The present study was undertaken to investigate morphologically and cytologically the process of growth of mice of NC and KK strain.
The data presented in this study are summarized as follows :
1) Body weight is separated into three stages in the process of growth. Remarkable change in growth of body weight occurred in 35 to 40 days (male) and 55 to 60 days (female) of age after birth.
2) Age of descensus testis was found to be 22 to 25 days in NC strain and 24 to 28 days in KK stram.
3) Age of vaginal opening was 30 days in NC strain and 32 to 35 days in KK strain.
4) Sexual cycle of mice generally was variable by strain. NC strain showed a considerably normal sexual cycle in contrast to KK strain.
5) Ovary weight reached a maximum of growth in 30 to 35 days of age， while testis weight reached a maximum growth in 50 to 55 days in both strains.
6) Seminiferous tubules of mice of both strains showed a marked development by 25 days after birth and slowed down from 25 days to 60 days. The diameter of seminiferous tubules showed a constant value in 60 days of age.
7) Microscopical observations revealed that healthy spermatozoa were observed in seminiferous tubules and epididymis in 40 days of age in NC strain and in 45 days in KK strain. Normal majure follicles proceeding prior to the maturation course is abundant in 25 days of age in NC strain and in 30 days of age in KK strain. Polyovular follicles were rare in occurrence showing no difference by strain. Especially， immature mice showed a comparative large number of polyovular follicles and polynuclear follicles in comparison with mature mice.