ハツカネズミ

The reseaches on the sexual cycle and breeding season of Mus musculus have been made by many investigators (Hinton, 1918 ; Kirkham, 1920 ; Parkes, 1924 ; Kalabukhov, 1937 ; Fenyuk, 1937 ; Makino, 1941 ; Southern and Laurie, 1946 ; etc.). In the Japanese mouse, Mus molossinus Temminck and Schlegel, there are some reports concerning the breeding season and its accompanying phenomena (Iliraiwa and Hamajima, 1957 ; Hamajima, 1958 ; Hiraiwa and Hamajima, 1958 ; Hiraiwa and Hamajima 1960a, b ; Hamajima, 1961). So far as I know, however, no report was made regarding the sexual cycle in the mouse. Since the classical description of Allen (1922), vaginal smear has been always used as an index to ovarian activity. So that, the detection of the sexual cycle in female has been carried out by means of the vaginal smear technique in this study. My observations were made basing on twenty three unmated adult mice derived from eight litters in our laboratory. Vaginal smear was taken by cotton swab method between August, 1956 and July, 1957. The length of one estrous cycle was determined by the mean duration, and the relative length of each stage in one cycle was estimated by Astwood's (1939) method. The summary is as follows : 1. Photographs of the vaginal smears taken at each stage of estrous cycle and anoestrous stage were shown in the Plate 2. 2. There are two types in the properties of vaginal smears at anoestrous stage. One of them consists of epithelial cells, cornified cells and leukocytes (Plate 2, Fig. 5), and another type epithelial cells, leukocytes and mucus (Plate 2, Fig. 6). 3. T he average duration of the estrous cycle in summer (June-Aug.) is 6.63 days at a maximum, in winter (Dec.-Feb.) 6.07 days, in spring (Mar.-May) 5.96 days, in autumn (Sept.-Nov.) 5.66 days at a minimum. 4. The cycle length which shows the most high ratio in frequency distributions is 6 days type. 5. Number o f estrous cycle observed per day in autumn is 1.83 at a maximum, in spring 1.57, in winter 0.78, in summer 0.74 at a minimum. 6. The length of proestrus in autumn and spring is longer than that in the other seasons. Contrary to it, the relation reverses in the length of estrus. On the other hand, the length of metestrus in winter and autumn is longer than that in the other seasons. On the contrary, the length of diestrus in winter and autumn is shorter than that in the other seasons. 7. Estrous cycles were observed mainly in autumn and spring, and there are many mice being anoestrous stage in summer and winter. On the other hand, we made a study on the breeding season of the mouse under the natural environments and reported preliminarily that breeding activity in autumn and spring is higher than in winter and summer (Hiraiwa and Hamajima, 1957, 1958 ; Hamajima, 1961). The fact was endorsed by the data concerning the sexual cycle in captivity obtained from the present study.