Medical Definition of gastrula
: 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 (as in reptiles, birds, and mammals) by differentiation of the upper layer of the blastodisc into the ectoderm and the lower layer into the endoderm and by the inward migration of cells through the primitive streak to form the mesoderm—compare morula
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