The fluid-filled cavity within a developing blastula (early embryonic stage); also called the segmentation cavity.
From Greek 'blastos' (bud, embryo) + 'koilia' (cavity, belly). The term was created in the 1800s as microscopy revealed embryonic structure.
When scientists first saw embryos under microscopes, they discovered this tiny hollow sphere of cells—the blastocele—which felt like discovering an entire universe inside a speck too small to see with your eyes!
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