_Second day_. There is a very noticeable increase in size. The vibrissae are longer, but touching them still fails to cause a reaction. No hairs are visible on the body. The mother danced rapidly for periods of a minute several times while the record was being made. She ate very little to-day. _Third day_. Scales began to appear on the skin to-day. The animals are rapidly increasing in strength; they can now crawl about the nest easily, but they are too weak to stand, and constantly roll over upon their sides or backs when they are placed on a smooth surface. Because of their inability to progress it is impossible to determine with certainty whether they have a tendency to move in circles. The mother was seen out of the nest dancing once to-day. She now eats ravenously. _Fourth day_. One of the six young mice was found under a corner of the nest this morning dead, and the others were scattered about the nest box. I gathered them together into a nest which I made out of bits of tissue paper, and the mother immediately began to suckle them. They are very sensitive to currents of air, but they do not respond to light or sound and seldom to contact with the vibrissae. _Fifth day_. When placed on a smooth surface, they tend to move in circles, frequently rolling over. When placed on their sides or backs, they immediately try to right themselves. They do not walk, for their legs are still too weak to support the weight of the body; instead they drag themselves about by the use of the fore legs. Fine colorless hairs are visible over the entire body surface. When the vibrissae are touched, the head is moved noticeably. The mother dances a great deal and eats about twice as much as she did before the birth of the litter. _Sixth day_. Certain regions of the skin, which were slightly darker than the remainder on the fourth and fifth days, are now almost black. It is evident that they are the regions in which the black hair is to appear. The movement in circles is much more definite today, although most of the individuals are still too weak to stand on their feet steadily for more than a few seconds at a time. Most of their time, when they are first taken from the nest, is spent in trying to maintain or regain an upright position. The hair is now easily visible, and the skin begins to have a white appearance as a result.