How a blastocyst is engendered
On the 4th day after insemination the outermost cells of the morula that are still enclosed within the pellucid zone begin to join up with each other (so-called compaction). An epithelial cellular layer forms, thicker towards the outside, and its cells flatten out and become smaller. The cells contact one another by means of tight junctions and gap junctions. A cavity forms in the interior of the blastocyst into which fluid flows (the so-called blastocyst cavity). The two to four innermost cells of the preceding morula develop into the so-called inner cell mass of the blastocyst. The actual embryo will develop solely from these cells (embryoblast). These cells are concentrated at one pole, the embryonic pole of the blastocyst. What has thus been formed is an outer cell mass (the trophoblast), consisting of many flat cells, and the embryoblast, formed from just a few rounded cells. The ratio between the number of embryoblast cells to those making up the trophoblast amounts to roughly 1:10. From the trophoblast the infantile part of the placenta and the fetal membranes will arise.