102 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



plexus (which has been away from the heart) to a direction towards the heart. 

 The vein on the left side quickly extends to the marginal vein, making the single 

 posterior vein of the yolk-sac of Popoff , which lies a little to the left of the mid-line, 

 as shown in Popoff 's plate vm. The two lateral veins form an arch around the 

 posterior end of the embryo; this arch is just cephalic to the point where the stalk 

 of the allantois will develop. 



On the third day of incubation there is a very extensive capillary plexus on 

 either side of the posterior end of the gut, and beginning at the very caudal tip of 

 the gut on either side are symmetrical ventral veins, which unite in a loop just 

 cephalic to the base of the allantois and then run forward, at first as two veins 

 and then as a single ventral vein in the ventral wall of the yolk-sac. The sub- 

 intestinal vein is thus the primitive vein for the entire posterior end of the gut, 

 for the caudal tip of the gut, the allantois, and the entire rectum and intestine 

 up to the margin of the yolk-sac. Caudal to the allantois these vitelline veins 

 receive the most caudal branches of the posterior limb-bud. This relation has 

 been described by Evans (Anat. Record, 1909, iii). On the third day the um- 

 bilical artery develops around the somatopleure in connection with the posterior 

 limb-bud and anastomoses with the primitive allantoic capillary plexus in the 

 wall of the allantoic stalk. By the beginning of the fourth day the vessels in the 

 stalk of the allantois show an exceedingly interesting relation. On either side 

 there is one large artery coming from the aorta and now running in the somato- 

 pleure instead of in the splanchnopleure; but this artery is fed also from a capillary 

 plexus in the wall of the splanchnopleure, which completely surrounds the stalk 

 of the allantois and the caudal tip of the gut, and by a few capillaries of the 

 somatopleure from the tail of the embryo, which capillaries, however, tend to 

 drain more and more into the posterior cardinal veins. 



These relations are clear in the light of the development of these vessels. 

 There is at first a plexus of capillaries arising from the aorta and running in 

 the stalk of the allantois, in which arise the primitive allantoic arteries; and 

 secondarily, a capillary plexus in the somatopleure of the caudal end of the 

 embryo, in which an umbilical artery develops. The umbilical artery joins the 

 original allantoic artery in the fused area of allantois, somatopleure, and amnion 

 (see text-fig. 6), and then the primitive allantoic arteries from the aorta become 

 reduced again to a capillary plexus. Thus the allantois has a double arterial 

 supply and a double venous drainage, the former in the wall of the gut and the 

 latter hi the somatopleure. The primitive allantoic arteries arise in a plexus of 

 the splanchnopleure, and the corresponding venous return is through the sub- 

 intestinal vein; the subintestinal vein anastomoses with the allantoic veins, but 

 the direct continuation of the allantoic veins is into the umbilical veins, which 

 develop in the somatopleure. Finally the umbilical arteries develop in the 

 somatopleure, connect with the allantoic arteries, and soon bring most of the blood 

 to the allantois. 



