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.