22 The Dancing Mouse 



and the only safe statement, in the light of my observations, 

 is that the mother dances less than usual for a few days after 

 a litter is born to her. 



The development of the young, as I have observed it in 

 the cases of twenty litters, for ten of which (Table i) sys- 

 tematic daily records were kept, may be sketched as follows. 

 At birth the mice have a rosy pink skin which is devoid of hair 

 and perfectly smooth; they are blind, deaf, and irresponsive 

 to stimulation of the vibrissae on the nose. During the first 

 week of post-natal life the members of a litter remain closely 

 huddled together in the nest, and no dance movements are 

 exhibited. The mother stays with them most of the time. 

 On the fourth or fifth day colorless hairs are visible, and by 

 the end of the week the body is covered with a coat which 

 rapidly assumes the characteristic black and white markings 

 of the race. For the first few days the hind legs are too weak 

 to support the body weight, and whatever movements ap- 

 pear are the result of the use of the fore legs. As soon as the 

 young mice are able to stand, circling movements are exhib- 

 ited, and by the end of the second week they are 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 re- 

 sumes 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 



