Facts: Week 1, 4 - 5 days, size 0.1-0.2 mm
Features: zona pellucida, trophoblast shell, inner cell mass, blastoceol
Blastocyst hatches from the zona pellucida shell.
The blastocyst has a trophoblast shell of flattened cells, a blastoceol which is a fluid-filled cavity, an inner cell mass which is a small number of cells attached to the trophoblast wall and protruding into the blastoceol.
Image source: Klimanskaya I, Chung Y, Becker S, Lu SJ, Lanza R. Human embryonic stem cell lines derived from single blastomeres. Nature. 2006 Aug 23 Adapted by permission from Macmillan Publishers Ltd (copyright 2006)