Blastocyst

A blastocyst exists during the early stages of mammal development, normally at the range of 5 to 6 days, where only around 70-100 cells have been produced since fertilisation. The blastocyst possesses a thin outer shell which aids the transfer into the uterine cavity and is mostly made of fluid meaning each individual cell is viewable under a miscroscope.

Reference:

http://www.ivfcliniclondon.com/en/treatments/blastocyst-transfer/what-is-a-blastocyst.html