PART I._The genetic and the operative evidence relating to secondary sexual characters

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THE GENETIC AND THE OPERATIVE EVIDENCE

RELATING TO SECONDARY SEXUAL

CHARACTERS

BY T. H. MORGAN

[Illustration: CARNEGIE INSTITUTION OF WASHINGTON 1902]

PUBLISHED BY THE CARNEGIE INSTITUTION OF WASHINGTON

WASHINGTON, 1919

CARNEGIE INSTITUTION OF WASHINGTON

PUBLICATION NO. 285

PRESS OF GIBSON BROTHERS, INC.

WASHINGTON, D. C.

CONTENTS.

PART 1. PAGE.

Introduction 5

Castration of Sebrights 6

A male Sebright that did not become cock-feathered after castration 10

Transitional feathers 10

Castration of F₁ hen-feathered males from Sebright by game 11

Castration of F₂ hen-feathered males 13

Hewitt’s Sebright hen that became cock-feathered in old age 14

Heredity of hen-feathering 14

Heredity of color in the cross between Sebright and Black-Breasted

Game bantam 17

A. The F₁ birds 18

B. Description of F₂ birds 19

C. Back-cross of F₁ to game 21

D. The number of color factors involved 22

E. Back-cross of F₁ ♀ to Sebright ♂ 23

F. Review of the heredity of the color of the plumage in poultry 23

Endocrine cells in ovary and testes of birds 32

Luteal-cells in the testes of the male Sebright 34

Endocrine cells in the testes of mammals 35

Cyclical changes in the interstitial cells in hibernating mammals 36

Hermaphroditism in poultry and the secondary sexual characters 37

PART II.

Darwin’s theory of sexual selection 43

Other theories to account for secondary sexual characters 45

Display of the male 50

PART III.

The genetic and the operative evidence 62

A. Evidence from mammals 64

B. Evidence from birds 73

C. Evidence from amphibia 86

D. Evidence from crustaceans 89

E. Evidence from insects 92

PART IV.

Summary and conclusions 95

Bibliography 100

Description of plates 106

THE GENETIC AND THE OPERATIVE EVIDENCE RELATING TO SECONDARY SEXUAL

CHARACTERS.

BY T. H. MORGAN.

PART I.

There are a few races of poultry that have two kinds of males, one with

the feathering of the ordinary cock, the other with the feathering of

the hen. The Hamburgs and the Campines are perhaps the best known races

of this sort. Convention amongst breeders, in certain countries, has

determined that the cock-feathered bird shall be the standard, and at

other times and places that the hen-feathered males shall be the show

birds. In one breed, at least, viz., the Sebright bantams, the

hen-feathered cock is the only known type. Cock-feathered Sebrights have

never been seen, so far as I know. This breed is pure for

hen-feathering. As shown in plate 1, figure 3, the male Sebright lacks

the long, pointed saddle feathers at the base of the tail of the common

cock, also the peculiar back and neck feathers (hackles) of the cock

bird, as well as the male feathering on the bow of the wing. His

feathers in these parts are almost exactly like those of the hen (plate

4, fig. 4). The long sickle feathers covering the true tail are also

absent, although the two median ones sometimes occur in males of this

race.

The Sebrights seemed excellent material for studying the heredity of

this type of plumage in the male. In 1911 I began to study this problem,

and crossed Sebrights to Black-Breasted Game bantams. The latter race

was chosen not only because the males have the typical cock-feathering,

but also because the coloration of these birds resembles very closely

that of the jungle-fowl, from which many, perhaps all, of our

domesticated races have sprung.

In dissecting some of the F₂ birds from this cross I noticed that the

testis of the male was often more flattened than is the testis of the

typical male bird, that it was often somewhat pear-shaped, and that

frequently it was in part or entirely black. Recalling that male

Sebrights are said to be often partially sterile, the idea naturally

suggested itself that these birds are hen-feathered because the testes

have assumed some of the characteristics of the ovary. It had long been

supposed, and had been finally established by Goodale, that the presence

of the ovary in the female suppresses her potential development of

plumage, for when the ovaries of the hen are diseased or removed she

develops the plumage of the male. This reasoning led me to try the

experiment of castrating the hen-feathered males in order to see if

they would become cock-feathered. The outcome was immediately apparent;

the new feathers were those of the cock bird. While the “reasoning” that

led to the experiment is open to serious question, nevertheless the

“hint” furnished by the unusual condition of the testis led finally to

the discovery that luteal cells were present in abundance in the testes

of the male Sebright like those present only in the females of other

breeds. Whether or not the shape of the testis of the Sebright, that is

sometimes like that of the ovary, is connected with the unusual

abundance of luteal cells in the testis I do not know. If so, then the

hint that came from their shape was not so unreasonable as appears at

first sight.

The birds first operated upon were adult F₁ and F₂ hen-feathered birds.

The first one done by myself died, but a few, whose testes were removed

by Dr. H. D. Goodale at my request, lived and changed to cock-feathered

birds. Since then I have operated successfully on a number of F₁ and F₂

birds, as well as Sebright males. In these operations I have had

throughout the assistance of Dr. A. H. Sturtevant and for two years the

assistance of Dr. J. W. Gowen also. I wish to express my appreciation of

their help and advice, for without it I doubt whether I could have

carried out the work successfully. Since the main interest attaches to

the Sebright experiments, they will be described first, although they

were the last to be performed.

CASTRATION OF SEBRIGHTS.

Except for the similarities of the plumage, the male Sebright differs as

much from the female as do cocks of other races. The rose comb is very

large in the male, small in the female (plate 4, figs. 3, 4). The

wattles also are longer in the male. The cock carries himself erect, as

do the males of other breeds. His spurs are well developed and he shows

the aggressive behavior of his sex. On the other hand, the shortness of

the feathers on the back of the neck (the hackles), the absence of the

pointed feathers on the back and rump, and the usual absence of long

sickles and other tail-covert feathers make him hen-like. The detailed

account of the feathers in these critical regions will be given when

comparisons are made with the feathers of the castrated birds (plates 6

and 8).

Six males have been successfully operated upon and with one apparent

exception have all given the same results. The birds were of somewhat

different ages; they had been hatched about July, and were operated upon

about November of the same year, when they were either half grown or had

nearly reached maturity. At the time of the operation a few feathers

were removed from different regions of the body, and the new feathers

that regenerated in the course of 3 or 4 weeks showed all the

characteristics of those that came in later to replace the juvenile or

first adult coat. These regenerated feathers do not, therefore, call

for special notice. All of the new feathers were in shape, pattern, and

general coloration strikingly different from the original feathers, some

of which were at first still present, the old feathers of course showing

no change.

After completely molting, the appearance of the birds may be gathered

from the photographs (plate 5) and from the colored drawings (plates 1

and 3). The male now has in all points the plumage of a typical

cock-feathered male bird of other breeds. This is startlingly apparent

in the hackle, back, rump, sickle, and tail-covert feathers. Instead of

the laced feathers that are characteristic of both male and female, the

whole upper surface of the bird appears reddish or yellowish, the black

marginal edging of the feathers having disappeared. A detailed

comparison of the feathers of the different regions will show how great

a change has taken place. (See page 8.)

In plates 6 and 8 the feathers from characteristic regions of the normal

Sebright and of the castrated Sebright are shown in pairs.

One of the first Sebrights that was castrated was a lighter bird than

the others. Its lighter color was partly due to the narrower outer band

of the laced feathers, (plate 6, figure 1,) and partly to the lighter

color of the yellow-brown center of the feathers. The bird had a single

comb, but as this crops up occasionally in some stocks of Sebrights, it

need not be interpreted to mean that the bird was impure for color

factors. After being castrated the bird changed over completely to

cock-feathering and has remained in that condition for two or more

years. As shown in plate 5, figure 2, the plumage is even more fully

developed than in cock birds of some other breeds. The comb and wattles

are, however, shrunken and pale, as in a capon. The bird is timid and

scarcely or never crows. When killed (May 1919) no pieces of testes and

no trace of testicular tissue at the old situs were found.

The details of the feathers are shown in plate 6, figures 1 and 1a,

where, in each instance, one of the old and one of the new feathers from

the same region are placed side by side. The feathers on the head and

hackle are yellow, even to the base. At the base of the hackle--the

so-called cape--a few feathers have a small black tip. The feathers of

the back are entirely yellow, except that where the fluff begins there

is some dark pigment. The saddle feathers are for the most part all

yellow, but a few have at the base, near the fluff, black on each side.

The tail coverts are long, with a black margin at their tip. The tail

feathers are long, mossy, and have a black tip. The wing-bow feathers

are all yellow, except the black fluff at the base. The feathers on the

crop are mostly yellow with black margin around the end. Those on the

breast lower down are yellow with black tip and black fluff.

There was another Sebright operated upon at the same time that was a

darker bird (as the original feathers show, plate 8, figs. 1 to 4). It

had a rose comb. The feathers that were plucked at the time of

operation were replaced at once by new feathers of the cock-feathered

type. The new feathers that came in as the old ones were molted were

also cock-type, and the bird soon assumed the complete characteristic

cock-feathering. The comb was shrunken as in castrated birds (plate 5,

figure 5).

SEBRIGHT.

Plate 6 and 8.

1. On the head (_a_) the feathers are small, dull black with

lighter margin and reddish quill.

2. On the hackle (_b_) the feathers are yellow bordered with black,

especially at the base, and at the tip outside of this border there

is a narrow yellow border (broader at base). The border is absent

at tip.

3. In the middle of the back (between the wings) the feathers are

yellow with black margin at the tip. At the base there is some dark

color.

4. The saddle is made up of typical laced feathers with black where

the fluff begins.

5. The tail coverts are short; the upper ones, especially the short

sickles, are slightly curved. The sickle feathers extend up only

about half the length of the tail. They are yellow, laced, and have

a black margin, tending to be lost at tip.

6. The yellow tail is short and erect.

7. The feathers on the wing bow are practically like those on the

back, but shorter.

8. Over the crop and lower breast the feathers are laced.

CASTRATE.

Plates 6 and 8.

1. Feathers entirely yellow and more slender. Those on each side of

the shrunken comb stand up from the head.

2. Hackles on upper part of neck have a black base with red tip.

The outer edge, without barbules, is narrow, then broader than at

tip. Farther down the neck the edge with barbules is yellow with a

narrow black margin.

3. In the part of the feather with barbules there is a yellow

center bordered by a broad black band, especially at base. In the

part without barbules the feathers are yellow and more pointed.

This region especially is deeper yellow than in the original

Sebright.

4. Saddle consists of long, slender laced feathers, except at tip,

which is red. Barbules are absent along edge of outer third of

feather.

5. Tail coverts long, covering the tail as in cock birds of other

breeds. The sickle feathers, especially the upper ones, are much

curved, with black tips; the black margin is largely gone. The

feathers are mossy, sometimes splotched (this is also sometimes

noticeable in normal birds).

6. The tail feathers themselves are almost twice as long as in

normal bird; the upper feathers are more curved.

7. The feathers of wing bow are like those of the back of the same

bird, but shorter.

8. The crop feathers are narrower, with a wider black margin, and a

few may be also entirely black. Feathers on lower breast much like

those in normal, but a little more pointed.

During the spring of 1917 it was noticed that the bird was going back

towards the hen-feathered type, and by the end of the summer he was in

the intermediate condition, as shown by the photograph and by the

individual feathers (plate 8, 1_b_, 2_b_, 3_b_, 4_b_). The comb had

begun to enlarge also. The bird was opened again (1918) and pieces of

testis about as big as peas were found on one side. Evidently a piece of

the old testis had been left behind and had regenerated. As it enlarged

the new feathers were affected so that the plumage returned towards the

normal type. The pieces of testis were removed and a few feathers

plucked out. The new feathers that came in were typically

cock-feathered, and, as the molting proceeded during the winter and

spring, the bird became cock-feathered for a second time as shown in

photograph (plate 5, figure 6) and by the feathers in plate 8, 1_c_,

2_c_, 3_c_, 4_c_. Here, then, is an excellent example of the connection

between the gonad and the condition of the plumage. On opening this bird

(May 1919) no pieces of testes were found. There was a very small

whitish lump at the situs of the old testes, which, when sectioned,

showed some glandular-like tissue, not in tubules, and no evidence of

testicular tissue.

Three other younger Sebrights of the same stock were successfully

castrated. They were hatched in June or July and castrated in November

of the same year. They remained quite small birds, despite their

elongation due to the long tail and tail coverts that they developed.

One of these birds in his cock-feathered plumage is shown in plate 3,

figure 1. One has died, the other two are alive and markedly

cock-feathered, as shown in plate 6, figure 2_a_. All three birds were

dark red-brown, much more so than the two preceding cases, especially

the first case. This color difference might be attributed to the earlier

age of the three birds when operated upon, or to a more complete (or

less complete) operation involving perhaps neighboring parts, or to the

birds having a somewhat different genetic composition (_i. e._,

modifying factors). There is no special reason why the operation if

performed early should have a different result on feathers that develop

after the bird is of adult age. Goodale has suggested that there may be

organs in the vicinity of the testis that have some influence on the

kind of plumage produced, and if there are such organs they might be

removed in one bird and accidentally left in another. It would not,

however, be probable that the bird operated on at first had received one

treatment and the later ones the other treatment. It seems to me more

probable that the birds have come from different genetic strains, and

that this genetic difference gives a more plausible explanation of the

darker cock-feathered plumage. Goodale observed, for the first time I

think, that the largest wing coverts of the castrated cock become

longer. I looked, therefore, with some interest at the condition of

these same feathers in the castrated Sebrights. As shown in plate 10,

figures 1, 1_a_, these feathers are also longer and narrower in the

castrated Sebright than in the normal bird.

The true tail feathers of the capon are said to be longer than those of

the cock. This holds also for the tail feathers of all of my castrated

Sebrights. Their true tail feathers are considerably longer than those

of the normal male, as seen by pulling them out and comparing the two.

Their length is concealed while on the bird by the excessively long

coverts that appear after castration.

In 1916 I operated on a Sebright male that lived for some months, but

died in the summer of 1917. At the time of his death he had assumed a

partial cock-feathering, as shown by the feathers in plate 9, figure 3,

3_a_. Dissection showed that some of the testes had been left, and as is

then to be expected, the change was incomplete.

A MALE SEBRIGHT THAT DID NOT BECOME COCK-FEATHERED AFTER CASTRATION.

One of the males that had been castrated with the others did not become

cock-feathered even after a year. Taking for granted that the castration

had been incomplete, the bird was opened, but as no pieces of the testes

were to be found in the normal position he was killed and carefully

dissected. There were no pieces of testes found in the normal situs. A

small whitish patch of material from this region was cut into sections,

but no testicular material was found in it. Then a large piece of the

back from the region of the attachment of the testes was prepared, but

as yet this piece has not been sectioned. Even were a small piece of

tissue to be found, it would seem unlikely that it would suffice to hold

back all indications of the cock-feathering, for after incomplete

removal of the testis there are nearly always at first some indications

of the lack of material. The most plausible view here is either that

some other gland may have assumed, provisionally, the function of the

missing testes, or else a detached piece has not yet been found.

Glandular cells like the luteal cells of the ovary have in fact been

described by some observers in other organs of the body. As yet I have

not found time to make a thorough histological study of the tissues of

this bird.

TRANSITIONAL FEATHERS.

In several birds new feathers had begun to develop at the time of the

operation under the influence of the testicular secretion. After the

removal of the testes, these feathers continued to grow and in the

absence of the original conditions changed over to the other type. The

outer end of these feathers shows the original or normal shape and

color, while the inner end shows the new characteristics. Such feathers

have been seen in nearly all of my castrated birds; a few from the

Sebright will suffice by way of illustration. In plate 10, figure 2_a_,

four such transitional feathers are shown. In _a_ and _b_ two feathers

from the hackle are photographed. The first (_a_) had begun as a normal

Sebright hackle feather, as seen in the condition of its tip; the rest

of the feather is the same as the feather of the castrated bird. For

comparison with this feather, two (2_b_) from the same bird are shown

that began to develop after the testes were removed, _i. e._, at the

same time as the change occurred in the former feather. At the time the

latter feather (_b_) had not yet completed its full growth. On the bow

of the wing a few intermediate feathers, like the one shown in 2_a_,

were present. (For comparison with normal and castrated feathers see

those on plates 6 and 8.)

An intermediate feather from the back is shown in 2_a_. For comparison

with the old feathers from the same region see plate 6, fig. 2. An

intermediate saddle feather is shown in 2_a_. For comparison with normal

feathers from the same region see figure 2. A still later feather from

the castrated bird is shown in 2_b_. The last was not yet complete when

removed from the bird.

It will be noticed that the change after castration involves the color,

the shape, and the presence and absence of barbules in those parts of

the bird that are peculiar in the last respect. The transition in these

characters is quite sharp--as sharp in fact as is compatible with the

passage from one structure to that of an entirely different kind without

any discontinuity of growth. Owing to the quickness of the response

shown by the feather, it will be possible to study more in detail the

length of time the secretion remains in the body of the bird after the

testes have been removed.

CASTRATION OF F₁ HEN-FEATHERED MALES FROM SEBRIGHT BY GAME.

Hen-feathering is dominant to cock-feathering. As shown in plate 2, the

F₁ male is almost as completely hen-feathered as is the male Sebright.

There is a somewhat greater color difference between the F₁ male and F₁

female than between the Sebright male and female. Two F₁ birds were

castrated for me by Goodale. At the time of operation, in the autumn of

1916, both birds were full grown, (plate 2, figure 1). After molting the

old feathers, both birds appeared as shown in plate 2, figure 4. Each is

completely cock-feathered. The plumage has also undergone a remarkable

change in color. In general, the color change is from yellow and black

to reddish yellow. The greatest change is over the upper surface. The

sickle, covert, and tail feathers are well formed and have now become

iridescent black. The breast has changed least of all. One bird died

February 12, 1919. When opened there was found on the left side a small

white lump; on the other side almost nothing. The lump was found to

consist of testicular tubules with loose glandular cells on its walls.

The extent to which the change has taken place is best shown by

comparison of individual feathers from identical regions--one before and

one after the new feathers (taken out two years later) have come in

(plate 7). The contrast between the old and new feathers of the hackle,

saddle, back, and wing-bow are the most striking. In all of these the

new feathers have become red on the exposed portion and the margin is

free from barbules, as in the cock bird. The increase in size and

change in shape of these feathers is remarkable. Equally great is the

change in the tail-coverts that grade into those of the saddle at the

base. The two median coverts or sickle feathers are longer than the tail

and much arched. They are jet black with a purplish iridescence and with

a yellow-red shaft. The tail itself has also changed; it has lost its

stippling, and has become black like the coverts. The increase in length

of the tail feathers is as remarkable as the increase in length of the

coverts. A detailed account of these changes follows:

F₁.

1. The head feathers are yellow with black base that shows through

on the head.

2. The hackle feathers are yellow with black base (showing through

on neck).

3. The red feathers of the back are penciled. There is a black

band, especially around tip just inside of the margin.

4. The saddle feathers are much like those of the back, much

stippled in center. The border is more distinct.

5. The tail coverts are similar to those of the hen.

6. The tail feathers are black, and with exception of the lower

feathers they are partly stippled.

7. The wing-bow feathers are penciled like those of the back.

8. Feathers on crop yellow with black spot at tip; those lower down

on breast have a bigger spot.

CASTRATE.

1. The feathers are entirely red.

2. The hackle feathers are entirely red.

3. Feathers of back are red except for black at base. Barbules

absent at end and side. Tip pointed.

4. The saddle feathers are red with black base. They are long and

pointed.

5. The coverts are blue-black, with brown shaft. They are long,

pointed, and curved.

6. The tail feathers are black, not stippled, and have a black

shaft.

7. The wing-bow feathers are red with black base.

8. Over the crop the feathers are orange-brown; on the breast they

have the same color and a small black tip.

The F₁ bird from which the colored drawing (plate 2) was made and from

which the normal F₁ feathers were pulled was lent to Dr. Goodale in the

summer of 1917. The bird died in April 1918, and his skin was sent to

me. He also had begun to change over to cock-feathering (plate 9,

figures 2, 2_a_). Goodale recorded that the testis had dwindled to small

bodies only about 10 by 5 mm. This accounts for the change to

cock-feathering. For comparison I have added a third set of feathers to

the two former sets, showing the new hackle, back, saddle, wing, and bow

feathers of this bird. The feathers show that the change is in the same

direction as that shown by the castrated cock, but it has not gone so

far in the direction of cock-feathering. The tail is still short and the

feathers are black. The sickle feathers are not longer than the tail and

are stippled. It is probable that this is the old tail whose feathers

have not been molted since the testis dwindled. In fact, elsewhere the

old and the new feathers are both present, showing that a complete molt

had not taken place. The old feathers still present are practically like

those of the original F₁ bird, showing that the change was of recent

date, and due to the decrease in the testis which was probably caused by

disease.

CASTRATION OF F₂ HEN-FEATHERED MALES.

The F₂ hen-feathered males from this cross could not be utilized until

they had begun to assume the adult plumage, since before that time they

were like the cock-feathered F₂ males. Consequently, the operation is

more difficult and more dangerous to the bird. A good many birds have

died in consequence of the operation, but enough successful operations

(five) were made to show what the color of certain types of

hen-feathered bird would be when changed to cock-feathering.

A hen-feathered male (No. 292) that was darker than the F₁ male--in

fact, almost black, except for a yellow center in some of the dorsal

feathers that were mossy or penciled--was castrated. The details of

characteristic feathers may be gathered from the feathers in plate 7,

figure 2. A corresponding set of the new feathers after castration,

2_a_, are paired with the former. The castrated male in his new plumage

is shown in plate 2, figure 3. His dorsal surface is colored very much

as is the same region in the F₁ bird, but the breast is very much

darker, so that the bird as a whole presents a very different appearance

from the F₁ castrated male. A very small white mass was found when the

bird was killed in place of the old testis, composed, in sections, of a

reticulated mass of cells that look like old broken-down follicles of

testicular tubules with a few cell-layers lining the tubules.

An F₂ male (68) also had dark feathers (plate 3, figure 2, and plate 9,

figure 1). The castrated male in his new plumage is represented in plate

3, figure 3. Here again the upper surface is much like that of the last

castrate, and also like that of the F₁ castrate. The breast has changed

much less than the back; the centers of the feather are brown with a

black margin and a black band at the tip. The exposed portion of the

secondaries and the coverts are not so brown as in the last bird. The

spurs of this bird were bent back, looking like the horns of a ram. When

killed and examined, several small white pieces, that looked like pieces

of testes, were found in the abdominal cavity near the old attachment of

the testis. A histological study showed that these pieces contained

tubular tissue apparently testicular, but without germ-cells.

Another F₂ male (Band No. 221) was yellow in general color, the feathers

being irregularly penciled. After castration (plate 3, figure 4) the

bird became red above and deep brown below; the tail and coverts were

black.

A pale-yellow hen-feathered bird (No. 218) was also castrated. Here also

the change was most conspicuous over the upper surface, not only in a

greater depth of color than elsewhere, but in the shape, etc., of the

feathers. On the breast the original yellow color remains, but is

slightly deepened. When killed and opened (May 14, 1919), a few small,

whitish pieces were found. When these were sectioned it was seen that

they were made up, for the most part, of tubules looking like those of

the epididymus and also a few testicular tubules. At the old situs there

were some regenerated lumps, which in sections appeared to be loose

glandular tissue. No germ-cells were present and the tissue just

referred to may be old testicular tubules.

HEWITT’S SEBRIGHT HEN THAT BECAME COCK-FEATHERED IN OLD AGE.

Darwin records in Chapter XIII of Animals and Plants under Domestication

a change that took place in an old female Sebright:

“Mr. Hewitt possessed an excellent Sebright gold-lace bantam hen,

which, as she became old, grew diseased in her ovaria and assumed

male characters. In this breed the males resemble the females in

all respects except in their combs, wattles, spurs, and instincts;

hence it might have been expected that the diseased hen would have

assumed only those masculine characters which are proper to the

breed, but she acquired, in addition, well-arched tail

sickle-feathers quite a foot in length, saddle-feathers on the

loins, and hackles on the neck--ornaments which, as Mr. Hewitt

remarks, would be held to be abominable in this breed.”

This is the only record I know of showing the change that takes place in

the Sebright hen when the influence of her ovary is removed. There can

be no doubt from the above description that she changes in the same way

as does the castrated Sebright male.

Concerning the origin of the Sebright bantam Darwin states that the race

“originated about the year 1800 from a cross between a common bantam and

a Polish fowl, recrossed by a hen-tailed bantam, and carefully selected;

hence there can hardly be a doubt that the sickle feathers and hackles

which appeared in the old hen were derived from the Polish fowl or

common bantam; and we thus see that not only certain masculine

characters proper to the Sebright bantam, but other masculine characters

derived from the first progenitors of the breed, removed by a period of

about 60 years, were lying latent in this hen bird ready to be evolved

as soon as her ovaria became diseased.” To-day the problem appears to us

in a somewhat different light, since the secondary sexual characters

referred to by Darwin have simply been kept under for more than a

hundred years by the secretion produced in the ovary of the hen (as in

all breeds) and in the testis of the male Sebright.

HEREDITY OF HEN-FEATHERING.

In 1913 I found that hen-feathering as seen in the Sebright is a

dominant non-sex-linked character. A preliminary statement was given in

the first edition of my book on Heredity and Sex (1913), which treated

the character as a recessive one. This was a mistake due to a male

having been obtained that was like the game race, which subsequent work

showed must have been due to a sperm having been retained in the

oviduct of the female during her isolation period. In the second edition

published a few months later the mistake, having been found out, was

corrected.

If _one_ dominant suffices to produce hen-feathering, the F₂ ratio would

be 3 hen-feathered to 1 cock-feathered bird. The numbers found were 31

to 28. This realized ratio departs too far from a 3:1 ratio to make it

probable that the results are due to a single factor.

The F₂ expectation for _two_ dominants, both necessarily present to

produce hen-feathering, is 9 hen-feathered to 7 cock-feathered birds. If

the dominant factors are represented by H and H´ and their wild-type

(recessive) allelomorphs by h and h´, the expected F₂ recombinations are

given in the following table:

+-----+-----+-----+-----+-----+

| | HH´ | Hh´ | hH´ | hh |

+-----+-----+-----+-----+-----+

| HH´ |{HH´ | Hh´ | hH´ | hh |

| |{HH´ | HH´ | HH´ | HH´ |

| +-----+-----+-----+-----+

| Hh´ |{HH´ | Hh´ | hH´ | hh |

| |{Hh´ | Hh´ | Hh´ | Hh´ |

| +-----+-----+-----+-----+

| hH´ |{HH´ | Hh´ | hH´ | hh |

| |{hH´ | hH´ | hH´ | hH´ |

| +-----+-----+-----+-----+

| hh |{HH´ | Hh´ | hH´ | hh |

| |{hh | hh | hh | hh |

+-----+-----+-----+-----+-----+

There are 9 classes containing both H and H´, 6 containing one or the

other, and one containing neither H nor H´. The realized numbers, 31 to

28, are in close approximation to 9:7.

In classifying the F₂ hen-feathered males, an attempt was made to divide

them into two classes, viz, type 1, hen-feathered to the same extent as

the Sebright, and type 2, intermediate between hen and cock feathering.

The line between intermediate and cock-feathering is sharp, all the

intermediates belonging distinctly to the hen-feathered group, but the

line between the two subdivisions of hen-feathered birds is not sharp,

and occasionally a bird is found that is difficult to place. These

statements hold also for the F₁ birds, whose skins I now have. Five of

these are classified as intermediates and one as completely

hen-feathered. The difference between these two classes, then, is

environmental or due to other modifying genetic factors, for which

either the Sebright or the game is not pure. Under these circumstances

it would not be profitable to attempt to find out (without additional

evidence) what genetic differences, if any, lie behind the hen-feathered

and intermediate-feathered birds in the F₂ classes.

Concerning the back-cross (F₁ by game) the expectation, for one dominant

factor-difference, is 1 hen-feathered to 1 cock-feathered male. There

were obtained 2 hen-feathered (intermediates) to 7 cock-feathered

birds. The numbers are too small to be significant, taken by themselves.

The expectation for 2 dominants, both essential to hen-feathering, is 1

to 3, and this is in agreement with 2 to 7 as found. It seems, then,

more probable from the evidence of the F₂ and of the back-cross combined

that there are two dominant factors present in the Sebright that make

the male hen-feathered, and since the race breeds true to

hen-feathering, both factors must be present in homozygous condition

unless an undetected lethal destroys some of the classes.[1]

+------------------------------------------+---------------------------------------------+

| | |

|_A_ |_C_ |

| GOLD MALE X SILVER FEMALE | |

| | (Product of English| |

| (Belgian imported | type males for 7 | |

| from Brussels.) | generations in my| SILVER MALE X GOLD FEMALE |

| +-------------+--------------+ | +----------------+---------------+ |

| | | | | | |

|6 Silver Males 6 Gold Females |6 Silver Males 6 Silver Females |

|(Belgian type) |(English type) |

+------------------------------------------+---------------------------------------------+

| | |

|_B_ |_D_ |

| | |

| SILVER CROSS-BRED X GOLD CROSS-BRED | SILVER CROSS-BRED X SILVER CROSS-BRED |

| | | | |

| +--------+-------+-----+---------+ | +---------------+---------------+ |

| | | | | | | | | |

|4 Silver 2 Gold 2 Silver 5 Gold | Silver Males Silver Females Gold Females|

| Males Males Females Females | (English type) |

|(Belgian (Belgian | |

| type) type) | |

+------------------------------------------+---------------------------------------------+

| |

|_E_ |

| |

| SILVER MALE (offspring of C) X GOLD FEMALE (pure) |

| | |

| +-------------+-------------+----------------+-------------+-------------+ |

| | | | | | | |

|Silver Males Silver Males GOLD MALES Gold Males Silver Females Gold Female. |

|(Eng. type) (Belgian type) (ENG. TYPE 1912) (Belgian type) |

| ============== |

+----------------------------------------------------------------------------------------+

| |

|_F_ |

| |

| SILVER MALE (offspring of C) X SILVER FEMALE (pure) |

| | |

| +------------------------------------+------------------------+ |

| | | | |

|Silver Males (English type) Silver Females Gold Females |

+----------------------------------------------------------------------------------------+

Smith and Haig have reported the following curious case of

hen-feathering. Smith had a breed of White Leghorns with cocks of two

classes--those that assumed cock plumage at 6 months and those that are

like the hens for 8 months, after which they slowly assumed the

cock-feathering. The difference is hereditary and appears to segregate.

Possibly this breed had one factor at least for hen-feathering that is

effective for young birds, but not for older ones, or some of the birds

pass through a stage when they produce an internal secretion that

disappears later. But it is also possible, and perhaps more probable,

that the young birds, not cock-feathered, have remained longer in the

juvenile stage than the others, so that they might be said to be falsely

hen-feathered.

The results published by the Rev. E. Lewis Jones in 1914, describing

crosses between two breeds of Campines, one called Belgian (which has

hen-feathered males), the other English (that has cock-feathered males),

are summarized in the table on page 16. They show the dominance of

hen-feathering with some probability. The table given there is the

original, to which the author has kindly added the numbers here prefixed

to some of the classes. The numbers are not large enough in all cases to

be satisfactory, but the dominance of the hen-feathering is, I think,

apparent, as well as its non-sex-linked transmission. The golden female

in C must have been English type, or at any rate heterozygous for

English-type feathering, for if Belgian her sons would have been Belgian

type.

Punnett and Bailey (1914) have published the result of a cross with

hen-feathered Silver Sebrights and Hamburgs. The dominance of

hen-feathering in the male is shown in the figures that illustrate their

paper, but as the paper deals solely with the inheritance of weight the

account of inheritance of hen-feathering was deferred to a later paper,

that has not yet appeared.

HEREDITY OF COLOR IN THE CROSS BETWEEN SEBRIGHT AND BLACK-BREASTED GAME

BANTAM.

The cross between the Sebright and the Black-Breasted Game bantam was

undertaken primarily to study the inheritance of hen-feathering. The

Sebright was chosen, on the one hand, because this race is pure for

hen-feathering, whereas in other races, such as the Campines, both kinds

of males are known. The hen-feathered birds of such races are, I

believe, frequently not pure for hen-feathering. The game race was

chosen because the cock has the typical plumage of the wild bird,

_Gallus bankiva_, and although his feathers are remarkably short, they

show the characteristic cock-feathered type.

Only secondarily was the experiment concerned with color inheritance.

The two breeds differ so markedly in coloration and pattern that the

very complex results that appeared in F₂ were to be expected. In

addition to the differences involving hen-feathering _versus_

cock-feathering, and Sebright plumage _versus_ game plumage, the game is

strongly dimorphic in the plumage, while in the Sebright the coloration

of the two sexes is closely similar. But the castration experiments have

shown that this difference is the result of hen-feathering in the

Sebright cock, and that the race carries the same potential dimorphism

as do other races of poultry.

The game cock is shown in plate 1, figure 1, and plate 4, figure 1. The

wattles and comb had been removed from the bird. The yellow-red back and

saddle are to be noted. The upper tail coverts and sickle feathers are

black, as is the tail. These parts are shorter in the game than in other

races, being one of the points selected for. The dorso-anterior edge of

the wing is black, this color meeting across the middle of the back.

Below this black area comes the red wing bow, followed by a double row

of blue-black feathers. The exposed portions of the secondaries are

brown, of the primaries black with green margin. The breast and entire

lower surface is black. The legs are greenish, the bill black and

yellow, the iris yellow.

The hen of the Black-Breasted game (plate 1, figure 2) is light

yellowish-brown. The back, saddle, and wing coverts are golden brown,

finely penciled with darker brown or black. The hackle is penciled; it

has a yellow border (without barbules); the back is more brown, the

forepart of the breast is salmon, the more posterior parts lighter

salmon. The sides of the body under and below the wings are stippled

gray.

The Sebright male is represented in plate 1, figure 3. Photographs of

the male and the female are given in plate 4, figures 3 and 4. Most of

the feathers have a yellow center and a black border. Such feathers are

said to be laced. The details of the different regions are shown in the

feather plates, 6 and 8.

A. THE F₁ BIRDS.

The F₁ birds were remarkably uniform. The sexual dimorphism is slight,

as a comparison of the male and female in plate 4, figures 5, 6, will

show. In the female the body feathers are penciled but very mossy, and

this holds for the male too, except that in the hackle, back, and

saddle, a change in color accompanies the change in shape, as seen in

the individual feathers in the feather chart (plate 7, figure 1). If

there are any sex-linked factors involved in the cross, we should expect

different types of F₁ hens in the direct cross and its reciprocal,

because in one case the F₁ hen gets her single X chromosome from one

father, and in the other case, the reciprocal cross, from the other.

Unfortunately no careful comparison can now be made, because the crosses

were carried out in different years and the changes due to age may have

affected the color sufficiently to obscure such slight difference that

may have existed. But the effects of such factors, if present, are very

small, since the birds seemed to be the same, regardless of the way in

which the cross was made. In the F₂ counts, although an attempt has been

made to keep apart the birds obtained in the two crosses (_i. e._, the

direct and the reciprocal crosses), it is very doubtful if the two

groups show any significant differences.

B. DESCRIPTION OF F₂ BIRDS.

All together there are 72 hens, 29 hen-feathered males, and 26

cock-feathered males, as shown in table 1:

TABLE 1.

+-+-----------------------------------------------+-------------------------------------------+

| | F₂ from Sebright ♀ by Game ♂. |F₂ from Sebright ♂ by Game ♀. |

| +---------+----------+--------------+-----------+---------+----------+----------+-----------+

+-+---------+----------+--------------+-----------+---------+----------+----------+-----------+

|A| 15 | | 3 | 2 | 1 | 1 | 4 | |

|B| 1 | 1 | | | 5 | | | 1 |

|C| 3 | 1 | | | 4 | 1 | | |

|D| 3 | 2 | | 2 | 2 | | | 1 |

|E| 6 | | | | | 1 | | 1? |

|F| 6 | 1 | 2 | 3 | | 2 | 1 | 1 |

|G| 11 | 1 | 1 | 1 | 2 | 1 | | 1 |

|H| 5 | | | | | | | |

|I| 1 | | | 2 | 1 | | | 2 |

|J| 2 | 1 | | 2 | 2 | 1 | 1 | 2 |

|K| | | 2 | 1 | | | | 2 |

|L| | | | | 1 | | | |

|M| | | | | 1 | | | |

|N| | | | | | | | |

|O| | | | | | | | 1 |

|P| | | | | | | 1 | |

| +---------+----------+--------------+-----------+---------+----------+----------+-----------+

| | 53 | 7 | 8 | 13 | 19 | 7 | 7 | 13 |

+-+---------+----------+--------------+-----------+---------+----------+----------+-----------+

F₂ HENS.

_A._ 8 females that are like F₁; 7 others resemble them below, but

have stippled back and rump feathers (4 of these have yellow necks,

and 3 black necks like those of F₁). Here, then, are two or three

subdivisions, or perhaps main classes.

_B._ 1 female is very close to game, having the characteristic

stippling above and salmon breast below. She is darker colored than

the game females, therefore more like the Leghorn female.

_C._ 3 birds resemble the Sebright in plumage, but would not pass

muster for real Sebrights.

_D._ 3 others have spangled breast feathers like the Sebright, but

a great amount of stippling on the back.

_E._ 6 birds are yellow on breast, with stippled back.

_F._ 6 others are yellow, stippled birds with a little yellow

penciling on the breast.

_G._ 11 black birds with some stippling on the back of the wing,

and sometimes with traces of yellow in the hackle.

_H._ 6 other birds are dark, but not as black as the last. There is

some stippling, especially on breast. The hackle is always striped.

(4 of these have yellow necks above and below; 2 have dark necks.)

The series of feathers photographed in plate 10, figure 4, from a

bird of class _H_, show all gradations between a spangled and a

barred condition. It is practically certain that the barring seen

here (as well as that under class _M_) is quite different from that

of the Barred Plymouth Rock.

_I._ 1 yellow hen with a little black (as a band) on wings and

tail.

_J._ 2 other yellow birds with a little black penciling on the

back, the tail, and with long wing feathers.

F₂ MALES.

_K._ 2 intermediate males with a black-splotched red breast and

black tail. The saddle is coarsely stippled. The corresponding

male-feathered bird is red above.

_L._ A Sebright-like bird with black stippled feathers on back; the

rest of the plumage heavily laced. Posterior part of breast and

thigh black. The tail stippled.

_M._ Yellow neck, the back, tail, and lower half of each secondary

coarsely stippled. Posterior part of breast barred; cape and

anterior breast penciled. The series of feathers photographed in

plate 10, figure 5, from the breast of a bird in class _M_, shows

all possible gradations from a penciled to a barred condition.

_O._ 1 cock-feathered, Red-Breasted game with somewhat stippled

feathers. The lower half of each secondary is penciled (Hamburg

type).

_P._ 1 intermediate male with yellow hackle that is black striped.

He has a peculiar saddle, the general color of which is reddish

brown. Each feather has a faint black edge, and is clear yellow

along shaft; the rest of the feather is finely dusted on a yellow

background (plate 10, fig. 3). Breast feathers (in front part) are

laced with an outer black band edged with yellow. In the posterior

region of the breast the feathers are broadly laced. Wing-bow and

coverts red, laced with black. The exposed edges of primaries and

secondaries are red-brown, the covered parts black. The tail is

black.

BACK-CROSS HENS.

_A._ Two dark F₁ types. The breast is between stippled and

penciled, the head is black.

_C._ (1) Sebright type. Very dark with much stippling. Some

penciling on back. Breast dark; neck like that of Sebright.

(2) Sebright type like (1), but not such clear yellow. Secondaries

and tail feathers and coverts stippled (with black tips).

(3) Yellow Sebright. Neck and breast yellow with black base and tip

to feathers. Cape, breast, and wings (except bow) penciled to

barred.

_C._ to _D._ (1) Pale yellow, breast spangled, back lightly

penciled, tail same. Secondaries yellow and little stippled. Upper

web of primaries stippled.

(2) Breast spangled, rest as in (1).

(3) Same as (2).

_L._ Dark Sebright. Back-feathers broadly laced and a little

penciled. Neck black with yellow centers to feathers and yellow

edges (reversed Sebright).

All of the preceding hens except A are in general Sebrights. The last

three are pale stippled Sebrights.

BACK-CROSS HEN-FEATHERED COCKS.

_A._ One cock like F₁ male, but rather paler on back.

_C._ Four cocks. Light Sebrights, but spangled, in general, instead

of laced. Feathers clear, not stippled.

_G._ One black.

_L._ Dark Sebright. Back and rump black. Feathers with narrow

center, not stippled.

_Q._ Dark Sebright nearer to hen C (1). Thoroughly stippled with

game tail. Neck and breast dark Sebright. Probably a new class

nearer to (C).

_R._ Two cocks. Pale yellow instead of reddish, and much less black

than are other yellows. No class of hens to match.[2]

In regard to color inheritance the preceding 19 birds are too few to add

anything of significance to the other results, except that they serve to

emphasize the dominance of the factors making for Sebright coloration.

The hen-feathered cocks confirm the other results as to the dominance of

the factor or factors in question.

There can be little doubt that some of these classes are complex. They

almost merge into each other and in one part of the body individuals

may grade off into one class, in other parts into other classes. An

almost continuous series of types might be arranged from black to pale

yellow.

The difficulty of matching the hen-feathered males to their genetic

mates is almost insuperable. In table 1 an attempt was made to put these

males with their respective females. The difficulty is, of course,

greater for the cock-feathered birds, even with the castration evidence

(that is too meager at present for the purpose), but a few of the males

may be placed with certainty, and the rest guessed at.

One bird appears to be a _hen-feathered_ game male resembling in many

respects the female game, but darker and redder. There is more shafting

on cape and wing-bow. The breast is unusually dark-salmon. The hackle is

darker than is the game female. Upper wing-coverts broadly laced with

black. (Plate 10, fig. 3.)

The occurrence of this hen-feathered jungle-fowl is so unique and the

coloration of the bird so interesting that I have added to the plates

three feathers of such a bird, viz., a stippled saddle feather, a

feather from the back, a hackle feather, and a wing covert with stippled

center and a black border. The neck hackle departs somewhat from the

hackle of the jungle-fowl hen, but in the same direction as does the

neck hackle of the Sebright cock from his hen.

Looking over the F₂ group, the most noticeable thing is the large number

of blacks (E and G), all of which are stippled. Probably the factor came

from the game, because group E was present in the back-cross as well as

in F₂, and because these black birds are always stippled. The yellow

color (I and J) may have come from both, each breed having then a black

factor that, as a pattern, covers over most of the yellow. It is

difficult to distinguish penciling from stippling in the F₂ yellows.

Without figuring each of these types, their description in detail is not

of much value. The skins will be deposited for reference in the

Zoological Laboratory of Columbia University.

C. BACK-CROSS OF F₁ TO GAME.

As the back-cross of the F₁ to the game might appear more likely to

reveal the kinds of germ-cells present in the individual, the results

from such a cross may be given before discussing the genetic data. If it

were certain that the “game” contained all of the recessive factors that

are involved in the experiment, this method of testing the result would

be ideal, but there is no way of determining _a priori_ whether this is

the case. The question will be taken up later. The presence of two kinds

of males with corresponding but largely uncorrelated differences in

their plumage makes their classification as a group impossible. It is

simpler, therefore, to put the females into their classes first, after

which the hen-feathered males may be expected to fall into the same

groups (or nearly so), while the identity of the cock-feathered males,

_i. e._, their class relationship can only be determined for the classes

that resemble the F₁ and the P₁ birds. The F₂ hen-feathered males can in

part be further identified by means of the evidence that castration of

these types affords.

Two of the F₂ classes of hens can be identified in this back-cross, viz,

(_a_) 4 hens like the F₁ birds, (_b_) 3 hens like the game; (_c_) there

were 3 other hens with plain yellow, _i. e._, not stippled backs. The

upper surface was like that of the game female, but much lighter. The

first two classes (_a_), (_b_) might be again split into two types.

There were only two hen-feathered males, one nearly like the F₁ male,

the other blacker; they probably belong to different classes.

Of the 7 cock-feathered males, one was like the F₁ _castrated_ males;

another had a similar back, but a darker and differently marked breast;

2 were game-cock type; 3 were odd birds much like the game cock above

except for absence of black, with reddish heads without any black. The

males may be approximately classified as follows:

+----------+-----------------------------------------------+

| | Back-cross F₁ ♀ by game ♂. |

| +---------------+--------------+----------------+

| | ♂. | ♂. | ♂. |

+----------+---------------+--------------+----------------+

| F (or K) | | 1 | 1 |

| (C) | | 1 | 2 |

| B | | | 2 |

| A | | | 2 |

| | | +----------------+

| | | | 7 |

+----------+---------------+--------------+----------------+

Four or five types may then be recognized in this rough grouping. None

of the groups seem uniform and probably might be split again.

D. THE NUMBER OF COLOR FACTORS INVOLVED.

The theoretical expectation for two pairs of factors calls for 4 classes

in the back-cross, but this assumes that the parent type used for

back-crossing contains all (here 2) recessives. But this simple

assumption can not be true in this case, for the F₁ bird would have been

like the Sebright. On the 3-factor assumption the expectation for the

back-cross is 8 classes, but this would apply only if the game were the

triple-recessive form, which, again, it is not, as shown by the F₁

cross. But if the dominance of one or more of the Sebright color factors

is incomplete, then either a 2 or 3 factor assumption might apply to the

back-cross.

If only 2 pairs of factors are present we should expect to recover the

game type once in 16 cases in F₂. But, as will be shown, only 1 game was

recovered out of the 49 F₂ females. This result fits better with a

3-factor assumption, for even with the small number in the back-cross

the indications are that more than 4 classes are present.

In the F₂ birds at least 11 classes may be distinguished, and some of

these appear composite. For 3 factors the maximum number of possible

classes (including heterozygotes) is 27. We can recognize at least 11 F₂

classes amongst the females alone, and a few others are doubtfully

present in the males.

In favor of the view that the heterozygous classes are here different

from the homozygous, the following evidence may be utilized:

(1) The F₁ birds are entirely different from either parent and they are

heterozygous for all the factor differences between the two types. The

only alternative explanation for the intermediate condition of F₁ would

be that each race carries one or more completely dominant factors. But

the latter view is improbable because more of each parent type would

then be expected in the F₂ generation.

(2) In the F₂ generation the F₁ type is not as frequent as would be

expected on the view that the heterozygotes could not be distinguished.

E. BACK-CROSS OF F₁ ♀ TO SEBRIGHT ♂.

It is possible to add, now, while this paper is passing through the

press (June 1919), the results of a back-cross of 4 F₁ females to a

Sebright male carried out during the summer of 1918. The birds being now

mature their permanent colors are evident. Making the back-cross in this

direction is much less advantageous than the reciprocal described above,

because the Sebright contains most of the dominant color factors. The

group of birds obtained appeared to be less variable in color than those

from the other back-cross, and one can see at a glance that more of them

approach the Sebright type; some quite closely.

All of the males are hen-feathered, as expected. No evidence was found

that two types of males exist, which would have been expected if the two

types noted in F₂ had any hereditary significance. If, then, as the F₂

results suggest, two factors for hen-feathering are present both are

dominant, and no genetic distinction is found between individuals in

which one or both of the dominant factors are duplex or simplex.

There were 9 adult hens and 10 hen-feathered cocks. An attempt is made

below to refer them to their corresponding F₂ classes.

F. REVIEW OF THE HEREDITY OF THE COLOR OF THE PLUMAGE OF POULTRY.

In poultry there are perhaps more different colors and color-patterns

than in any other species of domesticated animals. The genetic work has

advanced far enough to show that many of the differences depend on

Mendelian factors. It is probable that, in addition to the main factors,

there are many contributory, minor, or modifying factors that give the

finer details to “show birds.”

It is generally supposed that the wild bird from which some at least of

the domesticated races have come is _Gallus bankiva_ of India and

Indo-China, or else one or another of its subspecies. In any case, the

wild type of coloration is approximately known, since the known wild

races are colored alike in all essential respects. Even were the color

of the wild type not known, the original plumage could be deduced with

some degree of probability from the atavism that appears when some of

the races are hybridized. It is interesting to find that many of the new

plumage characters are dominant to the wild type. The same relation also

holds rather generally for other characters of poultry, such as the

comb, etc.

Amongst the uniform or single-colored races, the whites, blacks, reds,

and buffs have been studied. Bateson and Punnett were the first to show

that the white of the White Leghorn is dominant. They also showed that

the white of the White Rose Comb bantams is recessive. Another white,

that of the White Silky, is also recessive, but due to a different

factor from the white factor of the Rose Comb bantams; for, when these

two whites are bred together they give colored birds in the first

generation. Hurst showed later that the white of the Leghorn is dominant

over the black of the Hamburg and the buff of the Cochin. The dominance

is often not complete, since tints of black or of buff or even patches

of these colors may occur. The latter may be confined to the head, neck,

and breast. The black plumage of the Hamburg is dominant over the buff

of the Cochins, but incompletely so, as the black background may be

marked and shaded with brown. Whether we are dealing here with one pair

of factors, or two pairs, could only be determined by an F₂ ratio;

whether it is 3:1 or 9:3:3:1.

The blue color of the Andalusian is known not to be a simple color, but

to be a fine mosaic of splashed white and black. The color is produced

in birds that are heterozygous for splashed white and black, or at least

for certain kinds of white and black. This relation was first

demonstrated by Bateson and Punnett (1902 and 1905) and later Saunders

(1906). It appears also from certain crosses made by Davenport that some

of the whites (such as that of the Leghorn) and black (such as that of

the Minorca) may at times also give some blue birds when crossed.

Whether there are also other races with dominant white color different

from that of the Andalusian white (and the same holds for black races

also) or whether a special (recessive) white was present in this cross

when the blue appeared, was not made out by Davenport.

Lippincott has recently studied the Andalusian cross and obtained

essentially the same results as his predecessors. He calls attention to

an interesting fact in the splashed whites, namely, that the color

splashes are blue when they are found in those parts of the body where

the color is blue in the Andalusian. Although the Andalusian is always

spoken of as a blue bird, the hen only is entirely blue, while the male

is black above and blue below. The splashes on a white male correspond

to the black and blue of the Andalusian male, and are black if above and

blue if below.

Lippincott found also that the blue birds differ from the black in two

characteristics, viz, in the blues the pigment is in larger masses, _i.

e._, it is more clumped, leaving more white between the clumps than in

the blacks, and in the blues the pigment is absent in the extremities of

the barbules. If the clumping and the condition of the barbules are

treated as separate entities, each gives a 3:1 ratio. Lippincott

concludes, therefore, that the Andalusian cross is a 2-factor case. If

each of these characteristics was independent of the other in the sense

that some birds had clumped pigment and others deficiencies in the

barbules, then one might conclude that he was dealing with a 2-factor

case; but if these two characters are only different aspects of the same

gene, and when one is present the other is also, the situation is not

different from those that are very common, viz, two or more effects

produced by the same genetic factor.

Davenport has recorded results of crossing several breeds of different

colors (1906 and 1909). The white of the Leghorn was found dominant to

the black of the Minorca breed, although the hybrids, “at least the

females,” had some black feathers. This white was also found to be

dominant to the mottled Houdan and to the “Red-backed game.” On the

other hand, a male Tosa with wild-type plumage by recessive White Cochin

female gave “barred” males in F₁; the barring coming in, no doubt, from

the Cochin and although not at the time recognized by Davenport as

sex-linked inheritance, the statement that barring is “associated with

maleness” (as already pointed out by Darwin) indicated that the barring

that appeared within the cross was probably the sex-linked barring shown

by other breeds.

In Davenport’s cross of White Leghorn by Minorca two blues appeared (as

stated above), indicating that the same factors were here present that

in the Andalusian white and black strain gives the same result,[3] but

why only some of the F₁ appear as blue, while others are not blue, is

not yet made clear, unless two factors for white were present. White of

the Leghorn breed was found not to be as completely dominant over buff

as over black. Black was found dominant over the wild-type

(Black-Breasted game), but red is present in F₁ birds also to some

extent in those places where red is found in the game. Lacing, as shown

by the Dark Brahma, is dominant to the plumage of the Tosa. Penciling

also is said to be dominant, as shown in females of the cross between

the Dark Brahma and Tosa fowl.

In his later paper (1909) Davenport gives fuller information in regard

to some of the F₁ cases reported in his first paper, as well as the F₂

results. Thus, in the cross of Silky to Minorca, that gives black F₁

birds, the F₂ count gave 210 black, 57 game, and 95 white--approximately

the expectation for two pairs of factors, one of them giving white

(9:3:4). Silky by White Leghorn gave white F₁’s, but the males developed

red on the wing bow and saddle when they became mature, and the female a

faint blush of salmon (“red”) on the breast. In F₂ there were whites,

games, and blacks, approximating to expectation for three pairs of

factors, one being a dominant white (52:9:3). Silky by Buff Cochin gave

a washed-out buff, but with the jungle coloration partly developed in

the tail (black) and hackles and wing bow (redder buff). Davenport

represents the Buff Cochin as having lost the jungle patterns and

coloration, while the Silky retains it. The heterozygous condition of

the genes for the wild-type color in F₁ is made responsible for the part

development of color. The White Silky is represented as carrying the

factor for black (N), hence in F₂ both black and game-colored birds are

expected and they were obtained. When Black Cochin is crossed to Buff

Cochin, the F₁ males are in general like the game (black and red) while

the females are black (except for some red on the hackle). In this case

Davenport represents the Black Cochin as showing a factor for

jungle-fowl pattern, but lacking the color that is assumed in his other

formulæ to go with this pattern. What is meant by this change is not

quite clear to me, unless Davenport supposes there is an independent

factor for the jungle-fowl pattern which may be filled in by other

colors determined by other factors. But were there enough F₁ birds to

exclude the possibility that jungle-fowl birds would not appear in this

cross?

Davenport has reported a cross between a female White Cochin and a male

Tosa (wild type) from which the daughters were Tosa, except that the

shafting was broadened, and the saddle feathers and proximal secondaries

were obscurely barred (black and buff); the sons were also like the

Tosa, but every feather was repeatedly barred (see above). In F₂ there

were 15 white, 25 game, and 16 barred birds. Davenport concludes that

“barring is clearly heterozygous and confined to the male sex,” and in a

footnote he adds that the sex-linked barring factor of the Plymouth Rock

is different from that of this Cochin-Tosa cross, but Goodale informs me

that the barring that appeared in this cross is probably the same as

that in Barred Rocks.

As pointed out, an interesting feature of color inheritance in poultry

is the large number of cases of sex-linked inheritance. It might seem

probable here, as in the case of _Drosophila_, that this is due to a

well-recognized difference between sex-linked and autosomal characters,

namely, that a recessive mutation in one of the sex chromosomes of a

sperm-cell of the male bird will have a chance of showing its effect

immediately if that sperm-cell unites with an egg without a Z to form a

daughter, whereas it would not immediately show up in the offspring if

the mutation were autosomal.[4] In consequence the recessive mutant

would have a greater chance of being observed and selected if it

appeared in a sex chromosome. But dominant sex-linked characters,

however, have the same chance as dominant autosomal ones and the

question turns therefore on the kinds of characters shown in the cross.

The first indication of sex-linkage in fowls was furnished by evidence

that Spillman published in 1903 on information supplied by

poultry-men--information that has been proven subsequently to have been

accurate. Spillman pointed out clearly the similarity between the facts

he quoted and the then known cases of sex-linkage in the canary and in

the currant moth. The case referred to by Spillman was a cross between

Barred Plymouth Rock and Black Langshan. Goodale and I repeated the

cross, using both Plymouth Rock and American Dominques, publishing the

results in 1912. In addition to the F₁ results evidence was obtained for

the F₂ generation. The theory was also tested by back-crossing. The

results of such a cross that are typical for all cases of the sort are

briefly as follows: Plymouth Rock cock by Langshan hen gives F₁ barred

sons and barred daughters. These inbred give F₂ barred cocks and barred

and black hens (2:1:1).

In the following schemes the sex chromosomes are represented by Z and W,

while the exponents stand for the factors involved, viz, B for barred

and b for not-barred, which here means a black bird.

Barred♂ Black♀

P₁ ZᴮZᴮ ZᵇbW

\ /

ZᴮZᵇ ZᴮW

F₁ Barred♂ Barred♀

\ /

ZᴮZᴮ ZᴮZᵇ ZᴮW ZᵇW

Barred♂ Barred♂ Barred♀ Black♀

In the reciprocal cross, a black cock was mated to a barred hen. The

sons were barred, the daughters black (F₁). These inbred gave (F₂)

barred males and females, black males and females in the ratio of

1:1:1:1. The chromosome scheme of inheritance is as follows:

Black♂ Barred♀

P₁ ZᵇZᵇ ZᴮW

\ /

ZᴮZᵇ ZᵇW

F₁ Barred♂ Black♀

\ /

ZᴮZᵇ ZᵇZᵇ ZᴮW ZᵇW

Barred♂ Black♂ Barred♀ Black♀

One back-cross test consists in mating the F₁ barred males ZᴮZᵇ (from

both crosses) to a pure black female. The expectation is for equal

numbers of barred and black males and females, and the result was

realized. The F₁ barred hen of the first cross (ZᴮW) back-crossed to a

black cock is expected to give only barred males and black females, and

this result also was obtained. The explanation of the last cross, based

on the sex chromosomes, is as follows:

Black♂ F₁ Barred ♀

ZᵇZᵇ ZᴮW

\ /

ZᴮZᵇ ZᵇW

Barred♂ Black♀

Before these experiments were finished Goodale had made other crosses

involving the barring factor, and had obtained results that showed the

sex-linked inheritance of this factor (1909). For example, he crossed

Buff Rock male (not barred) to white Plymouth Rock females. The sons

were barred and the daughters not barred. The reciprocal cross gave

barred sons and daughters. A White Rock male (carrying barring) mated to

a Brown Leghorn female gave barred sons and daughters. Reciprocally, the

chicks were of two kinds as to their down, viz, black chicks and chicks

with the down pattern of the barred rock. All these results with Barred

Plymouth Rocks show that they carry a sex-linked dominant factor for

barring. Its wild-type allelomorph would be game-color (jungle-fowl),

but since, when the dominant barring is absent in some of the

individuals in these crosses, they are black, it would seem to follow

that another dominant factor, one for black, that is not sex-linked, is

also present.

Pearl and Surface have also carried out crosses with Plymouth Rocks on a

much larger scale. Their results conformed in every way to the

foregoing. They crossed Barred Plymouth Rocks and Cornish Indian games.

The plumage of the male of the latter race is black with dark red on the

back and wing-bows; the females are also black laced with mahogany

ground-color on back, breast, wing, and tail coverts. When the male game

is mated to the barred hen the sons are barred and the daughters are

black. In the reciprocal cross both sons and daughters are barred. The

back-cross tests conformed to expectation. The results were the same as

those already stated above for the Langshan-Rock cross.

Sturtevant crossed Columbian Wyandottes and Brown Leghorns. The F₁ sons

were alike, whichever way the cross was made. They were fairly typical

Wyandottes, which race carries therefore more of the dominant plumage

characters (two or three?). There were two types of daughters, depending

on the direction in which the cross was made. When the father is

Wyandotte, the daughters are like him (except for stippling of the

Leghorn type). When the father is Brown Leghorn the daughters are

somewhat stippled red birds. In the former case the daughters getting

their Z chromosome from their Wyandotte father resemble him; in the

latter case the daughters getting their Z chromosome from their Leghorn

father look more like him. Their failure to look exactly like him must

be due to autosomal factors derived from the Wyandotte mother that

dominate other autosomal factors from the father.

Hagedoorn crossed Black Breasted Game bantams (like those used in my

Sebright crosses) to Brown-Breasted bantams. In the latter the black

breast feathers of the male are bordered by lemon; the hens are nearly

black. Black-breasted male to “brown-red” female gave both

black-breasted sons and daughters. In the reciprocal cross all the sons

were black-breasted (like the mother) and all the daughters were brown

red like the father. Evidently the factor here for Brown Breasted game

is sex-linked and recessive. In this case the new mutant sex-linked

character is recessive to the wild type.

Davenport (1912) crossed Brown Leghorns to Dark Brahmas. In the cross

and its reciprocal all the sons are alike. Two dominant sex-linked

factors were found,[5] viz, the white background characteristic of the

Dark Brahmas and the red upper wing-coverts (and back) characteristic of

the Brown Leghorns. On the other hand, the _daughters_ differ in the two

crosses, in each case resembling their father in their hackle color.

When two sex-linked characters are involved in a cross it is possible to

determine by suitable matings whether an interchange between the

chromosomes that bear them has taken place. In the case of the sex

chromosomes only one sex, the male, has both like chromosomes, viz, ZZ,

and we expect from analogy with the _Drosophila_ work that crossing-over

would be found between the sex chromosomes only in the male. Goodale has

recently (1917) made the important discovery that in poultry

crossing-over takes place between the sex chromosomes (ZZ) in the male,

but not in the female (ZW or ZO). This relation, therefore, is the

reverse in birds and flies, for, in the one, crossing-over takes place

in the female and in the other in the male. Whether this difference

extends also to the other chromosomes in birds as it does in flies is as

yet not known.

Several years ago some crosses between gold and silver Campines were

reported by Rev. E. Lewis Jones. The results are consistent with the

view that a sex-linked factor pair is responsible for this difference in

color, although the author does not apply this view to his results. The

results may be seen in the table on page 16, to which Jones has

prefixed the number of individuals. The cross also involved

hen-feathering _versus_ cock-feathering, which appears here (as in other

cases) to be a non-sex-linked dominant factor. As stated above there are

in the results a few apparent inconsistencies with this interpretation,

due possibly to heterozygous females having been used in the crosses.

Lefevre crossed Silver Spangled Hamburgs and Brown Leghorns. The

spangling was found to be a sex-linked dominant factor. A spangled cock

bred to a Leghorn hen gives spangled sons and daughters; a spangled hen

by a Leghorn male gave spangled sons and not spangled daughters. The

daughters do not transmit spangling. Other factors may obscure the

results, especially factors for black, or the localization of the

pattern. Lefevre says “it would seem probable that multiple factors for

black, introduced by the Brown Leghorns, are present, and that these

factors may have a cumulative effect, with the result that pigmentation

is developed to varying degrees of extension.” Whether the factors for

black spoken of as coming from the Leghorns are dominant wild-type

factors that have mutant allelomorphs in the Silver Spangled Hamburg is

not entirely clear from the quotation.

Baur gives in his Introduction to the Study of Heredity (1914, pp.

202-203) some results (unpublished) that Hagedoorn had obtained by

crossing gold and silver races of Assendelver birds. The factor is

sex-linked and is no doubt the same factor reported by Jones for gold

and silver Campines and by Sturtevant for Columbian Wyandottes. Silver

dominates gold and the sex relations are the same as those already

reported by others for poultry, viz, the male is ZZ, the female ZW. Gold

hens by a heterozygous silver[6] gave 162 silver cocks, 163 silver hens,

168 gold cocks, 160 gold hens, expressed graphically (_g_ for gold, _s_

for silver):

Zᵍ--W♀ × Z--Zᵍ♂

-----------------

ZˢZᵍ--ZᵍZᵍ--ZˢW--ZᵍW

Silver Gold Silver Gold

male male female female

When a silver hen was united to a gold cock there were 246 silver cocks

and 243 gold hens--crisscross inheritance.

_Summary._

From the standpoint of the Brown Leghorn type representing the wild

type, the following colors and patterns represent dominant mutations

from that type:

_Dominants._

White of White Leghorn.

Silver of Dark Brahma.

Black of Minorca.

Lacing of Brahma.

Barring of Plymouth Rock.

Black (?) of Plymouth Rock.

Buff (or red).

Each of these (in heterozygous condition of course) is dominant; in some

cases completely so, in others incompletely dominant. At three different

loci in the sex chromosome a dominant mutation has occurred; at three

loci in other chromosomes dominant mutant changes have also occurred.

_Recessives._

White of Rose Comb bantam.

White of Silky.

White of White Rock.

Brown of Brown-breasted game.

Penciling.

Whether the recessive white that is sometimes found in dominant White

Rock stock is different from both of the other recessive whites is not

known. There are, then, 5 or 6 recessive characters that are not

sex-linked and 1 recessive sex-linked character.

Owing to the relatively large number of color dominants in poultry, some

unnecessary confusion has arisen concerning the relation of the

dominants to the wild type, and especially to other mutant characters to

which they are said to be dominant, in the sense, however, of being

epistatic. An imaginary example will illustrate this. For example, if at

some locus in the wild type a mutation occurred that gave a dominant

black (_i. e._, a black that shows up when one gene for it is present)

and at the same time this black also showed up even when other recessive

mutant characters were present in homozygous form, then F₁ birds would

be black when black is crossed to such pure recessive stocks. Such cases

have indeed been described as dominant, but a knowledge of F₂ would have

shown at once the error of such a system. For, if black had been a real

dominant, the F₂ would have given 3 blacks to 1 of the other type (such

as the wild type), but if the case were one of epistasis, then there

would have been 9:3:3:1 classes in F₂ (or some modification of that

ratio). In this sense, then, epistasis may be defined as a result that

appears when one member of the pair of genes produces its effect

regardless of the constitution of the individual with respect to another

gene (or other pairs of genes). It is curious at least to note that in

the case of dominant white the term epistatic has been much less often

used than in the case of black. Theoretically the two situations are

exactly alike, but because black could so obviously conceal things

beneath it, while white is not thought of as doing so, it seemed

“natural” to make such a distinction. In reality it is not a question of

covering up at all, but a case of a dominant character (white or black)

preventing other colors from appearing.

In the case of recessive white the situation is somewhat different and

no one, so far as I know, has gone so far as to speak of such a white as

epistatic, although when the animal is white it certainly hides, when

completely effective, all the other effects of color-producing factors,

but allows them to “show through” in some of the cases. This means not

that they do “show through,” but that they only develop to a “lower”

degree. The difference between dominant and recessive whites rests on

the fact that in one case one member of a pair of factors gives white

and in the other both members are necessary. But obviously such a

distinction is not important, and if it were worth while the case might

be argued for recessive whites being also epistatic. The whole tangle

goes back to a false interpretation of presence and absence of

characters and presence and absence of factors. As I have gone over this

ground recently in my paper on the Theory of the Gene, I need not repeat

here what I tried to make clear there.

ENDOCRINE CELLS IN OVARY AND TESTES OF BIRDS.

The occurrence of gland-like cells with an internal secretion in the

ovary and testes of fowls has been described by a number of writers and

denied, at least for the testes, by others. The work of Boring and Pearl

has done much to bring this question to a satisfactory solution, for

they have tested out and made use of the best reagents that their

predecessors had discovered and have used a much greater amount of

material. As they have reviewed very fully the literature of the

subject, it will not be necessary to go over the ground again in detail.

In the follicles of the ovary there are present, according to Boring and

Pearl, groups or nests of cells lying among the connective tissue of the

inner theca. The cells are about three times as large as the ordinary

connective-tissue cells of the ovary. The cytoplasm is clear and

vacuolated, “only occasionally containing a few acidophile granules

which stain with the fuchsin in Mallory’s stain or the eosin of Mann’s

stain, while the real interstitial cells are crowded with granules.”

When the egg is set free from its follicle, the latter collapses and the

rupture becomes closed. A mass of cells collects in the center of the

collapsed structure which develop yellow pigment. The cells, lying in

the puckered edge of the follicle, may also develop such yellow color.

The cells that produce the yellow pigment come from the nests of cells

that lay originally mainly in the theca interna. Either by migration or

by division they come to fill up the central cavity. The yellow

substance in the cells is not fat, since it does not dissolve in the

clearing oils, nor can it be protein, for it does not take acid stains

as normal secretion granules of protein. It does not dissolve in HCl,

HNO₃, or H₂SO₄, nor in strong KOH, although the latter turns the pigment

a bright red color. Many other substances were also tried by Boring and

Pearl, but none of them dissolved the yellow pigment, which reacts in

this respect in the same way as does the yellow pigment in the luteal

cells of the mammal. The similarity in the nature of the pigments in the

two cases is an argument in favor of the view that the cells that

produce the pigment are the same in both groups. In the mammal the

yellow corpus luteum is a large, gland-like organ that develops after

the ovum is discharged; in the bird there is also a yellow spot on the

ovary, due to the pigment in the collapsed follicle, but it is smaller

and much less conspicuous than in the mammal. The evidence concerning

luteal cells in the testes of the bird is conflicting. One of the

difficulties in the situation is the identification of the cells, which

are sometimes regarded merely as the general connective-tissue stroma of

the testis that is undoubtedly present; at other times special secretory

cells are discerned embedded in the connective tissue, as individual

cells or in islands. Boring states (1912) that in newly hatched chicks

about half of the tissue of the testes is interstitial connective

tissue; the other half consists of tubes or cords whose principal

function is the development of the germ-cells. In the paper of 1912

Boring reached the conclusion that there are no “interstitial cells in

the testes of the domesticated chicken in the sense that this term has

been previously used,” and states that no evidence has been found that

an internal secretion of any kind is formed by any cells of the

interstitial tissue.

It is not necessary to discuss whether or not connective-tissue cells

are present in the testes of birds, for is it generally conceded that

they are found at least in certain stages, but it is important to look

into the question as to whether among these interstitial cells there are

others that have an endocrine function. Mazzetti gives pictures of such

gland-cells between the seminal tubules of the cock bird, but says that

they are rare, “even though this bird has very marked secondary sexual

characters” (Boring and Pearl). It may be remarked parenthetically that

if they had been more abundant the bird might have had no secondary

sexual plumage since it will be pointed out below that such glandular

cells may have as their special function the suppression of these

characters.

According to Des Cilleuls, interstitial cells are first found in males

about 30 days old and at this time the secondary sexual characters put

in their appearance. If, as will be shown in the sequel, he means by

interstitial cells the endocrine cells that suppress the development of

the male plumage in the female, the appearance of these cells at this

time would be significant; but if he implies that their occurrence in

the male incites the development of the secondary sexual characters, his

interpretation is open to serious doubt. Reeves found interstitial cells

in testes of cocks 3, 5½, 9, and 18 months--more in the earlier stages.

In a later communication by Boring and Pearl the whole question is taken

up again with improved methods, etc. Previously 21 male birds had been

studied, just hatched to 12 months old. More sections of this same

material were made which were stained according to Mann’s and Mallory’s

methods. In addition, a whole new series of preparations was made. A few

interstitial cells, i. e., granule containing-cells were found in newly

hatched chicks, but not in any of the 60 mature birds examined.

LUTEAL-CELLS IN THE TESTES OF THE MALE SEBRIGHT.

Finding that the testes of F₂ hen-feathered birds were often flat and

pear-shaped instead of rounded and cylindrical, as in ordinary cocks,

and that they were often black in color, suggested, as already stated,

that the testes of the Sebright might be hermaphrodite in some element.

It seemed not impossible that egg-cells might be found. I made a

considerable number of sections of the testes of these birds and

examined them under the microscope; not finding any egg or egg-like

bodies, the slides were laid aside, but the idea that in some other way

the Sebright’s testes might correspond to the ovary of the female next

recurred to my mind. Consequently, when in the summer of 1918 I had some

new material derived from a castrated Sebright male that had partly

regenerated its testes and was again going back to hen-feathering, and

pieces from one of the old testes of a castrated bird, I asked Miss

Boring, who was then in Woods Hole, to make some preparations and

examine them to see if she could detect any such elements in them as she

had found in the female. Miss Boring reported the occurrence of luteal

cells in the testes from hen-feathered males, and the results have been

published in a brief preliminary paper (1918). The abundance of these

clear cells, supposedly gland-cells with endocrine influences, in the

testes of hen-feathered birds is in sharp contrast to their absence in

the normal adult cock birds. It seems to follow, therefore, that the

hen-feathering in the Sebrights is due to the presence of these cells,

whose function is the same as of the similar cells in the female, _i.

e._, the suppression in both of cock-feathering. Castrating the Sebright

produces its effect by the removal of these cells that are responsible

for the suppression of cock-feathering.

The occurrence of luteal cells in young stages of other races of poultry

raises the question as to whether in these races the first or juvenile

plumage, that resembles that of the hen rather than that of the cock,

may not also be due to an internal secretion from these cells, or

whether this juvenile plumage is only the plumage of a characteristic

stage in development. Castration of young chicks ought to settle this

point. Such castration experiments have been made by Goodale. The

absence of any reference to any effect on the juvenile plumage in these

early castrated birds probably meant that they did not develop

precociously cock-feathering, and he writes me that he examined them

carefully and that their plumage is like that of the normal chicks.

Geoffrey Smith has reported the occurrence of two kinds of males in a

race of Leghorns, the males of one of which become cock-feathered before

the other. May not this difference depend on the length of time

endocrine cells remain or begin to develop? A histological study of the

two types would be of the greatest interest.

ENDOCRINE CELLS IN THE TESTES OF MAMMALS.

In man and other mammals it has long been recognized that in addition to

the germinal cells of the testis there are also present other cells,

sometimes called interstitial cells, that, so far as known, have no

immediate function in connection with the germ-cells, or at least that

have other important functions outside the relation to the reproductive

organ. That some internal secretion from these cells has an important

influence on the secondary sexual characters rather than anything done

by or produced by the germinal cells has been very clearly shown by

evidence derived from three separate sources, namely, from the operation

known as vasectomy, from an exceptional condition known as

cryptorchidism, and more indirectly from X-ray treatment. Vasectomy

involves either cutting the vasa deferentia in such a way that the cut

ends do not reunite. In consequence of the closure of the outlet of the

testis the germinal cells slowly degenerate, and finally completely

disappear. How such an effect is produced we do not know. That this

result does take place is borne out by the unanimous testimony of all

those who have successfully performed the operation. Ancel and Bouin

showed (1903) that breaking the continuity of the vas deferens

suppressed spermatogenesis in 8 to 12 months. Both the Sertoli cells

(the nourishing cells of the germinal epithelium) and the interstitial

cells persist. Such animals remain sexually active and their secondary

sexual characters are not affected. Marshall states that in the hedgehog

the remarkable periodic enlargement of the testis takes place even after

vasectomy, although the germ-cells have disappeared.

In mammals the testes fail at times to pass through the inguinal canal,

and, in consequence of their retention in the body-cavity, the

germ-cells fail to develop. On the other hand, the interstitial cells of

the testis develop normally. Cryptorchid individuals show the normal

secondary sexual characters of their species. How retention of the sperm

should give rise to the same result as cutting the duct, viz, absorption

of the germinal cells, is not known. A possible solution may be found in

the pressure exerted on the testes, both when retained in the abdomen

and when their outlets are stopped by tying or cutting the ducts.

Finally, it has been long known that continued or repeated exposure to

X-rays or to radium causes the destruction of the germ-cells, but leaves

the interstitial cells intact and presumably functional. Destruction of

the germ-cell by X-rays has no effect on the secondary sexual

characters.

This threefold evidence demonstrates that in the male of the mammalia

most, perhaps all, of the secondary sexual characters that are affected

by castration are not affected by the destruction of the germ-cells.

This conclusion supports very strongly the view that the interstitial

cells are the cellular element in the testes that influence through

internal secretion the development of the secondary sexual characters of

the male.

Equally important are the results that relate to the accessory organs of

reproduction, such as the glands that open into the vas deferens

(prostate, Cowper’s gland, etc.) and the copulatory organs also. In the

castrated mammals these organs diminish in size. On the other hand,

after destruction of the germ-cells in the testes (or even when they

fail to develop as in cryptorchid individuals) these accessory parts are

unaffected. In birds, as will be shown, the situation is entirely

different.

CYCLICAL CHANGES IN THE INTERSTITIAL CELLS IN HIBERNATING MAMMALS.

The changes that take place in the interstitial cells in mammals that

hibernate and in which there is a definite rutting season following

hibernation have been examined by several workers. The mole has been

studied by Regaud (1904), Lécaillon (1909), Tandler and Grosz (1911);

the marmot by Hauseman (1895) and Gaugini (1903); the hedgehog by

Marshall (1911); and the woodchuck by Rasmussan (1917). In the mole the

interstitial cells are most abundant when the tubules in which the

spermatogenesis is taking place are least developed, and _vice versa_.

In the hedgehog the increase in both tissues takes place at the same

time. In the woodchuck both tissues increase rapidly after hibernation

(during March and April), after which the spermatogenesis continues

actively for the two following months (May and June), while the

interstitial cells retrograde rapidly during April and remain at a low

level for the rest of the year. Retrogression in the germinal epithelium

begins in July, after the rutting season is past. It appears from this

evidence that the activity of the two tissues does not always run the

same course. Since the secondary sexual characters of the male, which

are not well developed in these animals, are not so far as known

affected by the condition of the testes, the evidence does not have any

very direct bearing on our present topic. How far the sexual behavior of

these mammals is determined by the quantity or by the activity of the

interstitial cells is not very clear from the evidence, although there

is a very noticeable increase in the amount of this tissue just before

and during the rutting season. In the mole also the interstitial cells

begin to increase just before the mating season, and the increase

continues for several months after mating has taken place. It is

difficult to judge how great or how little the change amounts to unless

the whole organ is considered, for the relative volumes of the seminal

tubes and the interstitial tissues does not give a measure of the total

volume of these tissues, since the testes may decrease greatly in size

when the seminal tubes retrograde, and the apparent increase of the

interstitial cells at the time may not increase the total amount of that

tissue present.

Probably more important than the ratio of interstitial tissue to tubules

is the activity of the former. Rasmussan states that in the woodchuck

the interstitial cells not only increase in number immediately after

hibernation, but the increase in amount of this tissue is largely due to

increase in the cytoplasm, in which there appears an accumulation of

fatty globules in the more peripheral parts of the cells. In the central

cytoplasm an abundance of fine lipoid granules develops.

Marshall has made some interesting experiments on the hedgehog at

different seasons. Castration in March prior to the breeding-season has

an influence on the accessory generative organs (vesiculæ seminales,

prostates, and Cowper’s glands). They remain in the same undeveloped

stage in which they were at the time of operation. If castration is

carried out very early in the breeding-season, when the accessory

reproductive organs are about half developed, their further enlargement

is prevented. In so far as the accessory organs rank as secondary sexual

organs, their complete development is thus shown to depend on the

testes. Transection of the vasa deferentia before the beginning of the

breeding-season affects somewhat the enlargement of the testes, but

produces no effect on the accessory organs.

HERMAPHRODITISM IN POULTRY AND THE SECONDARY SEXUAL CHARACTERS.

Several hermaphrodite birds have been described (Brandt, 1889; Shattock

and Seligman, 1906; Pearl and Curtis, 1909; Smith and Thomas, 1913;

Bond, 1914; etc.). The most recent and complete account of such birds is

that by Boring and Pearl. They examined in all 8 hermaphrodites, or at

least 8 birds that showed in their plumage, or other secondary sexual

characters, peculiarities of both sexes. Five of the birds came from

Herr Houwink in Meppel, Holland, who had a stock in which there

appeared, in 1911, two hermaphrodites out of 80 birds, and in 1912,

three out of 80 birds. These were the birds studied by Boring and Pearl.

In addition, when Pearl saw Herr Houwink’s birds in 1910, “there were

then on hand a considerable number of these supposed hermaphrodite

birds.” An anatomical study of the Holland birds showed that one of them

was nearly a normal female; three, the authors say, were “evidently

undeveloped females. They have infantile oviducts and embryonic

ovaries.” It should be added that there was a tumor more than twice the

size of the ovary attached to or part of the ovary. If the ovary itself

was affected by the tumor, or the tumor was a part of the ovary, the

slightly unusual condition of the birds might be accounted for. Of the

other 3 birds, 2 are also suspected to have ovarian tumors, while in the

third bird streaks of a secretion which resembles the substance of the

tumor of the other two were found. The change towards male plumage in

these 5 birds is probably due either to the incomplete development of

ovary or to the effect of the tumor on the ovary. Although luteal cells

are described as present, it seems probable that their total number

might be less than in a normal bird, and hence their insufficient

secretion would fail to suppress the development of male plumage. From

this point of view these birds are no more hermaphrodites than is a hen

with her ovary taken out.

The remaining Holland birds were entirely different. On the left side

there was an ovary in an inactive condition; on the right side there was

a testis, producing spermatozoa. Sections of the testis show that it is

normal, consisting of a mass of tubules with very little connective

tissue between them. In both ovary and testis there are “a few nests of

luteal cells near the surface. The ovary contains eggs, but is abnormal

to some extent.” The authors state:

“In external appearance it is more like a male than the others,

which fact correlates well with the active condition of the testis

and inactive diseased ovary, with only one corpus luteum scar. The

interstitial cells can scarcely be held accountable for the male

secondary sex characters, as the only ones in an active secreting

condition are a few in the ovary.”

It is not quite clear what is meant in this quotation by the statement

that the interstitial cells can scarcely be held accountable for the

male secondary characters unless to suggest that they cause the

development of these characters in the male, as they are supposed to do

in mammals--a view that the authors do not seem at other times to hold.

Another hermaphrodite (Atwood’s black) had an infantile oviduct and an

ovotestis. A second bird, too, had an ovotestis--mostly testis--as well

as a rather large oviduct. Collections of luteal cells are described

between the tubules of the testicular portion. If, as suggested by the

Sebright cases, these cells tend to suppress the female plumage, their

presence here in excess might at least be made to account for the female

part of the plumage of this bird. Comparing the last two birds (that

showed active sex-behavior as males) with the best of the Holland birds,

Boring and Pearl point out that the active sex behavior of the two

former can not be due to “interstitial cells that are absent in these

but present to a slight extent in the former.” They then add”

...though the differences can not be laid to the lutear cells, as they

are present in all three.” That the relative amounts of the latter or

their activity might still be accountable for the difference would not

seem entirely excluded from the evidence so far as it is given.

A fourth hermaphrodite (Dexter’s) laid 12 eggs and had a large coiled

oviduct. There was present “a large, lobulated reproductive organ on the

left,” which proved to be an ovotestis. Several ovarian tumors were

present and there was testicular tissue.

It is fairly evident, then, that four of these birds described by Boring

and Pearl were females with abnormal ovaries. The incomplete development

of the latter, or their abnormal condition due to tumors, may

sufficiently explain the occurrence of male secondary sexual characters.

That these tumors affect, to different degrees, such characters is

expected from what is shown by imperfectly spayed females of normal

breeds.

There are a few statements in the summary of this paper that call for

comment. The statement that the “development of comb, spurs, and wattles

does not stand in direct quantitative relation to the sex of the gonad,”

appears to be only intended as a statement of fact based on the author’s

observation. But in what sense is there an expectation that they should

stand in such relation beyond the obvious fact that in the cock the comb

and wattles are larger than in the hen, and that spurs are generally

present only on the cock. But if the expression “sex of the gonad”

implies the germ-cells it is not at all certain that there is any

expectation of a quantitative relation, and there is some probability at

least that other cells than the sex-cells are involved in the

development of combs, wattles, and possibly spurs. A castrated cock has

a small comb resembling that of the female bird. On the other hand,

removal of the ovary sometimes leads to an increase in the comb and

wattles. Here we have, to say the least, a paradoxical situation, for

the result looks superficially as though something in the ovary keeps

down the hen’s comb, while something in the testes keeps up the cock’s

comb, yet when the ovary is removed the hen develops a cock’s comb; when

the testes are removed the cock develops a hen’s comb. The real meaning

is, I think, that the genetic complex for femaleness (one Z or else ZW)

stands in itself for a full-sized comb, while the genetic complex for

maleness (two Z’s) stands in itself for small comb.

Boring and Pearl state that “body-shape and carriage have a genetic

relation to the sex of the gonad.” This statement means, I think, that

the amount of testicular matter present stands in some direct relation

to the shape of the body and carriage of the male. Castration, both of

the normal cock and the Sebright, seems to change the carriage somewhat

and perhaps the shape. Both lose something of the peculiar attitude of

the male, but I have not been able to my own satisfaction to analyze

what this means. As has been pointed out, and as the pictures show, the

castrated Sebright changes his attitude, but whether this is a change

due to his new contour, or to a new balance resulting from a large tail,

or to a let-down resulting principally from effects on the nervous

system, is difficult to determine. The same statements apply in part to

the castrated cock of ordinary breeds, but not to the same degree, since

the change after castration, in feathering and in carriage at least, is

slight.

The conclusions that the “amount of lutear cells or pigment (?) is in

precise correlation with the degree of external somatic femaleness

exhibited by the individual” is of especial interest in connection with

the Sebright evidence. It is difficult, however, to gather from the body

of the paper what the absolute amount of luteal cells is that is

present, for even in some of the more male-like birds with an ovotestis

the description leads one to suppose that there may be as much luteal

material present as in some of the more female birds with infantile

ovaries or cystic tumors.

Pearl and Curtis (1909) described “a case of incomplete hermaphroditism”

in a Barred Plymouth Rock fowl. Externally the bird looked like a hen,

but “the head and neck resembled these parts in the cockerel,”

especially the comb and wattles. The bird was never seen to tread a hen,

nor did it ever crow normally. An ovary and oviduct were found on the

left side, the former no larger than that of a laying hen after removal

of the large yolks. No eggs were visible on its surface. On the right

side a testis (9 mm. by 6 mm.) and vas deferens were present. No eggs

were found in the ovary, and it gave every indication of being in a

degenerating condition, with no eggs or egg follicles in it. The testis

had no “normal seminiferous tubules”, but indications of cellular rods

were present. The organ is in all probability a degenerating testis.

A Leghorn 2 years old has been described by Shattuck and Seligmann

(1906) that had the full-developed comb and wattles of the cock, but the

former drooped slightly to one side as in the hen. Well-developed spurs

were present. The plumage was mainly female, with neck-hackles

moderately developed, and with “saddle-hackles” practically absent. The

tail, though not typically female, lacks sickle feathers. The bird

excited no notice from other birds of either sex. A large left oviduct

and the distal end of a right oviduct were present. Two vasa deferentia

were also present. In the left side a flattened sex-gland (3 cm. high)

was found, made up of testicular tubules. Two small ova were found in

its posterior end. The right gonad was also tubular (testis).

The occurrence of real testicular tissue in one of the Holland birds and

in three others described by Boring and Pearl, as well as in one

described by Pearl and Curtis, and in another by Shattuck and Seligmann

calls for special comment, since the presence of both testicular and

ovarian tissue in the same bird is the essence of hermaphroditism. In

general there are two ways of looking at such a result. Either the

sex-determining factors have been changed so that in one part of the

body, where the reproductive organs are laid down, one condition can

prevail, in other parts other conditions; or a mixup of the sex

chromosomes has taken place. Until we get some more evidence concerning

such cases it is useless to speculate, although the former view might

seem the most probable of the two if the Holland birds of Herr Houwink’s

flock were in a high degree true hermaphrodites.

But in fact three of the four described by Boring and Pearl were due to

tumors of the ovary, which, if they suppress the normal development of

this organ, would be expected to call forth the appearance of the

secondary sexual characters of the cock. If the likelihood of developing

a tumor were inherited, the frequent occurrence of hen-feathered birds

in this flock would be explained. However, one true hermaphrodite in 4

birds is surprisingly high for a chance result, since hermaphrodite

birds are very rare.

The second interpretation suggested above is one that has been advanced

and established by genetic evidence in _Drosophila_, viz., dislocation

of the sex chromosomes. In the case of birds the male is supposed to be

duplex for the sex factors (ZZ), the female simplex (ZW), and

consequently the chromosome-dislocation hypothesis must be worked out

contrawise in birds and insects. We should have to suppose that such

birds start as males (ZZ), and that at some division of the cells of the

embryo one of the Z’s became lost (left at the cell-wall for example).

All the cells that got ZZ would be male; all that got Z would be female.

If the reproductive region included cells of these two kinds, an

ovotestis would result. The rest of the body should be the same, or

nearly so, since the soma of male and female birds is alike whether ZZ

or Z, except in so far as it is affected by the secretions from the

ovaries (in most races of poultry), or from the testes if the race be

Sebright, Campines, or Hamburgs. Birds with ovotestis might,

nevertheless, be expected, on this view, to show at times an

intermediate condition of the secondary sexual characters, according to

how much internal secretion is produced in the ovotestis. In other

words, the chromosome loss might involve much more extensive regions

than the reproduction organs, but show its effects first in that organ

and then indirectly other parts of the body be affected by the luteal

cells of the testis. There is one rather good piece of evidence that

seems opposed to this interpretation. In the hermaphrodites the oviduct

is present in all cases. Its conspicuous presence in the four

hermaphrodites would seem, therefore, to indicate that the birds

started as females (ZW), which is inconsistent with the dislocation

hypothesis. The alternate would be that in all these cases the Z part

always included the region of the oviduct, which seems improbable.

There is another possibility, viz, that in birds a sex-factor is carried

by the W chromosome, and ZW is a female not because of one Z, but due to

the presence of W. If so, then one Z or two Z’s might give the same

result, viz, female. If a bird started as female, (ZW) and chromosomal

dislocation occurred, then the Z parts would be female and the male part

W. Until we get evidence on this point it is not worth elaborating.

Without genetic evidence from hybrids, the interpretation of

hermaphrodites in birds can have at present only a speculative interest.

We may hope some day to get the same kind of evidence as in the case of

_Drosophila_. Hermaphrodite hybrid pheasants that have been often

described might seem to furnish a hopeful field, for they appear to be

quite common and to show characteristics of both races. As yet, however,

no one has, I think, succeeded in finding a simple interpretation of the

results. It is also not unlikely that many of the pheasant cases are not

true hermaphrodites, but due to failure of normal development of the

reproductive gland, which gives an intermediate or mixed type of

secondary sexual characters.

The genetic and the operative evidence relating to secondary sexual characters · 第 1 / 4 章