KMBRYOLOGY OP THE MARSUPIALIA. 9 



the capillaries being on the whole larger. As in that stage, 

 owing to the projection of the superficial capillaries, the 

 syncytial surface presents an irregularly ridged appearance 

 (PL 2, fig. 7, syn. c). 



(6) Syncytium beyond Allantoic Placental Area. — 

 The portion in contact with the vascular omphalopleure is 

 also of the same average thickness as in Stage D, viz. '09 mm. 

 In other respects also the same features are visible. The 

 supei'ficial capillaries are fairly abundant, but on the whole 

 very much smaller than those of the placental area, and not 

 quite so well developed as in Stage D. 



The syncytium in contact with the bilaminar omphalopleure 

 is a thinner and much less regular layer, and less vascular. 



II. FcETAL Membranes. — (a) Chorionic Ectoderm. — 

 Over the placental area usually single, much degenerated, and 

 deeply staining chorionic ectoderm cells are still to be found, 

 but are not numerous. Of the marginal zone of chorionic 

 ectoderm, seen persistent in Stage D, only traces now remain, 

 in the form of isolated and often much altered cells. Occa- 

 sionally such cells are found forming the transition to the 

 ectoderm of the zone of pure chorion (PI. 2, fig. 6, ch.) which 

 exists between the outer margin of the allantois and the 

 vascular omphalopleure (PI. 2, fig. 6, ch. ect.) . The ectoderm 

 cells of this chorionic zone are usually enlarged close to the 

 margin of the placental area, but rapidly diminish in size as 

 they pass outwards. 



(6) Allantois and Allantoic Placenta. — The allantoic 

 stalk is more or less rounded in section, and where it lies 

 enclosed by the umbilical stalk has a diameter of '5 mm. 

 The allantoic canal (fig. 4, all. cl.) is no longer perfectly con- 

 tinuous, and its entodermal lining is only recognisable in 

 places. Curiously enough, the allantoic vessels appear to be 

 smaller than in Stage D, but the difference in size is not so 

 disproportionate as in the case of the vitelline vessels in the 

 two stages. As regards the vesicular portion of the allantois 

 and the allantoic placenta, I have little of importance to add 

 to the description already given of the same parts in Stage 



