ABSTRACT
Following fertilization, the egg cell (zygote) divides into a number of cells called blastomeres that surround a central cavity known as a blastocoel (Figure 2.2). A group of blastomeres that aggregate above the cavity is called the blastoderm. This entire structure-that is, the cells and the hollow cavity they surround-is known by different names, depending on the species: the blastula in amphibians, the blastocyst in many mammals, and the blastodisc in birds, fish, and some mammals. The difference in terminology refers to the morphology of the cells. In birds, for example, the cells form a disc-like structure that is distinct from the cyst-like morphology observed in mice. In many cases, the term “blastula” is used as a generic term for all species.
