This idea was originally proposed by the Greek philosopher Aristotle. Modern biologists have found that the organism's development is mostly determined by the zygote's genome and the organization of the egg's cytoplasm. As the zygote undergoes cleavage, the cytoplasm is compartmentalized causing the nuclei of the different cells to be exposed to different cytoplasmic environments. These different cytoplasmic environments result in the expression of different genes in different cells. Inherited traits then emerge, in an orderly fashion, in space and time by mechanisms controlling gene expression.
Embryonic development involves cell division, differentiation, and morphogenesis.
Cell division results in an increase in the number of cells. All the cells result from mitotic divisions beginning with the zygote. Differentiation is the development of specialized cells that are organized into tissues and organs. This differentiation is controlled by the cell's expression of different genes brought on by its contact with the environment. Morphogenesis includes the physical processes that give shape to the animal's body and organs.