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Stage 2 begins with the division of the zygote into two cells and ends with the appearance of the blastocystic cavity after the embryo is composed of more than 16 cells. The embryo has an estimated postfertilization age of two to three days and is approximately 0.1 to 0.2 mm in diameter. The zygote undergoes cleavage during this period whereby it divides mitotically into an increasing number of cells that become progressively smaller with each division. True growth does not occur during cleavage since the volume of the entire protoplasmic mass actually diminishes. However, the DNA content increases dramatically duplicating with each cell division.


Section 5

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Beginning cleavage

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Embryo after first cleavage is completed

Embryo after first cleavage is completed

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