Aball of fertilized cells with a cavity in the center is called

A ball of fertilized cells with a cavity in the center is called blastula.

The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage.

A single-celled zygote will undergo multiple rounds of cleavage, or cell division, in order to produced a ball of cells, called a blastula, with a fluid-filled cavity in its center, called a blastocoel.

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Blastocyst

Explanation:

Blastocyst is a structure formed during the early embryonic development of mammals that consists of an inner group of cells (inner cell mass),  that later becomes the embryo, and an outer group of cells (trophoblast), that develops later into membrane structures that provides protection and nutrient to the embryo. The blastocyst consists of more than 100 cells formed together as a complex cellular structure which forms into the embryo that is later implanted in the uterus.

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