GRADIENT THEORY -
It was given by T. Boveri & was supported by M.C. Child.
Child called it as metabolic axial gradient theory.
According to it, the egg has distinct animal & vegital poles.
The cytoplasm of the animal pole divides repidly but the rate of cleavage in the vegital pole is slow. It indicates that the rate of metabolism is more in the animal pole than the vegital pole which is called as metabolic axial gradient.
Horstadius & Runnstrom called it as double gradient thoery. They explain egg development on the basis of animal pole gradient & vegital pole gradient.