TlfB PLACENTATION OV PK RAM ELKS. 413 



but is soinewliat thicker (averaging '12 mm.) than in that 

 stage. It is also much more highly vascular (cf., e. g., fig. 15 

 with fig. 7). The capillaries entering between the syncytial 

 lobules ramify in great abundance at and just beneath the 

 surface, where they form a rich network. These superficial 

 capillaries are large, and vary considerably in diameter, averaging 

 about *01 mm., some attaining a width of '028 mm. The 

 surface of the syncytium is here by no means smooth, but 

 owing to the bulging of the capillaries on the surface it pre- 

 sents an irregularly ridged structure (figs. 16 and 17, pi. syn.). 



{b) Syncytium beyond Allantoic Placental Area. — 

 That portion of it in contact with the vascular omphalopleure 

 forms a fairly uniform layer, averaging '09 mm. in diameter. 

 It is thus nearly double as thick as the corresponding portion 

 of the syncytium in Stage C, and is also rather more vascular. 

 These facts are significant in view of what has been said above 

 on the high functional importance of the yolk-sac placenta 

 prior to the complete formation of the allantoic. 



The superficial capillaries of this portion of the syncytium 

 are markedly developed ; many of them lie at the surface 

 (fig. 24, syn. c), which here presents a wavy contour, but is 

 not ridged, as is the allantoic placental syncytium. 



That area of the syncytium in relation with the bilaminar 

 omphalopleure presents the same features as were described 

 for the corresponding region in Stage C. It is, however, 

 somewhat thicker than in that stage. 



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

 The chorionic ectoderm has now almost completely disappeared. 

 It is still, indeed, recognisable as a continuous layer of cells at 

 the margin of the allantoic placental area (fig. 15, ch. e.ct.), 

 but over the remainder of the latter it is represented only by 

 more or less isolated degenerating cells (figs. 19 — 21, ch. ect'.). 



The persistent marginal zone of ectoderm (fig. 15, ch. ect.) 

 is narrow but of very variable width. Its most peripheral 

 cells, adjoining the ectoderm of the vascular omphalopleure, 

 are the least altered, but the remainder are irregular, and 



