Chick embryos were treated in four ways; the first group of embryos was given a single dose of 0.5% urethane alone at the definitive primitive streak stage; the second group was irradiated with 300 R X-ray alone at the same stage; the third group was treated with urethane (0.5%) at the definitive primitive streak stage and following 6 hours incubation was irradiated with 300 R X-ray; the fourth group was irradiated with 300 R x-ray at the definitive primitive streak stage and after 6 hours incubation was treated with 0.5 % urethane. The fifth, untreated group was used as control. 2. After urethane treatment alone, the embryos did not show any abnormalities. 3. X-ray treatment alone also did not cause any malformation in the embryos. 4. After successive administration of two agents irrespective of which was first, significant incidence of gross deformation of various organs including brain, heart, somites, etc. was observed. 5. The significance of these observations is discussed.
