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.
Fourteenth day. An opening into the ear is visible to-day. When tested with the Galton whistle, all five responded with quick, jerky movements of the head and legs. They evidently hear certain tones. During the past two days the ears have changed rapidly. In one of the females, which seems to be a little in advance of the others in development, certain peculiarities of behavior appeared to-day. She jumped and squeaked sharply when touched and sprang out of my hand when I attempted to take her up. This is in marked contrast with her behavior previously.
Fifteenth day. The eyes are partly opened. All of the members of the litter came out of the nest box this morning and ran around the cage, dancing frequently and trying to eat with the mother. Three out of the five gave auditory reactions on first being stimulated; none of them responded to repetitions of the stimulus. All appeared to be less sensitive to sounds than yesterday. The quick, nervous, jerky movements are very noticeable.
Sixteenth day. The eyes of all five are fully opened. They dance vigorously and are outside the nest much of the time.
Seventeenth day. No reactions to sound could be detected to-day. The sense of sight gives evidence of being well developed. The nervous jumping movements persist.
Eighteenth day. The young mice continue to suck, although they eat of the food which is given to the mother. They are now able to take care of themselves.
Nineteenth day. There are no noteworthy changes except increase in size and strength.