Feeding, Breeding, and Development 27 



Tenth day. To-day teats are plainly visible in the case of 

 four of the five individuals of the litter. Up to this time I 

 had thought, from structural indications, that there were 

 three males and two females ; it is now evident that there are 

 four females and one male. The external ear, the pinna, is 

 well formed, and has begun to stand out from the head, but 

 no opening to the inner portion of the ear is present. The 

 eyelids appear to be almost fully formed. 



Eleventh day. There are no very noticeable changes in ap- 

 pearance except in size, which continues to increase rapidly. 

 They are able to regain their normal upright position almost 

 immediately when they happen to roll over. The mother 

 dances as usual. 



Twelfth day. It appears to-day as if the eyes were about 

 to open. The ears are still closed, and there is no evidence 

 of a sense of hearing. They squeaked considerably when in 

 the nest, but not at all when I took them out to note their 

 development. The mother stays outside of the nest box 

 much of the time now, probably to prevent the young ones 

 from sucking continuously. 



Thirteenth day. One of the little mice came out of the 

 nest box while I was watching the litter this morning, and 

 was able to find his way back directly despite the lack of 

 sight. The mice are still dependent upon the mother for 

 nourishment. I have not seen any of them attempt to eat the 

 food which is given to the mother. They are extremely neat 

 and clean. I watched one of them wash himself this morning. 

 Each foot was carefully licked with the tongue. There seems 

 to be special care taken to keep the toes perfectly clean. 



Fourteenth day. An opening into the ear is visible to-day. 

 When tested with the Galton whistle, all five responded with 

 quick, jerky movements of the head and legs. They evi- 

 dently hear certain tones. During the past two days the ears 



