132 THE PHYSIOLOGY OF TWINNING 



THE CAUSES OF TWINNING IN MAN 



The most nearly direct evidence bearing on the cause 

 of monozygotic twinning in man is derived from certain 

 data presented by Arey (19226), already referred to 

 above. He has shown that monochorial twins are many 

 times as numerous in the Fallopian tubes as in the uterus. 

 The tubes are far from being a normal locality for the 

 placentation of the embryx) and there is reason to believe 

 that even the makeshift placentation that does take 

 place is greatly belated. If this assumption be warranted 

 we have a situation quite similar to the ''period of 

 quiescence" in the armadillo, and the consequence would 

 be the same: partial loss of axiate organization and 

 a physiological isolation of two secondary apical points 

 or points of gastrulation. 



Thus we might be able to account for monochorial 

 twinning in the Fallopian tubes, but we would still have 

 to explain uterine monochorial twinning. The nearest 

 approach to direct evidence of the causes of uterine 

 monochorial twinning is furnished by Stockard (192 1). 

 A case of triplets came to his attention in which one 

 individual was born a normal female baby. 



After delivering the child the physician, Dr. Erdwurm, noted 

 that a second chorionic sac ruptured and discharged its fluid. 

 Later two dead twin female fetuses were delivered. These lay- 

 in a common amnion with their umbilical cords twisted around 

 one another in such a way that they had probably cut off both 

 blood connections. 



In further comment on this case, Stockard goes on to say: 



My interpretation of this triplet condition is as follows: 

 The mother Uberated from the ovary two eggs, both of which 

 became fertilized and began development. One became implanted 



