Fig. 5

Fig. 5 Initiation of gastrulation is independent of the degree of epiboly. In this UV-treated embryo at the 30 % epiboly stage, a labeled involuted cell (arrowhead) is seen migrating upwards towards the animal pole. Untreated controls (not shown) were at the 50% epiboly at this time and had also begun involution. The frames shown were taken at 10 minute intervals. The embryos were irradiated at early cleavage stages and then labeled by injecting a single blastomere at the 32-cell stage with TRITC-dextran. Animal poles are up, the dorsal side could not be identified at these early involution stages.