VARIATION AND HEREDITY IN TWINS 149 



general factor for doubling; (2) several minor factors 

 determining in double scutes the particular kind of 

 doubling; (3) a factor determining extent of doubling, 

 whether one element or many are involved in doubling; 

 (4) a localizing factor, determining more or less precisely 

 the exact position of the doubling in the individual. 

 It may be supposed by way of illustration that a mother 

 has the doubhng factor, together with the extension 

 factor; the father lacks the doubhng and extension 

 factor, but has the locahzing factor. The factors could 

 be variously distributed among the four fetuses so as to 

 give all sorts of combinations. It seems quite evident, 

 then, that doubling is a complex matter, not at all a 

 simple unit factor, and it must therefore be explained 

 on the basis of the interaction of several factors. 



Somatic and germinal segregation. — That band and 

 scute doubling are definitely heritable is proved by the 

 fact that doubling is always present in some form in the 

 offspring of mothers that show doubling. The real 

 problem is to find a mechanism to explain why it so 

 often happens that some of the fetuses derived from 

 a single egg exhibit the character and others do not. 

 In monozygotic quadruplets we should expect the same 

 genetic constitution in each individual unless there 

 exists some segregative mechanism, resulting during 

 early ontogeny in an irregular distribution of the factors 

 responsible for doubling. It has been suggested that 

 the real differentiating factors are environmental; 

 but the only environmental differences conceivable for 

 monochorial quadruplets are nutritional differences 

 due to more or less extensive placentation. It can 

 be shown, however, that nutritional differences, so 



