WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 75 



doubtless forms at a slightly more advanced stage. It is at the chorionic end of 

 the body-stalk that the greatest transformation would be required to bring about 

 separate arterial and venous channels, instead of the single set of vessels which 

 occupies this location at present. 



The stroma of the body-stalk varies somewhat in different localities. It is 

 densest and most cellular close to the embryo'; at a little distance its cellular content 

 becomes much less marked, while the cytoplasmic reticulum is very conspicuous. 

 Along the posterior border, near the allantois and the umbilical arteries, this tissue 

 is always much less rich in nuclei than around the plexus in front. Often the wide 

 space between the arteries, allantois, and the investing mesothelium is bridged by a 

 very delicate network of cytodesmata almost devoid of nuclei. At the smaller, 

 chorionic end of the stalk, where the venous plexus is running out, the stroma 

 gradually loses its delicate reticular appearance, becomes denser, and assumes a 

 more fibrous aspect much like the mesoderm of the chorionic wall. The vessels in 

 this part of the stalk, which establish the slender and circuitous connection between 

 the plexus and the chorionic vessels, are quite small and thick-walled, and so in 

 both respects more like those of the chorion than those of the body-stalk. They 

 may be looked upon, therefore, as ingrowths from the chorion to meet the inde- 

 pendently formed vessels of the stalk. Over the distal part of the body-stalk the 

 mesothelial investment is gradually lost, and in fact close to its attachment to the 

 chorion the limits of the stalk fade almost imperceptibly into the adjacent coagulum 

 in the exoccelom. From the mesothelium there are found short, tubular or funnel- 

 shaped ingrowths, but these are few in number and have no relation to the under- 

 lying vessels. 



YOLK-SAC. 



On account of the extreme distortion of the yolk-sac, due to folding and partial 

 collapse, it is not always possible to locate exactly many of the vascular rudiments. 

 Blood-islands are most numerous in the region of the fundus and are also scattered 

 sparsely over the remainder of the vesicle, being rather more frequent laterally 

 and posteriorly, while a few are found quite close to the embryo. Vessels in various 

 stages of development are most conspicuous in these same areas, but no special 

 attempt has been made to trace them or to determine their relations more definitely. 

 On what appears to be the posterior surface of the sac, there are a considerable 

 number of large, well-defined channels which are not even remotely connected by 

 open vessels with those close to the embryo. As the embryo is approached, all the 

 vascular features become less frequent, more scattered, and more difficult to recog- 

 nize, especially lateral to the anterior part of the embryo. Any connection between 

 the vessels or heart of the embryo and those of the adjacent yolk-sac anteriorly by 

 way of the vitelline (omphalomesenteric) veins is at best very attenuated and 

 indirect; a continuous, open endothelium-lined path is not present; for, whatever 

 interchange of fluids may be going on between the embryo and yolk-sac in this 

 region, there are no corresponding morphological features to be recognized. 



Posteriorly, however, nearer the attachment of the body-stalk, conditions pre- 

 vail which may be described as a vitelline plexus (fig. 4), much better developed 



