76 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



here than farther forward. This plexus is made up of short, open endothelial 

 channels, solid cords, scattered angiocysts, and occasional small blood-islands, with 

 the better-developed vessels exhibiting a tendency to assume a course at right 

 angles to the axis of the embryo. It is impossible to determine the extent to which 

 these various vascular primordia may be united at this stage to form a more or less 

 continuous network. Conditions here are more like the "rete periintestinale" of Felix 

 (1910) than those shown by Bremer (1912) in the rabbit. The posterior and mesial 

 limit of the vitelline plexus on the right side is formed by a large, fairly definite 

 channel, for the most part quite unconnected with the general plexus in front of and 

 lateral to it. Near its origin on the yolk-sac it exhibits a small, patent, but not yet 

 definitely walled communication with the umbilical artery in the body-stalk. 

 Traced inward and forward in the splanchnopleure it becomes progressively larger, 

 its proximal portion being a distinct endothelium-lined tube. It possesses here a few 

 short sprouts and then, considerably reduced in size, turns inward to join what 

 seems to be the posterior end of the dorsal aorta (fig. 4) . Just where it passes inward 

 to meet the aorta, a small branch runs forward which is apparently destined to 

 establish a second, more anterior connection between the aorta and vitelline plexus. 

 Both the aortic and the vitelline ends of this latter vessel are in evidence, but the 

 intervening portion is rather problematical, at best a slender cellular connection. 

 We have here a nascent vitelline artery, the second of the series of roots of the future 

 umbilical artery. On the left side no indication can be discovered of any connection 

 between the aorta and the plexus. There is, however, a well-marked blood-channel 

 already laid down on the periphery of the plexus, but still lacking its connections 

 with the aorta in front and the body-stalk behind. At the aortic end a communica- 

 tion may be in process of formation by means of single cells, but the line of section 

 is much less favorable for determining this than on the right side. 



As just noted, the anterior part of the vitelline plexus is much less developed 

 than the posterior. A connection between the former, the radicles of the vitelline 

 or omphalomesenteric veins, and the proximal portion of the vein as it lies in the 

 septum transversum, is either absent or so small as to escape detection. If there is 

 an interruption it occurs just at the lateral border of the embryonic body. Sub- 

 stantially the same conditions are present on both sides. If not already present 

 there will be established here very soon a second connection between the intra- 

 embryonic and extraembryonic vessels, in this case by way of the venous end of 

 the heart. Apparently the posterior (aortic) connection antedates the anterior or 

 omphalomesenteric union. 



HEART AND EMBRYONIC VESSELS. 



In considering the cardiac plexus (figs. 4, 7, and 8) we may begin with the 

 omphalomesenteric roots, single on either side, beginning in the septum transversum 

 near the lateral border of the embryo. Passing inward and forward through the 

 septum, beneath the dorsal recesses, they turn cephalad under cover of the myoepi- 

 cardial mantle and unite with each other to form a single median mass. Of the two 

 vessels, without regard to their structure, whether hollow or otherwise, the left is 

 rather larger than the right. Small where they may be called the omphalomesen- 



