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.
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 410+415 2 4 5th day 10th day 13th day
14th day 14th day 16th day 420+425 0 2 4th day
10th day 12th day 14th day 14th day 16th day 210+215
4 1 — — 17th day
13th day 17th day 15th day 210+215 3 3 5th day
11th day 11th day 14th day No 16th day 212+211
1 3 4th day 10th day 15th day 14th day No
15th day 220+225 2 4 4th day 10th day 16th day
14th day No 15th day 220+225 3 3 4th day
10th day 17th day 13th day No 15th day
A course of development very similar to that just
described was observed by Alexander and Kreidl (3
p. 565) in three litters of dancing mice which contained
3, 5, and 7 individuals respectively. These authors,
in comparing the development of the dancer with that
of the common mouse, say that at birth the young in
both cases are about 24 mm. in length. The young
common mouse grows much more rapidly than the dancer,
and by the ninth day its length is about 43 mm. as
compared with 31 mm. in the case of the dancer.
According to Zoth (31 p. 148) the adult dancer has
a body length of from 7 to 7.5 cm., a length from
tip of nose to tip of tail of from 12 to 13 cm., and
a weight of about 18 grams. The movement of the
dancer from the first tends to take the form of circles
toward the middle of the nest; that of the common
mouse has no definite tendency as to direction.
When the common mouse does move in circles, it goes
first in one direction, then in the other, and not
for any considerable period in one direction as does
the true dancer. Neither the young dancer nor
the common mouse is able to equilibrate itself well
for the first few days after birth, but the latter
can follow a narrow path with far greater accuracy
and steadiness than the former. The uncertain
and irregular movements of the common mouse are due
to muscular weakness and to blindness, but the bizarre
movements of the young dancer seem to demand some
additional facts as an explanation.
