DETEBMINATION OF AGE 



17 



In observations on activity in other laboratories the superior 

 performance of the female has generally appeared. Dr. Wang 

 ('23) has pointed out that activity in the female is rhythmic 

 and correlated with the oestrous cycle, showing a maximum at 

 oestrus. 



TABLE 1 



Total number of miles run during life 



Determination of age. To determine the approximate age of 

 a rat when the date of conception or of birth is not known, there 

 are a few changes and events which are useful. The references 

 for these data will be found at the appropriate places in the 

 text. 



Before birth: 



Eyelids of fetus fuse at the 17th day 



After birth: 



Ears open at 2^-3§ days 



Incisors erupt at 8-10 days 



Eyes open at 14-17 days 



Hair obscures genitalia 16 days 



1st molar erupts 19 days 



2nd molar erupts 21 days 



3rd molar erupts 35 days 



Testes descend 40 days 



Vagina opens 72 days 



The menopause occurs at 15-18 months 



In rats more than a year old the angle of the nasals with 

 the frontals increases, giving a "Roman nose" effect. Under 

 unusual care and feeding the events preceding puberty tend to 

 come earlier and the changes characteristic of old age are de- 

 ferred. 



2. Span of life. On the assumption that dating from birth, 

 the life span of the albino rat is three years, then such a rat 

 may be regarded as corresponding to a man of ninety years. 



