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. Cytokinesis is limited to the original fluid cytoplasmic region. There is no fixed cleavage plan.