_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. _Twentieth day_. No auditory reactions were obtained today, but other forms of stimulation brought about unmistakable responses. _Twenty-first day_. They are now about half grown and there is no other change of special interest to be recorded. Growth continues for several weeks. The statement made by Alexander and Kreidl to the effect that the dancer is almost full grown by the thirty-first day of life is false. At that age they may be sexually mature, but usually they are far from full grown. CHAPTER III BEHAVIOR: DANCE MOVEMENTS