Gastrulation in Amphibians
Amphibians comprise a large and moderately telolecithal egg. Cleavage is holoblastic and unequal generating a spherical blastula along with a reduced eccentric blastocoel shifted towards the animal pole. The blastocoel has a thin roof of various layers of small yolk-poor cells and a floor of large yolk-laden cells various layers deep extending down to the vegetal pole. The procedure of gastrulation starting with such a blastula is complicated since it is impossible for the enormous mass of large yolk cells in the floor of the reduced blastocoel of the cells to invaginate from the vegetal pole inwards as takes place in sea urchin. In place, invagination begins on the future dorsal side of the embryo where cells are small and less yolky. Furthermore, invagination is very limited and only initiates gastrulation; rest of the gastrulation process involves a variety of cell movements involving convergence, divergence.