Ace the 2026 OSAT Bio Sciences Test – Dive into Dynamic Discovery!

During early embryonic development, the stage characterized by a solid ball of 16 cells is known as a morula. This stage follows cleavage, where the zygote undergoes several rounds of cell division without significant growth, resulting in a compact mass of cells. The morula typically forms around the third to fourth day after fertilization, with a structure that appears solid as the cells are tightly packed together.

In contrast, the blastula is a later stage in embryonic development, where the morula transforms into a hollow structure filled with fluid, signaling the beginning of a differentiation process. The zygote refers to the fertilized egg just after sperm entry, and the gastrula represents a later phase where cell differentiation leads to the formation of germ layers. Thus, the correct terminology for the solid ball of cells prior to further developmental changes is indeed the morula.