64 THE BIOLOGY OF TWINS 



of arrangements; but, after a careful examination with 

 the idea of finding out just how definitely fixed this kind 

 of orientation is, I am surprised to discover a consider- 

 able range of variability in fetal relations. 



One not infrequently finds the point of attachment 

 of fetuses II or III much nearer the mid-dorsal fine than 

 that shown in Fig. 20; similarly, fetuses II or IV may 

 be much nearer the mid-ventral line. In such cases 

 the placental vessels of the fetus which is attached 

 near the edge of one or other placental disk invade both 

 right and left placentae. This may be readily shown 

 by injections. 



Cases of this sort point to the conclusion that the 

 apparent bilaterality of the blastodermic vesicle is 

 simply a semblance of bilaterality which has been 

 imposed upon a more fundamental symmetry of the 

 vesicle itself by the uterine blood supply. The two 

 heavy placental disks that develop respectively on the 

 right and left sides of the uterus occupy their positions 

 because it is in those places that placental growth is 

 favored by the proximity of the uterine bilateral blood 

 trunks. The embryos are usually paired so that one 

 pair draws nutriment from one placental disk and the 

 other pair from the other disk; but there are many 

 exceptions, as has already been indicated. 



In previous papers I have laid much stress on the 

 paired arrangement of embryos and have been inclined 

 to underemphasize those cases in which pairing was 

 absent; a few cases of non-pairing are accordingly 

 cited. In some advanced sets it is noteworthy that, 

 when the ventral bridge between the two lateral placental 

 areas is severed, three fetuses appear to be attached 



