What makes the hollow part in a blastula?

When the embryo consists of a hundred or more cells post fertilization, it forms a morula, a ball of cells which resemble a mulberry. In most species the mass then resolves itself into a single layer of cells forming a hollow sphere, the blastula. Through the process called cavitation, the morula becomes hollow; this is where the cells in the interior of the morula actively move apart from each other, creating a fluid-filled cavity called the blastocoel. This process is triggered by changes in cell shape and changes in the distribution of substances within the cells. The resulting blastocoel then allows for further growth and differentiation of the cells into the various tissues and organs of the developing embryo.

More info on cavitation is found here: https://en.wikipedia.org/wiki/Cavitation_(embryology)

I have already read this.

What confuses me is, as I've said, I have read before that the blastula is the same size as the zygote.... The morula is hollow... and 'the cells move apart'. How do they move apart without making the whole structure bigger?

Hello Naithi O_Rea,

Just a few corrections. Blastula is not the same size as the zygote. The zygote is a single fertilized cell! That is, a zygote is the initial cell formed when two gamete cells (sperm and egg) fuse during fertilization (day 0).

The morula is not hollow, it is a solid bunch of cells clumped together without a distinct shape other than how you'd describe a mulberry to look, but microscopic of course.

Once the morula is ready to form into the blastula (around day 5 of fertilization), it is made up of 24 to 32 cells.  The outermost cells secret fluid, which accumulates to a fluid-filled cavity, the blastocoel, giving rise to the blastocyst.