Let us review a few important aspects of an important topic in embryonic development, cleavage patterns. Cleavage patterns differ along the lines of two major lineages in the animal kindgom. With vertebrates, starfish and a few others, cleavage is radial, while with worms, insects, molluscs, cleavage is spiral. In mammals, cleavage produces the morula, then the blastula, a hollow sphere. Cytoplasmic distribution during cleavage determines the spacial arrangement of control substances (derived from the mother's genes).

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