 . The sex of the tadpoles of this race is appar-
ently not affected by temperature.

On the other hand, tadpoles of the Irschenhausen race
reared at 20° C. gave 241 undifferentiated females, and 6
lots reared at 10° C. gave 25 $ and 438 9 . From this re-
sult Witschi concludes that cold is a male determining
factor, but it should not be overlooked that many of these
so-called females would later develop into male frogs.
In a later account of these experiments he states that
"cold changes the males into protogynous juvenile her-
maphrodites as is in general normal for undifferentiated
races. ' '

It seems questionable, therefore, whether there is any-
thing more here than a retardation of the definitive male
condition.

In so far as it is possible to reach a provisional conclu-
sion from the evidence available at present, it appears that
in the undifferentiated races the germ-cells, that are pres-
ent in half the individuals that would normally become
females, may be changed over into sperm-cells, or else
be replaced by cells from a different source that, in turn,
become sperm-cells. In other words, the balance of the

264

THE THEORY OF THE GENE

genes that ordinarily suffices in frogs to give males or
females may be "overridden" by environmental factors
and testes may develop in an individual whose internal
chromosomal balance would produce a female. Stated in
another way, this may mean that each frog is capable of
developing both testes and ovary ; that under normal cir-
cumstances the XX individual develops only the ovary

a b c

Fig. 146.
Three hermaphroditic frogs. (After Crew and Witschi.)

and the XY individual a testis, but under exceptional con-
ditions a female of the XX type may develop a testis.
The possibility of the reciprocal change has not been
demonstrated.

There are many records of "hermaphrodite" adult
frogs (Fig. 146). Crew has listed 40 recent cases. Whether
these hermaphrodites are related in any way to the in-
versions just described is unknown. It is significant per-
haps that a few individual hermaphrodites have also been
reported from those experiments. On the other hand, it is
possible that some of the hermaphrodites have a different
origin. There is not much evidence that they can be ex-
plained as gynandromorphs or mosaics due to elimination

SEX REVERSALS 265

of the sex-chromosomes, because only very rarely is there
indication of asymmetry of the accessory organs outside
the gonads, and the gonad tissue is frequently irregu-
larly distributed. Furthermore, if the evidence that the
sperm and eggs of hermaphrodites are both homogametic
is valid, the ground of a possible explanation due to chro-
mosome elimination is removed.

From a hermaphrodite (Hh) Witschi was able to ob-
tain ripe sperm and eggs. He tested these with sperm
and eggs from a differentiated race with the following
results

(1) Eggs dif. 9 by sperm from herm.= $ 2

(2) Eggs herm. by sperm from dif. 3 = 50%9 + 50%3

The eggs of the hermaphrodite were also fertilized by
sperm of the same individual and gave 45 $ and one her-
maphrodite, thus

(3) Eggs herm. by sperm from herm.=45 $ -fl herm.

These results can be interpreted to mean that the original
hermaphroditic female was XX. Each ripe egg carried
one X. Likewise each functional sperm must also have
carried one X. There seems to be no escape from one or
the other conclusion, either that every sperm carries an
X, or else half carry X, half no X, but the latter die in the
female (i.e., never become functional).4

4 Crew (1921) has also reported the result of successful fertilization of
the eggs of a hermaphrodite (Fig. 14) with its own sperm. In each tadpole
the development of the gonad was direct. All the offspring (774) that were
sufficiently developed to determine the sex were female. The mother may be
regarded as a true XX female that produced eggs and sperm, each with an
X-chromosome.

It is conceivable, but perhaps not probable, that the testis in the her-
maphroditic females is due to elimination of one of the X-chromosomes in a
somatic division, and that the no X-sperm die. (See above.)

266

THE THEORY OF THE GENE

The Transformation of Bidder's Organ of the
Male Toad into an Ovary.

The anterior part of the testis of the male toad is com-
posed of rounded cells that resemble young egg-cells

Fig. 147.

Bidder's organ at the anterior end of the testis of a half -grown
male of a California species of Bufo. The lobes of the fat bodies
lie at the sides, the kidney beneath. The testes are indicated by the
branching blood vessels in their walls, the Bidder's organs lie in
front of the testes each consisting of several lobes.

(Fig. 147). It is conspicuous in the young toad even be-
fore the germ-cells in the more posterior part or testis
proper have differentiated. The anterior part is called
Bidder's organ and has for many years excited the inter-

SEX REVERSALS

267

est of zoologists, who have proposed many views as to its
possible functions. The most frequent interpretation is
that the Bidder's organ is an ovary and the resemblance
of its cells to eggs lends strong support to this interpre-
tation; but the presence of a Bidder's organ at the ante-

FiG. 148.
Toad in third year from which the testes have been removed at
an early stage. Bidder's organ has developed into an ovary. In the
figure (to the right) the ovary is turned to one side in order to
show the enlarged oviduct. (After Harms.)

rior end of the true ovary in the young female is difficult
to bring into accord with the view that in the male the
same organ is an ovary, for, if so, the female has a rudi-
mentary or perhaps ancestral rudimentary ovary in
front, and a functional one behind it.

The recent experimental work of Harms, and especially
that of Miss K. Ponse, has shown that when the testes are
completely removed from a young toad, the organ of Bid-
der develops after two or three years into an ovary with

268 THE THEORY OF THE GENE

eggs (Fig. 148). The eggs have been deposited and
fertilized and observed to develop. There can be here
no doubt but that a female has arisen after removal
of the testes, but whether the individual operated upon
is to be called a male or an hermaphrodite is perhaps a
question of definition. Personally, I should call the above
toad a male, and interpret the result to mean that a male
has been transformed into a female by removal of the
testes. It seems to me a matter of secondary importance
that the male toad carries an organ whose cells are poten-
tially capable of developing into egg-cells, for, in general,
even when sex is determined by a chromosomal mecha-
nism, there is no implication that under changed condi-
tions undifferentiated cells situated in that part of the
body where the gonads develop might not become egg-
cells even with the chromosomal complex that gives rise
to a male under other circumstances. In terms of genes,
this means that in the toad the balance of the genes is
such that under the normal conditions of development
one part of the gonad (the anterior end) begins to de-
velop into an ovary, while another part (the posterior
end) begins to develop into a testis. The latter overtakes
the former as development proceeds and holds its further
development in check. If the testicular end is removed,
however, this control is lost, and the cells of Bidder's
organ proceed to develop into functional eggs. If this
interpretation is correct and if a sex-chromosomal mecha-
nism is present in toads (which has not as yet been
certainly demonstrated), the mature eggs that come from
Bidder's organ should have the same chromosomal com-
plexes (possibly an X- or a Y-chromosome each) as have
the ripe sperm of the male. Crossed to a normal male the
offspring would then be 1 XX+2 XY+1 YY. If the YY
embryo fails to develop there should be twice as many
sons as daughters.

SEX REVERSALS 269

Champy lias described a case of "total sexual inver-
sion" in Triton alpestris. A male triton that had func-
tioned as a fertile male was subsequently starved. Under
these circumstances the normal renewal of the sperm
does not take place, but the animal remains in a sort of
"neutral condition,'' characterized by the presence in the
testis of primitive germ-cells. It remains in this condition
throughout the winter. Two male tritons that had been
treated in this way, underwent, after they had been in-
tensively renourished, a change in color from male to
female. One of these examined several months later fur-
nished evidence that Champy interprets as sex inversion.
Since this case has been cited recently as furnishing com-
plete evidence of sex inversion, it may be worth while to
give a somewhat detailed statement as to what Champy
really records. In place of the ovaries he found an elon-
gated organ resembling somewhat a young ovary. When
sectioned it was found to contain young egg-like cells
("ovocytes") resembling those of a young triton at the
stage of metamorphosis. An oviduct was also apparent,
recognizable by its white color and sinuous course.
Champy concludes we have here an adult animal with
the ovary of a young female. The evidence seems to indi-
cate that the treatment led to the absorption of the sper-
matocytes and sperm. It does not indicate clearly whether
the new cells that come to replace them are enlarged
spermatogonia or primitive germ-cells or young ova. In
the light of other evidence in the Amphibia (Witschi,
Harms, Ponse) it may seem not improbable that these
cells are in reality young egg-cells and that a partial
inversion has taken place.

Sex Reversal in Miastor.

In flies belonging to the genera Miastor and Oligarces
there is a generation consisting of sexual winged males

270 THE THEORY OF THE GENE

and females that appear at the end of a long succession
of generations of maggots reproducing by partheno-
genesis.

The eggs laid by the winged females are supposed to
be fertilized by sperm from the winged males and develop
as far as the maggot (larval) stage. These maggots, with-
out passing on to the adult stage, produce eggs that de-
velop by parthenogenesis. From these eggs a new genera-
tion of maggots arises that repeats the process. This
continues throughout the year, the maggots living under
the bark of dead trees, and in some species on mushrooms.
In the spring or summer, winged males and females
appear from eggs laid by the last generation of maggots.
The appearance of the winged forms seems to be con-
nected with some change in the environment. Recently
Harris has shown that when the cultures become crowded,
owing to the presence of many maggots, the adult insects
appear if suitable conditions prevail, while if the mag-
gots are reared in isolation, or in small numbers, they
continue to reproduce in the larval stages (paedogene-
sis). The effective factors in crowding are not known.
If young from a single individual maggot are reared
together, and if their offspring in turn are kept in the
same culture, etc., it has been found by Harris that when
the adult flies appear they are of one sex in each such
culture. This seems to mean that each individual maggot
is either male or female in genetic constitution, and re-
produces by parthenogenesis the same sex. If this is the
correct conclusion, it follows that both the male-deter-
mined maggots and the female-determined maggots pro-
duce functional eggs. As yet we have no evidence relating
to the distribution of the sex-chromosomes in these flies.

There is, here, an example of male-determined indi-
viduals producing parthenogenetic eggs at one phase of
the life cycle and spermatozoa at another phase.

SEX REVERSALS 271

Sex Reversal in Birds.

It has long been known that old hens, and hens with
ovarian tumors, may develop the secondary plumage of
the male, and. that they sometimes show characteristic
male behavior. It was also known (Goodale) that after
the complete removal of the single left ovary of a young
chick, the bird, when mature, develops the secondary
sexual characters of the male sex. Both effects may be
interpreted on the hypothesis that the normal ovary of
the hen produces some substance that suppresses the full
development of the plumage. When the ovary is diseased
or removed the hen then develops the full possibilities of
her genetic composition as seen ordinarily only in the
male.

It is also known that hermaphroditic fowls occur in
which both ovaries and testes may be present, although
neither, as a rule, is fully developed, and it may or may
not be significant that in most of these cases the gonad
contains a tumor. There is some doubt here whether the
hermaphroditic condition came first, and the tumor later,
or, the ovary of a normal hen becoming tumorous, a testis
began later to develop. In none of these cases is there
evidence of sex reversal in the sense that the bird func-
tioned at one time as a female and later as a male. One
case has, however, been recently reported by Crew (1923)
in which a hen is said to have laid eggs and reared chicks
(from them?) and later to have become a functional male
that fertilized two eggs of a normal hen. Concerning the
second part of the story there seems to be no question,
since the results were obtained under controlled condi-
tions, but the previous history of this hen is not perhaps
above suspicion, since it was apparently an unrecorded
member of a small flock and no evidence by direct obser-
vation or by trap nesting is given that she was known to

272 THE THEORY OF THE GENE

lay eggs. When killed the bird was found to have exten-
sive tumor growths in the situation of the ovary. ' ' Incor-
porated in the dorsal aspect of this mass, there was a
structure exactly resembling a testis, while another, simi-
lar in appearance, was situated in like position on the
other side of the body." Every stage of spermatogenesis
was present in the testes. On the left side "a thin straight
oviduct could be identified having a diameter of 3 mm.
in its widest part near the cloaca. ' '

A second case has been recorded by Riddle. A ring
dove functioned first as a female, laying a series of eggs.
She ceased later to lay eggs, and frequently acted as a
male in courtship and copulation. Many months later she
died with very advanced tuberculosis. She was opened
and under misapprehension that she was her mate (a
male that had died YIV^ months earlier) was recorded as
a male. Later, when her number and record were deter-
mined, it was found that she had been the female, but the
"testes" had been thrown away. There is here no record
that the bodies identified as testes contained sperm.

The Effect of Ovariotomy in Birds.

The complete removal of the single left ovary of young
chicks is a rather difficult operation. In 1916 Goodale car-
ried out several successful operations of this kind. The
birds developed the full plumage of the male. Goodale
also reported the presence on the right side of a rounded
body with tubules which he compared with early ne-
phrogenous tissue. Benoit has also recently described the
effect of ovariotomy on young birds. In general, the effect
on the plumage, comb, and spurs is the same as in Good-
ale's birds, but in addition he describes the development
of a testis or testis-like organ in the situs of the rudimen-
tary right "ovary," and sometimes a similar organ in
the place of the left ovary removed. In one case germ-

SEX REVERSALS 273

cells in all stages of ripening and even spermatozoa
(pycnotic) were found. This single case calls for care-
ful scrutiny, since it is, so far, the only recorded case of
the presence in the testis-like organs of spermatozoa, or
even distinctive germ-cells. The left ovary had been re-
moved from a bird twenty-six days after hatching. At six
months its comb was red, turgid, stood upright, and was
as large as that of a cock. An organ "resembling a testis
had developed on the right side." Histologically it was
found to contain seminiferous tubules containing all
stages of spermatogenesis. The nuclei of the spermatids
were pycnotic and the spermatozoa, few in number, ap-
peared abnormal. The efferent canal of the male extended
from this body to the cloaca. There was also present at
the base of the testis a tubular structure resembling the
epididymis of the young cock. The presence of sperma-
tozoa in the testis-like organ is the only record of this
kind. In the other birds operated on by Benoit, in which
testis-like bodies developed, no germ-cells were found.
May it not be possible that in the above case a mistake
had been made and that the bird was in reality a male?
It should be added that Benoit found, after removing its
testis, the comb shrank and the bird came to resemble a
capon. In other cases, no such decrease of the comb has
been reported. Still, it is just possible that the presence
of testis-like organs that were present with sperm in
them may be held responsible for the full development
of the comb and wattles. Another bird, ovariotomized at
four days after hatching, described by Benoit, showed
at four months an unusual organ. An examination re-
vealed, on the right side, a testicle-like organ. No report
of its contents is made.

Benoit examined the histological structure of the right
rudimentarv ovary of a voung normal female. He de-
scribes it as identical with an epididymis of a young male

274 THE THEORY OF THE GENE

having efferent ciliated canals and "rete testis." He con-
cluded that the right gonad of birds is not a rudimentary
ovary, but a right rudimentary testis that enlarges when
the left ovarv is removed to become a testis. The evidence
does not, I think, necessitate this conclusion, for it is
known that in the early stage of development of the
reproductive organs in vertebrates, the essential acces-
sory organs of the male and the female are present in
both sexes. It is possible, therefore, that upon interfer-
ence with the normal process of development (removal
of the left ovary) these rudimentary organs may begin
to develop and produce a testis-like structure, which, in
most cases so far reported, does not contain sperm-cells.
The occurrence of globular organs (reported by Goodale
and Domm) on the left side also would seem to support
this view, rather than that advanced by Benoit.

A preliminary report of the results of ovariotomy in
young birds has recently been given by L. V. Domm
(1924). The birds when they become adult not only show
secondary male characters in their plumage, comb, wat-
tles, and spurs, but fight with normal cocks, crow, and
attempt to tread hens. One bird had a "white testis-like
organ" in the position of the normal ovary (removed).
Associated with the organ was also a small ovarian fol-
licle. On the right side there was also a testis-like organ.
A second bird was similar as to its gonads. In a third
bird a testis-like organ was present only on the right side.
In none of these cases are germ-cells or spermatozoa
reported as present.

Whether these cases are strictly sex reversals cannot
be definitely stated, unless Benoit 's observation on the
presence of sperm is confirmed. Aside from this unique
statement, the other results appear to show definitely
that, after removal of the ovary, a structure develops re-
sembling in its appearance a testis (except for the pres-

SEX REVERSALS 275

ence of germ-cells). The development of this organ, after
castration, can, I think, be provisionally at least accounted
for by a secondary growth and enlargement of the funda-
ments of the male organs that are known to be present in
the embryonic stage. The maintenance of a testis, even a
functional one, in a female body is not in itself surpris-
ing, since it is known that pieces of testis, grafted into
the body of a female, may continue to develop, and even
to produce sperm.

In general, it appears that the genetic composition of
the female bird (present both in the body-cells and in the
young ovary) creates a favorable situation for the de-
velopment of the ovary, rather than a testis. Conversely,
in the male the genetic composition is favorable for the
development of the testis. In the male, however, the early
removal of the testis does not suffice to call forth the
development of structures peculiar to the ovary.

The Sex of Parabiotic Salamander Twins.

The union of young salamanders by side-to-side fusion
has been brought about by several embryologists. The
young embryos taken from the egg, just after closure of
the medullary folds, have portions of one side of each
removed and are then brought in contact by the exposed
surface. Their union quickly follows. Burns has studied
the sex of the united (parabiotic) twins. He found that
members of a pair were always of the same sex ; 44 pairs
were both males, 36 pairs were both females. Random
union would give 1 pair of males to 2 pairs of male-
females to 1 pair of females. Since no double-sexed pairs
appear, it follows either that pairs of opposite sexes die,
or that the sex of one individual changes over that of the
other and, since both male and female pairs were found,
the influence is sometimes one way, sometimes the other
way. Unless some explanation can be found for such a

276 THE THEORY OF THE GENE

difference in the reciprocal effects, the results do not
convincingly demonstrate the probability of the latter
interpretation.

Sex Reversal in Hemp.

Many of the flowering plants develop both pistils con-
taining egg-cells and stamens containing pollen in the
same flower, sometimes in different flowers on the same
plant. It is not uncommon for the pollen to ripen before
the ovules, or, in other cases, the ovules before the pollen.
In other plants, the ovules may develop only on one plant,
and the pollen on another plant, i.e., the sexes are sepa-
rate, the species dioecious. In some of these dioecious
plants, however, the organs of the opposite sex may occur
as rudiments; occasionally they become functional. Cor-
rens has studied a few cases of this kind, and has at-
tempted to test the character of the germ-cells of such
exceptional cases.

More recently experiments with dioecious hemp (Can-
nabis sativa) by Pritchard, Schaffner, and McPhee have
shown that environmental conditions may change a pistil-
producing plant (or female) into one in which stamens
and even functional pollen are also produced, and, con-
versely, may change a staminate plant into one producing
pistils containing functional eggs.

When hemp seeds are planted at the normal time in
early spring they produce male (staminate) and female
(carpellate) individuals in about equal numbers (Fig.
149), but Schaffner has found that when planted in rich
soil accompanied by a changed light period, the plants
show "sex reversal" in both directions. "The amount of
reversal is approximately inversely proportional to the
length of daylight." That the same environment should
change carpellate into staminate, and staminate into car-
pellate plants is at first sight rather surprising, for

SEX REVERSALS

277

one might anticipate that identical conditions would tend
to bring each toward a neutral or intermediate condition
or one only toward the other. In fact, something like this
seems to take place, for on a carpellate plant stamens

Fig. 149.
Female plant, to left, and male plant, to right, of hemp. (After

Pritchard.)

appear; conversely, on a staminate plant, pistils may
appear. It is in this sense, in the main, that ' ' sex rever-
sal" occurs, although there are other cases still in which
a new branch of a pistillate plant may develop only sta-
mens, and a new branch of a staminate plant develop only
pistils. In these extreme cases ' ' sex reversal ' ' may almost
be said to take place in those new parts that develop

278 THE THEORY OF THE GENE

under changed conditions. McPhee, who has also studied
the effect of exposure to light for different lengths of
time, has found that male plants may produce branches
with pistils, and vice versa ; but he points out that many
intersexual flowers also appear as well as many abnormal
flowers. He states "that the changes produced are in
many cases relatively minor ones and a sweeping conclu-
sion that genetic factors are in no way concerned with
sex in these species is not warranted at the present
time. ' '

The question as to whether there is an internal sex-
determining factor system — possibly chromosomal — in
hemp, is at present unanswered, and as yet we have only
an oral report by McPhee concerning the genetic evi-
dence, but this report is significant. If the normal female
hemp plant is homogametic (XX) and the male hetero-
gametic, then we may expect when a female is trans-
formed into a male (or more accurately produces func-
tional pollen) that all the pollen grains will be alike as to
their sex-determining properties, i.e., such a male is
homogametic. McPhee 's oral report5 supports this view.
Conversely, if the male (XY) is transformed into a fe-
male, then two kinds of eggs are expected. This seems to
be realized.

Correns had earlier reported somewhat similar results
in other plants but the data relating to the kinds of
gametes produced are not satisfactory. It is to be hoped
that evidence will soon be available that bears on this
question. Assuming, in the meantime, that there is an
internal mechanism for sex-determination in hemp (pos-
sibly of the XX-XY type), there is nothing revolutionary
in the discovery that sex reversal may take place through
environmental agencies, and there is certainly nothing
in these results that is, in principle, in contradiction to

5 At the meeting of the Zoological Society, 1925.

SEX REVERSALS 279

the presence of a sex-chromosomal mechanism that is sex-
determining. Such a mechanism is an agent that tips the
scale one way or the other under a given set of environ-
mental conditions. The mechanism has never been under-
stood in any other way. It may be overborne by other
agents that turn the scale without thereby losing its
power to act in its usual way when the conditions return
under which it is accustomed to work. No better example
of this relation could we hope to find, if the tentative con-
clusions stated above are confirmed, namely, the change
of a homogametic female into a homogametic male in a
species in which the normal male is heterogametic. This,
in fact, would furnish another convincing proof of the
genetic explanation of sex-determination, and one that
would be especially instructive for those who fail to
understand the interpretation that geneticists place on
this mechanism and on Mendelian phenomena in general.

Another plant, Mercurialis annua, has separate sexes
but rarely a pistillate flower appears on a male plant,
and, conversely, a staminate flower on a female plant. A
male plant may have 25,000 male flowers and only from
1 to 47 pistillate flowers, while the staminate flowers on a
female may be as 1 to 32.

Yampolski has reported the sex of offspring produced
from both these kinds of plants after self-fertilization.
Offspring from selfed female plants are female or pre-
dominantly female. Offspring of selfed male plants are
male or predominantly male.

It is not possible at present to give a satisfactory ex-
planation of these results on the XX-XY formula unless
rather arbitrary assumptions are made. For instance, if
the female plant is XX, then all the pollen grains she
produces should carry one X, hence all the offspring
should be females, as was the case. But if the male plant
is XY, half the mature eggs should be X and half Y. Simi-

280

THE THEORY OF THE GENE

larly for the pollen. Self-fertilization should then give
1 XX+2 XY+1 YY. If YY dies there should be one fe-
male to two male offspring. This, however, was not the
result obtained. In order that the selfed male plants
should produce only males it must be assumed that the
X eggs die as gametes and the Y eggs only are func-
tional. As yet there is no evidence either for or against
this hypothesis. Until there is evidence bearing on this
question the case must be left open.

CHAPTER XVIII

STABILITY OF THE GENE

IN what has been said, so far, it has been implied that
the gene is a stable element in heredity, but whether
it is stable in the sense that a chemical molecule is
stable, or whether it is stable only because it fluctuates
quantitatively about a persistent standard, is a question
of theoretical and perhaps of fundamental importance.

Since the gene cannot be studied directly by physical
or chemical methods, our conclusions concerning its sta-
bility must rest on deductions from its effects.

Mendel's theory of heredity postulates that the gene
is stable. It assumes that the gene that each parent con-
tributes to the hybrid remains intact in its new environ-
ment in the hybrid. A few examples will serve to recall
the nature of the evidence for this conclusion.

The Andalusian race of poultry has white, black, and
blue individuals. If a white bird is mated to a black one,
the offspring are slate-colored or blue. If two of these
blue-colored birds are mated, the offspring fall into three
classes, black, blue, and white, in the proportion of 1 : 2 : 1.
The gene for white and the gene for black separate in the
blue hybrid. Half the mature germ-cells come to carry
the black-producing element and half the white-produc-
ing element. Chance fertilization of any egg by any sperm
will give the observed proportions 1:2:1 in the second
filial generation.

The test of the correctness of the assumption that the
germ-cells of the hybrids are of two kinds is as follows.
If a blue hybrid is back-crossed to a pure white bird, half

282 THE THEORY OF THE GENE

the offspring will be blue and half white. If a blue hybrid
is back-crossed to a pure black bird, half the chicks will
be black and half blue. Both results are consistent with
the postulate that the genes of the blue hybrid are pure,
half for black and half for white. Their occurrence in the
same cell has not resulted in contamination or mutual
infection.

In the example just given the hybrid is unlike either
parent and, in a sense, is intermediate between them. In
the next example the hybrid is indistinguishable from one
parent. If a black guinea pig is bred to a white one, the
offspring are black. If these are inbred, the offspring are
three blacks to one white. The extracted whites breed as
true as the original race of whites. The white gene has
not been contaminated by the black gene in their sojourn
together in the hybrid.

In the next example a case is chosen in which the two
original forms are much alike, and the hybrids, while
intermediate to some degree, are so variable that, at the
ends of the series, they overlap the parental types. The
types differ in a pair of genes.

If an ebony Drosophila is bred to a sooty one, the off-
spring are, as stated, intermediate, but variable. If these
are inbred, they produce an array of shades that give a
practically continuous series. There are ways, however,
of testing the grades. When this is done it is found that
the array is made up of individuals that are pure for
ebony, others that are hybrids, and others that are pure
for sooty, in the ratio of 1 : 2 : 1. Here again we have evi-
dence that the genes have not been mixed. The continuous
series of shades is merely due to overlapping variability
of the characters.

All this is simple and clear because we are dealing in
each case with a single pair of genes that act as differen-
tials. These cases serve to establish the principle at stake.

STABILITY OF THE GENE 283

In practice, however, the actual conditions are not
always so simple. Many types differ from each other in
several genes, each of which has an effect on the same
character. Consequently, when they are crossed simple
ratios are not found. For example, if a race of corn with a
short cob is crossed to a race with a long cob, the next
generation has cobs of intermediate length. If these are
inbred the following generation has cobs of all sizes.
Some are as short as the cobs of one of the original races,
others as long as the original long. These stand at the
ends. Between them is a series of intermediate sizes. A
test of the individuals of this generation shows that there
are several pairs of genes that affect the size of the cob.

Height in man is another such case. A man may be tall
because he has long legs, or because he has a long body,
or both. Some of the genes may affect all parts, but other
genes may affect one region more than another. The
result is that the genetic situation is complex and, as yet,
not unraveled. Added to this is the probability that the
environment may also to some extent affect the end-
product.

These are the multiple factor cases, and students of
heredity are trying to determine in each cross how many
factors are present. The results are complex only because
several or many genes are involved.

It is this sort of variability that in the earlier days,
before Mendel's discovery had been made known, sup-
plied natural selection with the evidence on which that
theory was based. This question will be considered later,
but first must be described the great advance in our
understanding of the limitations of the selection theory
that was made in 1910 by Johannsen's brilliant work.

Johannsen carried out his experiments with a garden
plant, the princess bean. This bean reproduces exclusively
by self-fertilization. As a result of long-continued in-

284

THE THEORY OF THE GENE

breeding each individual has become homozygous. This
means that the two members of each pair of genes are
identical. Hence such material is suitable to carry out

^vvv*

r\n

hoiSS

A-B

Fig. 150.
A-E groups of beans representing five pure lines. Below A-E the
group formed of the combination of the other five. (After Johann-
sen.)

critical experiments to determine whether individual dif-
ferences shown by the beans are affected by selection.
If selection changes the character of the individual, it

STABILITY OF THE GENE 285

must, under these conditions, do so by changing the gene
itself.

The beans produced by each plant are somewhat vari-
able in size, and when arranged according to sizes they
give the normal curve of probability. All the beans from
any one plant and all of the descendents of this plant
have the same distribution (Fig. 150), no matter whether
large beans are continually selected, or small beans are
picked out in each generation. The offspring always give
the same groups of beans.

Johannsen detected nine races of beans in those he
examined. He interpreted his results to mean that the
differences in size of the beans from a given plant are
due to its environment in the widest sense. It was pos-
sible to demonstrate this with material in which the mem-
bers of each pair of genes were identical when selection
began. Selection is shown to have no effect in changing
the genes themselves.

When sexually reproducing animals or plants are
selected that are not homozygous at the start, the imme-
diate outcome is different. There are numerous experi-
ments showing what happens, such as Cuenot's results
with spotted mice, or McDowell's results with ear-length
in rabbits, or East and Hay's experiments with corn.
Any of these might serve as an example of what takes
place under selection. One example will suffice.

Castle studied the effects of selection of the color-
pattern of a race of hooded rats (Fig. 151). Starting with
the offspring of commercial animals, he selected in one
direction those rats that had the broadest stripes, and
in the other direction the rats that had the narrowest
stripes, keeping these two lines apart. In the course of a
few generations the two populations became measurably
different — in one the dorsal stripe was broader, on the
average, than in the original group of rats ; in the other,

286

THE THEORY OF THE GENE

the stripe was narrower. Selection had in some way-
changed the width of the stripe. So far there is nothing
in the results to show that this change may not have been
due to the sorting out by selection of two sets of factors
that determine the width of the dorsal stripe. Castle
argued, however, that he was dealing with the effect of a

Fig. 151.
Four types of hooded rats. (After Castle.)

single gene, because when the striped rats are crossed to
a rat with uniform coat (all black or all gray) and the
hybrid (F2) rats are inbred, their offspring give three
uniform to one spotted coat. This Mendelian ratio does
show, in fact, that a spotted coat is due to a recessive
gene, but it does not show that the effects of this gene
may not also be influenced by other genetic factors that
determine the width of the stripe, and this is really the
question at issue.

STABILITY OF THE GENE 287

A later experiment, devised by Wright and carried out
by Castle, showed, in fact, that the results had been due
to the isolation of modifying genes for width of stripe.
The test was as follows : Each of the highly selected races
was back-crossed to wild rats, that is, rats with uniform
coat color, and a second (extracted) generation of
spotted rats obtained. The process was repeated with
the (F2) spotted rats obtained from the first back-cross.
It was found after back-crossing for two or three genera-
tions that the selected stock began to change back, so to
speak, to its original state. The selected race with a nar-
rower stripe changed toward a broader stripe and the
selected race with a broader stripe changed toward a
narrower stripe. In other words, the two selected races
became more and more like each other, and more like the
original race from which they had started.

This result is completely in accord with the view that
modifying factors are present in the wild rats that affect
the width of the stripe in animals that are already
striped. In other words, the original selection had
changed the character of the stripe by sorting out those
genes that made it broader or narrower.

At one time Castle went so far as to claim that the
results of the experiments with hooded rats reestablished
a view that he ascribed to Darwin, namely, that selection
itself brings about a change in the hereditary materials
in the direction in which the selection takes place. If this
were really Darwin's meaning, such an interpretation of
variability might seem greatly to strengthen the theory
of natural selection as the method by which evolution has
taken place. Castle said in 1915: "All the evidence we
have thus far obtained indicates that outside modifiers
will not account for the changes observed in the hooded
pattern, itself a clear Mendelian unit. We are forced to
conclude that this unit itself changes under repeated

288 THE THEORY OF THE GENE

selection in the direction of the selection; sometimes
abruptly, as in the case of our 'mutant' race, a highly
stable plus variation ; but much of tener gradually, as has
occurred continuously in both the plus and the minus
selection series."

In the following year he said : ' ' Many students of ge-
netics at present regard unit-characters as unchangeable.
. . . For several years I have been investigating this
question, and the general conclusion at which I have
arrived is this, that unit-characters are modifiable as well
as recombinable. Many Mendelians think otherwise, but
this is, I believe, because they have not studied the ques-
tion closely enough. The fact is unmistakable that unit-
characters are subject to quantitative variation. . . .
Selection, as an agency in evolution, must then be re-
stored to the important place which it held in Darwin's
estimation, an agency capable of producing continuous
and progressive racial changes."

A careful reading of Darwin's books will fail to fur-
nish a single clear statement to the effect that he believed
that the selection process determines or influences the
direction of future variation, unless we bring into the
field another theory held by Darwin, namely, the theory
of inheritance of acquired characters.

Darwin held strongly to the belief in Lamarck's theory.
He did not hesitate to make use of it whenever his theory
of natural selection was in difficulty. It would be logical,
therefore, for anyone who cared to do so (although Dar-
win himself does not appear to have put the two views
together, nor does Castle) to point out that whenever a
more advantageous type is selected its germ-cells are
exposed, so to speak, to the pangenes produced by its own
body, and might be expected to be changed in the direc-
tion of the character selected. Hence each new advance
would start from a new base, and if scattering variations

STABILITY OF THE GENE 289

occurred about this as a new mode that overstepped the
previous boundary, further advances would be expected
to appear in the direction in which the last advance took
place. In other words, selection would bring about further
advances in the direction in which each selection had
taken place.

But, as I have said, Darwin never made use of this
argument in favor of his selection theory, although it
might be claimed he did so in principle whenever he
found natural selection inadequate to explain a situation
and appealed to Lamarck's principle to carry through
the new advance.

Today we regard the selection process, whether natural
or artificial, as capable, at most, of causing changes only
to the extent to which recombination of the genes already
present may affect a change ; or, in other words, selection
cannot cause a group (species) to transcend the extreme
variations that it naturally shows. Rigorous selection can
bring a population to a point where all of the individuals
are nearer to the extreme type shown by the original
population, but beyond this it cannot go. Only by the
occurrence of a new mutation in a gene, or by a mass-
change in a group of old genes, is it possible, as it now
appears to us, for a permanent advance — a step forward,
or backward — to be made.

This conclusion is not only a logical deduction from
the theory of the stability of the gene, but rests on numer-
ous observations showing that whenever a population is
subjected to selection, a rather rapid change begins, but
quickly slows down and soon comes to a standstill at or
near the extreme type shown by a few individuals of the
original population.

So far the problem of the stability of the gene has been
examined with respect to gene-contamination in the hy-
brid, and from the point of view of selection. The possible

290 THE THEORY OF THE GENE

influence of the body itself on the constitution of the gene
has been only touched upon. The clean separation of the
genes in the hybrid, that is the basal postulate of Men-
del's first law, would not be possible if genes were sub-
ject to influences from the bodily characters of the hybrid.

This conclusion brings us face to face with the La-
marckian theory of the inheritance of acquired charac-
ters. It would take us too far afield to attempt to consider
the varied claims of this theory, but I may be allowed to
call attention to certain relations that would be expected,
if, as this theory postulates, the germ-cells are affected
by the body in the sense that a change in a character may
bring about corresponding alterations in specific genes.
A few examples will illustrate the essential facts.

When a black rabbit is bred to a white rabbit the hy-
brid young are black, yet the germ-cells produced in this
hybrid are black- or white-producing, in equal numbers.
The black hair of the hybrid has no influence on the white
germ-cells. No matter how long the genes for white are
carried by black hybrids, the white genes remain white.

Now if the white gene is interpreted as an entity of
some sort, it should show, if the Lamarckian theory
holds, some effect of the body character of the individual
in which the gene is carried.

Suppose, however, the white gene is interpreted as the
absence of the black gene. Then, of course, there is no
reason for supposing that the black color of the hybrid
could produce any influence on nothing. To anyone hold-
ing the presence and absence theory this argument
against Lamarck's theory is not cogent.

There is, however, another line of approach that may
be more to the point. A white f our-o 'clock, bred to a red
one, produces an intermediate hybrid with pink-colored
flowers (Fig. 5). If we interpret the white color as an
absence, the red must be due to a presence. The color of

STABILITY OF THE GENE

291

the flower of the hybrid — pink — is weaker than the red,
and if the character affects the gene, the red-producing
gene in this hybrid should be diluted by the color of the
flower. No such effects here, or elsewhere, have been re-
corded. The red and the white genes separate in the pink
hybrid without showing any somatic effects.

Fig. 152.
a, Abdomen of normal male; &, of "abnormal" male; c, of nor-
mal female ; d, of " abnormal ' ' female of Drosophila melanogaster.

The evidence from another source is perhaps even a
stronger argument against the theory of the inheritance
of acquired characters. There is a race of Drosophila —
called abnormal abdomen — in which the regular banding
of the abdomen is more or less obliterated (Fig. 152).
This condition, in its most extreme form, appears in the
first flies that emerge from a culture when the food is
abundant and the culture is moist and acid. As the cul-
ture gets older and dryer, the flies that emerge become
more and more normal in appearance, until at last they
cannot be distinguished from wild flies. Here we have a
genetic character that is extremely sensitive to the
environment. Such characters as these furnish a favor-
able opportunity to study the possible effects of the body
on the germ-cells.

If we breed the first hatched flies with very abnormal

292 THE THEORY OF THE GENE

abdomens, and at the same time and under like conditions
we breed the late hatching flies with normal abdomens, we
obtain exactly the same kinds of flies in the next genera-
tion. The first to emerge are abnormal, the later ones
more normal. It has made no difference whether the abdo-
men of the parent was normal or abnormal, so far as the
germ-cells are concerned.

If it be said that the effects might be too small to be
seen at first, then I may add that late-hatching flies have
been bred from for ten successive generations without
any observed difference in the results.

Another example is equally convincing. There is a
mutant stock of Drosophila called eyeless (Fig. 30). The
eyes are smaller than the normal eye and very variable.
By selection, a uniform stock has been produced in which
most of the flies are without eyes, but, as each culture
gets older, more and more flies have eyes, and larger
ones. If, now, we breed from the late-hatched flies, the
offspring are the same as when eyeless flies are used.

Here the presence of eyes in the older culture is a posi-
tive character and might be considered to furnish better
evidence than the abnormal abdomen, where the sym-
metry and pigmentation of the late hatching larvae is
less obviously a present character. The outcome is, how-
ever, the same in both cases.

It is quite unnecessary to attempt to consider here the
numerous claimants that have appeared in the last few
years, who have furnished " proof," as they say, of the
inheritance of acquired characters. I choose only one
case, that is the most complete of its kind, since it gives
the numerical and quantitative data on which the conclu-
sions are based. I refer to the recent work of Diirken. The
experiment seems to have been carefully made and ap-
pears to Diirken to furnish proof of the inheritance of
acquired characters.

STABILITY OF THE GENE

293

Diirken worked with the chrysalids (or pupae) of the
common cabbage butterfly (Colias brassicae). Since 1890
it has been known that when the caterpillars of some
butterflies pupate (that is, when they transform into the
resting chrysalis) the color of the pupa is to some extent
influenced by the background, or by the color of the light
that falls on it.

Fig. 153.
In the center four differently colored pupae of the cabbage butter-
fly. Around them is shown the characteristic arrangement of the
pigment cells in the epidermis in different color types. (After
Leonore Brecher.)

For example, the pupae of the cabbage butterfly are
quite dark, if the caterpillars live and transform in day-
light, or even in a faint light ; but if the caterpillar lives
in yellow or red surroundings or behind a yellow or red
screen the pupae are green. The green color is due to the
absence of superficial black pigment. In its absence the
greenish yellow color of the interior shows through the
skin (Fig. 153).

294 THE THEORY OF THE GENE

Diirken's experiments consisted in rearing caterpillars
in orange (or red) light, where the pupae assumed a light
or green color. The butterflies that emerged were reared
in open cages and their eggs collected. Some of the young
from these eggs were reared again in colored light, others
in bright light or in darkness. The latter are the controls.
The results are summarized in the chart (Fig. 154). The
number of dark chrysalids is indicated by the length of
the black band, and the green or light ones by the light
band in the chart. As a matter of fact, the pupae were
classified in five color groups. Three of these were then
lumped together as dark and the other two as light.

As shown in Fig. 154 at 1 (which gives the normal
coloration), nearly all pupae, collected at random or in
normal surroundings, are dark; only a few are light or
green. The caterpillars that came from these were reared
in an orange environment. When they transformed into
pupae there was a very high percentage of light-colored
types, 2. If the light-colored types only are now picked
out and reared, some in orange, some in the light, and
others in the dark, the results are shown in 3a and 3b. In
the former, there are more light pupae than before ; since
two generations have been in orange, the effect is aug-
mented. It is the other set, 3b, however, that is more sig-
nificant. As the bands show there were more light pupae
than in the wild pupae, 1, that were reared in the light or
dark. This increase Diirken attributes in part to the in-
herited effect of the orange light on the preceding genera-
tion, and in part to the new environment, whose effect is
in the opposite direction.

Now this interpretation, from the point of view of
genetics, is not satisfactory. The experiment shows, in the
first place, that not all caterpillars respond to the orange
light. If those that do respond are genetically different,
then of course when they — the light pupae in the experi-

STABILITY OF THE GENE 295

merit — are selected for the second orange trial and for
the control in light and dark, we are dealing already with
a more responsive type, a selected group, and these are
expected to again respond in the next generation, as they
do in fact.

G=3,73

Normal
Coloring

Light

Dark

Fig. 154.

Diagram illustrating the results of selection of dark and light
pupae of the cabbage butterfly. (After Diirken.)

Therefore, unless the material is genetically homo-
geneous at the start or unless other controls are used, the
evidence fails signally to establish the inherited effect of
the environment.

The same error runs through nearly all the work of
this sort that has been done. Modern geneties, if it had
accomplished nothing more, would have justified itself
in showing the worthlessness of such evidence.

We may pass now to a group of cases in some of which
it seems probable that the germ-cells themselves have
been directly injured by special treatment, and that the
injured germ-material is transmitted to later genera-
tions. Owing to this injury, malformations may appear in
successive generations. This means that the treatment

296

THE THEORY OF THE GENE

has not affected the germ-material by first causing de-
fects in the embryo, but has affected both the embryo and
its germ-cells at the same time.

Stockard carried out a prolonged series of experiments
on the effects of alcohol on guinea pigs. The guinea pigs
were treated by placing them in closed tanks over strong

Fig. 155.
Two abnormal young guinea pigs whose ancestors were alcoholics.

(After Stockard.)

alcohol. They breathed the air saturated with alcohol,
and after a few hours became completely stupefied. The
treatment was carried over a long time. Some of the
guinea pigs were bred while undergoing treatment, others
onlv at the end of the treatment. The results were essen-
tially the same. Many young were aborted or absorbed,
others were born dead, others showed abnormalities,
especially in the nervous system and eyes (Fig. 155).
Only those that themselves showed no defects could be
bred. From these, abnormal young continued to appear

STABILITY OF THE GENE 297

along with other individuals normal in appearance. In
later generations abnormals continued to appear, but
only from certain individuals.

If we examine the pedigrees of the alcoholic series
there is no evidence that the results conform to any of
the known Mendelian ratios. Moreover, the varied locali-
zation of the effects shown by the abnormals is not of a
kind that resembles what we meet with when single gene-
changes are involved. On the other hand, the defects have
many points of resemblance to the kind of changes that
we are familiar with in experimental embryology when
abnormal development is brought about by treating eggs
with toxic agents. Stockard has called attention to these
relations, and interprets his result to mean that an injury
of some sort to the germ-cells has been produced by the
alcohol — an injury to some part of the machinery that is
involved in heredity. The effects are localized only in so
far as they pertain to those parts of the body that are
most sensitive to any departure from the normal course
of development. These parts are most frequently the
nervous svstem and the sense organs.

More recently Little and Bagg have carried out a series
of experiments on the effects of radium on pregnant mice
and rats. When the treatment is properly administered,
the young mice in utero may develop abnormally. When
examined before birth many of them show hemorrhagic
areas (Fig. 156) in the brain and cord, or elsewhere
(especially in the leg rudiments). Some of these embryos
die before parturition, and are absorbed, others are
aborted. Still others are born alive and some of these sur-
vive and may procreate. The offspring often show serious
defects in the brain or in the appendages. One or both
eyes may be defective. Both eyes may be absent, or one
only may be present, much reduced in size. Bagg has bred
some of these mice and finds that they produce many

298

THE THEORY OF THE GENE

abnormal offspring that show defects similar, in a gen-
eral way, to those induced directly in the original em-
bryos.

How shall we interpret these experiments? Has the
radium first produced its effects on the brain of the de-

Fig. 156.

Young mice embryos with hemorrhagic areas whose mother had
been treated with radium when the young were in utero. (After
Bagg.)

veloping embryo, causing defects, and is it owing to the
presence of these defects that the germ-cells of the same
embryo become affected? There is an apparent objection
to this interpretation. We should expect when the brain
alone is affected, the next generation should show brain
defects ; when the eye is the principal organ affected, the
next generation should show only eye defects. So far as

STABILITY OF THE GENE 299

reported the results are not like this, for a mouse with
abnormal brain and full-sized eyes may produce offspring
that have defective eyes. In other words, there is not here
a specific effect, but a general one.

The other interpretation is that the germ-cells of the
young mouse in utero are affected by the radium. When,
in turn, these germ-cells produce a new generation, the
individuals are defective because the same organs whose
normal development was most disturbed are the organs
that are most easily affected by any alteration in the
course of development. They are, in a word, the weakest
or most delicately balanced phases of development, and
therefore the first ones to show the effect of any depar-
ture from the normal course of events. This is, I think,
at present the most plausible explanation of these and
similar experiments.

CHAPTER XIX

GENERAL CONCLUSIONS

THE preceding chapters have dealt with two main
topics : with the effects following a change in the
number of the chromosomes ; and with the effects
following a change within a chromosome (a point muta-
tion). The theory of the gene is broad enough to cover
both these kinds of changes, although its main concern is
with the gene itself. The term mutation also has come,
through usage, to include the effects produced in both
these wavs.

These kinds of changes have important bearings on
current genetic theories.

The Effects Produced by a Change in Chromosome
Number and by a Change in a Gene.

When the number of the chromosomes is doubled, tre-
bled, or multiplied any number of times, the individual
has the same kinds of genes as before, and they stand in
the same numerical ratio to one another. There is no
a priori expectation that this kind of change would affect
the character of the individual, were it not that the vol-
ume of the cytoplasm may not increase to correspond
with the increase in the number of the genes. Just what a
failure to attain a corresponding increase of cytoplasmic
volume means is not clear at present. At any rate, the
results show that triploid, tetraploids, octoploids, etc.,
do not differ markedly in any special characters (except
size) from the original diploid type. In other words, the
changes produced may be very numerous, but not strik-
ingly different from the original ones.

GENERAL CONCLUSIONS 301

On the other hand, the addition of a single chromosome
or of two members of the same pair, or of two or more
members of different pairs to the group, or the loss of a
whole chromosome from the group, may be expected to
produce more evident effects on the individual. There is
some evidence that such additions or losses are less ex-
treme when many chromosomes are present, or when the
change takes place in a small chromosome. From the
point of view of the theory of the gene, this result is what
would be anticipated. For instance, the addition of one
chromosome means that a large number of genes are now
present in triplicate. The balance of the genes is changed
in the sense that there are now present more genes of
certain kinds than before, but since no new genes are
added the effects would be expected to be distributed
amongst many of the characters that might be somewhat
enhanced or diminished in intensitv. This accords with
the facts as yet reported. It is interesting to note, how-
ever, that, as far as known, the general results are not
beneficial but, if anything, deleterious. This, too, is ex-
pected if the adjustments, both to internal and to exter-
nal relations, are as perfect as possible in the normal
individual as its long evolutionary history might lead
one to expect.

Because such a change affects many parts to a slight
degree, it does not follow that such effects are more likely
to lead to the establishment of a new viable type than
when changes are brought about one step at a time by
changes in single genes.

Furthermore, even the addition of two new chromo-
somes of the same kind, giving possibly a new stable type
of inheritance, does not improve the situation, but, as far
as we know, — the evidence is slight at present, — the mal-
adjustments are even further increased. For these
reasons it does not seem that a change from one chromo-

302 THE THEORY OF THE GENE

some group to another is easily brought about in this'
way, although the possibility of such a change cannot
be entirely excluded. We need, at present, more evidence
to decide this question.

The same arguments apply, though less strongly per-
haps, to those cases when parts of chromosomes are
added to, or subtracted from, the chromosome group.
The effect produced is the same in kind, but less in de-
gree, and it is correspondingly more difficult to determine
whether the final effect on viability is injurious or bene-
ficial.

The work of the last few years in genetics has made it
clear that, despite the occurrence of the same number of
chromosomes in related species and even in entire fami-
lies and orders, it is hazardous to assume that the chro-
mosomes, even in closely related species, are always iden-
tical as to their genes. The genetic evidence is beginning
to make clear that readjustments may take place both
within the chromosomes, where groups of genes may
come to lie in reversed order, and between different chro-
mosomes, where blocks of genes may be shifted, without
giving a measurable difference in size. Even whole chro-
mosomes might be recombined in different groupings
without changing the actual number. Alterations of these
kinds will affect profoundly the linkage relations, hence
the modes of inheritance of the various characters, with-
out, however, changing the total number or kinds of the
genes involved. Unless, therefore, the cytological obser-
vations are checked by genetic studies it will always be
unsafe to assume that identity in number of chromo-
somes means a correspondence in grouping of the genes.

Two methods by which changes in chromosome num-
bers take place are, first, the union of two chromosomes
to form one, as in the attached X's of Drosophila, and
the occasional breaking apart of chromosomes, as re-

GENERAL CONCLUSIONS 303

ported by Hance in Oenothera and in several other cases.
The temporary separation and reunion of certain chro-
mosomes in moths, described by Seiler, also come under
this heading, especially if, as he supposes, the separated
elements may sometimes recombine reciprocally.

In contrast with the effect produced when large num-
bers of genes are involved, the effects produced by a
change in a gene appear at first sight much more ex-
treme. This first impression may, however, be very mis-
leading. While it is true that many of the most striking
mutant characters studied by geneticists are markedly
different from the normal character with which they are
contrasted, these mutant characters have often been
chosen for study because they are sharply marked off
from the typical character, and can, in consequence, be
readily followed in succeeding generations. Their sepa-
ration is accurate, and the results more certain than in
cases where the differences are less marked, or where
there is an overlap between the characters of the con-
trasted pair. Moreover, the more bizarre and extreme
modifications, that sometimes amount to "abnormali-
ties," are the ones that are most likely to attract atten-
tion and interest, hence are utilized for genetic study,
while the less obvious modifications are overlooked or
neglected. Geneticists are familiar with the fact that the
more intensively any particular group is studied the
more mutant characters are found which had been, at
first, overlooked, and since these are those that more
nearly approach the normal type, it becomes increasingly
evident that the mutation process involves very small, as
well as very great, modifications.

In the older literature the extreme, abnormal types
were called sports, and for a long time it was supposed
that these sports were sharply separated from the small
or individual differences constantly present in all species

304 THE THEORY OF THE GENE

and commonly spoken of as variations. Today we know
that there is no such sharp contrast, but that sports and
variations may have the same kind of origin, and are
inherited according to the same laws.

It is true that many of the small individual differences
are due to the environmental conditions under which the
development takes place, and superficial examination
fails often to distinguish between this sort of variability
and that due to minor changes brought about by genetic
factors. One of the most important results of modern
genetics is the recognition of this fact, and the invention
of methods by which the smaller differences may be re-
ferred to one or to the other of these factors. If, as Dar-
win supposed, and if, as is generally accepted today, the
process of evolution has taken place by the slow process
of accumulation of small variations, it follows that it
must be the genetic variations that are utilized, since
these, and not those due to environmental effects, are
inherited.

It must not be supposed, however, from what has just
been said, that mutant changes produce only a single
striking or even a single small change in one particular
part of the body. On the contrary, the evidence from the
Drosophila work, which is in accord with that from all
other forms that have been critically studied, shows that
even in those cases where one part is especially modified,
other effects are commonly present in several or in all
parts of the body. The subsidiary effects not only involve
structural modifications, but physiological effects also,
if one may judge by the activity, the fertility, and the
length of life of the mutants. For example, the loss of
positive phototropism, characteristic of Drosophila, ac-
companied a change involving a very slight alteration
in the general color of the body.

The converse of this relation must also hold. Slight

GENERAL CONCLUSIONS 305

changes clue to a mutated gene that affect physiological
processes and reactions may frequently be accompanied
by alterations in external structural characters. If these
physiological changes are of a kind to better adjust the
organism to its environment, they may be expected to
persist, and, at times, lead to the survival of new types.
These types may then differ from the original type in
superficial characters that are constant but trivial in
themselves. Since many species differences appear to be
of this kind, it is plausible to interpret their constancy
as due not to their own survival value, but rather due to
their relation to some other deeply seated character that
is important for the welfare of the species.

In the light of what has just been said we can give a
reasonable explanation of the differences that follow
when a mutant change involves a whole chromosome (or
part of one) and when only a single gene is involved. The
former change adds nothing intrinsically new to the
situation. More or less of what is already present is in-
volved in the change, and the effects are small in degree
but involve a large number of characters. The latter
change — mutation in a single gene — may also produce
widespread and slight effects, but, in addition, it often
happens that one part of the body is changed to a strik-
ing degree along with other changes less striking. This
latter kind of change, as I have said, supplies materials
favorable for genetic study; these have been widely uti-
lized. Now it is these mutational changes that have occu-
pied the forefront of genetic publication, and have given
rise to a popular illusion that each such mutant character
is the effect of only one gene, and by implication to the
fallacy, more insidious still, that each unit character has
a single representative in the germ material. On the con-
trary, the study of embryology shows that every organ
of the body is the end-result, the culmination of a long

306 THE THEORY OF THE GENE

series of processes. A change that affects any step in the
process may be expected often to affect a change in the
end-resnlt. It is the final visible effect that we see, not
the point at which the effect was brought about. If, as we
may readily suppose, very many steps are involved in the
development of a single organ, and if each of these steps
is affected by the action of a host of genes, there can be
no single representative in the germ-plasm for any organ
of the body, however small or trivial that organ may be.
Suppose, for instance, to take perhaps an extreme case,
all the genes are instrumental in producing each organ
of the body. This may only mean that they all produce
chemical substances essential for the normal course of
development. If now one gene is changed so that it pro-
duces some substance different from that which it pro-
duced before, the end-result may be affected, and if the
change affects one organ predominatingly it may appear
that one gene alone has produced this effect. In a strictly
causal sense this is true, but the effect is produced only
in conjunction with all the other genes. In other words,
they are all still contributing, as before, to the end-result,
which is different in so far as one of them is different.

In this sense, then, each gene may have a specific effect
on a particular organ, but this gene is by no means the
sole representative of that organ, and it has also equally
specific effects on other organs, and, in extreme cases,
perhaps on all the organs or characters of the body.

To return now to our comparison. The effect of a
change in a gene (which if recessive means, of course, .a
pair of like genes) frequently produces a more localized
effect than a doubling or trebling of the genes already
present, because a change in one gene is more likely to
upset the established relation between all the genes than
is an increase in the number of genes already present. By
extension, this argument seems to mean that each gene

GENERAL CONCLUSIONS 307

lias a specific effect on the course of development, and
this is not inconsistent with the point of view urged
above, that all the genes or many of them work together
toward a definite and complicated end-product.

The best argument at present in favor of a specific
action of each gene is found in the series of multiple alle-
lomorphs. Here changes in the same locus affect pri-
marily the same end-result not only in one organ, but in
all the parts that are also visibly affected.

Is the Mutation Process Due to a Degradation

of the Gene?

In his mutation theory de Vries spoke of types that we
now call mutant recessive types as arising from the loss
or inactivation of genes. Such changes he regarded as
retrogressive. At about the same time, or a little later,
the idea that recessive characters are due to losses of
genes from the germ material became popular. At the
present time several critics interested primarily in the
philosophical discussion of evolution have attacked with
violence the idea that the mutant types studied by geneti-
cists have anything to do with the traditional theory of
evolution. With this latter assertion we are not much con-
cerned, and may safely leave the question at issue for the
future to decide; but the suggestion that the mutation
process, in so far as it involves an effect on single genes,
is limited to the loss of genes or to their partial loss or
degradation, as I venture to call such a change, is a mat-
ter of some theoretical interest; for, as Bateson elabo-
rated in his 1914 address, it leads logically to the idea
that the materials that we use in genetic work are due to
loss of genes; that absences, in a literal sense, are the
allelomorphs of wild type genes ; and that, in so far as
this evidence applies to evolution, it leads to the reductio

308 THE THEORY OF THE GENE

ad absurdum that that process has been a steady drain
on the original storehouse of genes wherever they existed.

In chapter VI the genetic evidence at hand that bears
on this question has been considered, and it is unneces-
sary to summarize again what was there said, but I may
be allowed to repeat that it is not justifiable to conclude
from the fact that many mutant characters are defective,
or even partial or complete losses, that they must, there-
fore, be due to absences of a corresponding gene in the
germ material. So far as there is any direct evidence that
bears on this question, quite aside from the arbitrariness
of the absence hypothesis, it does not, as I have at-
tempted to show, support such a point of view.

There remains, however, a problem of some interest,
namely, whether some or many of the changes in the
genes that lead to the occurrence of mutant characters
(whether recessive, intermediate, or dominant makes
little difference) may not be due to a breaking up of a
gene, or to its reconstitution into another element pro-
ducing somewhat different effects. There is, however, no
reason for assuming that such change, if it occurs, is a
downhill one rather than the development of a more com-
plex gene, unless it appears more probable, a priori, that
a highly complex stable compound is more likely to break
down than to build up. Until we know more concerning
the chemical constitution of the genes, and how they grow
and divide, it is quite futile to argue the merits of the two
sides of the argument. For the genetic theory it is only
necessary to assume that any kind of a change may suffice
as a basis for what is observed to take place.

It is equally futile to discuss, at present, whether new
genes arise independently of the old ones, and worse than
futile to discuss how the genes arose in the first instance.
The evidence that we have furnishes no grounds whatso-
ever for the view that new genes independently arise,

GENERAL CONCLUSIONS 309

but it would be extremely difficult, if not impossible, to
show that they do not arise. To the ancients it seemed not
incredible that worms and eels arose from the river's
slime, and that vermin in general arose in dark dusty
corners. The origin of bacterial life from putrefying sub-
stances was believed in only one generation ago, and it
was extremely difficult to prove that this does not happen.
It may be equally difficult to prove convincingly, to one
who insists on believing the contrary, that genes arise
independently of other genes ; but the genetic theory need
not be anxious concerning this question until it meets
with a situation where such a postulate becomes neces-
sary. At present we find no need of interpolating new
genes in the linkage series, or at the ends of the series.
If the same number of genes is present in a white blood
corpuscle as in all the other cells of the body that con-
stitutes a mammal, and if the former makes only an
amoeba-like cell and the rest collectively a man, it
scarcely seems necessary to postulate fewer genes for an
amoeba or more for a man.

Are Genes of the Order of Organic Molecules?

The only practical interest that a discussion of the
question as to whether genes are organic molecules might
have would relate to the nature of their stability. By
stability we might mean only that the gene tends to vary
about a definite mode, or we might mean that the gene is
stable in the sense that an organic molecule is stable. The
genetic problem would be simplified if we could establish
the latter interpretation. If, on the other hand, the gene
is regarded as merely a quantity of so much material, we
can give no satisfactory answer as to why it remains so
constant through all the vicissitudes of outcrossing, un-
less we appeal to mysterious powers of organization out-
side the genes that keep them constant. There is little

310 THE THEORY OF THE GENE

hope at present of settling the question. A few years ago
I attempted to make a calculation as to the size of the
gene in the hope that it might throw a little light on the
problem, but at present we lack sufficiently exact meas-
urements to make such a calculation more than a specula-
tion. It seemed to show that the order of magnitude of
the gene is near that of the larger-sized organic mole-
cules. If any weight can be attached to the result it indi-
cates, perhaps, that the gene is not too large for it to be
considered as a chemical molecule, but further than this
we are not justified in going. The gene might even then
not be a molecule but only a collection of organic matter
not held together in chemical combination.

When all this is given due weight it nevertheless is
difficult to resist the fascinating assumption that the gene
is constant because it represents an organic chemical
entity. This is the simplest assumption that one can make
at present, and since this view is consistent with all that
is known about the stability of the gene it seems, at least,
a good working hypothesis.

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BIBLIOGRAPHY 333

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334 THE THEORY OF THE GENE

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336 THE THEORY OF THE GENE

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INDEX

abnormal abdomen, 291.

Abraxas, 61, 62, 208, 209, 245.

absence of genes, 71-77, 85.

Acer, 170.

Agar, 253.

Aida, 222, 223, 226.

albinos, 65.

allelomorphs, multiple, 92, 93.

Allen, 128, 215.

amphibia, 205.

Andalusian fowl, 281.

Angiostomum, 231, 232,

Anthothrips, 149.

Antirrhinum, 97, 98.

Aphelopus, 252.

aphids, 228, 229, 230.

Aplocheilus, 223.

Archieracium, 167.

Artemia, 108, 109.

Artom, 108, 109.

Ascaris, 38, 39, 107, 108, 136, 219,

220.
atavism, 85.
Atriplex, 171.
attached-X, 56-58.
Avery, 177.
avian type, 206.

Babcock, 194.
Bagg, 297, 298, 299.
Baker, 249.
balanced genes. 124.
Baltzer, 139, 254.

Bank, 149.

Banta, 253.

bar-eye, 86-91.

bar-infra-bar, 90, 91.

barnacles, 253.

barley, 150.

Bateson, 10, 17, 307.

Baur, 97, 98.

bee. 106, 144-146, 214. 235-237.

Belaf, 213.

Bellamy, 255.

Belling* 119-122, 132-134. 177,

180, 182, 185, 186.
Benoit, 272-274.
bent wings, 86.
Bidder's organ. 266-268.
birds, 271-275.
Blackburn, 158, 214.
Blakeslee, 118-124, 133, 134, 142,

143. 177-186.
Boedyn. 110. '«->

Boneilia, 253, 254.
Boveri, 38, 108, 142, 145. 220, 231.
brachydactyly in man, 64.
Brecher, 293.
Bremer. 170.
Bridges. 79, 80, 135. 136, 175. 241.

242.
Bufo, 266.
bullfrog. 25S-265.
Burns, 275.
butterflies, 206, 293.

Callitriche, 171.
Campanula, 171.

338

THE THEORY OF THE GENE

Canina roses, 159, 163, 164.

Carina, 133.

Cannabis, 276.

Cape bees, 237.

Carex, 170, 171.

Carothers, 35, 36.

Castle, 285-289.

cattle, 249.

Cavia, 99.

Champy, 269.

Cbapin, 248.

chart of chromosomes, 22, 23.

chiasmatype, 41.

chimaera, 113.

chromosome continuity, 37-39.

chromosomes and genes, 45-58.

chromosomes of Oenothera gigas,

109.
chromosomes of 0. Lamarkiana,

109.
chromosomes of sweet pea, 36.
chrysanthemums, 168, 169.

Clausen, 143, 192, 193.

Cleland, 110, 111.

Cocklebur, 181, 182.

Colias, 293.

Collins, 194.

combs of fowls, 72, 73.

corn, 82, 83, 85, 124, 196, 285.

corn, flinty, 83, 84.

corn, floury, 83, 84.

Correns, 96, 214, 276, 278.

coupling, 17.

crayfish, 253.

Crepidula, 254.

Crepis, 194, 195.

crossing-over, 14-22, 24

Crew, 249, 264, 265, 271.

Cuenot, 285.

Cunningham, 254.

curly, 63.

daphnians, 253.

Darwin, 67, 287-289, 304.

Darwin's pangenesis, 28.

Datura, 118, 119, 121, 122, 133,
134, 142, 143, 177, 179, 182,
183.

Davis, 110, 111.

Delage, 28.

Delia Valle, 107.

Detlefsen, 98.

Dinophilus, 231.

Domm, 274.

Doncaster, 245.

double-bar, 87.

double-infra-bar, 87.

double-X, 56-58.

Drosera, 171, 191.

Drosophila melanogaster, 11-23,
40, 41, 47, 48, 50-52, 59, 60,
64-66, 70, 75, 84-93, 99-101,
104, 135, 136, 175, 201-203,
241-243, 282, 291, 292, 304.

D. obscura, 103, 104.

D. simulans, 100, 101.

D. virilis, 102, 104.

Durken, 293-295.

East, 96, 285.

Einkorn, 151-153, 156, 157.

Elodea, 212, 213.

Emerson, S. H., 71, 190.

Emmer wheat, 152-157.

endosperm, 82-84.

Essenberg, 255.

Euchlaena, 123, 187.

Ewing, 149.

eyeless, 48, 49, 51, 52, 75, 292.

INDEX

339

Farnham, 133, 177.

Federley, 136, 137, 198, 211.

ferns, 214.

fish, 205.

flowering plants, 212.

four-o'clock, 5-7, 290, 291.

fowls, 72, 73, 206, 208.

free martin, 247.

frog's eggs, 139.

Fumea, 209, 210.

Galton, 66.

gametophyte, 125, 126.
Gammarus, 253.
Gates, 131, 172, 173.
Geerts, 131.
Geinitz, 219.
Gelei, 43.
gemmules, 29.
genes, 45-58.
germ-plasm, 28, 29.
Giard, 251, 252.
gipsy moth, 243, 244.
Glarydichthys, 255.
Goldschmidt, 222, 243-246.
Goodale, 271, 272, 274.
Goodspeed, 192, 193.
Gould, 254.
grasshoppers, 35, 36.
Gregory, 112.

guinea pig, 74, 98, 99, 205,
296.

Habrobracon, 237-239.
Hance, 303.

haplo-IV, 47-49, 81, 176, 177.
haploids, 139-149.
Hargreaves, 148.

282,

Harman, 249.

Harms, 267, 269.

Harris, 270.

Harrison, 158.

hawthorns, 171.

Hays, 285.

Heilborn, 170.

Helix, 99.

hemp, 276-279.

hermaphrodites, 249, 253, 254,

264.
Hertwig, G., 140.
Hertwig, O., 140, 262.
Hertwig, R., 258, 261, 262.
Hertzenstein, 255.
Hesperotettix, 221.
heteroploids, 172-190.
Hieracium, 165-167.
Hindle, 149.
honey bee, 144.
hooded rats, 285-287.
horse, 205.
Hovasse, 107.
Humulus, 212, 213.
Hurst, 158, 163, 164.
Huxley, 253.
hyacinth, 132, 133.
Hydatina, 147.
Hymenoptera, 206.

ids, 29, 30.
Indian corn, 37.
infra-bar, 87, 89, 90, 91.
infra-bar-bar, 90, 91.
insect type, 199.
intersex, 136, 241.

Janssens, 41, 42.
Jimson weed, 118, 177.

340

THE THEORY OF THE GENE

Johannsen, 283-285.
jungle fowl, 75.
Junker, 255-257.

Keller, 247.

Kihara, 151, 152, 154-157, 212.

Kornhauser, 252.

Kuschakewitsch, 261.

Kuttner, 253.

Kuwada, 187.

Lactuea, 171.

Lancefield, 103, 104.

Lang, 99.

Lamarck's theory, 30.

Lamarck, 289, 290.

lata types, 70.

Lebistes, 222, 226.

leghorn fowl, 75.

Lepidoptera, 206.

Lillie, 247, 248.

linear order, 22.

linkage, 10-12, 14-20, 24.

linkage groups, 22, 23, 36, 48.

Little, 297.

liverworts, 128, 149, 214-216.

Ljundahl, 197, 198.

lobe, 62, 63.

Longley, 123, 124, 171, 196.

loss of gene, 94.

Lutz, Anne, 131, 173.

Lygaeus, 200.

Lymantria, 244.

m-chromosome, 105.
McClung, 221.
MacDowell, 285.

McPhee, 276, 278.

Magnussen, 247, 248.

maize, 124.

man, 203, 204, 205.

man, eye color, 4, 5.

Mann, 143, 194.

map of the chromosomes, 22, 23.

maples, 170.

Marchal, El. and Em, 125, 126,

128, 214, 216, 218.
maturation of germ-cells, 33, 34.
May, 86.
mechanism of crossing-over, 39-

44.
Mehling, 145.
Melandrium, 213, 214.
Mendel, 72.

Mendel's laws, 1-25, 59.
Mercurialis, 279, 280.
Metapodius, 105, 106.
Metz, 102, 104.
Meves, 146.
Miastor, 270.
mice, 285.
Mirabilis, 6.
mites, 149.
Myxine, 254.
Mohr, 79.

de Mol, 124, 132, 133.
Morgan, H. A., 149.
Morgan, L. V., 82.
Morgan, T. H., 88.
Morrill, 148.
Morus, 169.

mosses, 124-128, 149, 214.
moths, 206.
mulberry, 169.
Muller, 112.

mutant characters, 59-71.
mutation theory, 67, 68, 95.

INDEX

341

Nachtsheim, 145, 231.

Nansen, 254.

Narcissus, 124.

Nematodes, 206, 231.

Newell, 237.

Nicotiana, 96, 143, 144, 192, 193.

nightshade, 112-116.

non-disjunction, 53-55.

notch wing, 77-81.

0
oats, 150.

Oenothera, 105, 131, 132.
0. franciscana, 111.
0. gigas, 70, 109, 110.
O. Lamarckiana, 69, 71, 109, 110,

172, 187, 188.
0. lata, 172-174, 189.
O. semilata, 172, 189.
Oguma, 205.
Oligarces, 270.
Ono, 212.
opossum, 205.
Osawa, 169.
Overeem, van, 131.

Painter, 203, 204.

pangenesis, 28.

Papaver, 171, 197, 198.

parabiotic twins, 275.

Parker, 149.

particulate theory of heredity, 26-

31.
pea comb, 72, 73, 74.
pea, edible, 2, 7-10, 36, 37.
pea, sweet, 10, 11.
Peltogaster, 252.
Percival, 151.
Perkins, 149.

Perla, 255-257.
Pfliiger, 256, 261.
Philippi, 255.
Phylloxerans, 228-230.
physiological units, 28.
Phragmatobia, 105, 211.
Pick, 249.
pigs, 249.
Planaria, 44.

planarian crossing-over, 43.
Plantago, 171.
Platanthera, 171.
Poinsettia, 177, 181, 182.
pollen grains, 10.
Ponse, 267, 269.
polyploid roses, 158-165.
Polyploids, 150-171.
poppy, 197, 198.
poultry, 207.
Prange, 249.
Primula sinensis, 112.
Pritchard, 276, 277.
Protenor, 200.
protonema, 126.
Punnett, 10, 17, 36.
Pygaera, 136, 137, 198, 211.

rabbit, albino, 74.
rabbit, black, 75.
rabbits, 285, 290.
radium, 139.
Rana, 258-265.
raspberries, 171.
rats, 74, 285-287.
recessive characters, 74.
recurrent mutations, 66.
repulsion, 17.
retrograde variety, 69.
reverse mutations, 85.
Riddle, 272.

342

THE THEORY OF THE GENE

ring dove, 272.

Rosenberg, 165, 166, 167, 191.

roses, 158-165.

Rosa, 163.

rose comb, 72, 73, 74.

rotifer, 147, 214, 233, 234, 235.

round worms, 206.

Rumex, 212, 213.

rye, 150, 158.

Saccharum, 170.

Sacculina, 252.

Sakamura, 152.

Salamander, 275.

Santos, 212.

Sax, 152, 155, 156.

Schaffner, 276.

Schleip, 231, 232.

Scbmidt, 128, 222, 226.

Schrader, 148.

scute, bristles, 86.

sea urchins, 206.

Seller, 105, 209, 210, 220, 245,

303.
semi-gigas, 70.
semi-lata types, 70.
sesquiplex mutant type, 189.
sex, 199-218, 219.
sex-chromosomes, 32, 52-55, 199-

218.
sex-determination, 219.
sex-linkage, 52.

sex-linked inheritance, 207, 208.
sex reversals, 250-280.
Sexton, 253.
sheep, 249.
Shiwago, 206, 208.
Shull, A. P., 149.
Shull, G. H., 71, 189, 190.
single comb, 73, 74.

Sinnott, 180.

Smith, G., 251-253.

snail, 99.

snapdragon, 97.

Solenobia, 210.

Solenum, 112-114.

species, 68.

"Species and Varieties," 68.

Spemann, 139, 140.

Spencer, Herbert, 28.

spider crab, 251.

sporophyte, 125, 126.

stable type, 124.

Stevens, 208.

Stockard, 296, 297.

stone fly, 255.

Stomps, 70, 131.

Sturtevant, 88-91, 101, 102, 243.

sugar cane, 170.

superfemale, 56, 242.

supermale, 136, 241.

sweet peas, 10, 11, 36, 37.

Swingle, 258.

Tackholm, 158-163.

tadpole, 256-265.

Tahara, 167-169.

Talaeporia, 210.

Tandler, 247.

Taylor, 170.

teosinte, 123, 124, 187, 196.

tetraploids, 105-130.

tetra-type, 176.

Thelia, 252.

theories of heredity, 26-31.

theory of the gene, 25.

thrips, 149.

Tischler, 170.

toad, 266.

tobacco, 96.

INDEX

343

tomato, 112, 113, 115-117.
translocation, 80-82.
Trialeurodes, 148.
triplo-IV, 50, 51, 175, 176.
triploid Drosophila, 84.
triploid endosperm, 82.
triploids, 131-138.
trisomic type, 177-189.
Triticum, 151, 152.
Triton, 140, 269.
twins, 247, 275.

Vallisneria, 213, 214.
vermilion genes, 81, 82.
vestigial, 75.
Viola, 171.

de Vries, 67, 68, 69, 70, 71, 95,
109, 131, 174, 187, 188, 189.
Vulgare wheat, 152, 154-157.

walnut comb, 72, 73, 74.
wasp, 237.

W-chromosome, 245, 246.
Wedge, 182.
Weinstein, 102, 104.

Weismann, 28-30.

Wettstein, 128, 149, 216, 217, 218.

wheats, polyploids, 150-158.

White, O. E., 37.

Whiting, Anna R., 239.

Whiting, 237-239.

Whitney, 147, 233, 234.

Williams, 148.

Willier, 248.

Winge, 214, 222, 223, 226, 227.

Winiwarter, 203-205.

Winkler, 112-118.

Wiry, 181, 182.

Witschi, 258-265, 269.

Wright, 287.

Xiphophorus, 254, 255.

Yampolski, 279.

Y-chromosome, 52, 105, 222, 239.

yellow mice, 64.

Zea mays, 123.
Zeleny, 86.

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