Practice question

Chick egg telolecithal organization with large yolk mass prevents complete cytokinesis. Early cleavages occur only in blastodisc, small cytoplasmic cap at animal pole separated from yolk, mitotic furrows do not penetrate yolk but remain open basally connected to yolk. This incomplete division defines discoidal meroblastic cleavage characteristic of birds, reptiles, bony fish and monotremes. Holoblastic cleavage seen in mammals and amphibians divides entire egg, spiral holoblastic in molluscs, superficial meroblastic in insects. Chick pattern reflects yolk-imposed meroblastic division. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.