Feeding, Breeding, and Development 23 



noteworthy external changes until sexual maturity is at- 

 tained, 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 



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. Accord- 

 ing 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 



