74 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



more definitely lined vessel, it becomes continuous with the small offshoot from the 

 right aorta, the first vitelline (umbilical) artery. On the left side we can make out 

 no connection between the umbilical artery and the vitelline plexus, although the 

 latter shows much the same condition as on the right side except for the communi- 

 cation with the artery. There may well be some exchange of fluid through the loose 

 t issue on the left side, but the more open anastomosis on the right may help explain 

 the smaller size of the artery on this side, since a free flow to or from the yolk-sac is 

 possible. 



Traced away from the embryo, the blood-islands in the umbilical arteries 

 gradually become smaller and looser, their connections with the vessel-walls grow 

 less conspicuous, while the individual elements become rounded and more distinct 

 and also fewer in number until both vessels are entirely empty. These early blood- 

 cells, where they are not especially numerous, are attached to the anterior wall of 

 the vessel, i.e., on the side toward the embryo. In this location the wall is often 

 less sharply defined than where there are no blood-cells forming, although such 

 cells may be connected with a perfectly definite vessel- wall. 



Both arteries present, low down, a few very short branches which are usually 

 much less distinctly lined than the main vessels. A little farther from the embryo 

 a number of ventral branches are found which anastomose and increase in size 

 until there is formed a plexus of large, irregular vessels entirely filling the body- 

 stalk between the amnion in front and the umbilical arteries and allantois behind. 

 Distal to the giving off of these branches, the arteries are somewhat reduced and 

 also more nearly equal in size. They become, however, progressively larger, espe- 

 cially the left, but give off very few branches until, near the free end of the allantois, 

 they rapidly lose their identity by extensive anastomoses with the venous plexus in 

 front. Small, scattered masses of blood-cells are found in both arteries, being more 

 numerous in the left. At the very distal extremity of the left artery, in the channels 

 which connect it with the ventral plexus, there is again blood-island formation. The 

 right artery, although it has many connections with the plexus, can be followed 

 farther toward the chorion than the left. 



Ventral to the umbilical arteries and the allantois which courses between 

 them lies the dense, irregular vascular plexus referred to above. Beginning low 

 down in the body-stalk, where it is connected with the arteries, it increases rapidly 

 in size and complexity and at a higher level has incorporated in it the two arteries. 

 Beyond this point of union the plexus dwindles very markedly, so that the distal 

 portion of the body-stalk is occupied by but a few small vessels, through which a 

 narrow connection is set up with the vessels in the chorion. The walls of the plexus 

 are not always as sharply defined as in the arteries, being also distinctly thinner and 

 much less conspicuous. Small anastomosing channels, offshoots from the plexus, 

 are especially in evidence immediately around the allantois. Blood-cells are present 

 to about the same extent as in the arteries. No direct connections exist between 

 the plexus (the future umbilical veins) and either the yolk-sac or the beginning 

 umbilical veins which form in the body of the embryo. This latter communication 



