'Blastocyst' definitions:

Definition of 'blastocyst'

noun

The blastula of a placental mammal in which some differentiation of cells has occurred [syn: blastocyst, blastodermic vessicle]

From: GCIDE

Blastocyst \Blas"to*cyst\, n. [Gr. blasto`s sprout + E. cyst.] (Biol.) The germinal vesicle. [1913 Webster]