388 H. H. Newman and J. T. Patterson. 



The tip of one of the branches of an arborescent villus is shown 

 in fig. 11. The terminal knot of cells is seen to be practically, 

 naked, while farther down in the villus are shown blood vessels 

 containing nucleated blood cells. 



Although the formation of villi occurs at first over almost the 

 entire area of the Trager, somewhat more advanced stages clearly 

 show the beginning of a tendency for them to become restricted 

 into four distinct patches near the boundary line between the 

 Trager and yolk-sa*c and around the umbilicus of each embryo. 

 The villi of other regions cease to grow and remain short, as in 

 fig. 3, even flattening down into small rounded prominences 

 which probably serve no nutritive function. Small patches of 

 these flattened villi are scattered over the central area of the Tra- 

 ger as well as between the newly formed placental discs of the 

 various embryos. 



During this period the Trager area of the vesicle has been grow- 

 ing more rapidly than the yolk-sac region, the boundary between 

 the two remaining at all times definitely marked. In fig. 3 is 

 shown semidiagrammatically the conditions in vesicle 11 in which 

 four discoid placentae are clearly marked off from the surrounding 

 areas of scattering flat villi. At this stage the placentation is 

 obviously discoid for each embryo. 



In vesicle 14, (fig. 4) a decided change is in evidence. The four 

 formerly quite separate discs have undergone a considerable in- 

 crease in diameter and have come into very intimate contact 

 along contiguous margins. This fusion is more complete between 

 the placentae of embryos I and II and between III and IV than 

 between II and III or I and IV. The significance of tiais is dis- 

 cussed later. A further change is seen in that the villous margin 

 of the Trager region has overgrown the yolk-sac region (not fusing 

 at this time with the latter) and has extended the placental area 

 of the vesicle along the sides of the cervix cavity as far as the os 

 uteri. Judging by the size and abundance of the arborescent villi 

 in this placental annex it seems obvious that it plays the jDrinci- 

 pal nutritive role at this period. One might compare this over- 

 growing fringe of branching villi to the cricoid placenta of Dasy- 

 pus sexcinctus. 



