Illustration of a blastocyst in sectional view. Approximately three to four days after fertilization, the blastocyst cavity is formed. The outer wall of the blastocyst is enclosed by the zona pellucida, the original envelope of the oocyte. The outer layer of the blastocyst consists of a single layer of cells (trophoblasts) which unite together with the maternal tissue to form the placenta. Over time, the original embryoblasts develop into a simple endoderm at the inner area, which becomes the yolk sac.