All multicellular organisms begin as a single cel. Within four or five days, that first cell multiplies into a ball-like structure of about 100 cells. What is this structure called?​

The right answer is a blastocyst.

The blastocyst Stage of embryo development is characterized by the formation in the center of the morula (embryonic cell group) of a cavity isolated from the external environment: the blastocoel.

In embryonic development, this step precedes gastrulation.

Blastocysts are indeed composed of a hundred cells; they have the shape of a hollow sphere at the bottom of which is the mound of the "embryonic bud" from which the embryonic stem cells are derived

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