Two-gradient hypothesis of primary embryonic induction.

The "two-gradient" hypothesis of primary embryonic induction was developed by Sulo Toivonen and his school in 1938--1968. The model postulates two inductive principles distributed as gradients in the inductor tissue. The prospective neuroectoderm becomes first uniformly neuralized by one of the factors, and only afterwards segregation is brought about by cells mesodermalized by the other inductor. The short review summarizes the experimental evidence for the "two-gradient" hypothesis.