Hypophyseal control of genetic expression during chick feather and skin differentiation.

The normal sequence of molecular events which occurs during feather and skin differentiation in the chick embryo is interrupted by pituitary gland ablation. The characteristic pattern of development is reestablished in hypophysectomized embryos treated with pituitary extract and in hypophysectomized embryos in parabiotic union with their twins within a double-yolked egg. These results suggest that genetic expression in differentiating chick feather and skin after day 12 of incubation is regulated by hormones.