4 JAS. P. HILL. 



mesially, forms a single loop upon itself, hence it passes over 

 tlie right side of the embryo to reach the centre of the allan- 

 toic placental area (fig. 1, jpl. a.), a portion of which is visible 

 in the figure, just behind the back of the embryo. In fig. 2, 

 PI. 1, the embryo has been removed to expose the placental 

 area. In both uteri the placental area is roughly oval in 

 shape, as in Stage D, and situated on the mesial side of the 

 uterus. It is bound by a continuous ridge-like thickening of 

 the mucosa, so that the area itself appears depressed below 

 the general level of the inner surface of the uterus. Through 

 the thin walls of the vesicular portion of the allantois the 

 irregularly ridged surface of the placental syncytium could 

 be made out. The inner surface of the mucosa of the rest of 

 the uterus also presents a ridged appearance. 



The placental area oi the right uterus measures approxi- 

 mately 8 mm. in length by 6 mm. in breadth ; that of the left 

 uterus was somewhat larger, measuring 6*5 mm. by 9 mm. 

 The allantoic stalk has now a length of about 16 mm., i. e. just 

 double the length of the stalk in the preceding Stage D, and 

 it is now also just about double as thick as in that stage. 



As in previous stages, the outer or placental surface of the 

 vesicular part of the allantois is found not to occupy the 

 entire extent of the original chorionic area, but there is left 

 outside its margin, i. e. outside the margin of the placental 

 area, a narrow annular zone of persistent pure chorion, which 

 surrounds the placental area, and which is attached on the 

 one side around the margin of that area, and on the other 

 passes into the vascular omphalopleure, along the line of 

 junction of the latter with the yolk-sac splanchnopleure 

 (PL 2, fig. 6, c/^.). 



The placental area of the present stage as seen in surface 

 view is essentially similar to that of Stage D. The allantoic 

 stalk (PI. 1, fig. 2, all. s.) is seen to join the inner surface of 

 the flattened vesicular portion of the allantois near its centre. 

 In the stalk run the two allantoic arteries and the single vein. 

 The latter is situated above the two arteries, and is hence not 

 visible in the figure. At the distal end of the stalk the 



