All multicellular organisms begin as a single cell. within four or five days, the first cell multiplies into a ball-like structure of about 100 cells. what is this structure called?

The right answer is blastocyst.

The blastocystic 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 blastocoele.

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.

Blastocyst

Explanation:

not always but somethings can be reproduced in a laboratory.