Pronunciation: (gas'troo-lu), [key]
—pl. -las, -laePronunciation: (-lē"). [key] Embryol.
a metazoan embryo in an early state of germ layer formation following the blastula stage, consisting of a cuplike body of two layers of cells, the ectoderm and endoderm, enclosing a central cavity, or archenteron, that opens to the outside by the blastopore: in most animals progressing to the formation of a third cell layer, the mesoderm.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.