_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