blastocyst blas·to·cyst (blās'tə-sĭst') n. The modified blastula stage of mammalian embryos, consisting of the inner cell mass and a thin trophoblast layer enclosing the blastocoel. Also called blastodermic vesicle.
(blās'tə-sĭst') The modified blastula that is characteristic of placental mammals. It has an outer layer, known as a trophoblast, that participates in the development of the placenta. The inner layer of cells develops into the embryo.