ACTION OF SEX HORMONES IN FOETAL LIFE 393 



rarely this does not occur either because the chorions fail to fuse 

 (cases 40 and 56) or because a slender connection is not vascu- 

 larized (case 46). The significance of the exceptions is very 

 great. 



The nature and extent of the vascular connections may now be 

 illustrated by a detailed study of two cases. 1. Case no. 33 

 Males, 10.5 cm. long (figs. 2 and 3). The entire arterial system of 

 both chorions was injected from one umbilical artery of one 

 partner; the mass easily passed the constriction between the 

 two halves of the chorion, and penetrated even into the umbili- 

 cal arteries of the other; every cotyledon was injected on both 

 sides. The venous system was also injected from one of the 

 umbilical veins of the same specimen; the injection mass also 

 passed the constriction far into the chorion of the opposite 

 side, but the blood present in the veins prevented as complete 

 an injection of the veins as of the arteries. 



The two umbilical arteries of each foetus have a cross connec- 

 tion at the distal end of the umbilical cord, so that an injection 

 from one artery outwards flows both centrally and distally. The 

 two veins lack such an anastomosis. 



The arterial anastomosis (fig. 3). The main artery from the 

 right of the drawing divides in three branches 1, 2, and 3 as it 

 approaches the center. Branch 3 need not be farther considered 

 as it does not anastomose with the opposite side. Branch 1 can 

 be followed directly through into communication with the arterial 

 system of the other side, branch 2 has a strong anastomosis with 

 the through trunk 1-1 (at 1-2), but branches for the most part 

 within its own venous territory. The side branches of the 

 through trunk 1-1 are of considerable interest, inasmuch as some 

 are oriented in the direction of the blood flow from the right, and 

 others from the left. Thus following the trunk from the right 

 the first branches that we meet are directed against the blood 

 stream coming from this direction (la); immediately after pass- 

 ing the anastomosis {1-2) we meet a branch Ih, directed with 

 the blood flow from the right; the next two branches, Ic and Id, 

 are directed similarly, but the large branch le immediately 

 beyond has the reverse orientation. If we suppose the blood 



