390 ■ H. H. Newman and J. T. Patterson. 



The fundus end of the vesicle is still villous to some extent, but 

 the villi are so small and scattered as to interfere only slightly with 

 the transparency of the membrane. One can readily view the em- 

 bryos in situ through this end. Subsequently the villi of this region 

 disappear entirely with the exception of occasional small tufts 

 that might readily be overlooked. In several vesicles (nos. 116 

 and 117) this region was seen to be four-lobed owing to the pre- 

 sence of two thickened bands of tissue crossing each other at 

 right angles (figs. 37). These may indicate a demarkation of the 

 several embryonic primordia earlier than that seen in the differ 

 entiation of the embryos themselves. 



Stages intermediate between that shown in fig. 4 and the defini- 

 tive condition can best be shown by a series of photographs. 



Fig. 34 shows a somewhat older vesicle, in which the area at 

 the fundus end is seen to be smooth and almost free of villi. 

 The lobing of the composite zonary placenta is only slightly 

 marked. 



In fig. 35 is shown the cervix end of a stage slightly more ad- 

 vanced than the preceding one. The heavy coating of arbores- 

 cent villi is seen to cover the entire cervix end of the vesicle with 

 the exception of the small area that lies across the mouth of the 

 uterus. 



The dorsal surface of another vesicle, approximately of the same 

 age as the last, is seen in fig. 36. The vesicle is attached to the 

 shrunken cervix of the uterus. Here is evidenced the tendency 

 on the part of the composite zonary placenta to divide into two 

 double lateral discs. The deep notch occurs between the placental 

 areas of embryos II and III. The small lobe (d. b.) is destined to 

 persist as a bridge between the two lateral discs. 



Two farther steps in the development of the definitive placenta 

 are seen in figs. 38 and 39. The vesicle has grown to be several 

 times the size of that shown in fig. 34. Coincident with this great 

 increase in surface the villi in the composite zone have increased 

 ,in functional importance while those that previously over- 

 grew the yolk-sac region of the vesicle have degenerated, leaving 

 a membraneous area at the cervix pole, which in time becomes as 

 large or even larger than that at the other end of the vesicle. 



