If there is little yolk in the vegetal hemisphere cleavage will proceed equally. The first two cleavage divisions are vertical and divide the embryo into 4 cells. The third cleavage plane is horizontal and produces an 8 cell embryo with two levels. Deuterostome cleavage forms 2 tiers of cells one exactly above the other. Protostome cleavage the upper tier of cell align directly over the grooves of the lower tier.
Continual cleavage produces a solid ball of cells called the morula .
A fluid filled cavity, called the blastocoel develops within the morula forming a hollow ball of cells called the blastula . The three layers produced by gastrulation are embryonic tissues called embryonic germ layers. The ectoderm is the outer layer of the gastrula. The nervous system and the outer layer of the skin in adult animals develop from the ectoderm. The mesoderm partially fills the space between the ecto and endoderm.