SPAN OF LIFE — PUBERTY — PERIOD OF GESTATION 21 



Slonaker ('12, '12 a) 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 unexpected result. 



3. Puberty — Ovulation and Menopause. Sexual maturity as 

 indicated by the structure of the gonads usually occurs in both 

 males and females at the age of about two months or less. 



According to our observations, puberty in the female may oc- 

 cur at 60-70 days after birth — although the females usually be- 

 gin to breed at 90-100 days. On the other hand there are oc- 

 casional instances of remarkable precocity. In the breeding Al- 

 bino it is found that impregnation most readily follows 1^4 daj-s 

 after a litter has been cast. This accords with the time of ovu- 

 lation (Kirkham, '10; Sobotta and Burckhard, '10; Kirkham 

 and Burr, '13). During the breeding season of the female ovu- 

 lation occurs at intervals of about three weeks, but only from 

 April to October do the females regularly ovulate 20-48 hours 

 after parturition (Kirkham and Burr, '13). The menopause 

 commonly appears at the age of 15-18 months, but King (MS., 

 '15) reports a female 22 months old — crossed with a male of 

 like age — giving birth to a Utter of one. 



4. Period of gestation. The gestation period of the non-lac- 

 tating albino rat is usually stated to be about 21-22 days. In 

 the cases where the gestation period has been exactly recorded in 

 our colony the exact time of copulation and of birth having been 

 observed, Stotsenburg (MS '14) has found it to be from 21 days 

 and 15 hours to 22 days and 16 hours. 



