ACTION OF SEX HORMONES IN FOETAL LIFE 395 



The arterial anastomosis is a single strong vessel, the relation- 

 ships of which are shown clearly in the figure and require no 

 further description. The stage is much more advanced than the 

 preceding case, and the cotyledons are much more developed. 

 Most of the arterial branches are distributed directly to the coty- 

 ledons. The venous anastomosis is much less viable than the 

 arterial; macroscopically it consists exclusively of a connection 

 between the two veins of one cotyledon (2, fig. 4) one of which 

 returns to the male side and the other to the side of the free- 

 martin. This is the only cotyledon that appears to be connected 

 with the umbilical veins of both sides; therefore any other 

 venous anastomisis must be through the capillary circulation 

 of the extra-cotyledonary chorion if it exists. 



The circulation in this case must be according to the same 

 principles as in the preceding: whenever the arterial pressure is 

 higher on one side than the other blood must be distributed from 

 the side of higher pressure to that of the lower pressure; it will 

 thus reach the veins and the foetus of the opposite side; varia- 

 tions in pressure on the two sides must constantly occur, if there 

 is any difference in the time of occurrence of systole and diastole 

 of the twin hearts. The blood of the twins must therefore in- 

 termingle intimately, and internal secretions of either must reach 

 the other. 



These cases adequately illustrate the time and nature of the 

 vascular anastomosis; we may therefore turn to the question of 

 duration of the intermingling of the blood during foetal life. 



We have seen that the vascular anastomosis probably begins at 

 the stage of about 19-20 mm. The two cases we have consid- 

 ered in detail indicate a strengthening of the arterial anastomo- 

 sis, and a weakening of the venous anastomosis after a certain 

 stage as development proceeds. This is to be expected because 

 the arterial flow is stronger and toward the center, whereas the 

 venous flow is slower and away from the center. The circula- 

 tion itself tends therefore to strengthen any primitive arterial 

 connection, and to diminish relatively any venous connection. 

 Moreover as development proceeds the cotyledons increase in 

 size, and the intercotyledonary circulation in the chorion becomes 



