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Updated: May 14, 2025
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.
And now we have a perfect cell-structure, differing from an ordinary cell only in having the inner sac, inclosing the dot, on the side, instead of in the centre. The outer membrane corresponds to the Ectoblast, or outer cell sac, the Purkinjean vesicle to the Mesoblast, or inner cell sac, while the dot in the centre answers to the Entoblast.
In the language of our science, the outer envelope is called the Ectoblast, the inner sac the Mesoblast, and the dot the Entoblast.
At this period every yolk cell is itself like a whole yolk; for each cell is as full of lesser cells as the yolk-bag itself. When the mesoblast has become thus infinitely subdivided into hundreds of minute spheres, the ectoblast bursts, and the new generations of cells thus set free collect in that part of the egg where the embryonic disk is to arise.
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