An early stage in animal development where the embryo forms a simple two-layered structure with an opening (blastopore) that will become the mouth or anus.
From German 'Gastrula,' coined by naturalist Ernst Haeckel in the 1870s, derived from Greek 'gastro-' (stomach) because the structure resembles a simple stomach with a mouth-like opening.
The gastrula is essentially nature's template—almost all animals from sea urchins to humans go through this same basic shape during early development, which is evidence that we're all distant cousins in the evolutionary family tree.
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