BLASTOCOELE, the name applied to the "segmentation cavity" formed in the fertilized ovum by its division into numer ous cells. In the blastula (q.v.) the blastocoele is often large, par ticularly in eggs without much yolk; in heavily-yolked eggs there may be no blastocoele visible. On the formation of the gastrula (q.v.) the blastocoele is largely obliterated and the remains of it become filled up with mesoderm. See EMBRYOLOGY.