Feeding, Breeding, and Development 25 



First day. The skin is pink and hairless, several vibrissae 

 are visible on the nose and lips, but there is no definite re- 

 sponse when they are touched. The mice are both blind 

 and deaf, but they are able to squeak vigorously. The 

 mother was not seen to dance or eat during the day. 



Second day. There is a very noticeable increase in size. 

 The vibrissae are longer, but touching them still fails to cause 

 a reaction. No hairs are visible on the body. The mother 

 danced rapidly for periods of a minute several times while 

 the record was being made. She ate very little to-day. 



Third day. Scales began to appear on the skin to-day. 

 The animals are rapidly increasing in strength ; they can now 

 crawl about the nest easily, but they are too weak to stand, 

 and constantly roll over upon their sides or backs when they 

 are placed on a smooth surface. Because of their inability 

 to progress it is impossible to determine with certainty whether 

 they have a tendency to move in circles. The mother was 

 seen out of the nest dancing once to-day. She now eats 

 ravenously. 



Fourth day. One of the six young mice was found under 

 a corner of the nest this morning dead, and the others were 

 scattered about the nest box. I gathered them together into 

 a nest which I made out of bits of tissue paper, and the 

 mother immediately began to suckle them. They are very 

 sensitive to currents of air, but they do not respond to light 

 or sound and seldom to contact with the vibrissae. 



Fifth day. When placed on a smooth surface, they tend 

 to move in circles, frequently rolling over. When placed on 

 their sides or backs, they immediately try to right themselves. 

 They do not walk, for their legs are still too weak to support 

 the weight of the body; instead they drag themselves about 

 by the use of the fore legs. Fine colorless hairs are visible 

 over the entire body surface. When the vibrissae are touched, 



