pronounced. Somewhere about the tenth day the appearance of the teats in
the case of the females serves to distinguish the sexes plainly. Between
the tenth and fifteenth days excitability, as indicated by restless jerky
movements in the presence of a disturbing condition, increases markedly;
the auditory meatus opens, and, in the case of some individuals, there are
signs of hearing. On or after the fifteenth day the eyes open and the
efforts to escape from the nest box rapidly become more vigorous. About
this time the mother resumes her dancing with customary vigor, and the
young, when they have opportunity, begin to eat of the food which is given
to her. They now dance essentially as do the adults. From the end of the
third week growth continues without noteworthy external changes until
sexual maturity is attained, between the fourth and the sixth week. For
several weeks after they are sexually mature the mice continue to increase
in size.



TABLE I

DEVELOPMENT OF THE YOUNG


NUMBER JERKY REACT
IN HAIR TEATS MOVE- EARS TO EYES
PARENTS LITTER VISIBLE VISIBLE MENTS OPEN SOUND OPEN
M F APPEAR

152+151 5 0 4th day -- 13th day 14th day 14th day 16th day
152+151 1 3 4th day 9th day 10th day 12th day 13th day 15th day
410+415 4 1 5th day 11th day 14th day 15th day 15th day 17th day