394 FRANK R. LILLIE 



flow to come from the left leis directed with the current, Id, 

 Ic, and lb against it, etc. The orientation of these branches 

 seems to indicate an alternation in direction of flow in the main 

 trunk, as is to be expected ^\dth a beating heart at each end of 

 it. It is significant that this is the only place in the arterial 

 system of the membranes where reversal of orientation of 

 branches is found in the course of a single trunk. 



The venous anastomoses. The venous anastomoses are two in 

 number (4 and 6 in figure 3). The larger one 4 was on the 

 opposite side from which the drawing was made, and is there- 

 fore represented as a broken line; 5 comes from the same main 

 venous stem. There is no reversal of orientation of side 

 branches. 



The circulation. It is obvious that any arterial blood that is 

 pumped by either foetus into the capillary system situated be- 

 yond about the line A-B will be taken up by the venous system 

 to the other foetus. There must therefore be a constant inter- 

 change of blood between the two foetuses, which, considering 

 the size of the arterial intercommunications, must be very con- 

 siderable. The venous anastomoses are not significant for the 

 intermingling of the two circulations. The direction of flow 

 along the main arterial trunk (1-1) mil depend on the blood 

 pressure on the two sides. If for any reason an excess of blood 

 is received by one of the two foetuses, this will have a tendency 

 to raise the blood pressure on that side and thus to equalize the 

 distribution. There is of course the possibility that the beat 

 may alternate on the two sides, but nothing is known of this, 

 and the effect of such an arrangement would not be easily 

 deduced. 



2. Case no. 47. cT 22.75 cm. and $^22.25 cm. (fig. 4). Ar- 

 teries injected yellow; veins blue. The injection was made first 

 into an umbilical artery and vein of the male. The arterial 

 injection flowed regularly into the opposite chorion and through 

 to the free-martin; the venous injection also flowed into the op- 

 posite chorion, but not so freely. The injection was then com- 

 pleted from an umbilical artery and vein of the free-martin in 

 order to fill the vessels on this side more completely. 



