An early stage in the development of an embryo where it has two main cell layers and begins to form a stomach-like cavity.
From German 'Gastrea,' coined by biologist Ernst Haeckel in the 1870s from Greek 'gaster' (stomach). Haeckel theorized this was a universal stage in animal development.
Haeckel's gastrea theory was revolutionary—he suggested that all complex animals passed through this two-layered stage, connecting human development to our evolutionary past with simple sea creatures!
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