CLEAVAGE
After fertilization, the zygote goes through cleavage, a series of quick mitotic divisions with DNA synthesis but no growth phase. The zygote divides to form a 2-celled embryo. These two cells divide, resulting in 4 cells (blastomeres). At about the 32-cell stage, the embryo is a hollow ball of blastomeres (a morula). Anywhere from 64 to several hundred blastomeres form the blastula, usually a ball with a fluid-filled cavity, the blastocoel.

Most invertebrates and simple chordates have isolecithal eggs with comparatively small amounts of yolk evenly dispersed within the cytoplasm. These eggs divide completely (holoblastic cleaveage). Cleavage of these eggs can be radial or spiral. Radial cleavage is characteristic of echinoderms (including sea urchins) and chordates; spiral cleavage is generally found in the embryos of annelids, arthropods, and mollusks.

Many vertebrate eggs may be telolecithal, with large quantitites of yolk at one end of the cell, known as the vegetal pole, and a small quantity of cytoplasm (the blastodisc) at the animal pole. Cell division is limited to the blastodisc (meroblastic cleavage).

In moderately telolecithal (mesolecithal) amphibian eggs, divisions in the vegetal hemisphere are hampered by the presence of the inert yolk. Thus the blastula consists of several small cells in the animal hemisphere and fewer larger cells in the vegetal hemisphere.