Читать книгу Ridley's The Vulva - Группа авторов - Страница 19

The point of fertilisation occurs on the first post‐ovulatory day in which the human zygote, with its XX sex chromosome constitution, is conceived in the distal third of the uterine tube. An acellular envelope, the zona pellucida, encases the zygote. The first cleavage division occurs 24–30 hours after fertilisation, and the two‐cell zygote increases to 8–16 blastomeres.

A blastocyst then develops with a fluid‐filled cavity. There are 16–32 blastomeres which start to form an inner cell mass (embryonic pole) and outer cell mass (mural and polar trophoblast). The blastocyst eventually comes to lie free within the reproductive tract as the surrounding zona pellucida degenerates (Figure 1.1).