DEVELOPMENTAL HAZARDS OF HUMAN TWINS 143 



contribution of one is less than that of the other, a serious 

 situation is sure to arise and the degree of seriousness 

 depends on the degree of asymmetry. Which of the 

 twins in any case is the more damaged depends upon the 

 particular conformation of the asymmetry. The usual 

 result of placental anastomosis is that the amount of 

 blood which flows from the first twin to the second is 

 not entirely the same as that which flows from the second 

 to the first. There therefore exists in most twin placentae 

 a dynamic asymmetry of the third circulation which is 

 not equalized by venous anastomoses and must there- 

 fore be equalized by functional adjustments in the bodies 

 of the twins. 



Figures 50 and 51 (pp. 144, 145) represent typical 

 placental relations in separate one-egg twins. In both 

 there is a decided asymmetry. In Figure 50 the twin 

 A is favored by the circulation and in Figure 51 the 

 twin B is favored. 



Although the word ''favored" has been used we must 

 understand that this term is merely relative, for it would 

 be much better for both twins if no anastomoses of their 

 placental circulations occurred or if the balance of 

 exchange were equalized within the placental circulation 

 itself. It is somewhat more immediately harmful for 

 a twin to be robbed of part of its blood than to gain a 

 constant access of blood from the other twin; but too 

 much blood is in the end decidedly harmful: If we may 

 speak of the twin which gains additional blood through 

 the asymmetry of the third circulation as the favored 

 twin and the twin which loses blood as the injured twin, 

 we may discuss separately the effects of these disturb- 

 ances in the two kinds of twins. 



