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The cavity of the blastosphere, or segmentation cavity.

In the course of a series of divisions the eight cells come to form a hollow sphere—the blastosphere—enclosing a cavity known as the cleavage or segmentation cavity.

The central part of the blastosphere is called the segmentation cavity or blastocoel.

The sharp angle by which the cells met at the centre has become rounded off, and has left a little space, the segmentation cavity, filled with fluid in the middle of the embryo.

In the gastrula stage, the segmentation cavity in which the mesoblast is formed lies between the entoblast and ectoblast.