126 CALVIN P. STONE 



The appearance of these dissociated elements is not confined to 

 the young animal alone; it is sometimes seen in the adult as well. 

 The difference is one of relative frequency only. 



Following the first period of copulation the young animal is 

 less discriminating in the mounting of other animals than the 

 adult. He mounts without regard for sex or condition of recep- 

 tivity. Here again the difference is largely one of degree char- 

 acterized by persistence in mounting attempts, for the adults 

 also have been observed to mount non-receptive females and 

 males at various times after a period of copulation was brought 

 to an end. 



In two instances young males performed the first sexual act 

 while mounted at the head end of the female and with the fore- 

 limbs clasping the head and shoulders rather than the sides. 

 The pelvic movements were directed toward the face in these 

 cases. 



Licking the penis is, so far as observations in this study go, 

 an invariable element of the complete copulatory act providing 

 vaginal entrance has been effected. In the case of the young 

 male the licking is oftimes omitted at the completion of the first 

 sexual act. This probably, is due to incomplete erection and 

 failure to make vaginal entrance. 



b. The age of functional sexual maturity and the strength 

 of the copulatory impulse. The age at which copulatory ability 

 normally appears has never been determined under sufficiently 

 controlled conditions to establish a reliable norm. Kirkham 

 ('13) reported that a pair of rats bred prior to the age of sixty 

 days. Hewer ('14) considered rats of seventy days of age as 

 sexually mature, for at that time she was able to find spermata- 

 zoa in the epididymis. My own observations show, however, 

 that copulatory ability does not run exactly parallel to produc- 

 tion of spermatazoa. Therefore, her criterion is not a reliable 

 index of the age at which copulatory ability appears. 



In table 12 are given data bearing on the age at which the 

 first sexual act appeared in seventeen normal rats reared under 

 conditions as nearly uniform as possible. The age and number 

 of minutes given to testing each rat prior to the appearance of 



