a fertilized egg after about 5 or 6 days, when it is a ball of rapidly dividing cells, 1876, from blasto- + cyst.
Entries linking to blastocyst
blasto-
before vowels blast-, word-forming element used in scientific compounds to mean "germ, bud," from Greek blasto-, combining form of blastos "sprout, germ," which is of unknown origin.
cyst n.
"bladder-like bag or vesicle in an animal body," 1713, from Modern Latin cystis (in English as a Latin word from 1540s), from Greek kystis "bladder, pouch," which is of uncertain etymology.