DEVELOPMENTAL HAZARDS OF HUMAN TWINS 141 



deformed twins which are interpreted as the result of 

 such an original inverse symmetry. It is interesting to 

 note that symmetry reversal, besides being diagnosed 

 as a definite criterion of one-egg twins, is considered as 

 a hazard of twinning. Further discussion of reversed 

 symmetry is to be found in chapter xii. 



INTERINFLUENCES OF SEPARATE ONE -EGG TWINS 



Human one-egg twins have a common discoid 

 placenta to which are attached the two umbilical cords. 

 As a rule the two cords are symmetrically placed upon 

 the placenta, though there are some noteworthy cases 

 of asymmetrical attachment. Sometimes the two cords 

 are near together in the center of the placenta; sometimes 

 they are on opposite sides and near the margin. There 

 is even some evidence that the attachment of the two 

 cords more or less closely coincides with the right and 

 left sides of the uterus, reminding one of the situation 

 in the armadillo. Whatever may be the point of attach- 

 ment of the two cords on the single placental area, the 

 twins divide this area more or less equally between them. 

 There is opportunity for competition here; for the twins 

 may develop at sKghtly different rates and the one that 

 first develops a placenta is likely to acquire more than 

 its fair share of placental area, and hence more than half 

 of the available nutriment. This may account for a 

 part of the marked size difference between one-egg twins 

 during the middle period of pregnancy. 



A far more important condition leading to interin- 

 fluence arises probably as a direct result of a competition 

 for placental area. In the zone of competition the 

 separate placental circulations of the twins come very 



