/ Dictionary / Index B blastocyst: Meaning and Definition of Find definitions for: blas•to•cyst Pronunciation: (blas'tu-sist), [key] — n. Embryol. the blastula of the mammalian embryo, consisting of an inner cell mass, a cavity, and an outer layer, the trophoblast. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: blastocyst (Thesaurus) blastocoel blastoderm Related Content Daily Word Quiz: paradox Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language