Gastrulas

Part of speech: noun

Definitions

1. A structure formed during early embryonic development, particularly in animals, characterized by a hollow ball of cells that subsequently invaginates
2. An early developmental phase in organisms whereby the embryo transitions from a blastula to a more complex arrangement of tissues
3. The stage in embryogenesis marked by cell rearrangement and formation of the germ layers, essential for further development into mature forms

Etymology: "Gastrulas" is the plural of "gastrula," a term coined in 1877 by the German zoologist Ernst Haeckel. He built it from the Greek "gastēr," meaning "stomach" or "belly," which also gives English "gastric" and "gastropod," plus the Latin diminutive suffix "-ula," meaning "little." A gastrula is thus, literally, a "little stomach," named for the pouch-like cavity that forms when the embryo's cell sheet folds inward. Haeckel proposed the idea behind the stage in his gastraea theory, which argued that all multicellular animals pass through a stage resembling a simple two-layered sac, and he needed a name for that sac. The word therefore records a theory rather than an ancient observation: the Greeks had no term for it, because the embryonic process was not understood until the nineteenth century.