Blastocyst

Overview
The blastocyst is an early stage of the mammalian development that occurs at the beginning of pregnancy. It 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 whoch 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. Other definitions consider the blastocyst an intermediate stage of the embryo, and begin using the term blastocyst at day 5 (16-20 cells.)

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