I50 THE PHYSIOLOGY OF TWINNING 



a) slight 1 



b) moderate > asymmetry in the third circulation 



c) great J 



,. . . r . • 1 •!• 1 . f and simultaneous 



d) interruption of current in umbi ica artery _^^ asymmetry of 



e) mterruption of current m umbilical vein | ^^^ ^^^^^ circulation 

 /) primary heart-death in one twin (with pronounced asymmetry 



of third circulation) 

 g) destruction of one-half of the placenta 



The effects upon the favored twin (the one that acquires 

 an excess of blood) differ in every case from those upon 

 the injured twin (the one that suffers a diminution of 

 blood). In all there would appear to be fifty-six per- 

 mutations and combinations of the possible variables in 

 physiological interrelations between one-egg twins. In 

 nearly half of the combinations either the favored, or 

 rarely the injured, twin shows no noticeable departure 

 from the normal; but it seems to be quite probable that 

 many of these apparently normal individuals suffer 

 physiologically so as to acquire certain functional heart 

 weaknesses or disorders, and it may well be that the very 

 common difference in vigor or vivacity between one-egg 

 twins is the result of an intra-uterine injury of the same 

 kind but of lesser degree than those that are clearly 

 recognized. The most serious results are found in con- 

 nection with placenta- type D and the vascular condi- 

 tions c, d, e for both twins, and / and g for the injured 

 twin. Moderately serious results appear in connection 

 with placenta-type A, and conditions b, c, d, e, for the 

 favored twin, and conditions c, d, e, for the injured one; 

 and in placenta-type B, and conditions b, c, d, e, for the 

 favored twin, and c, d, e for the injured one. So few 

 examples of placenta- type C occur that the situation is 

 less clear and may be omitted from the present discussion. 



