GASTRULA

[gas.tru.la] n, pl -las or -lae [NL, fr. gastr-] (1877): an early metazoan embryo in which the ectoderm, mesoderm, and endoderm are established either by invagination of the blastula (as in fish and amphibians) to form a multilayered cellular cup with a blastopore opening into the archenteron or by differentiation of the blastodisc (as in reptiles, birds, and mammals) and inward cellular migration--compare blastula, morula -- gas.tru.lar adj