Explanation
A: Differentiation is the beginning of the formation of three germinal layers in gastrula.
During gastrulation, the blastula undergoes a series of changes that lead to the formation of three germinal layers: the endoderm, mesoderm, and ectoderm. These germinal layers give rise to all the tissues and organs of the developing embryo. The process of differentiation begins as the cells in each germinal layer start to develop specialized functions.
Option B is incorrect because involution is a process that occurs during gastrulation, in which cells move inward from the surface of the blastula to form the germinal layers.
Option C is incorrect because neurulation is a later stage of embryonic development, in which the neural plate folds to form the neural tube, which eventually develops into the brain and spinal cord.
Option D is incorrect because gastrulation is the process by which the blastula is transformed into a gastrula, which consists of three germinal layers.
