142 THE PHYSIOLOGY OF TWINNING 



closely into contact and more or less extensive anasto- 

 moses of capillaries, arteries, and veins take place between 

 the two circulations. Four types of vascular inter- 

 communication are distinguished in these twin placentae : 



A. In almost all one-egg twins there occur in the 

 competitive zone twenty or more villous trees which are 

 occupied in common by the circulations of the twins. 

 The arteries of one twin occupy half of such a villous 

 tree and the veins of the other twin occupy the other half. 

 The real connection between the circulations here is 

 through capillaries only. 



B. In addition to the villous transfusion there may 

 exist cases with one or more superficial arterial anas- 

 tomoses. 



C. Instead of arterial, there may be one or more 

 venous anastomoses. 



D. Many placentae show, in addition to villous 

 transfusion, both venous and arterial anastomoses. 



Types A and C are rare; types B and D are frequent. 

 In brief, there are nearly always at least superficial 

 arterial anastomoses, either with or without compensat- 

 ing venous anastomoses. This region of intercommuni- 

 cation between the placental vascular systems constitutes 

 what Schatz calls the third circulation. This third circu- 

 lation has a volume only about one-tenth or even one- 

 twentieth as great as that of the general circulation of 

 one twin. Small in volume as this may be, much of the 

 welfare of the twins hinges upon whether it is symmetrical 

 or asymmetrical. If it is symmetrical with reference to 

 the volume of blood exchanged between the twins, all 

 is well with both twins; but if the arterial contribution 

 of one is greater than that of the other, or the venous 



