Main points of our observation on estrous cycle, copulation, pregnancy and parturition of Japanese mouse, Mus molossinus Temminck and Schlegel are summarized as follows : 1. The estrous cycle of this wild mouse is also divided into the following four stages, i. e. proestrus, estrus, metestrus and diestrus. The average duration of the estrous cycle is 6.001 ± 0.079 days. The average duration of the estrous is 0.68 day, estrus 2.30 days, metestrus 0.63 day and diestrus 2.39 days respectively. 2. The copulation occurs usually between proestrus and estrus at midnight. 3. At the beginning of the second week after copulation the vaginal smears contain goblet-shaped mucus-secreting epithelial cells with the nucleus at the apex (Fig.1 ). 4. The appearance of blood in the vagina, so called "placental sign " is noticed within the period of 8 to 13 days after copulation. The average duration elapsing between copulation and placental sign is 11.16 ± 0.133 days. The blood is a positive indications of pregnancy. 5. The majority of parturitions takes place at midnight. The average period of gestation in the non-suckling mouse is usually 20.24 ± 0.122 days. But in the case of the suckling mouse it is somewhat lengthened.
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