[3H]thymidine autoradiographic study in the transit part from the spinal cord to the medulla oblongata of the chick embryo--the ontogenetic relation between the reticular formation and the spinal cord.

In order to examine the ontogenetic structural continuity of the reticular formation to the spinal gray matter, the time of origin of neurons was investigated in the transit part from the spinal cord to the medulla of chick embryos with tritriated thymidine autoradiography. The reticular formation was found to be neurogenetically divided into parvocellular reticular formation (RP), dorsal part of the magnocellular reticular formation (RME) and ventral part of the magnocellular reticular formation (RMI). Neurons in the RP differentiated synchronously with those in the neck and base of the dorsal horn, neurons in the RME with those in the zona intermedia, and neurons in the RMI with those in the ventral horn, suggesting that each synchronously developing region forms an ontogenetic structural unit.