_Seventh day_. Although they are strong enough to move about the nest readily, none of the young has attempted to leave the nest. They huddle together in the middle of it for warmth. The epidermal scales, which have increased in number since the third day, are dropping off rapidly. Contact with the vibrissae or with the surface of the body, frequently calls forth a motor reaction but neither light nor sound produces any visible change in behavior. The black and white regions of the skin are sufficiently definite now to enable one to distinguish the various individuals by their markings. The mother was seen to dance repeatedly today, and she ate all the food that was given to her. _Eighth day_. A fold is plainly visible where later the eyelids will separate. The black pigment in the skin has increased markedly. _Ninth day_. The eyelids are taking form rapidly, but they I have not separated. The body is covered with a thick coat of hair which is either pure white or black. Standing on the four legs is still a difficult task. _Tenth day_. To-day teats are plainly visible in the case of four of the five individuals of the litter. Up to this time I had thought, from structural indications, that there were three males and two females; it is now evident that there are four females and one male. The external ear, the pinna, is well formed, and has begun to stand out from the head, but no opening to the inner portion of the ear is present. The eyelids appear to be almost fully formed. _Eleventh day_. There are no very noticeable changes in appearance except in size, which continues to increase rapidly. They are able to regain their normal upright position almost immediately when they happen to roll over. The mother dances as usual. _Twelfth day_. It appears to-day as if the eyes were about to open. The ears are still closed, and there is no evidence of a sense of hearing. They squeaked considerably when in the nest, but not at all when I took them out to note their development. The mother stays outside of the nest box much of the time now, probably to prevent the young ones from sucking continuously. _Thirteenth day_. One of the little mice came out of the nest box while I was watching the litter this morning, and was able to find his way back directly despite the lack of sight. The mice are still dependent upon the mother for nourishment. I have not seen any of them attempt to eat the food which is given to the mother. They are extremely neat and clean. I watched one of them wash himself this morning. Each foot was carefully licked with the tongue. There seems to be special care taken to keep the toes perfectly clean.