FECUNDITY AND WEIGHT AT BIRTH 23 



Kolazy (71) reports litters consisting of 5-17 young. Crampe 

 ('84) records for 2503 young represented by 394 litters, an av- 

 erage of 6.3 per Utter. From 1911-1913, 275 Utters (1928 in- 

 dividuals) in our colony gave an average of 7.0 individuals per 

 Utter, and in 1914, 814 litters (5691 individuals) gave an average 

 of 6.99 individuals per Utter. Litter size does not appear to be 

 influenced by season (King and Stotsenburg, '15). 



Under certain food conditions the size of the Utters is much 

 modified. When an exclusive diet of ox flesh is given to Albi- 

 nos — 2-4: months of age at the beginning of the experiment — 

 and these are compared with control rats fed on bread and milk, 

 Chalmers Watson ('06 a) finds in the meat fed Albinos preg- 

 nancy less frequent, the weight of the mammae less, and the 

 average number of young in a Utter, as well as the average weight 

 of the young, both smaUer than in the controls. Such an ex- 

 clusive meat diet is therefore unfavorable both for breeding and 

 for early growth. On the other hand, Stotsenburg (MS '15) 

 found that mothers fed on a table scrap diet produced a larger 

 number of fetuses than those fed on bread and milk. 



As to the size of the Utters at different periods in the life of 

 the female, there are a few observations. Lloyd ('09 a) in his 

 studies on two strains of the house rat, pubUshed tables which 

 he interpreted to mean that the number of individuals in a Utter 

 was independent of the body weight of the mother. Pearson 

 ('10) however was able to show from Lloyd's data that in both 

 groups the number in a Utter increased with the body weight of 

 the mother. 



It seems probable however that the heavier rats were also 

 older, as Pearson suggests, and that the proper interpretation of 

 the increase in the size of the litter is to relate it with the age of 

 the mother. In these groups none of the animals were beyond the 

 prime of life and hence the explanation is very probably correct. 



There is now available some detailed information on the rela- 

 tion between the weight and age of the mother and the charac- 

 ters of the young. 



A study of 11 litters of common albino rats containing 91 

 young bred by King (MS '15) at The Wistar Institute, gives 



