CHAPTER VII 

 VARIATION AND HEREDITY IN TWINS 



A. VARIATION AND HEREDITY IN ARMADILLO 

 QUADRUPLETS 



The known laws of heredity and theories as to the 

 mechanism of inheritance are at present apphcable 

 only to those usual reproductive modes which involve 

 the development of but a single offspring from a fertilized 

 egg. A new situation appears when an egg gives rise 

 to two or more offspring, and new modes of inheritance 

 are involved. 



If polyembryonic offspring were absolutely identical 

 within a monozygotic set, the problem would be to 

 account for the identity. Since they are not identical, 

 but show a definite variability within a set, the problem 

 is to account for the differences that exist among them. 



Only in one character are the members of a poly- 

 embryonic set always identical: they are always of the 

 same sex. In all other respects intra-set differences of a 

 more or less radical character exist. Some of these 

 differences are purely extrinsic in nature or origin; 

 others are strictly intrinsic or due to factors inherited 

 from the parents. 



The following problems connected with heredity in 

 monozygotic quadruplets present themselves for solu- 

 tion: (a) What kinds of character are inherited? (b) 

 Which ones are inherited by all and which are subject 

 to unequal distribution among the fetuses of different 



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