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The Genetic and the operative evidence relating to secondary sexual characters

The Genetic and the operative evidence relating to secondary sexual characters
Title: The Genetic and the operative evidence relating to secondary sexual characters
Release Date: 2018-07-07
Type book: Text
Copyright Status: Public domain in the USA.
Date added: 27 March 2019
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By T. H. Morgan

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Published by the Carnegie Institution of Washington
Washington, 1919


Publication No. 285



Part 1.
Castration of Sebrights6
A male Sebright that did not become cock-feathered after castration10
Transitional feathers10
Castration of F₁ hen-feathered males from Sebright by game11
Castration of F₂ hen-feathered males13
Hewitt’s Sebright hen that became cock-feathered in old age14
Heredity of hen-feathering14
Heredity of color in the cross between Sebright and Black-Breasted Game bantam17
      A. The F₁ birds18
      B. Description of F₂ birds19
      C. Back-cross of F₁ to game21
      D. The number of color factors involved22
      E. Back-cross of F₁ ♀ to Sebright ♂23
      F. Review of the heredity of the color of the plumage in poultry23
Endocrine cells in ovary and testes of birds32
Luteal-cells in the testes of the male Sebright34
Endocrine cells in the testes of mammals35
Cyclical changes in the interstitial cells in hibernating mammals36
Hermaphroditism in poultry and the secondary sexual characters37
Part II.
Darwin’s theory of sexual selection43
Other theories to account for secondary sexual characters45
Display of the male50
Part III.
The genetic and the operative evidence62
      A. Evidence from mammals64
      B. Evidence from birds73
      C. Evidence from amphibia86
      D. Evidence from crustaceans89
      E. Evidence from insects92
Part IV.
Summary and conclusions95
Description of plates106



By T. H. Morgan


There are a few races of poultry that have two kinds of males, one withthe feathering of the ordinary cock, the other with the feathering ofthe hen. The Hamburgs and the Campines are perhaps the best known racesof this sort. Convention amongst breeders, in certain countries, hasdetermined that the cock-feathered bird shall be the standard, and atother times and places that the hen-feathered males shall be the showbirds. In one breed, at least, viz., the Sebright bantams, thehen-feathered cock is the only known type. Cock-feathered Sebrights havenever been seen, so far as I know. This breed is pure forhen-feathering. As shown in plate 1, figure 3, the male Sebright lacksthe long, pointed saddle feathers at the base of the tail of the commoncock, also the peculiar back and neck feathers (hackles) of the cockbird, as well as the male feathering on the bow of the wing. Hisfeathers in these parts are almost exactly like those of the hen (plate4, fig. 4). The long sickle feathers covering the true tail are alsoabsent, although the two median ones sometimes occur in males of thisrace.

The Sebrights seemed excellent material for studying the heredity ofthis type of plumage in the male. In 1911 I began to study this problem,and crossed Sebrights to Black-Breasted Game bantams. The latter racewas chosen not only because the males have the typical cock-feathering,but also because the coloration of these birds resembles very closelythat of the jungle-fowl, from which many, perhaps all, of ourdomesticated races have sprung.

In dissecting some of the F₂ birds from this cross I noticed that thetestis of the male was often more flattened than is the testis of thetypical male bird, that it was often somewhat pear-shaped, and thatfrequently it was in part or entirely black. Recalling that maleSebrights are said to be often partially sterile, the idea naturallysuggested itself that these birds are hen-feathered because the testeshave assumed some of the characteristics of the ovary. It had long beensupposed, and had been finally established by Goodale, that the presenceof the ovary in the female suppresses her potential development ofplumage, for when the ovaries of the hen are diseased or removed shedevelops the plumage of the male. This reasoning led me to try theexperiment of castrating the hen-feathered males in{6} order to see ifthey would become cock-feathered. The outcome was immediately apparent;the new feathers were those of the cock bird. While the “reasoning” thatled to the experiment is open to serious question, nevertheless the“hint” furnished by the unusual condition of the testis led finally tothe discovery that luteal cells were present in abundance in the testesof the male Sebright like those present only in the females of otherbreeds. Whether or not the shape of the testis of the Sebright, that issometimes like that of the ovary, is connected with the unusualabundance of luteal cells in the testis I do not know. If so, then thehint that came from their shape was not so unreasonable as appears atfirst 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 removedby Dr. H. D. Goodale at my request, lived and changed to cock-featheredbirds. Since then I have operated successfully on a number of F₁ and F₂birds, as well as Sebright males. In these operations I have hadthroughout the assistance of Dr. A. H. Sturtevant and for two years theassistance of Dr. J. W. Gowen also. I wish to express my appreciation oftheir help and advice, for without it I doubt whether I could havecarried out the work successfully. Since the main interest attaches tothe Sebright experiments, they will be described first, although theywere the last to be performed.


Except for the similarities of the plumage, the male Sebright differs asmuch from the female as do cocks of other races. The rose comb is verylarge in the male, small in the female (plate 4, figs. 3, 4). Thewattles also are longer in the male. The cock carries himself erect, asdo the males of other breeds. His spurs are well developed and he showsthe aggressive behavior of his sex. On the other hand, the shortness ofthe feathers on the back of the neck (the hackles), the absence of thepointed feathers on the back and rump, and the usual absence of longsickles and other tail-covert feathers make him hen-like. The detailedaccount of the feathers in these critical regions will be given whencomparisons are made with the feathers of the castrated birds (plate 6and plate 8).

Six males have been successfully operated upon and with one apparentexception have all given the same results. The birds were of somewhatdifferent ages; they had been hatched about July, and were operated uponabout November of the same year, when they were either half grown or hadnearly reached maturity. At the time of the operation a few featherswere removed from different regions of the body, and the new feathersthat regenerated in the course of 3 or 4 weeks showed all thecharacteristics of those that came in later to replace the juvenile orfirst adult coat. These regenerated feathers do{7} not, therefore, callfor special notice. All of the new feathers were in shape, pattern, andgeneral coloration strikingly different from the original feathers, someof which were at first still present, the old feathers of course showingno change.

After completely molting, the appearance of the birds may be gatheredfrom the photographs (plate 5) and from the colored drawings (plate 1and plate 3). The male now has in all points the plumage of a typicalcock-feathered male bird of other breeds. This is startlingly apparentin the hackle, back, rump, sickle, and tail-covert feathers. Instead ofthe laced feathers that are characteristic of both male and female, thewhole upper surface of the bird appears reddish or yellowish, the blackmarginal edging of the feathers having disappeared. A detailedcomparison of the feathers of the different regions will show how greata change has taken place. (See page 8.)

In plate 6 and plate 8 the feathers from characteristic regions of the normalSebright and of the castrated Sebright are shown in pairs.

One of the first Sebrights that was castrated was a lighter bird thanthe others. Its lighter color was partly due to the narrower outer bandof the laced feathers, (plate 6, figure 1,) and partly to the lightercolor of the yellow-brown center of the feathers. The bird had a singlecomb, but as this crops up occasionally in some stocks of Sebrights, itneed not be interpreted to mean that the bird was impure for colorfactors. After being castrated the bird changed over completely tocock-feathering and has remained in that condition for two or moreyears. As shown in plate 5, figure 2, the plumage is even more fullydeveloped than in cock birds of some other breeds. The comb and wattlesare, however, shrunken and pale, as in a capon. The bird is timid andscarcely or never crows. When killed (May 1919) no pieces of testes andno 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 fromthe same region are placed side by side. The feathers on the head andhackle are yellow, even to the base. At the base of the hackle—theso-called cape—a few feathers have a small black tip. The feathers ofthe back are entirely yellow, except that where the fluff begins thereis some dark pigment. The saddle feathers are for the most part allyellow, 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 tailfeathers are long, mossy, and have a black tip. The wing-bow feathersare all yellow, except the black fluff at the base. The feathers on thecrop are mostly yellow with black margin around the end. Those on thebreast lower down are yellow with black tip and black fluff.

There was another Sebright operated upon at the same time that was adarker bird (as the original feathers show, plate 8, figs. 1 to 4). Ithad{8} a rose comb. The feathers that were plucked at the time ofoperation were replaced at once by new feathers of the cock-featheredtype. The new feathers that came in as the old ones were molted werealso cock-type, and the bird soon assumed the complete characteristiccock-feathering. The comb was shrunken as in castrated birds (plate 5,figure 5).

Plate 6 and 8.
Plates 6 and 8.
1.On the head (a) the feathers are small, dull black with lighter margin and reddish quill.1. Feathers entirely yellow and more slender. Those on each side of the shrunken comb stand up from the head.
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 an arrow yellow border (broader at base). The border is absent at tip.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 middle of the
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