Figure 1.

Schematic representation of a compacted morula and blastocyst (adapted from ref. 1). Blastocyst formation is the morphological consequence of cavitation. During this process the newly differentiated epithelial TE cells, with their apical surface facing outward, engage in vectorial fluid transport so that the resultant blastocyst consists of TE cells surrounding a fluid filled cavity (blastocoel) and a small group of totipotent inner cell mass cells eccentrically placed at the side of one pole of the TE.