Effects of thyroxine on prehatching developmental rate and behavior in the chick.

Development of chick embryos was studied following injection of thyroxine or the goitrogen thiourea and compared with that in control saline-injected eggs. The three stages recorded (onset of beak clapping, clicking, and hatching movements) all occurred late in embryos treated with thiourea on the 16th day of incubation. Thyroxine-treated eggs hatched significantly early only when incubated in groups in contact. This suggests that an acceleration effect of thyroxine reported in previous work may have represented an unreliable effect of thyroxine coupled with the synchronizing effect of contact with other eggs. Changes in hatching times following hormone injection are compared with these associated with effects of clicks, which are known to regulate developmental rate and to play a part in the synchronization of hatching.