DIFFERENTIATION OF GASTRULAS AND FEATHER GERMS 1003 RESULTS Measurements of the treated embryos 3 days after irradiation, show that the size of the treated embryos decreases when the intensity of the irradiation increases (Fig. 3). The findings from histological 3| Fig. 3—Effects of irradiation of the ectodermal cap (3 days after irradiation). (1) Control; (2) 1050 R; (3) 800 R; (4) 550 R; (5) 300 R; and (6) 150 R. sections of the anterior brain, posterior brain, and trunk of these embryos are summarized in Fig. 4. After doses of 150, 300, 550, and 800 R, spinocaudal structures remain normal though a little reduced. They begin to disappear after 1050 R in 50% of the cases. In contrast, the archencephalic and deuterencephalic structures begin to disappear very early. After 550 R, they are very rudimental and never develop sense organs; after 800 R, in 82% of the cases they are not differentiated; after 1050 R, they are always absent (Fig. 5). So, the loss of competence of irradiated ectodermal cells toward inductors occurs according to a determined sequence: the deuterencephalic and archencephalic structures are the first to disappear, then the spinal cord,! I have checked these findings using another model system for tissue interaction, a system less complex than the whole egg and one in mee tee EE ieUNOlie te