An embryo is called a(n) when it reaches around 100 cells, and appears to be a hollow, ball-like structure.

The answer is blastocyst. It contains a mass of cells in the inside and a blastocoele which is fluid-filled and an enveloping layer of cells (trophoblast).The inner mass will differentiate into  the endoderm and consequently the internal organs.  The formation of this blastocyst from an embryo is called gastrulation.

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explanation:

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igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.