Pregastrulation Amphibians go through a mid-blastula transition, where the cell cycle slows down and cell division becomes asynchronous. Notice that the dorsal lip of the blastopore forms first and the ventral lip later. Use this figure to help you figure out Plate 1. Observe and draw each phase and label them. The blastopore itself begins to form just below the grey crescent.
Gastrulation occurs in roughly two forms, depending on the amount of yolk present.
The overall outcome is that the single layer of the blastula has been rearranged to form two layers, with the outer layer termed the ectoderm, the inner layer called the endoderm. The upper surface of the endoderm includes cells destined to form the middle layer, the mesoderm. The second form of gastrulation occurs in birds and reptiles where the yolk is not cleaved at all but does affect the cleavage pattern. We will study this later. It is characterized by a profound and well-ordered rearrangement of the cells in the embryo.