The blastocyst is the structure formed in early embryogenesis, after the formation of the blastocoel, but before implantation. It possesses an inner cell mass, or embryoblast which subsequently forms the embryo, and an outer cell mass, or trophoblast which forms the placenta. The human blastocyst comprises 70-100 cells. It is preceded by a zygote, the fertilized egg cell, and succeeded by an embryo. Blastocyst formation begins at day 5.

Recently the inner cell mass has become a source for embryonic stem cells.

Blastocyst photos at different stages of development:

 http://www.advancedfertility.com/blastocystimages.htm