blastosphere

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.

Such a sphere is called a blastosphere, and may be regarded as a spherical mass of protoplasm, of which the central portion is so much vacuolated that it seems to consist entirely of fluid.

The segmentation is complete; one side of the hollow blastosphere invaginates and forms a gastrula.

If we compare this with the typical blastosphere of the lower type, we see that it is, as it were, flattened out on the yolk.