88 The Dancing Mouse 



obvious on the twenty-second day, but thereafter they be- 

 came indefinite. 



Still another litter, which consisted of one female and four 

 males, began to exhibit the quick, jerky movements, already 

 mentioned, on the fourteenth day. On the morning of the 

 fifteenth day three members of the litter definitely reacted to 

 the tone of the steel bar, and also to the hammer blow when 

 the bar was held tightly in the hand of the experimenter. 

 My observations were verified by another experimenter. 

 Two individuals which appeared to be very sensitive were 

 selected for special tests. Their reactions were obvious on 

 the sixteenth, seventeenth, and eighteenth days ; on the nine- 

 teenth day they were indefinite, and on the twentieth none 

 could be detected. Some individuals of this litter certainly 

 had the ability to hear for at least five days. 



A sixth litter of four females and two males first gave indi- 

 cations of the change in behavior which by this time I had 

 come to interpret as a sign of the approach of the period of 

 auditory sensitiveness, on the seventeenth day. I had tested 

 them almost every day previous to this time without obtaining 

 evidence of hearing. The tests with the steel bar and the 

 Galton whistle were continued each day until the end of the 

 fourth week without positive results. To all appearances 

 the individuals of this litter were unable to hear at any time 

 during the first month of life. 



Practically the same results were obtained with another 

 litter of four females. The change in their behavior was 

 obvious on -the eighteenth day, but at no time during the 

 first month did they give any satisfactory indications of 

 hearing. 



In the accompanying table, I have presented in condensed 

 form the results of my auditory tests in the case of twelve 

 litters of young dancers. 



