DEVELOPMENTAL HAZARDS OF HUMAN TWINS 139 



available area to the complete or partial exclusion of 

 the other. Stockard describes one case which he inter- 

 prets in that way, in which the later egg, failing to 

 gain a good placental attachment, underwent twinning, 

 probably as the result of retarded development. Sub- 

 sequently at about the middle of pregnancy the twin 

 embryos died through a complete shutting off of nutri- 

 tion, while the original single fetus went on to full term. 

 It seems probable then that the main influences exercised 

 by two-egg twins upon each other are the result of com- 

 petition for placental surface. In addition to the extra 

 hazards due to competition, twins seem to fall heir to 

 all of the ordinary hazards met with by single fetuses, 

 such as loosened placenta, twisted and knotted umbiHcal 

 cord, stricture or breaking of umbilical blood vessels, 

 rupture of amnion, loss of amniotic fluid, and the result- 

 ant adhesions. The period of uterine gestation is at 

 best a hazardous one, but, quite in addition to all of the 

 hazards that are met by single embryos and those that 

 are shared also by two-egg twins, there are. certain very 

 serious special dangers that fall upon one-egg twins by 

 reason of their close genetic relationship. 



THE SPECIAL HAZARDS OF ONE-EGG TWINS 



For the data herewith presented I am indebted to the 

 numerous contributions to our knowledge of the develop- 

 mental physiology of one-egg twins by Friedrich Schatz. 

 These papers all appeared in the Archiv fur Gynaekolo- 

 gie between the years. 1882 and 1900. This author had 

 the advantage of studying abundant material well pre- 

 served and adequately injected. No question seems to 

 exist in his mind as to the occurrence of human one-egg 



