During which of the following stages of development do individual tissue layers begin to form?

The correct answer relates specifically to the process of gastrulation, a crucial phase in embryonic development where individual tissue layers are established. During gastrulation, the single-layered structure of the blastula reorganizes into multiple layers, known as germ layers. These layers include the ectoderm, mesoderm, and endoderm, each of which will develop into different tissues and organs in the organism.

Gastrulation sets the foundation for organogenesis, as it determines the body’s basic layout—where different types of cells will migrate to and form specific structures later in development. This stage is integral because it transitions the embryo from a simple structure into a more complex organization with distinct layers dedicated to developing specific systems and functions.

In contrast, during the earlier stages of zygote development, the fertilized egg undergoes a series of cell divisions known as cleavage. This process does not involve the formation of tissue layers but rather creates a multicellular ball without specific organization. Embryo formation is a broader term that encompasses the entire initial phase of development rather than pinpointing the specific step of tissue layer formation. Differentiation follows gastrulation and involves cells becoming specialized for particular functions, which occurs after the germ layers have already formed.