The early growth and development of the woodland jumping mouse, Napaeozapus, have not been described in adequate detail. Many litters have been born to captive females (Hamilton, 1935) and to females bred in captivity (Sheldon, 1938) but the young are usually eaten by the mother or deserted soon after birth. Sheldon (1938) succeeded in raising one litter to 16 days of age and another to more than ten weeks, but made observations only at rather infrequent intervals. On June 5, 1952, we took two adult females at Stony Clove (alt. 2000 feet), Hunter Mountain, Greene County, New York. They were kept in large aquaria in the laboratory and gave birth to litters of three and four young each on June 13 and June 20, respectively., One new born young (CU No. 6849) was, removed from the first litter to be described and preserved. The remaining two were examined on June 16. These were subsequently deserted by the female and found dead three days later. They had not been mutilated in any way. In the second litter, one young, noticeably smaller than the others, died on the sixth day; another was killed at 64 days to observe skeletal development. The adults, and the young after weaning, were fed shelled whole corn, oat flakes, rabbit pellets, strawberries, mulberries, raspberries, lettuce, and apples. The berries were particularly favored. The aquaria were kept covered with several layers of cheesecloth to prevent disturbing the animals with strong light and movements in the laboratory. Shredded toweling was provided for nesting material while the young were small so that they would not easily become separated from the mother. Measurements were obtained up to 26 days of age from photographs in which a centimeter rule was included (Svihla, 1933). Weights of the entire litter were taken at intervals of two to six days until 40 days of age and less frequently thereafter. The weights were divided by the number of young to obtain average weights of individuals. This was done to reduce handling of the young and to permit more time for actual observations during the time they were removed from the nest. New Born Young.-The umbilical cord is a dried stub about .5 mm. long. The skin is rugose, loose, pinkish in color, and fairly transparent, so that the sutures between frontals and parietals, and the ribs, sternum, and viscera are distinct. Except for vibrissae, the body is hairless, and no dark pigmentation that would indicate aggregation of pigment in hair follicles is seen. In both sexes, the four pairs of mammae are visible as small light spots on the venter. Facial vibrissae are present, although once considered to be absent in zapodids at birth. Quimby (1951) has confirmed their presence in new born Zapus. The most posterior mental bristles are about .2 to .3 mm. and grade down in size anteriorly. Those at the edge of the chin are not yet visible. The mystacial vibrissae are arranged in six major rows and show a similar size gradation. Those nearest the eye are the longest, measuring about .3 mm. (fig. la). 242