First step after fertilization. This a direct consequence of the rapid cell division cycle and competition between replication and transcription. There is basically no yolk present in sea urchin oocyte, therefore the cleavage is holoblastic . There is a fixed radial cleavage plan.
Unequal cytokinesis leads to daughter cells of different sizes.
Blastula is composed of one cell layer surrounding a cavity, the blastocoele. Cells are epithelial in nature, in that they are polar and connected with each other. This leads to superficial meroblastic cleavage. This leads to the formation of a cellular blastoderm with central yolk and no blastocoele. Small region of more fluid cytoplasm containing nucleus is found on top of the yolk.