24 The Dancing Mouse 



common mouse does move in circles, it goes first in one direc- 

 tion, 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. 



A brief account of the development of the dancer given by 

 Zoth (31 p. 149) adds nothing of importance to the descrip- 

 tion given by Alexander and Kreidl. As my own observa- 

 tions disagree with their accounts in certain respects, I shall 

 now give, in the form of a diary, a description of the important 

 changes observed from day to day in a normal litter. The 

 litter which I have selected as typical of the course of develop- 

 ment in the dancer grew rapidly under favorable conditions. 

 I have observed many litters which passed through the various 

 stages of development mentioned in this description anywhere 

 from a day to a week later. This was usually due to some 

 such obviously unfavorable condition as too little food or 

 slight digestive or bowel troubles. According to the nature 

 of the conditions of growth the eyes of the dancer open any- 

 where from the fourteenth to the twentieth day. This state- 

 ment may serve to indicate the degree of variability as to 

 the time at which a given stage of development is reached 

 by different litters. 



On July 14, 1906, No. 151 (female) and No. 152 (male) 

 were mated, and on August 3 a litter of six was born to them. 

 The course of the development of this litter during the first 

 three weeks was as follows : 



