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Cleavage is defined as a rapid succession of mitotic divisions resulting in the production of a progressively larger number of increasingly smaller cells called blastomeres.

Cleavage occurs in the zygote as it makes its way through the uterine tube to the uterine cavity. No true growth occurs since there is no increase in protoplasmic volume but only an increase in the number of cells.

The mass of cells reaches the uterine cavity when it is composed of approximately 16 cells. It has the appearance of a mulberry and is referred to as a morula. The cells in the center of the mass are called collectively the inner cell mass and will give rise to the embryo proper. The surrounding cells at the surface are called collectively the outer cell mass and will give rise to the extraembryonic membranes.

Source: Atlas of Human Embryos.