Thymidine: inhibitor of differentiation in the young chick blastoderm in culture.

Differentiation in the young chick blastoderm is affected by thymidine at concentrations higher than 8.2 x 10(-4) M. Blastoderms at the hypoblastic island stage cultured continuously in the presence of thymidine form an atypical primitive streak which is not capable of inducing the embryonic axis. However, blastoderms with a mature streak escape the effect of thymidine and develop normally.