Definition of gastrula
plural gastrulas
or gastrulae
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\ˈga-strə-ˌlē, -ˌlī\
: 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
