Cleavage - Metazoa

The unicellular zygote begins cell division (cleavage). First, the single cell divides forming two cells, these redivide further to form four, then eight cells and so on till it gets converted into a ball of cell. The cells are called blastomeres. The planes of the first and second cleavage are vertical passing through the axis, but at right angles to one another. These two cleavages together result in four blastomeres lying side by side around the axis.

The plane of the third cleavage is at right angles to the first two planes and to the axis and is horizontal and hence parallel to the equator of the zygote. This results in eight blastomeres. Of the eight blastomeres four lie on top of the other four. The pattern of cleavage and arrangement of blastomeres around an imaginary central axis in a zygote can be one of two types radial or spiral.