Blastocyst

The blastocyst is the development stage of the embryo characterised by the formation of a cavity isolated from the external environment, the blastocoele, in the centre of the morula (the group of embryonic cells).

This stage in embryonic development precedes gastrulation.

Blastocysts are composed of around one hundred cells with a hollow sphere at the bottom of which is the "embryonic button" from which the embryonic stem cells derive.