The miotic divisions of a fertilized egg give each newly cleaved cell a genotype identical to that of the parental cell. Yet, as the embryo develops, cell specialization occurs so that only a portion of any given cell's genetic potentialities are activated. Thus, cells that share the same chromosomes begin to serve different functions within the developing organism. The interaction of genetic materials with non-genetic factors controls this cellular specialization.
Determination is the initial step in cell specialization and occurs due to the interaction between the zygote's genetic material and non-genetic factors within the fertilized egg itself. Determination begins in vertebrates with the third cleavage of the zygote. The cells produced by the first and second cleavages, called indeterminate cleavages, have the full developmental potential of the original zygote because the planes of the initial cleavages insure that each new cell receives the same proportion of cytoplasmic substances. Physical factors such as heat, light, pH balance, and gravity, which cause the polar gradient within the distribution of an egg's cytoplasmic materials, or such as the point of the sperm's penetration into the egg, cause a non-homogeneous distribution of cytoplasmic materials within the egg. Because the cytoplasm of the egg is not homogeneous, by the third cleavage, the new nuclei of the different cells receive different cytoplasmic environments. Different cytoplasmic substances produce different regulatory effects on the genes in the nuclei
