Blastula Stages

This rapid series of cleavage divisions continues beyond the 32-cell stage, resulting in the formation of progressively smaller cells. Blastomeres are larger at the vegetal pole than the animal pole, a consequence of the rule that cleavages occur away from the more yolky region of cells. The first tangential cleavage occurs at the early blastula stage (stage 7), and this changes the previously single-cell-layered morula into a double-layered embryo. What was the cleavage cavity is now called the blastocoel, and this enlarges by osmotic uptake of water (see ref. 10).

This pattern of development—one cleavage every 30 min—lasts until cycle 13, when division becomes asynchronous, and it slows down significantly (7). This point, the midblastula transition (MBT), is also marked by the onset of cell motility and zygotic transcription. It corresponds to stage 8 of (6) and presages the next important stage in Xenopus development gastrulation.

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