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.