Final answer:
A sphere of embryonic cells surrounding a fluid-filled cavity is called a blastula. It's a key developmental stage where cells are arranged in a hollow ball containing a cavity known as the blastocoel, and it's critical for the formation of embryonic stem cells.
Step-by-step explanation:
The early embryonic stage characterized by a fluid-filled cavity with an inner cell mass surrounded by a layer of cells is called a blastula. After fertilization, cells divide and form a hollow ball of cells, known as the blastula, which contains the blastocoel. This stage precedes gastrulation, where the cells reorganize to form the three germ layers (ectoderm, mesoderm, and endoderm) that will develop into different organ systems. The blastula provides an important source for embryonic stem cells.