378 HELEN DEAN KING. 







rats will, perhaps, make the scheme somewhat clearer. 6B . 2 7 

 for example, denotes a female belonging to the sixth generation 

 of rats that descended from female B. She was a member of the 

 second litter borne by her mother, and her individual number in 

 the series of rats belonging to the second litters of the sixth 

 generation was 57. 



It is part of the daily routine work in the rat colony to ex- 

 amine the cages containing breeding animals and to record the 

 births of litters. The birth of a litter is ascribed to the day on 

 which the litter is discovered, unless it is evident from the 

 appearance of the young rats that their birth had been over- 

 looked at a previous visit. In the latter case an approximate 

 date is assigned for the birth of the litter, and on the record 

 cards this date is followed by an interrogation mark. As the 

 rat colony is usually visited in the morning it is very probable 

 that, in the majority of cases, a litter is discovered shortly after 

 its birth, since observations made at various times indicate that 

 in the rat parturition occurs most frequently in the morning, 

 although it may take place at any time during the day. All of 

 the dates of the births of litters given in the present paper are 

 correct within a few hours, since no cases have been included 

 in which a litter was obviously more than a day old when dis- 

 covered. 



As a rule the male is removed from the breeding cage just 

 before, or immediately after, the birth of a litter in order to 

 guard against the possibility of his destroying the young rats. 

 This is seemingly a needless precaution, since infanticide is 

 comparatively rare in albino rats, although it is very common 

 among brown rats kept in captivity according to the observations 

 of Miller ('i i). When young albino rats are destroyed it is done, 

 as a general thing, by the female, either because her nest is 

 disturbed during parturition or because she is not in a physical 

 condition to suckle her offspring and is annoyed by their attempts 

 to obtain food. In only one case, as yet, have I found a male 

 eating the young, and in this instance the female was equally 

 as guilty as her mate. 



The anomalies in gestation that are noted in the present paper 

 are of three kinds: (i) Prolongation of the period of gestation; 

 (2) cases of superfecundation ; (3) cases of superfcetation. 



