In our laboratory we have been breeding the Japanese mouse, Mus molossinus Temminck and Schlegel since 1955. Concerning the postnatal development of the sucking young, we have observed the changes of external characters from birth to weaning. And also in twenty five mice of seven litters we have measured the postnatal growth of several parts of the body, i. e. body weight, length of head & body, tail, hind foot and ear. The results are as follows : The mice are born hairless but with claws. At four days after birth body color becomes clear. And the external ears open at about five days. Primary hairs grow at about seven days after birth, and secondary hairs at eight days. In females at this age, the nipples become evident. Incisors of lower jaw erupt at about nine days after birth, and those of upper jaw ten days. At the age of about fourteen days eyes open, and the young begin to eat solid food shortly later. The average body weight at birth is the same in both male and female. The period in which growth rate is highest is from birth to thirty days. Growth becomes somewhat sluggish from forty days to sixty days, thenceforward the body weight increases gradually as well as in length of head & body, tail, hind foot and ear. Hind foot and ear attain their approximate maximum length by ninety days. Tail coefficient becomes maximum from twenty five to thirty days. It seems that there is little difference between adult males and females in the growth of the length of tail, hind foot and ear, though statistical treatment is not made yet.
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