(blas'tŏ-sist‵ )

[ blasto- + cyst ]

In mammalian embryonic development, the stage that follows the morula. It consists of an outer layer (the trophoblast) and an inner cell mass (the blastocoel). At this stage, implantation in the endometrium (lining of the uterus) occurs. In mammalian development, the blastocyst corresponds to the blastula of lower forms.

SEE: blastocoel; blastula; morula; trophoblast; fertilization for illus.