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

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.