414 JAS. p. HILL. 



vary both iu size and shape. A comparison of fig. 15 with 

 fig. 7 shows at a glance the marked change iu the character 

 of the marginal chorionic ectoderm iu this stage ; for the 

 process of degeneration and absorption which had set iu^ iu 

 the central part of the placental area in Stage C, has now 

 extended nearly to the margin. 



In connection with the disappearance of the chorionic 

 ectoderm at this stage it may be noted that the straggling 

 isolated cells of it which yet persist have become greatly 

 hypertrophied. In some cases they are multinucleated (figs. 

 20 and 21, ch. ect' .), or the single nucleus is also hypertrophied 

 and vesicular (figs. 15 and 19, ch. ect'.). Fig. 20 is worthy of 

 remark as showing how such isolated and hypertrophied cells 

 may be gradually undermined by the ingrowth below them of 

 an allantoic capillary ; the area of contact of the larger cell 

 shown in the figure with the syncytium is in this way greatly 

 reduced, while the smaller cell is entirely separated from the 

 syncytium, and awaits its resorption in isolation in the allanto- 

 chorionic mesenchyme. 



{b) Allantois and Allantoic Placenta. — In its general 

 features the allantois is essentially similar to that of Stage C. 

 The allantoic stalk with its vessels now, however, is just about 

 half as thick again as that of the preceding stage (cf. fig. 23 

 with fig. 10), thus showing that the allantoic circulation has 

 increased considerably in volume. In correlation with this 

 increase in the blood-supply, the capillary network (fig. 22, 

 all. cap.) in the allanto-chorionic mesenchyme is now much 

 more richly developed than in Stage C. 



Now that the chorionic ectoderm has almost wholly dis- 

 appeared, the capillaries are able to attach themselves closely 

 to the syncytial surface. We have already laid emphasis on 

 the characters of the allantoic placental portion of the maternal 

 syncytium, and have pointed out that its surface is both highly 

 irregular and very vascular, and in certain patches exhibits an 

 irregular system of interlacing vascular ridges separated by 

 depressions. With this irregularly ridged and highly vascular 

 surface the allantoic capillaries are in most intimate contact; 



