At around 6 days, the blastocyst is surrounded by a degenerating zona pellucida. The blastocyst is a structure that forms early in embryonic development after the fertilization of an egg. It is a hollow ball of cells with an inner cell mass and an outer layer of cells called the trophoblast. The inner cell mass will eventually give rise to the embryo, while the trophoblast will form the placenta and other supporting tissues.
At this stage, the blastocyst has not yet attached to the uterine wall and is free-floating in the uterine cavity. The hypoblast, which will eventually form the yolk sac, forms a layer of cells within the inner cell mass. The amniotic cavity, which will eventually become the fluid-filled sac that surrounds the embryo, forms later on as the inner cell mass divides and differentiates into various cell types.