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This process goes on till all the cells are so filled by the mesoblast, with its myriad brood of cells, that the outer sac or ectoblast becomes a mere halo around it.

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

The former is often also called the ectoblast, or epiblast, and the latter the endoblast, or hypoblast.