The
blastocyst is a structure formed in the early development of mammals. It possesses an
inner cell mass (ICM) which subsequently forms the
embryo. The outer layer of the blastocyst consists of cells collectively called the
trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the
blastocoele. The trophoblast gives rise to the
placenta. The name "blastocyst" arises from the
Greek βλαστός
blastos ("a sprout") and κύστις
kystis ("bladder, capsule").