26 



BIOLOGY 



of King. The general conclusion which King reaches is that 

 increasing weight or increasing age of the mother (the two being 

 correlated) give a heavier birth weight, while the increase in the 

 number in a litter tends to diminish the individual birth weight. 

 There is to be observed also a diminution in birth weight in 

 those litters born of mothers below the standard in size, or suffer- 

 ing from infectious disease. With the larger material just men- 

 tioned, it is also possible for King and Stotsenburg ('15) to show 

 a modification of the birth weight in relation to the place of the 

 litter in the series of litters born by a given female, see table 7. 



TABLE 7 



Showing the sex ratios and average number oj young in 75 litters of stock albino 

 rats. Data arranged according to the position of the litters in the litter series 



The observations indicate that the number of individuals in 

 the litter generally increases from the first to the second litter, 

 and after that decreases. These results would quite accord 

 with Crampe's conclusions. According to Crampe ('84) the 

 second litter of albino rats is the best. The majority of albino 

 females do not produce more than four or five litters. 



7. Recognition of sex. The recognition of sex through ex- 

 ternal characters in the young rat has been studied by Jackson 

 ('12). He finds in brief that the male, as contrasted with the 

 female, may be recognized by (1) The larger size of the genital 

 papilla; (2) the greater ano-genital distance (see table 8); (3) 

 the absence of clearly marked nipples. (This test is applicable 

 only up to the age of 16 days, i.e., before the development of 

 hair on the ventral surface.) (4) Small extent of the bare area 

 just ventral to the anus (test applicable only after the 16th day). 



