In the specimens from Michigan of age groups 3, 4, 5, and 6 the average length of the body is 80.9, 83.7, 89.0, and 83.6, respectively. According to Quimby (_loc. cit._), the average length of the tail for three _Z. hudsonius_ at birth was 9.2 mm. (8.5-10.0). During the first four weeks of development the tail grew rapidly and reached an average length of 92.0 mm, which was 10 times the length at birth. By the end of 13 weeks of development the average length of the tail for these three individuals was 119.6 mm or 12 times the average length at birth. The most rapid growth was early in development: 80 per cent of the growth of the tail occurred during the first month, after which growth proceeded at a much slower rate. Quimby (_loc. cit._) records an average dimension of 4.7 mm (4.5-5.0) for the length of the hind foot in three newly born _Z. hudsonius_. The hind foot grew rapidly in length and by the fourth week had increased 5.6 times in its length and averaged 26.3 mm. Growth was much less rapid from the fourth to the thirteenth week when the hind foot averaged 27.7 mm, only five per cent more than in mice four weeks old. Assuming the average length of the hind foot of the adults to be 29.0 mm, the hind foot in individuals 13 weeks old is 96 per cent of the adult size. According to Quimby (_loc. cit._), the pinna of the ear at birth is small and folded over the external auditory meatus. The length of the ear increases proportionately more (29 per cent) than any other external dimension after the first four weeks of growth. If the average length of the ear (measured from the crown) of adults is 14.7 mm, the animals from Michigan in age groups 1 and 2 are 91.8 per cent and 96.5 per cent as large as adults. TABLE 1.--Average Dimensions (in Millimeters) for Specimens of Z. h. hudsonius of Various Ages (Specimens from Michigan). ==============+=======+=======+=======+=======+=======+======= Age groups | 1 | 2 | 3 | 4 | 5 | 6 --------------+-------+-------+-------+-------+-------+------- No. examined | 4 | 13 | 33 | 12 | 3 | 3 --------------+-------+-------+-------+-------+-------+------- Body | 70.0 | 74.8 | 80.9 | 83.7 | 89.0 | 83.6 --------------+-------+-------+-------+-------+-------+------- Tail | 113.8 | 118.5 | 122.9 | 125.0 | 125.0 | 118.3 --------------+-------+-------+-------+-------+-------+------- Hind foot | 28.8 | 28.6 | 28.9 | 29.1 | 28.9 | 29.3 --------------+-------+-------+-------+-------+-------+------- Ear | 13.5 | 14.2 | 14.7 | 14.8 | 15.0 | 14.3 --------------+-------+-------+-------+-------+-------+-------