18 BIOLOGY 



So far as this assumption has been tested it appears to be a 

 useful approximation. 



Slonaker ('12, '12a) working at Leland Stanford University 

 under the favorable climatic conditions of California, has made 

 some direct tests. 



Four albino rats living in revolving cages attained an average 

 age of 29.5 months, while three control animals reared in sta- 

 tionary cages, but under conditions otherwise similar, attained 

 an average age of 40.3 months. In all these cases, death was 

 reported as due to 'old age.' 



The average age of these seven individuals was about 34 

 months, while the greatest age, attained by one of the controls, 

 was 45 months. The three controls all lived longer than any 

 of the four in the revolving cages. It appears from this that 

 living in the revolving cage shortened the span of life — an un- 

 expected result. 



The oldest rat reported by Slonaker would be equivalent in 

 age to a man of 113 years. 



During the last few years the oldest rats of both sexes in the 

 Institute colony have attained three years and somewhat over — 

 and with the present conditions it is probable that still greater 

 ages will be reached occasionally.^ 



3. Puberty — ovulation — oestrous cycle — menopause. Sexual ma- 

 turity, as indicated by the structure of the gonads, usually oc- 

 curs in both males and females at the age of about two months or 

 less. 



According to Long and Evans ('22) the vagina opens at about 

 72 days (range 34-109 days) and ovulation begins at about 77 

 days (range 45-147 days). These observations are based on 

 200 rats. 



A tendency of the females to breed at an earlier age than they 

 did in 1915 has been noted in The Institute colony and the same 

 has been reported from other laboratories. In a specially well 



^ Instances of unusual longevity occur. Thus an albino used for show pur- 

 poses was kept by Mr. Harris for 4 years and 8 months and was assumed to have 

 been about a year old when purchased in Shanghai in 1913. (Archives of The 

 Wistar Institute.) 



