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.