What is the term for a zygote composed of 16 or more cells?

A zygote that has developed into a mass of 16 or more cells is referred to as a morula. This term specifically describes the early stage of embryonic development following fertilization, when the zygote divides repeatedly through cleavage, resulting in a solid ball of cells, typically around 16 to 32 cells in total. This structure resembles a mulberry, which is where the name "morula" is derived from.

After the morula stage, if the process continues, cells will begin to reorganize and form a hollow structure known as a blastocyst, which is crucial for implantation in the uterine wall. While an embryo can refer broadly to a developing organism, the morula is a specific developmental phase that occurs earlier in the embryonic timeline. The gastrula stage follows the blastocyst stage, representing further differentiation and development of tissues and organs.

Understanding the specific stages of embryonic development helps clarify the progression from a single zygote to a multicellular organism, emphasizing the unique characteristics and names associated with each critical step.