Fig. 1. Three-dimensional views of normal frog development. Normal embryos from gastrulation stages are presented in mid-sagittal view. Internal events of gastrulation are clearly visible, including blastopore groove formation (A), archenteron formation (B), archenteron elongation (C) and archenteron inflation (D-F). Mesendoderm migration over the blastocoel roof initiates on the dorsal side (A), and spreads laterally (B) and ventrally (C) before meeting at the animal pole. In a corresponding manner, the blastocoel shrinks and shifts ventrally. (G-I) Three embryos with very similar blastopores, but with significantly different degrees of archenteron elongation. bc, blastocoel; ac, animal cap; dbpl, dorsal blastopore lip; cleft, cleft of Brachet; vbpl, ventral blastopore lip; arc, archenteron; mm, mesodermal mantle; v, ventral; d, dorsal.