An histological study reveals that embryos homozygous for the mutation tail-short (Ts) become abnormal as morulae, although a small amount of cell division may continue to take place during the following day. Homozygotes can be identified at 3 1/2 days post coitum (p.c.) by several criteria including low cell number, absence of cavitation, and weak cytoplasmic staining with haematoxylin. A similar pattern of developmental arrest is exhibited during culture, although an initial degree of cavitation occurs more frequently.