DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 101 



has been traced back as a vitelline vessel to the stage of 5 somites by Felix (1910), 

 and to the stage of 8 somites by Dandy (1910). This artery in the wall of the 

 gut, which is the primitive allantoic artery in the pig, has been called the um- 

 bilical artery in the human embryo on account of the insignificance of the allan- 

 tois and the earlier vascularization of the chorion. The relations of these two 

 vessels in connection with the human embryo were summed up by Evans (1912, 

 page 595) in the phrase that the umbilical artery is merely a modified vitelline vessel. 

 The entire question of the relation of the arteries for the fetal membranes at the 

 caudal end of the embryo has centered around the position of the central end of the 

 arteries with reference to the coelom, as can be seen in text-figure 6; that is to say, 

 whether the artery is mesial or lateral to the coelom. In general, both in birds and 

 in mammals there is a primitive artery mesial to the coelom; that is to say, a 

 splanchnic vessel, and a secondary vessel, the umbilical artery, lateral to the coelom 

 running in the somatopleure. Thus the vessels develop in the same manner in 

 the different forms, for there is a primitive splanchnic artery followed later by an 

 artery in the somatopleure, but there are variations in the relative importance of 

 the allantois itself. 



Besides the two arteries of the allantois, the two sheets of capillaries of the 

 wall of the caudal end of the gut give rise to another artery. Extending forward 

 from the stalk of the allantois, as seen in plate 5, figure 1, the two plexuses meet 

 in a capillary plexus ventral to the caudal root of the yolk-sac. This plexus is 

 continued as a single, ventral, subintestinal artery which joins the omphalo- 

 mesenteric plexus opposite the fourteenth or fifteenth somite. The point where 

 the subintestinal artery joins the omphalo-mesenteric plexus is the well-known 

 intestinal landmark where the stalk of the yolk-sac joins the gut. A figure which 

 gives a very clear understanding of these relations is Tandler's figure 1 in the 

 Anatomische Hefte, 1904, I 23 , page 192. 



This subintestinal artery in the pig is the more interesting in view of the 

 corresponding subintestinal vein in the chick, discovered by Hochstetter in 1888 

 and accurately described by him. He described its relations not only to the 

 omphalo-mesenteric veins, but also to the intestinal and the allantoic vessels, 

 and noted that it disappeared and that the left vein was larger than the right. 

 A complete understanding of the development of this vein in the chick can be 

 gained from the figures of Popoff (1894). As was mentioned in connection with 

 the chick, during the early hours of the third day of incubation the entire capillary 

 plexus of the area vasculosa caudal to the omphalo-mesenteric arteries must be 

 regarded as an arterial capillary plexus down to the marginal vein, as shown in 

 Popoff's plate v. During the last hours of the third day, as seen in Popoff s 

 plate vi, branches of the omphalo-mesenteric vein gradually extend caudalward 

 on either side of the embryo in the wall of the yolk-sac, and arch around the 

 posterior end of the embryo; the left vessel starts ahead of the right and is always 

 larger than the right. As these veins gradually extend backward into the terri- 

 tory of the pre-existing arterial plexus, forming more and more new connections 

 with the plexus, they change the direction of the current of the blood in the 



