CHAPTER X 



DEVELOPMENTAL HAZARDS OF HUMAN TWINS 



SEPARATE TWINS 



ON THE INFLUENCES WHICH TWINS, ESPECIALLY ONE-EGG TWINS, 

 EXERT UPON EACH OTHER IN THE UTERUS 



A popular impression prevails that in human twins 

 one is usually stronger and more vigorous than the other. 

 Observations of the writer and of others who have inter- 

 ested themselves in these matters tend to bear out this 

 impression. Even in the case of so-called duplicate or 

 identical twins, the products of a single egg, there is 

 nearly always a more vigorous twin who is the dominant 

 member of the combination. There is also a somewhat 

 vaguely expressed feeling among families in which twin- 

 ning has occurred that one twin has in some way drawn 

 upon the vitality of the other or has inherited more than 

 his fair share of certain essential quahties, leaving the 

 other somewhat depleted in energy and vigor. A more 

 definite form of this type of idea, to wit, that one twin 

 is commonly sterile, has come to me several times of 

 late. This idea may have had its origin in the free- 

 martin situation among cattle, where a female calf born 

 twin to a male is nearly always sterile. The possibility 

 that human freemartins may occur has never been 

 adequately affirmed nor denied. 



It is my belief that these popular impressions are not 

 without foundation. There is abundant evidence, espe- 

 cially in the case of one-egg twins, that, as the direct 



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