Abstract
From 0.3-3.0 mg. of thiourea in a vol. of 0.5 ml. was introduced aseptically into fertile eggs after 7-17 days'' incubation This retarded the hatching of the embryos up to 10 days depending on the age of the embryo and the concn. of the thiourea. Thyroxin in non-toxic doses had no antagonizing effect on thiourea, but larger doses neutralized its action. Hence the inhibiting effect of thiourea may be due to its interference with normal thyroxine metabolism.