The development of the young, as I have observed it in the cases of twenty litters, for ten of which (Table I) systematic daily records were kept, may be sketched as follows. At birth the mice have a rosy pink skin which is devoid of hair and perfectly smooth; they are blind, deaf, and irresponsive to stimulation of the vibrissae on the nose. During the first week of post-natal life the members of a litter remain closely huddled together in the nest, and no dance movements are exhibited. The mother stays with them most of the time. On the fourth or fifth day colorless hairs are visible, and by the end of the week the body is covered with a coat which rapidly assumes the characteristic black and white markings of the race. For the first few days the hind legs are too weak to support the body weight, and whatever movements appear are the result of the use of the fore legs. As soon as the young mice are able to stand, circling movements are exhibited, and by the end of the second week they are pronounced. Somewhere about the tenth day the appearance of the teats in the case of the females serves to distinguish the sexes plainly. Between the tenth and fifteenth days excitability, as indicated by restless jerky movements in the presence of a disturbing condition, increases markedly; the auditory meatus opens, and, in the case of some individuals, there are signs of hearing. On or after the fifteenth day the eyes open and the efforts to escape from the nest box rapidly become more vigorous. About this time the mother resumes her dancing with customary vigor, and the young, when they have opportunity, begin to eat of the food which is given to her. They now dance essentially as do the adults. From the end of the third week growth continues without noteworthy external changes until sexual maturity is attained, between the fourth and the sixth week. For several weeks after they are sexually mature the mice continue to increase in size. TABLE I DEVELOPMENT OF THE YOUNG NUMBER JERKY REACT IN HAIR TEATS MOVE- EARS TO EYES PARENTS LITTER VISIBLE VISIBLE MENTS OPEN SOUND OPEN M F APPEAR 152+151 5 0 4th day -- 13th day 14th day 14th day 16th day 152+151 1 3 4th day 9th day 10th day 12th day 13th day 15th day 410+415 4 1 5th day 11th day 14th day 15th day 15th day 17th day 410+415 2 4 5th day 10th day 13th day 14th day 14th day 16th day 420+425 0 2 4th day 10th day 12th day 14th day 14th day 16th day 210+215 4 1 -- -- 17th day 13th day 17th day 15th day 210+215 3 3 5th day 11th day 11th day 14th day No 16th day 212+211 1 3 4th day 10th day 15th day 14th day No 15th day 220+225 2 4 4th day 10th day 16th day 14th day No 15th day 220+225 3 3 4th day 10th day 17th day 13th day No 15th day