Similarly, if we suppose that segregation of ovum and sperm occurs in the

female, the sperms must disappear and the ovum would contain no factors

derived from the male parent. But the theory supposes that the segregation

of male and female does occur in the female, that half the ova are female

and half are male. What meaning are we to attach to the words 'male ovum'

or even 'male producing ovum'? It is a fundamental principle of Mendelism

that the soma does not influence the gametocytes or gametes; we have

therefore only to consider the sex of the gametes themselves, derived from

a zygote which is formed by the union of two sexes. The quality of

maleness consists only in the size, form, and mobility of the sperm in the

higher animals and of the microgamete in other cases. In what sense then,

can an ovum be male? It may perhaps be said that though it is itself

female, it has some property or factor which when united with a sperm

causes the zygote to be capable of producing only sperms, and conversely

the female ovum has a quality which causes the zygote to produce only ova.

But since these qualities segregate in the reduction divisions, how is it

that the male quality in the _f_ ovum does not make it a sperm? We are

asked to conceive a quality, or the absence of a factor, in an ovum which

is incapable of causing that ovum to be a sperm, but which, when

segregated in the gametes descended from that ovum, causes them all to be

sperms. It is impossible to conceive a single quality or factor which at

different times produces directly opposite effects. The Mendelian theory

is merely a theory in words, which have an apparent relation to the facts,

but which when examined do not correspond to any real conceptions.

However, we have to consider a number of remarkable facts concerning the

relation of chromosomes to sex. In the ants, bees, and wasps the

unfertilised ovum always develops into a male, the fertilised into a

female. The chromosomes of the ovum undergo reduction in the usual way,

and are only half the number of those present in the nucleus before

reduction. We may call this reduced number _N_ and the full number _2N_.

The ova developing by parthenogenesis and giving rise to males segment in

the usual way, and all the cells both of soma and gametocytes contain only

_N_ chromosomes. In the maturation divisions reduction does not occur, _N_

chromosomes passing to one gamete, none to the other, and the latter

perishes so that the sperms all contain _N_ chromosomes. When

fertilisation occurs the zygote therefore contains _2N_ chromosomes and

becomes female. Here then we have no segregation of _Fxf_ in the ova. The

difference of sex merely corresponds to duplex and simplex conditions of

nucleus, but it is curious that the simplex condition in the gametes

occurs in both ova and sperms.

In Daphnia and Rotifers the facts are different. Parthenogenesis occurs

when food supply is plentiful and temperature high. In this case reduction

of the chromosomes does not occur at all, the eggs develop with _2N_

chromosomes and all develop into females. Under unfavourable conditions

reduction or meiosis occurs, and two kinds of eggs larger and smaller are

formed, both with _N_ chromosomes. The larger only develops when

fertilised and give rise to females with _2N_ chromosomes. The smaller

eggs develop without fertilisation, by parthenogenesis, and become males.

Here then we have three kinds of gametes, large eggs, small eggs, and

sperms, each with the same number of chromosomes. It is not the mere

number then which makes the difference, but we find a segregation in the

ova into what may for convenience be called female ova and male ova.

In Aphidae or plant lice a third condition is found. Here again

parthenogenesis continues for generation after generation so long as

conditions are favourable, _i.e._ in summer, and the eggs are in the same

condition as in Daphnia, etc., that is to say, reduction does not occur,

and the number of chromosomes is 2_N_. Under unfavourable conditions males

are developed as well as females by parthenogenesis, but the males arise

from eggs which undergo partial reduction of chromosomes, only one or two

being separated instead of half the whole number. The number then in an

egg which develops into a male is 2_N_-1, while other eggs undergo

complete reduction and then have _N_ chromosomes. The latter, however, do

not develop until they have been fertilised. In the males, when mature,

reduction takes place in the gametes, so that two kinds of sperms are

formed, those with _N_ chromosomes and those with _N_-l chromosomes. The

latter degenerate and die, the former fertilise the ova, and the

fertilised ova develop only into females. The chief difference in this

case then is that the reduction in the male to the _N_ or simplex

condition takes place in two stages, one in the parthenogenetic ovum, one

in the gametes of the mature male. In Hymenoptera and in Daphnia, etc.,

the whole reduction takes place in the parthenogenetic ovum, and in the

mature male, though reduction divisions occur, no separation of

chromosomes takes place: at the first division one cell is formed with _N_

chromosomes and one with none, and the latter perishes.

In many insects and other Arthropods which are not parthenogenetic the

male has been found to possess fewer chromosomes than the female. The

female forms, as in the above cases of parthenogenesis, only gametes of

one kind each with _N_ chromosomes, but the male forms gametes of two

sorts, one with N chromosomes, the other with _N_-l or _N_-2 chromosomes.

On fertilisation two kinds of zygotes are formed, female-producing eggs

with 2_N_ chromosomes, and male-producing eggs with 2_N_-1 or 2_N_-2

chromosomes. There is also evidence that in some cases, _e.g._ the

sea-urchin, the female is heterozygous, forming gametes, some with _N_ and

some with _N_+ chromosomes, while the male gametes are all _N_.

Fertilisation then produces male-producing eggs with 2_N_ chromosomes,

female-producing with 2_N_+.

Such is the summary given by Castle in 1912. [Footnote: _Heredity and

Eugenics_, by Castle and Others. University of Chicago Press, 1912.] It

will be seen that he treats the differences as purely quantitative, mere

differences in the number of the chromosomes. Professor E. B. Wilson,

however, who had contributed largely by his own researches to our

knowledge of sex from the cytological point of view, had already

published, in 1910, [Footnote: '_The Determination of Sex_,' _Science

Progress_, April 1910.] a very instructive _résumé_ of the facts observed

up to that time. The important fact which is generally true for insects,

according to Wilson, is that there is a special chromosome or chromosomes

which can be distinguished from the others, and which is or are related to

sex differentiation. This chromosome, to speak of it for convenience in

the singular, has been variously named by different investigators. Wilson

called it the 'X chromosome,' McCluny the 'accessory chromosome,'

Montgomery the 'hetero-chromosome,' while the names 'heterotropic

chromosome' and idiochromosome have also been used. For the purpose of the

present discussion we may conveniently name it the sex-chromosome. It is

often distinguished by its larger size and different shape. Wilson

describes the following different cases:--

(1) The sex-chromosome in the male gametocytes is single and fails to

divide with the others, but passes undivided to one pole. This may occur

in the first reduction division (Orthoptera, Coleoptera, Diptera) or in

the second (many Hemiptera). But it is difficult to understand what is

meant by 'fails to divide.' In one of the reduction divisions all the

chromosomes divide as in ordinary or homotypic nucleus division, but in

the other the chromosomes simply separate into two equal groups without

division. If there are an odd number of chromosomes, 2_N_-1, in all the

gametocytes of the male, as stated in most accounts of the subject, then

if one chromosome fails to divide in the homotypic division, we shall have

2_N_-2 in one spermatocyte and 2_N_-1 in the other. Then when the

heterotypic division takes place and the number of chromosomes is halved,

we shall have two spermatocytes with _N_-1 chromosomes from one of the

first spermatocytes and one with _N_ and one with _N_-1 from the other.

Thus there will be three spermatozoa with _N_-1 chromosomes and one with

_N_ chromosomes, whereas we are supposed to find equal numbers with _N_

and _N_-1 chromosomes. It is evident that what Dr. Wilson means is

that the sex-chromosome is unpaired, and that although it divides

like the others in the homotypic division, in the heterotypic division

it has no mate and so passes with half the number of chromosomes to one

pole of the division spindle, while the other group of chromosomes has

no sex-chromosome. Examples of this are the genera _Pyrrhocoris_ and

_Protenor_ (Hemiptera) _Brachystola_ and many other Acrididae, _Anasa,

Euthoetha, Narnia, Anax_. In a second class of cases the sex-chromosome is

double, consisting of two components which pass together to one pole.

Examples of this are _Syromaster, Phylloxera, Agalena_. In a third class

the sex-chromosome is accompanied by a fellow which is usually smaller,

and the two separate at the differential division. The sizes of the two

differ in different degrees, from cases as in many Coleoptera and Diptera

in which the smaller chromosome is very minute, to those (_Benacus,

Mineus_) in which it is almost as large as its fellow, and others

(_Nezara, Oncopeltus_) in which the two are equal in size. Again, there

are cases in which one sex-chromosome, say _X_, is double, triple, or even

quadruple, while the other, say _Y_, is single. In all these cases there

are two _X_ chromosomes in the oocytes (and somatic cells) of the female,

and after reduction the female gametes or unfertilised ova are all alike,

having a single _X_ chromosome or group. On fertilisation half the zygotes

have _XX_ and half _XY_, whether _Y_ is absence of a sex-chromosome,

or one of the other _Y_ forms above mentioned. The sex is thus determined

by the male gamete, the _X_ chromosome united with that of the female

gamete producing female individuals, while the _Y_ united with _X_

produces male individuals.

Professor T. H. Morgan has made numerous observations and experiments on a

single culture of the fruit-fly, _Drosophila ampelophila_, bred in bottles

in the laboratory for five or six years. He has not only studied the

chromosomes in the gametes of this fly, and made Mendelian crosses with

it, but has obtained numerous mutations, so that his work is a very

important contribution to the mutation doctrine. Drosophila in the hands

of Professor Morgan and his students and colleagues has thus become as

classical a type as Oenothera in those of the botanical mutationists.

Different branches of Morgan's work are discussed elsewhere in this

volume, but here we are concerned only with its bearing on the question of

the determination of sex. He describes [Footnote: _A Critique of the

Theory of Evolution_. Princeton University Press and Oxford University

Press, 1916.] the chromosomes of Drosophila as consisting in the diploid

condition of four pairs, that is to say, pairs which separate in the

reduction division so that the gamete contains four single chromosomes,

one of each pair. In two of these pairs the chromosomes are elongated and

shaped like boomerangs, in the third they are small, round granules, and

the fourth pair are the sex-chromosomes: in the female these last are

straight rods, in the male one is straight as in the female, the other is

bent. The straight ones are called the X chromosomes, the bent one the Y

chromosome. The fertilisations are thus XX which develops into a female

fly, and XY which develops into a male. Drosophila therefore is an example

of one of the cases described by Wilson.

Dr. Wilson (_loc. cit._) discusses the question of how we are to interpret

these facts, in particular, the fact that the X chromosome in

fertilisation gives rise to females. He remarks that the X chromosome must

be a male-determining factor since in many cases it is the only

sex-chromosome in the males, yet its introduction into the egg establishes

the _female_ condition. This is the same difficulty which I pointed out

above in connection with the Mendelian theory that the female was

heterozygous and the male homozygous for sex. Dr. Wilson points out that

in the bee, where fertilised eggs develop into females and unfertilised

into males, we should have to assume that the _X_ chromosome in the female

gamete is a female determiner which meets a recessive male determiner in

the _X_ chromosomes of the sperm. When reduction occurs, the _X_[female]

must be eliminated since the reduced egg develops always into a male. But

on fertilisation, since the fertilised egg develops into a female, a

dominant _X_[female] must come from the sperm, so that our first

assumption contradicts itself.

Dr. Wilson, T. H. Morgan, and Richard Hartwig have therefore suggested

that the sex-difference as regards gametes is not a qualitative but a

quantitative one. In certain cases there is no evident quantitative

difference of chromatin as a whole, but there may in all cases be a

difference in the quantity of special sex-chromatin contained in the _X_

element. The theory put forward by Wilson then is that a single _X_

element means _per se_ the male condition, while the addition of a second

element of the same kind produces the female condition. Such a theory

might apply even to cases where no sex-chromosomes can be distinguished by

the eye: the ova, in such cases (probably the majority), might also have a

double dose of sex-chromatin, the males a single dose. This theory,

however, is still open to the objection that the female gametes before

fertilisation, and half the male gametes, have the half quantity of

sex-chromatin which by hypothesis determines the male condition, so that

here again we have the male condition as something which is distinct from

the characteristics of the spermatozoon. But if this is the case, what is

the male condition? The parthenogenetic ovum of the bee is male, and yet

it is an ovum capable only of producing spermatozoa. If the single X

chromosomes is the cause of the development of spermatozoa in the male

bee, why does it not produce spermatozoa in the gametes of the female bee,

since when reduction takes place all these gametes have a single X

chromosome?

In biology, as in every other science, we must admit facts even when we

cannot explain them. The facts of what we call gravitation are obvious,

and any attempt to disregard them would result in disaster, yet no

satisfactory explanation of gravitation has yet been discovered: many

theories have been suggested, but no theory has yet been proved to be

true. In the same way it may be necessary to admit that two X chromosomes

result in the development of a female, and one X, or XY chromosomes result

in the development of a male. But Mendelians have omitted to consider what

is meant by male and female. The soma with its male and female somatic

characters has nothing to do with the question, since somatic

sex-differences may be altogether wanting, and moreover, the essential

male character, the formation of spermatozoa, is by the Mendelian

hypothesis due to descent of the male gametes from the original fertilised

or unfertilised _ovum_. The Mendelian theory therefore is that when an

ovum has two X sex-chromosomes it can only after a number of

cell-divisions, at the following reduction division, give rise to ova,

while an ovum containing one X sex-chromosome, or two different, XY,

chromosomes, at the next reduction division gives rise to spermatozoa. The

X sex-chromosome is not in itself either female or male, since, as we have

seen, either ovum or spermatozoon may contain a single X chromosome. The

ovum then with one X chromosome or one X and one Y changes its sex at the

next reduction division and becomes male. In parthenogenetic ova this

happens without conjugation with a spermatozoon at all: in other cases,

since the zygote is compounded of spermatozoon and ovum, we can only say

that in the XX zygote, the ovum developing only ova, the female is

dominant, in the X or XY zygote developing only spermatozoa the male is

dominant. Hermaphrodite animals, as has been pointed out by Correns and

Wilson, cannot be brought under this scheme at all. In the earthworms, for

instance, we have, in every individual developed from a zygote, ova and

spermatozoa developing in different gonads in different parts of the body.

The differentiation here, therefore, must occur in some cell-division

preceding the reduction divisions. Every zygote must have the same

composition, and yet give rise to two sexes in the same individual.

Further light on the sex problem, as in many other problems in biology,

can only be obtained by more knowledge of the physical and chemical

processes which take place in the chromosomes and in the relations of

these structures to the rest of the cell. The recent advances in cytology,

remarkable as they are, consist almost entirely of observations of

microscopic structure. They may be said to reveal the statics of the cell

rather than its dynamics. Cytology is in fact a branch of anatomy, and in

the anatomy of the cell we have made some progress, but our knowledge of

the physiology of the cell is still infinitesimal. The nucleus, and

especially the chromosomes, are supposed in some unknown way to influence

or govern the metabolism of the cytoplasm. From this point of view the

hypothesis mentioned above that the sex-difference in the gametes is not

qualitative but quantitative is probably nearer to the truth. Geddes and

Thomson and others have maintained that the sex-difference is one of

metabolism, the ovum being more anabolic, the sperm more katabolic. A

double quantity of special chromatin may be the cause of the greater

anabolism of the ovum. In that case the difficulty indicated in a previous

part of this chapter, that the ovum after reduction resembles the sperm in

having only one X chromosome, may be explained by the fact that the growth

of the ovum and its accumulation of yolk substances has been already

accomplished under the influence of the two chromosomes before reduction.

Other difficulties previously discussed also appear to be diminished if we

adopt this point of view. We need not regard maleness and femaleness as

unit characters in heredity of the same kind as Mendelian characters of

the soma. Instead of saying that the zygote composed of ovum and

spermatozoon is incapable of giving rise in the male to ova, or in the

female to sperms, we should hold that the gametocytes ultimately give rise

to ova or to sperms according to the metabolic processes set up and

maintained in them through their successive cell-divisions under the

influence of the double or single X chromosome. There still remains the

difficulty of explaining why the male gametocytes after reduction develop

into similar sperms, with their heads and long flagella, although half of

them possess one X chromosome each and the other half none. We can only

suppose that the final development of the sperms is the result of the

presence of the single X chromosome in the successive generations of male

gametocytes before the reduction divisions.

The Mendelian theory of sex-heredity assumed that in the reduction

divisions the two sex-characters, maleness and femaleness, were segregated

in the same way as a pair of somatic allelomorphs, but the words maleness

and femaleness expressed no real conceptions. The view above suggested

merely attempts to bring our real knowledge of the difference between ovum

and sperm into relation with our real knowledge of the sex-chromosomes and

their behaviour in reduction and fertilisation.

CHAPTER III

Influence Of Hormones On Development Of Somatic Sex-Characters

We have next to consider what are commonly called secondary sexual

characters. These are characters or organs more or less completely limited

to one sex. When we distinguish in the higher animals the generative

organs or gonads on the one hand from the body or soma on the other, we

see that all differences between the sexes, except the gonads, are

somatic, and we may call them somatic sexual characters. The question at

once arises whether the soma itself is sexual, that is to say, whether on

the assumption that the sex of the zygote is already determined before it

begins to develop, the somatic cells as well as the gametocytes are

individually and collectively either male or female. In previous

discussions of the subject I have urged that the only meaning of sex was

the difference between the megagamete or ovum, and the microgamete or

sperm. But if the zygote, although compounded of ovum and sperm, is

predestined to give rise in the gametes descended from it, either to

sperms only or to ova only, it may be suggested that all the somatic cells

descended from the zygote are likewise either male or female, although

they do not give rise to gametes. It is evident, however, that the somatic

cells, organs, and characters do not differ necessarily or universally in

the two sexes. On the one hand, we have extraordinary and very conspicuous

peculiarities in the male, entirely absent in the female, such as the

antlers of stags, and the vivid plumage of the gold pheasant; on the other

we have the sexes externally alike and only distinguished by their sexual

organs, as in mouse, rabbit, hare, and many other Rodents, most Equidae,

kingfisher, crows and rooks, many parrots, many Reptiles, Amphibia,

Fishes, and invertebrate animals. In the majority of fishes, in which

fertilisation is external and no care is taken of the eggs or young, there

are no somatic sexual differences. Moreover, somatic sexual characters

where they do occur have no common characteristics either in structure or

position in the body. It may be said that any part of the soma may in

different cases present a sex-limited development. In the stag the male

peculiarity is an enormous development of bone on the head, in the peacock

it is the enlargement of the feathers of the tail. In some birds there are

spurs on the legs, in others spurs on the wings. It is no explanation,

therefore, to say that these various organs and characters are the

expression of sex in the somatic cells.

As I pointed out in my _Sexual Dimorphism_ (1900), the common

characteristic of somatic sexual characters is their adaptive relation to

some function in the sexual habits of the species in which they occur.

There is no universal characteristic of sex except the difference between

the gametes and the reproductive organs (gonads) in which they are

produced. All other differences, therefore, including genital ducts and

copulatory or intromittent organs, are somatic. When we examine these

somatic differences we find that they can be classified according to their

relation to fertilisation and reproduction, including the care or

protection of the offspring. The precise classification is of no great

importance, but we may distinguish the following kinds to show the

chief functions to which the characters or organs are adapted.

1. GENITAL DUCTS AND INTROMITTENT ORGANS.--According to the theory of the

coelom which we owe to Goodrich, in all the coelomata the coelom is

primarily the generative cavity, on the walls of which the gametocytes are

situated, and the coelomic ducts are the original genital ducts. In

Vertebrates we find two such ducts in both sexes in the embryo, originally

formed apparently by the splitting of a single duct. In the male one of

these ducts becomes connected with the testis while the other degenerates:

the one which degenerates in the male forms the oviduct in the female,

while the one which is functional in the male degenerates in the female.

Intromittent organs are formed in all sorts of different ways in different

animals. In Elasmobranchs (sharks and skates) they are enlarged portions

of the pelvic fins, and therefore paired. In Lizards they are pouches of

the skin at the sides of the cloacal opening. In Mammals the single penis

is developed from the ventral wall of the cloaca. In Crustacea certain

appendages are used for this function. There are a great many animals,

from jelly-fishes to fishes and frogs, in which fertilisation is external,

and there are no intromittent organs at all.

2. ORGANS FOR, CAPTURING OR HOLDING THE FEMALE: for example, the

thumb-pads of the frog, and a modification of the foot in a water-beetle.

Certain organs on the head and pelvic fins of the Chimaeroid fishes are

believed to be used for this purpose.

3. WEAPONS.--Organs which are employed in combats between males for the

exclusive possession of the females. For example, antlers of stags, horns

of other Ruminants, tusks of elephants, spurs of cocks and Phasiamidae

generally, horns and outgrowths in males of Reptiles and many Beetles,

probably used for this purpose.

4. ALLUREMENTS.--Organs or characters used to attract or excite the

female. These might be called the organs of courtship, such as the

peacock's tail, the plumes of the birds-of-paradise, and the brilliant

plumage of humming birds and many others. The song of birds is another

example, and sound is produced in many Fishes for a similar purpose.

5. ORGANS FOR THE BENEFIT OF THE OFFSPRING: for example, the extraordinary

pouches in which the eggs are developed in certain Frogs. In the South

American species, _Rhinoderma darwinii_, the enlarged vocal sacs are used

for this purpose. Pouches with the same function are developed in many

animals, for instance in Pipe-fishes and Marsupials. Abdominal appendages

are enlarged in female Crustacea for the attachment of the eggs, the

abdomen also being larger and broader.

The argument in favour of the Lamarckian explanation of the evolution of

these adaptive characters is the same as in the case of adaptations common

to both sexes, namely that in every case the function of the organs and

characters involves special irritations or stimulations by external

physical agents. Mechanical irritation, especially of the interrupted

kind, repeated blows or friction causes hypertrophy of the epidermis and

of superficial bone. I have stated this argument and the evidence for it

in some detail in my volume on _Sexual Dimorphism_. It is one of the most

striking facts in support of this argument that the hypertrophied plumage

which constitutes the somatic sexual character of the male in so many

birds is habitually erected by muscular action for the purpose of display

in the sexual excitement of courtship. I doubt if there is a single

instance in which the male bird takes up a position to present his

ornamental plumage to the sight of the female without a special erection

and movement of the feathers themselves. Such a stimulation must affect

the living epidermic cells of the feather papilla. Even supposing that the

feather is not growing at the time, it is probable, if not certain, that

the stimulation will affect the papilla at the base of the feather

follicle, so as to cause increased growth of the succeeding feather. But

we have no reason to believe that erection in display occurs only when the

growth of the feathers is completed, still less that it did so always at

the beginning of the evolution.

The antlers of stags are the best case in favour of the Lamarckian view of

the evolution of somatic sexual characters. The shedding of the skin

('velvet') followed by the death of the bone, and its ultimate separation

from the skull, are so closely similar to the pathological processes

occurring in the injury of superficial bones, that it is impossible to

believe that the resemblance is only apparent and deceptive. In an

individual man or mammal, if the periosteum of a bone is destroyed or

removed the bone dies, and is then either absorbed, or separated from the

living bone adjoining, by absorption of the connecting part. In the stag

both skin and periosteum are removed from the antler: probably they would

die and shrivel of their own accord by hereditary development, but as a

matter of fact the stag voluntarily removes them by rubbing the antler

against tree trunks, etc. When the bone is dead the living cells at its

base dissolve and absorb it, and when the base is dissolved the antler

must fall off.

The adaptive relation is not the only common characteristic of these

somatic sexual characters. Another most important fact is not only that

they are fully developed in one sex, absent or rudimentary in the other,

but that their development is connected with the functional maturity and

activity of the gonads. There is usually an early immature period of life

in which the male and female are similar, and then at the time of puberty

the somatic sexual characters of either sex, generally most marked in the

male, develop. In some cases, where the activity of the gonads is limited

to a particular season of the year, the sexual characters or organs are

developed at this season, and then disappear again, so that there is a

periodic development corresponding to the periodic activity of the testes

or ovaries. Stags have a limited breeding or 'rutting' season in autumn

(in north temperate regions), and the antlers also are shed and developed

annually. In this case we cannot assert that the development of the antler

takes place during the active state of the testes. The antlers are fully

developed and the velvet is shed at the commencement of the rutting

season, and development of the antlers takes place between the beginning

of the year and the month of August or September. In ducks and other birds

there is a brilliant male-breeding plumage in the breeding season which

disappears when breeding is over, so that the male becomes very similar to

the female. In the North American fresh-water crayfishes of the genus

Cambarus there are two forms of males, one of which has testes in

functional activity, while in the other these organs are small and

quiescent: the one form changes into the other when the testes pass from

the one condition to the other.

It has long been known that the development of male sex-characters is

profoundly affected by the operation of castration. The removal of the

testes is most easily carried out in Mammals, in consequence of the

external position of the organs in these animals, and the operation has

been practised on domesticated animals as well as on man himself from very

ancient times. The effect is the more or less complete suppression of the

male insignia, in man, for example, the beard fails to develop, the voice

does not undergo the usual change to lower pitch which takes place at

puberty, and the eunuch therefore has much resemblance to the boy or

woman. Many careful experimental researches have been made on the subject

in recent years. The consideration of the subject involves two questions:

(1) What are the exact effects of the removal of the gonads in male and

female? (2) By what means are these effects brought about, what is the

physiological explanation of the influence of the gonads on the soma?

I have quoted the evidence concerning the effects of castration on stags

in my _Sexual Dimorphism_ and in my paper on the 'Heredity of Secondary

Sexual Characters.' [Footnote: _Archiv für Entwicklungesmechanik_, 1908.]

When castration is performed soon after birth a minute, simple spike

antler is developed, only two to four inches in length: it remains covered

with skin, is never shed, and develops no branches. When the operation is

performed on a mature stag with antlers, the latter are shed soon after

the operation, whether they have lost their velvet or not. In the

following season new antlers develop, but these never lose their velvet or

skin and are never shed.

CASTRATION IN FOWLS

The removal of the testes from young cocks has been commonly practised in

many countries, _e.g._ France, capons, as such birds are called, being

fatter and more tender for the table than entire birds. The actual effect,

however, on the secondary sexual characters has not in former times been

very definitely described. The usual descriptions represent the castrated

birds as having rather fuller plumage than the entire birds; but the comb

and wattles are much smaller than in the latter, more similar to those of

a hen. It is stated that the capon will rear chickens, though he does not

incubate, and that they are used in this way in France.

The most precise of the statements on the subject by the earlier

naturalists is that of William Yarrell [Footnote: _Proc. Linn. Soc.,

1857.] (1857), who writes as follows:--

'The capon ceases to crow, the comb and gills do not attain the size of

those parts in the perfect male, the spurs appear but remain short and

blunt, and the hackle feathers of the neck and saddle instead of being

long and narrow are short and broadly webbed. The capon will take to a

clutch of chickens, attend them in their search for food, and brood them

under his wings when they are tired.'

It would naturally be expected, on the analogy of the case of stags, that

when a young cock was completely castrated all the male secondary

characters would be suppressed, namely, the greater size of the comb and

wattles in comparison with the hen, the long neck hackles, and saddle

hackles, long tail feathers, especially the sickle-feathers, and the

spurs. As a matter of fact, the castrated specimen usually shows only the

first of these effects to any conspicuous degree. The comb and wattles of

the capon are similar to those of the hen, but he still has the plumage

and the spurs of the entire cock. Many investigators have made experiments

in relation to this subject, and most of them have found that complete

castration is difficult, and that portions of the testes left in the bird

during the operation become grafted in some other position either on the

parietal peritoneum, or on that covering the intestines, and produce

spermatozoa, which, of course hare no outlet. In such cases the secondary

male characters may fee more or less completely developed. Thus Shattock

and Seligmann (1904) state that ligature of the vas deferens made no

difference to the male characters, and that after castration detached

fragments were often left in different positions as grafts, when the

secondary characters developed. In one particular case only a minute

nodule of testicular tissue showing normal spermatogenesis was found on

post mortem examination attached to the intestine. In this bird there was

no male development of comb or wattles, a full development of neck

hackles, a certain development of saddle hackles, a few straggling badly

curved feathers in the tail and short blunt spurs on the legs. Lode

[Footnote: _Wiener klin. Wochenschr._, 1895.] (1895) found that testes

could easily be transplanted into subcutaneous tissue and elsewhere, and

that the male characters then developed normally. Hanau [Footnote: _Arch.

f. ges. Physiologie_, 1896.] (1896) obtained the same result.

The question, however, to what degree the male characters of the cock are

suppressed after complete castration is not so definitely answered in the

literature of the subject. Shattock and Seligmann in their 1904 paper make

no definite statement on the subject. Rieger (1900), Selheim (1901), and

Foges [Footnote: _Pfügers Archiv_, 1902.] (1902) state that the true capon

is characterised by shrivelling of the comb, wattles, _and spurs_; poor

development of the neck and tail feathers; hoarse voice and excessive

deposit of fat. Shattock and Seligmann, on the other hand, have placed in

the College of Surgeons Museum the head of a Plymouth Rock which was

castrated in 1901. It was hatched in the spring of that year. In December

1901 the comb and wattles were very small, the spurs fairly well

developed, and the tail had a somewhat masculine appearance. In September

1902, when the bird was killed, the comb and wattles were still poorly

developed, the neck hackles fairly well so; saddle hackles rather well

developed; the tail contained rather loosely-grouped long sickle feathers;

the spurs stout. The description states that dissection showed no trace of

either testicle, and I am informed by Mr. Shattock that there were no

grafts. The description ends with the conclusion that the growth of the

spurs, and to a certain extent that of the long, curved sickle feathers,

is not prevented by castration. With regard to the spurs this result does

not agree with that of the German investigators, but it must be remembered

that the latter speak only of the reduction of the spurs, not entire

absence. It is important in discussing the effects of castration in cocks

to bear in mind the actual course of development of the secondary sexual

characters. When the chicks are first hatched they are in the down:

rudimentary combs are present, wattles can scarcely be distinguished, and

there is no external difference between the sexes. The ordinary plumage

begins to develop immediately after hatching, the primaries of the wings

being the first to appear. The feathers are completely developed in about

five weeks, and still there is no difference between the sexes. The first

sexual difference is the greater size of the combs in the males, and this

is quite distinct at the age of six weeks. At nine to ten weeks in

black-red fowls, in which the cocks have black breasts and red backs with

yellow hackles, the black feathers on the breast and red on the back are

gradually developing, both sexes previously having been a dull speckled

brown, closely similar to the adult hens. The spurs are the last of the

male characters to develop, these at the age of four months being still

mere nodules, scarcely, if at all, larger than the rudiments visible in

adult hens. This is the age at which castration is usually performed, as

at an earlier age the birds are too small to operate on successfully. It

follows, therefore, that the spurs develop after castration, and it would

seem that their development does not depend upon the presence of the

sexual organs. It is a question, however, whether castration in the cock

is ever quite complete. In the original wild species and in the majority

of domesticated breeds the spurs are confined to the male sex, and are

typical secondary sex-characters, as much so as the antlers of stags or

the beard of man, yet the above discussion shows that there is some doubt

whether their development is prevented as much as in other cases by the

absence of the sexual organs. Even if it should be proved that in supposed

cases of complete castration, such as that of Shattock and Seligmann, some

testicular tissue remained at the site of the testes, it would still be

true that the development of the comb and wattles is more affected by the

removal of the sexual organs than that of the spurs or tail feathers.

My own experiments in castrating cocks were as follows: On August 20,

1910, I operated on a White Leghorn cock about five months old. One testis

was removed, with a small part of the end broken off, but the other, after

it was detached, was lost among the intestines. On the same day I operated

on another about thirteen weeks old, a speckled mongrel. In this case both

testes were extracted but one was slightly broken at one end, although I

was not sure that any of it was left in the body. An entire White Leghorn

of the same age as the first was kept as a control. On August 27 the two

castrated birds had recovered and were active. Their combs had diminished

in size and lost colour considerably, that of the White Leghorn was

scarcely more than half as large as that of the control. Such a rapid

diminution can scarcely he due to absorption of tissue, but shows that the

size of the normal cock's comb is largely due to distension with blood,

which ceases when the sexual organs are removed. In the following January,

the second cock, supposed to be completely castrated, was seen to make a

sexual gesture like a cock, though not a complete action like an entire

animal: this showed that the sexual instinct was not completely

suppressed. In February this same bird was seen to attempt to tread a hen,

while the white one, supposed to be less perfectly emasculated, had never

shown such male instinct.

The White Leghorn cock was killed and dissected on May 13, 1911, nine

months after castration. I found an oval body of dark, dull brown colour

loose among the intestines: this was evidently the left testis which was

separated from its natural attachment and lost in the abdomen at the time

of the operation. I examined the natural sites of the testes: on the right

side there was a small testis of considerable size, about half an inch in

diameter. When a portion of this was teased up and examined under the

microscope moving spermatozoa were seen, but they were not in swarms as in

a normal testis, but scattered among numerous cells. On the left side was

a much smaller testis, in the tissue of which I with difficulty detected a

few slowly moving spermatozoa. The vasa deferentia were seen as white

convoluted threads on the peritoneum, but contained no spermatozoa.

On July 29, 1911, a little more than eleven months after the operation,

I examined and killed the second of these castrated cocks, the speckled

mongrel-bred bird. I measured the comb and wattles while it was alive, in

case there might be reduction in the size of these appendages when the

bird was killed. The comb was 1-1/3 inches high by 2-3/8 inches in length.

The spurs were 1 inch long, curved and pointed. Saddle hackles short,

hanging only a little below the end of the wing. Neck hackles well

developed, similar to those of an entire cock. Longest tail feather 15-5/8

inches, blue-black in colour.

I had no entire cock of same breed, but measured the entire White Leghorn

for comparison. Comb 1-3/4 inches high by 3-3/4 inches in length. (It is

to be remembered that the comb and wattles are especially large in

Leghorns.) Wattle 1-1/4 inches in vertical length. Spur 1 inch long,

stouter and less pointed than in the capon. Longest tail feather 12 inches

long.

When killed the capon was found to be very fat: there were masses of fat

around the intestines and under the peritoneum, which made it impossible

to make out details such as ureter and vas deferens properly. I found a

white nodule about half an inch in diameter attached to mesentery. The

liquid pressed from this was swarming with spermatozoa in active motion.

Two other masses about the same size or a little larger were found on the

sites of the original testes. These also were full of mobile spermatozoa,

and must have grown from portions of the testes left behind at castration.

In ducks the sexual characters of the male differ from those in the fowl,

especially in the fact that they almost completely disappear after the

breeding season and reappear in the following season. In the interval the

drake passes into a condition of plumage in which he resembles the female;

and this condition is known as 'eclipse.' The male plumage, therefore, in

the drake has a history somewhat similar to that of the antlers in deer.

Two investigations of the effects of castration on ducks and drakes have

been recorded. H. D. Goodale [Footnote: 'Castration of Drakes.' _Biol.

Bulletin_, Wood's Hole, Mass., vol. xx., 1910] removed the generative

organs from both drakes and ducks of the Rouen breed, which is strongly

dimorphic in plumage. One drake was castrated in the early spring of 1909

when a little less than a year old. This bird did not assume the summer

plumage in 1909, that is, did not pass into eclipse. It was in the nuptial

plumage when castrated. This breeding or nuptial plumage is well known: it

includes a white neck-ring, brilliant green feathers on the head, much

claret on the breast, brilliant metallic blue on the wing, and two or more

upward curled feathers on the tail. The drake mentioned above was

accidentally killed in the spring of 1910. Another drake was castrated on

August 8, 1909: only the left testis was removed, the other being

ligatured. At this time the bird would be in eclipse plumage. It appears

from the description that it assumed the nuptial plumage in the winter of

1909, and did not pass into eclipse again in the summer of 1910. Thus in

drakes the effect of castration is that the secondary sexual character

remains permanently instead of being lost and renewed annually. Goodale,

however, does not describe the moults in detail. In the natural condition

the drake must moult twice in the year, once when he sheds the nuptial

plumage, and again when he drops the summer dress. Goodale insists, from

some idea about secondary sexual characters which is not very obvious,

that the eclipse or summer plumage is not the same as that of the female.

He states that the male in summer plumage merely mimics the female but

does not become entirely like her. In certain parts of the body there are

no modifications toward the female type. In others, i.e. head, breast, and

keel region, the feathers of the male become quite like those of the

female. 'It can hardly be maintained that this is an example of assumption

by the male of the female's plumage, especially as the presence of the

testis is necessary for its appearance.' The idea here seems to be that

since the eclipse plumage is only assumed when the testis is present,

therefore it must be a male character.

Out of five females on which the operation was performed only two lived

more than a few days afterwards. One of these (a) was castrated in the

spring of 1909 when a little less than a year old, the other (b) on August

13 when twelve weeks old. In October 1909 they showed no marked

modifications. In July 1910 it was noticed that they had the male curled

feathers in the tail, and (a) had breast feathers similar to those of the

male in summer plumage, (b) was rather more strongly modified: she had a

very narrow white neck-ring, and breast feathers distinctly of male type.

The next moult began in September, and in November was well advanced. On

the whole (a) had made little advance towards the male type, but (b)

closely resembled the male in nuptial plumage. It had brilliant green

feathers on the head, a white neck-ring, much claret colour on the breast,

and some feathers indistinguishable from those of the male, and also the

male sex feathers on the tail. Goodale concludes that the female owes her

normal colour to the ovaries or something associated with them which

suppresses the male characters and ensures the development of her own

type. He considers it is quite as conceivable that selection should

operate to pick out inconspicuously coloured females as that selection of

brilliantly coloured males should bring about an addition to the female

type. But as pointed out above, selection cannot explain the dimorphism in

either case.

It may be mentioned here that owing to the fact that the single (left)

ovary in birds is very closely attached to the peritoneum immediately

covering the great post-caval vein, it is generally impossible to remove

the whole of the ovary without cutting or tearing the wall of the vein and

so causing fatal hemorrhage. The above results observed by Goodale are

therefore all the more remarkable, and it may be assumed that he removed

at any rate nearly all the ovary.

The research of Seligmann and Shattock [Footnote: Relation between

Seasonal Assumption of the Eclipse Plumage in the Mallard _(Anas boscas_)

and the Functions of the Testicle.' _Proc. Zool. Soc._ 1914.] begins with

a comparison between the stages of the development of the nuptial plumage

and the stages of spermatogenesis. In the young pheasant the male plumage

is fully developed in the autumn of its first year, but no pairing occurs

and no sexual instinct is exhibited till the following spring. The wild

duck pairs in autumn or early winter, after the assumption of the nuptial

plumage, but copulation does not occur till spring is advanced. The

investigation here considered was made upon specimens of semi-domesticated

_Anas boscas_, such as are kept in London parks and supplied from game

farms. The testes attain their maximum size during the breeding season--

end of March or beginning of April. At this time each organ is almost as

large as a pigeon's egg, is very soft, and the liquid exuding from it when

cut is swarming with spermatozoa. The bird is of course in full nuptial

plumage. By the end of May, although the plumage is unchanged, the testes

have diminished to the size of a haricot bean, and spermatogenesis has

ceased. They diminish still further during June, July, and August, and

acquire a yellow or brownish colour, while microscopically there is no

sign of activity in the spermatic cells. The change from nuptial plumage

to eclipse takes place between the beginning of June and the middle of

July. The reappearance of the nuptial plumage takes place in the month of

September, and while this process takes place there is no sign of change

or renewed activity in the testes. During October and November, when the

brilliant plumage is fully developed, the testes increase slowly in size

but remain yellow and firm and exude no liquid on incision.

Spermatogenesis does not commence until the end of November or beginning

of December. The testes increase greatly in size in January and February,

and again reach their maximum size by the end of March. It is shown,

therefore, that the loss of the nuptial plumage takes place in June when

spermatogenesis has ceased and the testes are diminishing in size, but the

redevelopment of this plumage takes place in September without any renewed

activity of the testis and long before the beginning of spermatogenesis.

The case of the antlers in the stag is probably very similar.

The important statement is made with regard to castration (under

anaesthetics, of course) that it was found impossible to extirpate the

testes completely. When the bird was killed some months after the

operation, a greater or lesser amount of regenerated testicular tissue was

found either on the original site of the organs or engrafted upon

neighbouring organs. This experience, it will be noted, agrees with my own

in the case of fowls. There were, however, reasons for believing that the

results observed within the first six or eight months after the operation

are not much different from those which would follow complete castration.

Castration carried out when the drake was in nuptial plumage produced the

same effect which was observed by Goodale, namely, delay, and imperfection

in the assumption of the eclipse condition, but the observations of

Seligmann and Shattock are more precise and detailed. One example

described was castrated in full winter plumage in December 1906. On July

11, when normally it would have been in eclipse, the nuptial plumage was

unmodified except for a diffuse light-brown coloration on the abdomen,

which is stated to be due not to any growth of new feathers but to

pigmentary modification in the old. By September 1 this bird was almost in

eclipse but not quite; curl feathers in the tail had disappeared, the

breast was almost in full eclipse, the white ring was slightly indicated

at the sides of the neck, the top of the head and the nape had still a

good deal of gloss. After this the nuptial plumage developed again, and on

November 12 the bird was in full nuptial plumage, with good curl feathers

in the tail. The only trace of the eclipse was the presence of a few brown

feathers on the flanks. This bird was killed July 30, 1908, when the bird

was in eclipse, but not perfectly so, as there were vermiculated feathers

mixed with eclipse feathers on the breast, abdomen, and flanks. Dissection

showed on the right side a series of loosely attached nodular grafts of

testicular tissue, in total volume about the size of a haricot bean: on

the left side two small nodules, together about the size of a pea, and two

other grafts at the root of the liver and on the mesentery. Several other

cases are described, and the general result was that the eclipse was

delayed and never quite complete, while although the nuptial plumage was

almost fully developed in the following winter, it retained some eclipse

feathers, and was also delayed and developed slowly.

Several drakes were castrated in July when in the eclipse condition, and

although the authors state, in their general conclusions, that this does

not produce any constant appreciable effect upon the next passage of the

bird into winter plumage, they describe one bird so treated which on

November 18 retained many eclipse feathers: the general appearance of the

chestnut area of the breast was eclipse.

It must be remembered that not only was the castration in these cases

incomplete, but also that it was performed on mature birds. Birds differ

from Mammals, firstly, in the difficulty of carrying out complete

castration, and secondly, in the fact that the occurrence of puberty is

not so definite, and that immature birds are so small and delicate that it

is almost impossible to operate upon them successfully.

ASSUMPTION OF MALE CHARACTERS BY THE FEMALE

That male somatic sexual characters are latent in the female is shown by

the frequent appearance of such characters in old age, or in individual

cases. The development of hair on the face of women in old age, or after

the child-bearing period, is a well-known fact. Rorig, [Footnote: 'Ueber

Geweihbildung und Geweihentwicklung.' _Arch. Ent.-Mech._ x. and xi.] who

carefully studied the antlers of stags, states that old sterile females,

and those with diseased ovaries, develop antlers to some degree. Cases of

crowing hens, and female birds assuming male plumage have long been known,

but the exact relation of the somatic changes to the condition of the

ovaries in these cases is worthy of consideration in view of the results

obtained by Goodale after removal of the ovaries from ducks. Shattock and

Seligmann [Footnote: 'True Hermaphroditism in Domestic Fowl, etc.' _Trans.

Path. Soc._, Lond., 57. 1, 1906.] record the case of a gold pheasant hen

which assumed the full male plumage after the first moult: it had never

laid eggs or shown any sexual instincts. The only male character which was

wanting was that of the spurs. The ovary was represented by a smooth,

slightly elevated deep black eminence 1 cm. in length and 1-5 mm. in

breadth at its upper end. These authors also mention three ducks in male

plumage in which the ovary was similarly atrophied but not pigmented. They

regard the condition of the ovary as insufficient to explain the

development of the male characters, and suggest that such birds are really

hermaphrodite, a male element being possibly concealed in a neighbouring

organ such as the adrenal or kidney. This hypothesis is not supported by

observation of testicular tissue in any such case, but by the condition

found in a hermaphrodite specimen of the common fowl described in the

paper. This bird presented the fully developed comb and wattles and the

spurs of the cock, but the tail was quite devoid of curved or sickle

feathers, and resembled that of the hen. Internally there were two

oviducts, that of the left side normally developed, that of the right

diminutive and less than half the full length. The gonad of the left side

had the tubular structure of a testis, but showed no signs of active

spermatogenesis, but in its lower part contained two ova. The organ of the

right side was somewhat smaller, it had the same tubular structure, and in

one small part the tubules were larger, showed division of nuclei (mitotic

figures), and one of them showed active spermatogenesis.

In discussing Heredity and Sex in 1909, [Footnote: _Mendel's Principles of

Heredity_. Camb. Univ. Press, 1909.] Bateson referred to the effects of

castration as evidence that in different types sex may be differently

constituted. Castration, he urged, in the male vertebrate on the whole

leads merely to the non-appearance of male features, not to the assumption

of female characters, while injury or disease of the ovaries may lead to

the assumption of male characters by the female. This was supposed to

support the view that the male is homozygous in sex, the female

heterozygous in Vertebrates: that is to say, the female sex-character and

the female secondary sex-characters are entirely wanting in the male. This

argument assumes that the secondary characters are essentially of sexual

nature without inquiring how they came to be connected with sex, and it

ignores the fact that the influence of castration on such characters is a

phenomenon entirely beyond the scope of Mendelian principles altogether.

The fact that castration does affect, in many cases very profoundly,

somatic characters confined to one sex, proves that Mendelian conceptions,

however true up to a certain point, are by no means the whole truth about

heredity and development. For it is the essence of Mendelism as of

Weismannism that not only sex but all other congenital characters are

determined in the fertilised ovum or zygote. The meaning of a recessive

character in Mendelian terminology is one that is hidden by a dominant

character, and both of them are due to factors in the gametes,

particularly in the chromosomes of the gametes which come together in

fertilisation. For example, in fowls rose comb is dominant over single. A

dominant is something present which is absent in the recessive: the rose

comb is due to a factor which is absent from the single. The two segregate

in the gametes of the hybrid or heterozygote, and if a recessive gamete is

fertilised by another recessive gamete the single comb reappears. But

castration shows that the antlers of stags and other such characters are

not determined in the zygote when the sex is determined, but owe their

development, partly at least, to the influence of another part of the

body, namely, the testes during the subsequent life of the individual.

According to Mendelism the structure and development of each part of the

soma is due to the constitution of the chromosomes of the nuclei in that

part. The effects of castration show that the development of certain

characters is greatly influenced in some way by the presence of the testes

in a distant part of the body. The Mendelians used to say it was

impossible to believe in the heredity of somatic modifications due to

external conditions, because it was impossible to conceive of any means by

which such modifications could affect the constitution of the chromosomes

in the gametes within the modified body. It would have been just as

logical to deny the proved effects of castration, because it was

impossible to conceive of any means by which the testes could affect the

development of a distant part of the body.

But this is not all. The supposed fact that female secondary characters in

Vertebrates are absent in the male is completely disproved for Mammals by

the presence of rudimentary mammary glands in the male. It is true that

secondary sex-characters are usually positive in the male, while those of

the female are apparently negative, but in the case of the mammary glands

the opposite is the case. There is no room for doubt that the mammary

glands are an essentially female somatic sex-character, not only in their

function but in the relation between the periodicity of that function and

those of the ovaries and uterus, and it is equally certain from their

presence in rudimentary condition in the male that they are not absent

from the male constitution.

INFLUENCE OF GONADS DUE TO HORMONES

The existence and the influence of hormones or internal secretions may be

said to have been first proved in the case of the testes, for Professor A.

A. Berthold [Footnote: 'Transplantation der Hoden,' _Archiv. f Anat. u.

Phys._, 1849.] of Göttingen in 1849 was the first to make the experiment

of removing the testicles from cocks and grafting them in another part of

the body, and finding that the animals remained male in regard to voice,

reproductive instinct, fighting spirit, and growth of comb and wattles. He

also drew the conclusion that the results were due to the effect of the

testicle upon the blood, and through the blood upon the organism. Little

attention was paid to Berthold's experiment at the time. The credit of

having been the first to formulate the doctrine of internal secretion is

generally given to Claude Bernard. He discovered the glycogenic function

of the liver, and proved that in addition to secreting bile, that organ

stores up glycogen from the sugar absorbed in the stomach and intestines,

and gives it out again as sugar to the blood. In 1855 he maintained that

every organ of the body by a process of internal secretion gives up

products to the blood. He did not, however, discover the action of such

products on other parts or functions of the body. Brown-Séquard, in his

address before the Medical Faculty of Paris in 1869, was the first to

suggest that glands, with or without ducts, supplied special substances to

the blood which were useful or necessary to the normal health, and in 1889

at a meeting of the Société de Biologie he described the experiment he had

made upon himself by the injection of testicular extract. This was the

commencement of organotherapy. Since that time investigation of the more

important organs of internal secretion--namely, the gonads, thyroid,

thymus, suprarenals, pituitary, and pineal bodies--has been carried on

both by clinical observation and experiment by a great number of

physiologists with very striking results, and new hormones have been

discovered in the walls of the intestine and other organs.

Here, however, we are more especially concerned with the gonads and other

reproductive organs. A great deal of evidence has now been obtained that

the influence of the testes and ovaries on secondary sexual characters is

due to a hormone formed in the gonads and passing in the blood in the

course of the circulation to the organs and tissues which constitute those

characters. The fact that transplanted portions of testes in birds (cocks

and drakes) are sufficient to maintain the secondary characters in the

same condition as in normal individuals shows that the nexus between the

primary and somatic organs is of a liquid chemical nature and not

anatomical, through the nervous system for example. Many physiologists in

recent years have maintained that the testicular hormone is not derived

from the male germ-cells or spermatocytes, but from certain cells between

the spermatic tubuli which are known as interstitial cells, or

collectively as the interstitial gland.

The views of Ancel and Bouin, [Footnote: _C. R. Soc. Biol., iv._]

published in 1903, may be described in large part as theory. They state

that the interstitial cells appear in the male embryo before the

gametocytes present distinctive sex-characters. They conclude that the

interstitial cells supply a nutritive material (hormone?), which has an

effect on the sexual orientation of the primitive generative cells. In

addition to this function, the interstitial cells by their hormone also

give the sexual character to the soma. When castration is carried out at

birth the male somatic characters do not entirely disappear, because the

hormone of the interstitial cells has acted during intrauterine life. The

functional independence between the interstitial cells and the seminal

tubules is shown by the fact that if the vasa deferentia are closed the

seminal gland (_i.e._ tubules) degenerates while the interstitial cells do

not. In the embryo the interstitial gland is large, in the adult

proportionately small.

There is complete disagreement between the results of Ancel and Bouin on

the one hand, and those of Shattock and Seligmann on the other, with

regard to the effects of ligature of the vasa deferentia. The latter

authors, as mentioned above, found that after ligature not only the

somatic characters but the testis itself developed normally. The

experiments were performed on Herdwick sheep and domestic fowls. They

state that on examination the testes were found to be normally developed,

and spermatogenesis was in progress. The experiments of Ancel and Bouin

were carried out on rabbits seven to eight weeks old, and consisted in

removing one testis, and ligaturing the vas deferens of the other. About

six months after the operation the testis left _in situ_ was smaller, the

seminal tubules contained few spermatogonia, though Sertoli's cells (cells

on the walls of the tubules to which the true spermatic cells are

attached) were unchanged; while the interstitial cells were enormously

developed, by compensatory hypertrophy in consequence of the removal of

the other testis. At the same time the male instincts and the other

generative organs were unchanged. In a few cases, however, Ancel and Bouin

observed atrophy of the interstitial cells as well as the spermatic cells.

They believe this is due to the nerves supplying the testis being included

in the ligature. This is rather a surprising conclusion in view of the

fact that testicular grafts show active spermatogenesis. It is difficult

to understand why nerve connection should be necessary for the

interstitial cells and not for the spermatic, and, moreover, if the

interstitial cells are really the source of the hormone on which the

somatic characters depend, they must be acting in the grafts in which the

nerve connections have been all severed.

The facts concerning cryptorchidism, that is to say, failure of the

descent of the testes in Mammals, seem to show that the hormone of the

testis is not derived from semen or spermatogenesis, for in the testes

which have remained in the abdomen there is no spermatogenesis, while the

interstitial cells are present, and the animals in some cases exhibit

normal or even excessive sexual instinct, and all the male characteristics

are well marked. It may be remarked, however, in criticism of this

conclusion that the descent of the testes being itself a somatic sexual

character of the male, its failure when the interstitial cells are normal

and the spermatic cells defective, would rather tend to prove that the

defect of the latter is itself the cause of cryptorchidism.

Many investigators have found that the Röntgen rays destroy the spermatic

cells of the testis in Mammals, leaving the cells of Sertoli, the

interstitial tissue, nerves, and vessels uninjured. Tandler and Gross

[Footnote: _Wiener klinische Wochenschrift_, 1907.] found that the antlers

of roebuck were not affected after the testes had been submitted to the

action of the rays, showing that the interstitial cells were sufficient to

maintain the normal condition of the antlers. Simmonds, [Footnote:

_Fortschr. a. d. G. d. Röntgenstr._, xiv., 1909-10.] however, found that

isolated seminal tubules remained, and regeneration took place, and

concludes that both spermatic cells and interstitial cells take part in

producing the testis hormone. The conclusions of two other investigators

have an important bearing on this question--namely, that of Miss Boring

[Footnote: _Biol. Bull._, xxiii. 1912.] that there is no interstitial

tissue in the bird's testis, and that of Miss Lane-Claypon, [Footnote:

_Proc. Roy. Soc._, 1905] that the interstitial cells of the ovary arise

from the germinal epithelium, and are perfectly equipotential with those

which form the ova and Graafian follicles. It seems possible, although no

such suggestion has been made, that the interstitial cells might either

normally or exceptionally give rise to ova and spermatocytes. The

observations of Seligmann and Shattock on the relation of spermatogenesis

to the development of nuptial plumage in drakes probably receive their

explanation from the above facts. Spermatogenesis is not the only source

of the testicular hormone: changes in the secretory activity of the

interstitial cells or spermatocytes are sufficient to account for periodic

development of somatic sex-characters, and the same reasoning applies to

the antlers of stags.

THE MAMMARY OR MILK GLANDS

The milk glands in Mammals constitute one of the most remarkable of

secondary sexual characters. Except in their functional relations to the

primary organs, the ovaries, and to the uterus, there is nothing sexual

about them. They are parts of the skin, being nothing more or less than

enormous enlargements of dermal glands, either sebaceous or sudoriparous.

Uterine and mammary functions are generally regarded as essentially female

characteristics, and are included in the popular idea of the sex of woman.

Scientifically, of course, they are not at all necessary or universal

features of the female sex, but are peculiar to the mammalian class of

Vertebrates in which they have been evolved. Milk glands, then, are

somatic sex-characters common to a whole class, instead of being

restricted to a family like the antlers in Cervidae. There is not the

slightest trace or rudiment of them in other classes of Vertebrates, such

as Birds or Reptiles. They are not actually sexual in their nature, since

their function is to supply food for the young, not to play a part in the

relations of the sexes. What is sexual about them is--firstly, that they

are normally fully developed only in the female, rudimentary in the male;

secondly, that their periodical development and functional activity

depends on the changes which take place in the ovary and uterus. Many

investigators have endeavoured to discover the nature of the nexus between

the latter organs and the milk glands.

That this nexus is of the nature of a hormone is generally agreed, and may

be regarded as having been proved in 1874 when Goltz and Ewald [Footnote:

_Pflügers Archiv,_ ix., 1874.] removed the whole of the lumbo-sacral

portion of the spinal cord of a bitch and found that the mammae in the

animal developed and enlarged in the usual way during pregnancy and

secreted milk normally after parturition. Ribbert [Footnote: _Fortschritte

der Medicin,_ Bd. 7.] in 1898 transplanted a milk gland of a guinea-pig to

the neighbourhood of the ear, and found that its development and function

during pregnancy and at parturition were unaffected. The effective

stimulus, therefore, is not conveyed through the nervous system, but must

be a chemical stimulus passing through the vascular system.

Physiologists, however, are not equally in agreement concerning the source

of the hormone which regulates lactation. Starling and Miss Lane-Claypon

concluded from their experiments on rabbits that the hormone originated in

the foetuses themselves within the pregnant uterus. In virgin rabbits it

is difficult to find the milk glands at all. When found the nipple is

minute and sections through it show the gland to consist of only a few

ducts a few millimetres in length. Five days after impregnation the gland

is about 2 cm. in diameter. Nine days after impregnation the glands have

grown so much that the whole inner surface of the skin of the abdomen is

covered with a thin layer of gland tissue. In six cases by injecting

subcutaneously extracts of foetus tissue Starling and Lane-Claypon

obtained a certain amount of growth of the milk glands. The hormone in the

case of the pregnant rabbit is of course acting continuously for the whole

period of pregnancy, while the artificial injection took place only once

in twenty-four hours, and the amount of hormone it contained may have been

absorbed in a very short time. The amount of growth obtained

experimentally in five weeks was less than that occurring in pregnancy in

nine days. Extracts of uterus, placenta, or ovary produced no growth,

although the ovaries used were taken from rabbits in the middle of

pregnancy. In one experiment ovaries from a pregnant rabbit were implanted

into the peritoneum of a non-pregnant rabbit, but on post-mortem

examination of the latter eleven days later the implanted ovaries were

found to be necrosed and no proliferation of milk gland had taken place.

The conclusions of Starling and Lane-Claypon were confirmed by Foa,

[Footnote: _Archivo d. Fisiologia_, v., 1909.] and by Biedl and

Königstein, [Footnote: _Zeitschrift f. exp. Path. und Therap_., 1910.] Foa

states that extracts of foetuses of cows produced swelling of the mammae

in a virgin rabbit.

O'Donoghue, however, concludes from a study of the Marsupial _Dasyurus_

that the stimulus which upon the milk glands proceeds from the corpora

lutea in the ovary. In this animal changes in the pouch occur in

pregnancy, which are doubtless also due to hormone stimulation, but which

we will not consider here. The most important evidence in O'Donoghue's

paper [Footnote: _Quart. Journ. Mic. Sci_., lvii., 1911-12.] is that

development of the milk glands takes place after ovulation not succeeded by

pregnancy; that is to say, when corpora lutea are formed but no fertilised

ova or foetus are present in the uterus. In one case eighteen days after

heat, the milk gland was in a condition resembling that found in the

stages twenty-four and thirty-six hours after parturition. In another

specimen, twenty-one days after heat, the milk glands were still more

advanced, with distended alveoli and enlarged ducts. The alveoli contained

a secretion which was almost certainly milk, O'Donoghue states that the

entire series of growth changes in these animals up to twenty-one days

after heat in identical with that which occurs in normally pregnant

animals.

O'Donoghue's conclusion is in agreement with that of Basch,[Footnote:

_Monatesschr. f. Kinderh. V._, No. ix., Dec. 1909.] who states that

implantation of the, ovaries from a pregnant bitch under the skin of the

back of a one-year-old bitch that was not pregnant was followed by

proliferation of the mammary glands of the latter. After six weeks the

glands were considerably enlarged, and after eight weeks they were caused

to secrete milk by the injection of extract of the placenta. It has to be

remembered, however, that the milk glands undergo considerable growth,

especially in the human species, at puberty and at every menstruation, or

at oestrus in animals, which correspond to menstruation. In these cases

there is no question of any influence of the foetus, and experiment has

shown that if the ovaries are removed before puberty, the milk glands nor

the uterus undergo the normal development and menstruation does not occur.

According to Marshall to Jolly [Footnote: _Quart. Journ. Exp. Phys._, i.

and ii., 1906.] the symptoms of oestrus in castrated bitches were found to

result from the implantation of ovaries from other individuals in the

condition of oestrus.

Before considering further the question of the corpora lutea as organs of

internal secretion, we may briefly refer to the origin and structure of

these bodies and of other parts of the mammalian ovary. The mature

follicle containing the ovum differs from that of other Vertebrates in the

fact that it is not completely filled by the ovum and the follicular cells

surrounding it, but there is a cell-free space of large size into which

the ovum covered by follicular cells projects. In the wall of the follicle

two layers are distinguished, the theca externa, which is more fibrous,

and the theca interna, which is more cellular. In the connective tissue

stroma of the ovary between the follicles are scattered, or in some cases

aggregated, epithelioid cells known as the interstitial cells, and it is

stated that the cells of the theca interna are exactly similar to the

interstitial cells. According to Limon [Footnote: _Arch. d'Anat. micr._,

v., 1902.] and Wallart [Footnote: _Arch. f. Gynock_, vi. 271.] the

interstitial cells are actually derived from those of the theca interna of

the follicles. Numbers of ova die without reaching maturity, the

follicular cells degenerate, and the follicle becomes filled with the

cells of the theca interna, which have a resemblance to those of the true

corpus luteum. These degenerate follicles have been termed spurious

corpora lutea, or atretic vesicles. The interstitial cells are the remains

of these atretic vesicles. The true corpora lutea arise from follicles in

which the ova have become mature and from which they have escaped through

the surface of the ovary. As a result of the escape of the ovum and the

contents of the cell-free space, the follicle contracts and the follicular

(so-called granulosa) cells secrete a yellow substance, lutein, and

enlarge. Buds from the theca interna invade the follicle and form the

connective tissue of the corpus luteum.

Somewhat similar processes take place in the ovaries of Teleostean fishes,

as I know from my own observations, but no corpora lutea are formed in

these, although the degenerating follicles in course of absorption

correspond to corpora lutea. The spawning of Fishes, usually annual,

corresponds to ovulation in Mammals, and in the ovary after spawning the

numerous collapsed follicles containing the follicular cells may be seen

in all stages of absorption. [Footnote: Cunningham, 'Ovaries of

Teleosteans.' _Quart. Journ. Mic. Sci._, vol. xl. pt. 1., 1897.] At other

times of the year sections of the ovary show here and there ova which

after developing to a certain stage die and undergo absorption with their

follicles.

In the higher Mammals (Eutheria) the corpora lutea show a special relation

in their development to the occurrence of pregnancy, that is to say, they

have a different history when ovulation is followed by pregnancy to that

which they have when the ova, from the escape of which they arise, are not

fertilised. When fertilisation occurs the corpus luteum increases in size

during the first part of the period of gestation (four months, or nearly a

half of the whole period in the human species). It then remains without

much change till parturition, after which it shrinks and is absorbed. When

pregnancy does not occur the corpus luteum is formed, but begins to

diminish within ten or twelve days in the human species and is then

gradually absorbed. According to O'Donoghue, in the Marsupial _Dasyurus_

there seems to be no difference either in the development of the milk

glands or of the corpora lutea between the pregnant and the non-pregnant

animal. Sandes [Footnote: _Proc. Lin. Soc._, New South Wales, 1903.]

showed that in the same species the corpora lutea persisted not only

during the whole of pregnancy, which Professor J. P. Hill [Footnote:

_Anat. Anz._, xviii., 1900.] estimates at a little over eight days, but

during the greater part of the period of lactation, which according to the

same authority is about four months. In the specimens of _Dasyurus_

described by O'Donoghue, in which the milk glands developed after

ovulation without ensuing pregnancy, normally developed corpora lutea were

present in the ovary. Of the five females which he mentions, the first

three, one with unfertilised ova in the uteri, two five and six days after

heat, could not have been pregnant, but the other two killed eighteen and

twenty-one days after heat might, since pregnancy lasts only eight days,

have been pregnant, the young having died at parturition or before. To

make certain on this point it would have been necessary to examine the

ovaries and milk glands of females which had been kept separate from a

male the whole time. There is no doubt, however, about the development of

the milk glands in the first three specimens, which were certainly not

pregnant.

It is difficult to reconcile entirely the evidence described by O'Donoghue

from _Dasyurus_, with that obtained from higher Mammals, although on the

whole there is reason to conclude that the corpora lutea have an important

influence on the development of the milk glands. According to Lane-Claypon

and Starling, if the ovaries and uteri are removed from a pregnant rabbit

before the fourteenth day the development of the mammary gland ceases,

retrogression takes place, and no milk appears in the gland. If, on the

other hand, the operation be performed after the fourteenth day, milk

appears within two days after the operation. It is to be concluded from

this that the cause of _secretion_ of milk is the withdrawal of a stimulus

proceeding from ovary or uterus. But O'Donoghue believes that milk is

secreted in _Dasyurus_ when no pregnancy has occurred. Ancel and Bouin

[Footnote: _C. R. Soc. de Biol._, t. lxvii., 1909.] have shown that the

growth of the mammary glands was produced in rabbits by the artificial

rupture of egg follicles and consequent production of corpora lutea: the

growth of the glands continued up to the fourteenth day, after which

regression set in. This shows that the development of the milk glands in

rabbits is due to the corpora lutea. On the other hand, Lane-Claypon and

Starling state that in rabbits the corpora lutea diminish after the first

half of pregnancy, while the growth of the milk glands is many times

greater during the second half than during the first half of the period,

and during the second half the ovaries may be removed entirely without

interfering with the course of pregnancy or the normal development of the

milk glands. It is evident, therefore, that in rabbits, whatever influence

the corpora lutea may have in the first half of pregnancy, they have none

in the second half, and that at this period the essential hormone proceeds

from the developing foetus or foetal placenta. Again, if it is the

withdrawal of a hormone stimulus which changes the milk gland from growth

to secretion, it cannot be the corpora lutea which are exclusively

concerned even in _Dasyurus_, for they persist during lactation, while

secretion begins shortly after parturition.

Gustav Born suggested, and Fränkel tested the suggestion experimentally,

that the corpus luteum of pregnancy is a gland of internal secretion whose

function is to cause the attachment of the ovum in the uterus and the

normal development of uterus and placenta. Fränkel found that removal of

both ovaries in rabbits between the first and sixth days after

fertilisation prevented pregnancy, and that the same result followed if

the corpora lutea were merely destroyed _in situ_ by galvano-cautery.

Either process carried out between the eighth and twentieth days of

pregnancy causes abortion.

Lane-Claypon and Starling also found that removal of both ovaries in the

rabbit before the fifteenth day was apt to cause abortion, but at a later

stage the same operation could be performed without interfering with the

course of pregnancy. According to these authors numberless instances prove

that in women double ovariotomy does not necessarily interfere with the

course of pregnancy or the development of the milk glands. Parturition may

take place and be followed by normal lactation. This shows that a hormone

from the corpora lutea is not necessary either to the uterus or the milk

glands, at any rate in the last third of pregnancy, though of course this

does not prove that such a hormone is not necessary for the earlier stages

both of pregnancy and growth of the milk glands.

The results of Steinach, if confirmed, would prove conclusively that the

ovaries and testes produce hormones which determine the development of all

the sexual characters, not merely physical but psychical. He adopts the

view that the interstitial cells or gland are the source of the active

hormone. He claims by transplantation of the gonads in young rats and

guinea-pigs to have feminised males and masculised females. The females

are smaller, and hare finer, softer hair than the males. The testes were

removed and ovaries implanted in young males. The animals so treated grew

less than the merely castrated specimens, and therefore when full-grown

resembled females in size. In the young state both sexes have fine, soft

hair, the feminised males had the same character, like the normal females.

They also developed teats and milk glands like the females, and were

sought and treated as females by the normal males. When the implanted

ovaries are able to resist the influence of their new surroundings, the

female interstitial gland, which Steinach calls the puberty gland,

develops so much that an intensification of the female character takes

place: the animals are smaller than normal females, the milk glands

develop and secrete milk, which can be easily pressed out, and if young

are given to them they suckle them and show all the maternal instincts.

Why the ovary in normal circumstances only when in the gravid condition

calls forth this perfection of femaleness is to be shown in a later

publication. By acting with Röntgen rays on the region where the ovaries

lie, Steinach and his colleague Holzknecht brought about all the symptoms

of pregnancy, development of teats and milk glands, secretion of milk, and

great growth of the uterus in all its layers.

Masculising of females was much more difficult than feminising of males

because the testicular tissue was less resistent, and could not be grafted

so easily. When it succeeded, however, degeneration of the seminal tubules

took place, with increase of the interstitial or Leydig's cells. The

vaginal opening in rats disappeared, partly or completely. The sexual

instincts became male, the animals recognised a female in heat from one

that was not, and attempted to copulate.

Steinach considers that he has proved from results that sex is not fixed

or predetermined but dependent on the puberty gland. By sex here he

obviously means the instincts and somatic characters, for sex in the first

instance, as we have already pointed out, means the difference between

ovary and testis, between ova and spermatozoa. It is difficult to accept

all Steinach's results without confirmation, especially those which show

that the feminised male is more female than the normal female. Such a

conclusion inevitably suggests that the investigator is proving too much.

The subject of the influence of hormones from the gonads is mentioned, but

not fully discussed, in a volume by Dr. Jacques Loeb, entitles _The

Organism as a Whole_. [Footnote: Putnam's Sons, 1916.] Loeb entirely omits

the problem of the _origin_ of somatic sex-characters, and fails to

perceive that the fact that such characters are dependent to a marked

degree on hormones derived from the gonads, together with their relation

to definite habits and functions connected with the behaviour of the sexes

to each other, is proof are these characters are not gametogenic, but were

originally due to external stimulation of particular parts of the soma.

CHAPTER IV

Origin Of Somatic Sex-Characters In Evolution

In his _Mendel's Principles of Heredity_, 1909, Bateson does not discuss

the nature of somatic sex-characters in general, but appears to regard

them as essential sex-features, as male or female respectively. As

mentioned above, he argues from the fact that injury or disease of the

ovaries may lead to the development of male characters in the female, that

the female is heterozygous for sex, and from the supposed fact that

castration of the male leads merely to the non-appearance of male somatic

characters, that the female sex-factor is wanting in the male. He does not

distinguish somatic sex-characters from primary sex-factors, and discusses

certain cases of heredity limited by sex as though they were examples of

the same kind of phenomenon as somatic sex-characters in general. One of

these cases is the crossing by Professor T. B. Wood of a breed of sheep

horned in both sexes with another hornless in both sexes. In the _F1_

generation the males were horned, the females hornless. Here, with regard

to the horned character, both sexes were of the same genetic composition,

_i.e._ heterozygous, or if we represent the possession of horns by _H_,

and their absence by _h_, both sexes were _Hh_. Thus _Hh[male]_ was horned

and _Hh[female]_ was hornless, or, as Bateson expresses it, the horned

character was dominant in males, recessive in females. Bateson offers no

explanation of this, but it obviously suggests that some trace of the

original dimorphism of the sheep in this character was retained in both

horned and hornless breeds. We may suppose that the factor for horns had

disappeared entirely from the hornless sheep by a mutation, but in the

horned breed another mutation had been a weakening of the influence of the

sexual hormones on the development of the character, which, as in all such

cases, is really inherited in both sexes. In the _F1_, when the horned

character in the female is only inherited from one side, the hereditary

tendency is not enough to overcome the influence of the absence of the

testis hormone and presence of the ovarian hormone, and so the horns do

not develop. The Mendelian merely sees a relation of the character to sex,

but overlooks entirely the question of the dimorphism in the original

species from which the domesticated breeds are descended. Similarly, with

regard to cattle where it has been found that hornlessness is dominant or

nearly so in both sexes, no reference is made to the opposite fact that

wild cattle have horns in both sexes and are not dimorphic in this

character.

Bateson proceeds to consider colour-blindness as though its heredity were

of similar kind. He refers to it as a male character latent in the female,

remarks that we should expect that disease or removal of the ovaries might

lead to the occasional appearance of colour-blindness in females. He also

discusses the case of _Abraxas grossulariata_ and its variety

_lacticolor_, and other cases of sex-linked heredity, apparently with the

idea that all such cases are similar to those of sexual dimorphism. _A.

lacticolor_ occurs in nature only in the female sex, and when bred with

_grossulariata_ [male] produces [male]'s and [female]'s all

_grossulariata_, these of course being heterozygous. When the _F1

grossulariata_ [male] was bred with the wild _lacticolor_ [female] it

produced both forms in both sexes, and thus _lacticolor_ [male] was

obtained for the first time. When this _lacticolor_ [male] was bred with

_F1 grossulariata_[female] it produced all the [male]'s _grossulariata_

and all the [female]'s _lacticolor_. Bateson's explanation is that the

female, according to the Mendelian theory of sex, is heterozygous in sex,

the male homozygous and recessive, and that _lacticolor_ is linked with

the female sex-character, _grossulariata_ being repelled by that

character. Thus we have, the _lacticolor_ character being recessive,

lact. male, LL male male x F, gross. female, GL female male

Gametes L male + L male x G male + L female

_____________________|______________________

| |

GL male male LL male female

gross. male lact. female

It will be seen that although in the progeny of this mating all the

_grossulariata_ were males and all the _lacticolor_ females, yet this case

is by no means similar to that of sexual dimorphism in which the

characters are normally always confined to the same sex. For the

_lacticolor_ character in the parent was in the male, while in the

offspring it was in the female. We cannot say here that in the theoretical

factors which are supposed to represent what happens, the _lacticolor_

character is coupled with the female sex-factor, for we find it with the

male sex-character in the _lacticolor_ [male]. It is so coupled only in

the heterozygous _grossulariata_ [female], and at the same time the

_grossulariata_ character is repelled.

According to Doncaster [Footnote: _Determination of Sex_, Camb. Univ.

Press, 1914.] sex-limited, or as it is now proposed to call it sex-linked,

transmission in this case means that the female _grossulariata_ transmits

the character to all her male offspring and to none of the female, while a

heterozygous male _grossulariata_ mated with _lacticolor_ female transmits

the character equally to both sexes: that is to say, the heredity is

completely sex-limited in the female but not at all in the male. This is

evidence that the female produces two kinds of eggs, one male producing

and the other female producing.

With regard to the ordinary form of colour-blindness, Bateson's first

explanation was that it was like the horns in the cross-bred sheep,

dominant in males, recessive in females. About 4 per cent. of males in

European countries are colour-blind, but less than 1/2 per cent. of

females. Affected males may transmit the defect to their sons but not to

their daughters: but daughters of affected persons transmit the defect

frequently to their sons. Bateson gives [Footnote: _Mendel's Principles of

Heredity_, 1909.] a scheme of the transmission, but corrects this in a

note stating that colour-blindness does not descend from father to son,

unless the defect was introduced by the normal sighted mother also, _i.e._

was carried by her as a recessive. The fact that unaffected males do not

transmit the defect shows, according to Bateson, that it is due to the

addition of a factor to the normal, not to omission of a factor.

According to later researches as quoted by Doncaster, colour-blindness is

due to the loss of some factor which is present in the normal individual.

The normal male is heterozygous for this normal factor. If we denote the

presence of the normal factor by _N_ and its absence or recessive by _n_,

then the male is _Nn_, while the female is homozygous or _NN_. But in

addition to this it is the male in this case which is heterozygous

for sex, and _n_ goes to the male-producing sperms, _N_ to the

female-producing. Thus in the mating of normal man with normal woman the

transmission is as follows:--

Nn (male) x NN (female)

Gametes n (male) + N (female) x N + N

n (male) + N N (female) + N

| |

Nn (male) NN (female)

That is all offspring normal, but the males again heterozygous.

An affected male has the constitution _nn_, and if he marries a normal

woman the descent is as follows:--

nn (male) x NN (female)

Gametes n (male) + n (female) x N + N

n (male) + N N (female) + N

| |

nN (male) nN (female)

When a normal male is mated with a heterozygous _nN_ female we get

nN (male) x nN (female)

Gametes n (male) + N (female) x n + N

______________________|______________________

| | | |

nn (male) nN (male) nN (female) NN (female)

that is, half the sons are normal and half colour-blind, while half the

females are homozygous and normal, and the other half heterozygous and

normal.

T. H. Morgan [Footnote: _A Critique of the Theory of Evolution._] has

observed a number of cases of sex-linked inheritance in the mutations

which occurred in his cultures of _Drosophila_. The eye of the wild

original fly is red, one of the mutants has a white eye, _i.e._ the red

colour and its factor are absent. When a white-eyed male is mated to a

red-eyed female all the offspring have red eyes. If these are bred _inter

se_, there are, as in ordinary Mendelian cases, three red-eyed to one

white-eyed in the _F2_ generation, but white eyes occur only in the males,

in other wards half the males are white-eyed. On the other hand, when a

white-eyed _female_ is mated to a red-eyed male all the daughters have red

eyes, and all the sons white eyes. This has been termed crisscross

inheritance. If these are bred together the result in _F2_ is equal

numbers of red-eyed and white-eyed females, and equal numbers of red-eyed

and white-eyed males. The ration of dominant to recessive is 2 to 2

instead of the usual Mendelian ration of 3 to 1.

According to Morgan the interpretation is as follows: In the nucleus

of the female gametocytes there are two _X_ chromosomes related to sex,

in those of the male there is one _X_ chromosome and one _Y_ chromosome

of slightly different shape. The factor for red eye occurs in the

sex-chromosomes, that is to say, according to this theory, the

sex-chromosome does not merely determine sex but carries other factors

as well, and this fact is the explanation of sex-linked inheritance. The

factor for red eye then is present in both _X_ chromosomes of the wild

female, absent from both _X_ and _Y_ chromosomes of the white-eyed male.

The gametes of the female each carry one _X_ red chromosome, of those of

the male half carry an _X_ white chromosome, and half the _Y_ white

chromosome. The fertilised female ova therefore carry an _X_ red

chromosome + an _X_ white chromosome, the male producing ova one _X_ red

chromosome and one _Y_ white chromosome. They are all therefore red-eyed,

but heterozygous--that is, the red eye is due to one red-eye factor, not

two. When the _F1_ are bred together, half the female gametes carry one

_X_ red chromosome, the other half one _X_ white chromosome; half the male

gametes carry one _X_ red chromosome, the other half one _Y_ white

chromosome. The fertilisations are therefore one _X_ red _X_ red, one _X_

red _X_ white, one _X_ red _Y_ white, and one _X_ white _Y_ white. These

last are the white-eyed males. The two different crosses are represented

diagrammatically below, the dark rod representing the _X_ red chromosome,

the clear rod the _X_ white chromosome, and the bent clear rod the _Y_

white chromosome.

According to Morgan, the heredity of colour-blindness in man is to be

explained exactly in the same way as that of white eye in _Drosophila_.

A colour-blind man married to a normal (homozygous) woman transmits the

peculiarity to half his grandsons and to none of his grand-daughters.

Colour-blind women are rare, but in the few cases known where such women

have married normal husbands the defect has appeared only in the sons, as

in the second of the diagrams below.

Parents Red-eyed male White-eyed female

XR XR x XW YW

F1 Red-eyed male Red-eyed female

XR XW XR YW

F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female

XR XR XW XR XR YW XW YW

Homozygous. Heterozygous. Heterozygous. Homozygous.

White-eyed male Red-eyed female

XW XW x XR YW

F1 Red-eyed male White-eyed female

XW XR XW YW

F2 White-eyed male Red-eyed male White-eyed female Red-eyed female

XW XW XR XW XW YW XR YW

Homozygous. Heterozygous. Homozygous. Heterozygous.

It must be explained that according to this theory the normal male is

always heterozygous, because the _Y_ chromosome never carries any other

factor except that for sex; it is thus of no more importance than the

absence of an _X_ chromosome which occurs in those cases where the male

has one sex-chromosome and the female two. According to the researches of

von Winiwarter [Footnote: 'Spermatogénèse humaine,' _Arch. de Biol._,

xxvii., 1912.] on spermatogenesis in man, the latter is actually the case

in the human species. This investigator found that there were 48

chromosomes in the female cell, 47 in the male; after the reduction

divisions the unfertilised ova had 24 chromosomes, half the spermatids 24

and half 23, so that sex is determined in man by the spermatozoon.

Morgan believes that the heredity of haemophilia (the constitutional

defect which prevents the spontaneous cessation of bleeding) follows the

same scheme, and also at least some forms of stationary night-blindness--

that is, the inability to see in twilight.

We may mention a few other in animals, referring the reader for a fuller

account to the works cited. One example in the barred character of the

feathers in the breed of fowls called Plymouth Rock. In this the female is

heterozygous for sex as in _Abraxas grossulariata_, and the barred

character is sex-linked. When a barred hen is crossed with an unbarred

cock all the male offspring are barred, all the females plain. On the

other hand, if a barred cock is crossed with an unbarred hen, the barred

character appears in all the offspring, both and females. The female thus

transmits the character only to her sons. If we represent the barred

character by _B_, and its absence by _b_, we can represent the heredity as

follows:--

BARRED FEMALE WITH UNBARRED MALE

B female b male X b male b male

Bb male bb female

Barred male. Unbarred female.

Heterozygous. Homozygous.

B male B male X b female b male

B male b female b male b male

Barred female. Barred male.

Heterozygous. Heterozygous.]

This case is thus exactly similar to that of _Abraxas grossulariata_ and

_A. lacticolor_. The barred character like _grossulariata_ is dominant,

the unbarred recessive, and to explain the results it is necessary to

assume that the female is not only heterozygous for the barred character,

but also for sex, with the female sex-factor dominant. The recessive

character in this case is linked to the female sex chromosome, or,

as Bateson described it, the dominant character is repelled by the

sex-factor. We may make a diagram of the kind given by Morgan if we use

a rod of different shape for the female-producing sex-chromosome, and use

the black rod for the dominant character:--

BARRED female x unbarred male

BX uY uX uX

| \/ |

| /\ |

BX uX uY uX

BARRED male unbarred female

Heterozygous Homozygous

BARRED male x unbarred female

BX BX uX uY

| \/ |

| /\ |

BX uX BX uY

BARRED male BARRED female

Heterozygous Heterozygous

Another case is that of tortoise-shell, _i.e._ black and yellow cats. The

tortoise-shell with very rare exceptions is female, the corresponding male

being yellow, without any black colour. Doncaster found that a yellow male

mated to a black female produced black male offspring and tortoise-shell

females. When a black male is mated to a yellow female, the female kittens

are tortoise-shell as before, but the males yellow. The Mendelian

hypothesis which explains these results is that the male is always

heterozygous, or has only one colour factor whether yellow or black, and

transmits these colours only to his daughters, while the female has two

colour factors, either _BB_, _YY_, or _BY_. Thus the crosses are:--

YELLOW male x BLACK female

YO male BB female

| \/ |

| /\ |

YB female BO male

Tortoise-shell female BLACK male

BLACK male x YELLOW female

BO male YY female

| \/ |

| /\ |

BY female YO male

Tortoise-shell female YELLOW male

The sex must be determined therefore by the spermatozoa, as in the case of

colour-blindness, etc., in man, and the colour factor must always be in

the female-producing sperm.

SEXUAL DIMORPHISM

It is obvious from the above facts that however interesting and important

sex-linked heredity may be, it is not the same thing as the heredity of

secondary sexual characters, and does not in the least explain sexual

dimorphism. In the first place, the term sex-linked does not mean

occurring always exclusively in one sex, but the direct contrary--

transmitted by one sex to the opposite sex--and in the second place there

is no suggestion that the development of the character is dependent in any

way on the presence or function of the gonad. The problem I am proposing

to consider is what light the facts throw on the origin of the secondary

sexual characters in evolution. In endeavouring to answer this question

there are only two alternatives: either the characters are blastogenic--

that is, they arise from some change in the gametocytes occurring

somewhere in the succession of cell-divisions of these cells--or they

arise in the soma and are impressed on the gametocytes by the influence of

the soma within which these gametocytes are contained--that is to say,

they are somatogenic. That characters do originate by the first of these

processes may be considered to be proved by recent researches, and such

characters are called mutations. There can be little doubt that the so-

called sex-linked characters, of which examples have been given above,

have originated in this way, and that their relation to sex is part of the

mutation. According to T. H. Morgan, it is simply due to the fact that

the determinants for such characters are situated in the sex-chromosome.

Morgan, however, also states that a case of true sexual dimorphism arose

as a mutation in his cultures of _Drosphilia_. The character was eosin

colour in the eye instead of the red colour of the eye in the original

fly. In the female this was dark eosin colour, in the male yellowish

eosin. But this case differs from the characters particularly under

consideration here in two points: (1) there is no suggestion that it was

adaptive, (2) or that it was influenced by hormones from the gonads.