The Sense of Hearing 85 



sponded vigorously to sudden high tones and loud noises. 

 After the third week I have seen only doubtful signs of hear- 

 ing. I shall now describe in detail the method of experi- 

 mentation, the condition of the animals, and the nature of 

 the auditory reactions. 



Between the twelfth and the eighteenth day the auditory 

 canal becomes open to the exterior. The time is very variable 

 in different litters, for their rate of growth depends upon the 

 amount of nourishment which the mother is able to supply. 

 Without exception, in my experience, the opening to the ear 

 appears before the eyes are open. Consequently visual stimuli 

 usually are not disturbing factors in the auditory tests with 

 mice less than sixteen days old. There is also a sudden and 

 marked change in the behavior of the mice during the third 

 week. Whereas, for the first fourteen or eighteen days they 

 are rather quiet and deliberate in their movements when re- 

 moved from the nest, some time in the third week their be- 

 havior suddenly changes and they act as if frightened when 

 taken up by the experimenter. They jump out of his hand, 

 squeak, and sometimes show fight. This is so pronounced 

 that it has attracted my attention many times and I have 

 studied it carefully to determine, if possible, whether it is 

 due to some profound change in the nervous system which 

 thus suddenly increases the sensitiveness of the animal or to 

 the development of the sexual organs. I am inclined to 

 think that it is a nervous phenomenon which is intimately 

 connected with the sexual condition. Within a day or two 

 after it appears the mice usually begin to show auditory re- 

 actions and continue to do so for three to five days. 



I shall now describe the results obtained with a few typical 

 litters. A litter born of Nos. 151 and 152 gave uniformly 

 negative results in all auditory tests up to the fourteenth day. 

 On that day the ears were open, and the following observa- 



