THE BREEDING OF MICE IN LABORATORIES 49 



the young mouse subsequent to its metamorphosis into an 

 air-breathing creature, at times perhaps even before it is dry. 

 The first period of nursing may continue as long as fifteen 

 hours, if the mother is not interrupted. 



Under poor conditions some mice will not nurse their 

 young. In case a mouse refuses to nurse young of great 

 value to the investigator, they should be fostered. If young 

 mice have nursed it is evident, because milk contained in the 

 stomach shows through the left side as a cream-colored 

 crescent. 



Growth. As with most helpless infants, during the first 

 period of their lives, the reactions of young mice are con- 

 fined chiefly to nursing and sleeping, while their bodies grow 

 rapidly in size and differentiation progresses in the more 

 retarded organs such as those of special sense. The rods of 

 the retina, for example, are developed after the fifth day. 



The pigmented hair within the skin is faintly visible upon 

 the second or third day and is well developed by the eighth 

 day, when the skin becomes scaly, probably due to a shedding 

 of the external surface as dandruff. By this time the little 

 mice move blindly about the nest and even venture into the 

 world outside, only to be hustled back in the mouth of their 

 mother, who continually washes, feeds, and looks after them. 



By the thirteenth day the truancy of these little balls of 

 fur, still unsteady upon their feet, becomes too great for their 

 attentive mother, who still repeatedly carries them back to 

 the nest. They occasionally escape to a sequestered corner 

 of the cage, where they test things with their paws, noses, 

 tongues, and vibrissa?. They sit and wash their faces, nibble 

 morsels of food, and blink at the light with their newly 

 opened eyes. 



In the childhood of the mouse there has been observed no 

 period of play such as young rats enjoy when they mouth, 

 kick, tussle, clutch, and roll with each other. However, be- 

 tween the fifteenth and twenty -fifth days young mice unused 

 to handling are more restless and more active than usual, 

 squeaking, scampering, leaping, and seeking to hide them- 



