In which phase of development do the basic body plans of organisms begin to form?

The correct phase of development in which the basic body plans of organisms begin to form is during gastrulation. Gastrulation is a critical stage that follows cleavage and is characterized by significant cellular movements that lead to the rearrangement of the embryo. During this process, cells begin to differentiate and migrate, resulting in the formation of three primary germ layers: ectoderm, mesoderm, and endoderm. These germ layers serve as the foundations for the development of various tissues and organs in the organism.

The establishment of these germ layers is essential for determining the body plan, including the formation of the nervous system, the digestive tract, and other vital structures. Thus, gastrulation is pivotal in defining the overall layout and organization of the future organism, setting the stage for further differentiation and development in subsequent stages.