408 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



TABLE 10. 



Correlation between the Nipples of the Right and Left Sides of the 

 Female Pigs in 1000 Litters, 2946 Individuals. 



Regular and irregular playis. The proportions of pigs in which the 



nipples are arranged on the regular and the irregular plans (see p. 402) 



as seen in the whole population and in the sexes separately are given 



in Table 11. 



TABLE 11. 



Proportions in 1000 Litters of the Whole Population, of Males, and 



OF Females, that Exhibited the Regular and the Irregular 



Arrangement of the Nipples. 



From Table 11 it is clear that in the population as a whole the regular 

 arrangement occurs more frequently than the irregular one in the 

 ratio of about 6 to 4. The proportion of instances of the regular 

 arrangement is very slightly greater in the males than in the females. 



The details of the various combinations of nipples under the two 

 general plans, regular and irregular, are given for the population as a 

 whole and for the sexes separately in Table 12. 



What we have called the regular plan in the arrangement of the 

 nipples has led to the assumption on the part of some investigators 

 that the mammary glands are segmentally arranged, an opinion 

 especially advocated by Williams (1891). But the conditions pre- 

 sented by many of the lower mammals do not seem to us to favor this 



