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THE PHYSIOLOGY OF TWINNING 



THE INFLUENCE OF ASYMMETRY OF THE THIRD CIRCULATION UPON 

 THE DEVELOPMENT OF THE FAVORED TWIN 



The primary effect of excess blood is naturally pleth- 

 ora or an overfulness of the vessels. The consequences 



Fig. 50. — The common placenta and the hearts of a typical pair of 

 one-egg human twin fetuses. Note the symmetrical arrangement of the 

 umbilical cords, the superficial intertwin anastomoses of placental blood 

 vessels within the dotted areas. The arteries are stippled on the left, 

 unshaded on the right. The veins are cross-hatched on the left and solid 

 black on the right. The heart of the left twin (below) is greatly reduced, 

 that of the right is decidedly enlarged. The left-hand twin is the so- 

 called "injured" one and the right-hand twin, the so-called "favored" 

 one. See text for further explanation. (After Schatz.) 



