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 |
From these data, concerning growth of external parts, it seems that: growth is most rapid during the four weeks following parturition; specimens from Michigan, assigned to age groups 1 and 2 on the basis of tooth wear, are less fully developed and probably younger than mice from Minnesota, with a known age of 13 weeks; individuals with sufficient wear on the teeth to be placed in age group 3, if they were obtained in the late fall, may be young from the first litters of the year or, if they were obtained in early spring, may be at least one year old; individuals in age groups 4, 5, and 6 are at least one year old.
Skull.—The post-embryonic development of the skull is rapid. Animals in age groups 1 and 2 have skulls which average more than 80 per cent of the size that is here considered adult (an average size obtained from age groups 3, 4, and 5). The actual increase in size of certain cranial elements for various age groups is given in [table 2].
In age group 1 the rostrum is relatively short as it is in Neotoma micropus (J. A. Allen, 1894:235) and juveniles of Peromyscus truei (Hoffmeister, 1951:7). The rostrum lengthens rapidly and there is a general increase in actual and relative size of the entire preorbital region; the increase after age group 3 is slower and of lesser magnitude. Changes with age in the size of the braincase are slight. In age group 1 the average depth of the braincase is 99.6 per cent of the adult size; the average breadth of the braincase is 98 per cent of the adult size, and the average width across the mastoid region is 96.4 per cent of the adult size. These dimensions indicate that the braincase reaches full size early. The zygomatic arch, however, undergoes change with age; there is a gradual increase in breadth owing to lateral bowing and a gradual lengthening which is in keeping with a general elongation of the skull anterior to the braincase.
The incisive foramina in age group 1 are short (4.0 mm), broad (2.2 mm in the middle), and taper to a point at each end. In age group 2 the foramina have elongated (4.2 mm) and are less pointed posteriorly, but there is no change in breadth. In age groups 3, 4, 5, and 6 the foramina become progressively longer (4.5 mm in age group 6), have a relatively constant breadth (2.2 mm), and become more nearly truncate anteriorly.