An early stage of embryonic development in animals, consisting of a hollow ball of cells surrounding a central cavity.
From German 'Blastula' (diminutive of Greek 'blastos' meaning bud or germ). Coined in the 1870s by German embryologist Ernst Haeckel during early developmental studies.
Before you were a blastula, you existed as just a hollow ball of cells for a few days—it's the universal animal shape at week one of development, whether you're a sea urchin, fish, frog, or human.
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