Differentiation and degeneration in the motor horn of the foetal mouse

Abstract The development of the motor horn of the foetal mouse was investigated. A sampling technique was evolved using cresyl violet stained material. A decline in the number of motor neuroblasts occurred during the development, on each side, of four definitive motor regions from a single longitudinal column. The total number of motor horn cells fell from about 100,000 on the eleventh day after mating to about 25,000 on the fourteenth day. The early stages of this decline (between 11 and 12 days) are probably explained by the fact that not all neuroblasts in the region differentiate into motor cells; he later decrease can be entirely accounted for by the number of degenerations. Irradiation of the foetal mouse with a dose of 50 rads of cobalt‐60 gamma‐radiation resulted in an excess of about 20,000 differentiating motor cells on the thirteenth and fourteenth days. Irradiation increased the actual number, but not the percentage, of degenerations. This suggests that differentiation is a phase in the growth process which proceeds degeneation.