WITH SPECIAL REFERENCE TO THE VASCULAR SYSTEM. 73 



those in the chorion are widely open, even though many of them are very small. 

 That a few of them might be solid can not be excluded, but the appearance in all 

 cases is rather an extremely fine lumen or simply an approximation of the walls. A 

 second and quite conspicuous distinction in these vessels is their relatively thick 

 walls, due to the increase in number and greater condensation of the cytoplasmic 

 processes immediately around them, so that the vessels, aside from their lumina, 

 are quite prominent features in a section through the vesicle wall. Near the attach- 

 ment of the embryo these vessels are larger and rather more numerous and have 

 already established a slender connection with the vascular channels in the body- 

 stalk. Formed elements are found, with certainty, nowhere in the chorionic vessels 

 except in the immediate neighborhood of the body-stalk. They are present here 

 only in very small numbers, having made their way upward from the larger vessels 

 farther down in the stalk. Hofbauer cells are occasionally present in the villi and 

 abundant in the chorion, particularly near the attachment of the embryo. 



BODY-STALK. 



It is in the body-stalk that vascular channels show their greatest development 

 at this stage, where, as has been previously mentioned, they are largely responsible 

 for the size and shape of this structure (figs. 3 and 4). 



About the level of the origin of the allantois, we may place the lower or embry- 

 onic extremity of the two umbilical arteries; that is, the limit of those portions of 

 the arteries which are formed earlier than the part upon the yolk-sac, and in loco 

 within the body-stalk. The vessel on the left extends to a slightly lower level than 

 that on the right and is in adcUtion distinctly larger, although in both cases the 

 arteries end quite abruptly instead of running out gradually as one or more smaller 

 channels. For a very short distance these arteries are represented by spaces in the 

 mesenchyme, devoid of a definite endothelial coat, but containing each a well- 

 marked blood island connected with the surrounding stroma by numerous cyto- 

 plasmic processes. A recognizable limit between a definite vessel and the apparently 

 neutral mesenchyme is entirely absent, although the transition is to some extent 

 bridged by the presence of blood islands. Followed distally, the left artery rapidly 

 acquires a definite coat, although its lumen is yet almost entirely filled by a large, 

 rather compact blood island attached to the walls by numerous cytoplasmic strands 

 (fig. 9). Blood-islands have been noted in the body-stalk on many occasions, as by 

 Debeyre (1912), Grosser (1913), and Bremer (1914). In our embryo they resemble 

 the blood-islands of the yolk-sac much more than those of the body-stalk described 

 by the investigators just mentioned, at least as far as we can make out from their 

 descriptions or illustrations. 



The right artery is not only much smaller at first, but on the side toward the 

 embryo its endothelial wall is deficient for a short distance, where its lumen becomes 

 continuous with an extensive, irregular, but not definitely lined space, by means of 

 which an open connection is established between the artery and the posterior part 

 of the vitelline plexus (fig. 10). This channel can be traced forward and inward 

 through the wall of the yolk-sac and splanchnopleure until, as a much smaller but 



