The genetic and the operative evidence relating to secondary sexual characters

by Thomas Hunt Morgan

Testicular tissue transplanted into castrated cocks whose comb, wattles, etc., had undergone retrogressive changes brought about a return to the normal conditions after an interval during which the implanted nodules had begun to regenerate.

Testicular extract from the cryptorchid testes of swine was injected into castrated cocks. In one case this resulted in a rapid growth in size of the comb, which, after 2 months, had reached its full size. Cessation of the injections led immediately to a cessation of growth. Before injection the bird exhibited the pacifistic characteristics of the capon, but the injections brought out little by little the aggressive behavior of the normal male. The voice reappeared and “nous assistons á une véritable crise de puberte.”

A histological study of the testes of the fowl and of pheasants showed that much connective tissue is characteristic of young birds. In the adult cock, and during the mating season of the pheasant, the connective tissue becomes largely crowded out by the enlargement of the tubules. Pézard concludes that the “interstitial” cells in birds have nothing to do with the secondary sexual characters, but that these come rather under the influence of the germinal cycle of cells of the testes. The submergence of the connective-tissue cells of pheasants during the breeding-season and their reappearance during the rest of the year might appear to have some relation to the facts that I have recently described in Sebrights, but as the nuptial plumage of the male remains the same throughout the year we can not ascribe any direct influence to this tissue. Nevertheless, the different tissues of the testes in birds that show seasonal dimorphism of plumage should be carefully examined.

Pézard made a few observations on hens whose ovary had been removed. His results are in accord with those of Goodale, except that he thinks that the ovary has no influence on the erectile organs (comb, etc.) which acquire in the spayed bird the same length as that of the normal female.

Two hens showing male characteristics and a pheasant similarly affected are described. In all three cases an examination of the ovary was found to be undeveloped or abnormal.

PART IV.
SUMMARY AND CONCLUSIONS.

1. The two principal results obtained were: (a) that castration of hen-feathered Sebright males causes them to develop the full plumage characteristic of the cock-bird; (b) that complete hen-feathering is due to two dominant Mendelian genes.

2. A striking change takes place when the Sebright male is castrated ([plate 1], figs. 3, 4; [plate 3], fig. 1). The new feathers on the upper surface of the head, neck, back, wings, rump, and tail-coverts assume a different color and distribution of their pigment; they take on a new shape, and in those regions where in the cock the barbules are absent from a part of the margin of the feather, the same absence occurs in the castrated birds. Such feathers are present on the neck, back, wing-bow, and rump. The transition is shown in the figures in [plate 6], where for comparison one of the old and one of the new feathers lie side by side. The tail-coverts in the hen-feathered bird are short, and like those in the hen do not cover the true tail. After castration they become excessively long—longer, in fact, than in many cocks—and cover the true tail feathers. The tail feathers themselves, moreover, become increased in length, as do the posterior row of feathers of the wing-coverts. On the breast and sides the change is less marked. The castrated Sebright loses his erect carriage, but how far this is due to the changes in his plumage and how far is real (as a result of a new balance due possibly to the lengthening tail and its coverts) I can not decide.

3. While castration causes the hen-feathered male to make additions in color, length, and size of many feathers, it causes at the same time the other retrogressive changes characteristic of the capon (a castrated cock-feathered bird); the comb and wattles shrink and become pale, the birds almost cease crowing, and become timid. They do not make much effort to mate with the hens, but when they do they show the usual copulatory reactions.