H~3-thymidine was used in determining the neuronal mitosis of the traumati cerebral cortex in rats. Intraperitoneal injection of H~3-thymidine was given to the rats immediately after corticectomy of the parietal lobes. On the 4th day after operation, the cerebral tissue was removed, fixed in Carnoy, serial sectioned, and stained with cresyl violet.The results showed that in the surrounding area 100—200?m from the traumatic site, 2-5％ of the various types of cells in the cerebral tissue were labeled, i. e. silver granules were distributed within the nuclei. Macrophages, endothelial cells and pericytes of vessels make up most of the 2—5％, whereas neuroglia and neuron rank next. The nuclei of the majority of the labeled cells were in the interphase state and a few of them were in the mitotic state. The more the labeled cells are in the interphase state,, the more the mitosis occur. On the contrary, it is less. They are always in direct proportion. In the case of neurons the same situation occurs.By inference, the mitotic division of the neurons in the cerebral cortex may occur in situ. Possibly some kind of substance is produced after the trauma leads to the division of the mature neurons. The basis for this inference is that a certain number of the neurons in division have typical features of the pyramidal neurons.

After partial decortication of the parietal cortex of adult rats, division of neurons and glial cells as well as proliferation of macrophages, endothelial cells, pericytes, around the damaged cortical area were observed. The criteria for differentiating mitotic neurons from other cell components especially neuroglia are: (1) the cell body of a dividing neuron especially the pyramidal type is considerably larger than glial cells, and is slightly larger than the adjacent resting neurons of similar type; (2) a dividing neuron has characteristic cell processes, identifiable as axons and apical dendrites, a morphological feature of pyramidal cells; and (3) the dividing neurons contain Nissl granules.The neurons undergo mitosis were located around the lesion area, 37～500 ?m from its border of the lesion. The cortical lamina No. V has more mitotic neurons than any other layers; the explanation for which has been discussed.