means that two types of cells are forming, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the zona pellucida breaches. The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation.

Placental circulation system

The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate

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