VARIOUS malformations in rats and mice have been observed to be caused by administration of the vital dye, Trypan blue (Gillman, Gilbert, Gillman, & Spence, 1948; Fox & Goss, 1956, 1957; Hamburgh, 1952, 1954; Waddington & Carter, 1952, 1953). Recently Waddington & Perry (1956) reported a teratogenic effect of Trypan blue on Amphibian embryos. The present paper deals with the effects of this dye...