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Roughly two weeks after fertilization, more than half of the embryos had an embryonic disk — a flat mass of cells.

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.

Sole-shaped embryonic disk of the chick, in three successive stages of development, looked at from the dorsal surface, magnified about twenty times, somewhat diagrammatic.

At the beginning of germination the flat embryonic disk curves outwards, and separates on the inner side from the underlying large yelk-ball.

We may now disregard both the outer ovolemma and the greater part of the vesicle, and concentrate our attention on the germinative area and the four-layered embryonic disk.