THE rLACENTATION OB' PERAMKLES. 407 



syncytium (fig. 9). The true chorion at the margin of the 

 allantoic placental area yet remains to be mentioned (text fig., 

 ch.). It will be seen from figs. 6 and 7, representing sec- 

 tions through the opposite margins of the allantoic placental 

 area, that the allantois does not at this stage completely 

 extend over the whole of the chorionic area, but leaves outside 

 its periphery a narrow marginal zone of true chorion, which 

 intervenes between the allanto-chorion and the vascular om- 

 phalopleure. 



On the one side (fig. 6) the allantois does not even extend 

 completely to the outer limit of attachment of the chorionic 

 ectoderm, but leaves here a marginal attached portion of true 

 chorion. On the other side (fig. 7) the margin of the allantois 

 corresponds fairly accurately with the outer limit of attach- 

 ment of the chorionic ectoderm, so that here only a narrow 

 strip of free chorion remains. On both sides the ectoderm 

 cells of this marginal chorionic zone become gradually reduced 

 in size, and pass over into the thin ectoderm of the vascular 

 omphalopleure. Closely adherent to the inner surface of this 

 transitional chorionic ectoderm is the thin single layer of 

 somatic mesoderm of the chorion (figs. 6 and 7, som.), con- 

 tinuous on the one side with the allanto-chorionic mesen- 

 chyme [all. mes.), and on the other with the mesoderm of 

 the yolk splanchnopleure [y. spl.). 



(c) Yolk Splanchnopleure. — The line along which the 

 somatic mesoderm of the chorion is continuous with the meso- 

 derm of the yolk splanchnopleure marks the outer limit of the 

 splanchnoccfile (figs. 6 and 7, cce.), and thus also the com- 

 mencement of the vascular omphalopleure {vase. omph.). The 

 mesoderm of the yolk splanchnopleure is a thin layer, carrying 

 fairly numerous vessels, except in that portion of it included 

 between the two main factors of the vitelline vein, which is, 

 as Semon has pointed out (8), permanently non-vascular. Its 

 entoderm is similar to that of the vascular omphalopleure. 



Traced round to its connection with the embryo, the yolk 

 splanchnopleure narrows to form the yolk-stalk, or vitelline 

 duct, which opens into the gut at the apex of the intestinal loop. 



