DEVELOPMENT OF PRIMITIVE BL(X)I)-\ KSSKI.S. 103 



The fate of the subintestinal vein in the chick is very interesting. If an 

 injected chick of the fourth and fifth days be dissected so as to expose the caudal 

 end of the gut and the straight posterior segment of the gut which leads up to the 

 open bell of the yolk-sac, it will be seen that the entire wall of the gut is surrounded by 

 a capillary plexus. At the caudal end of the gut and just cephalic to the stalk of the 

 allantois the ventral vein has entirely disappeared in this capillary plexus, while far- 

 ther forward it is still clear in the ventral wall of the gut, though freely connected 

 with the plexus. It is clear also that this posterior segment of the gut is receiving 

 new arterial and venous connections which grow in along the dorsal border at the 

 cephalic end of the segment. The new artery is a branch of the omphalo-mesen- 

 teric artery given off just at the root of the yolk-sac; it extends caudal ward along 

 the dorsal wall of the gut and anastomoses with the aortic branches which are the 

 forerunners of the inferior mesenteric arteries. The new veins are branches of 

 the omphalo-mesenteric veins within the mesentery, the forerunners of the portal 

 system. The entire subintestinal vein gradually disappears as a single channel 

 by developing into the plexus of the wall of the gut. In this plexus it is clear 

 that the direction of the flow of the blood in the wall of the gut is from the ventral 

 toward the dorsal border, at right angles to the direction of the stream in the 

 subintestinal vein. 



It may seem curious that the pig should have a subintestinal artery in place 

 of the well-established subintestinal vein of the chick. As has been shown, the 

 subintestinal vein in the chick develops as a part of the process by which the 

 primitive circulation of the yolk-sac, with arteries and veins as far apart as possible, 

 becomes changed so that every zone of the area vasculosa is invaded by veins. 

 The pig of plate 5, figure 1, represents the more primitive condition for compar- 

 ison with Popoff 's plate iv, in which the caudal part of the yolk-sac is still arterial. 



The subintestinal artery of the pig can be seen in section in text-figure 5 

 from a pig of the same litter as the one shown in figure 1, plate 1; it receives 

 numerous ventral arteries from the aorta, as does the corresponding vein in the 

 chick; but it joins the omphalo-mesenteric arteries at the point of loop of the 

 mesenteric arteries instead of the veins. This same artery is still present in the 

 pig measuring 9 or 10 mm. after the caudal flexure has formed, at which stage it 

 is breaking up into the capillary plexus within the wall of the gut. By the time 

 the pig is 15 to 17 mm. long there is a new longitudinal artery in the dorsal wall 

 of the gut, extending from the superior mesenteric artery caudalward and anas- 

 tomosing with all the ventral aortic branches which represent the interior mesen- 

 teric artery. At the same time the accompanying venous plexus from the omphalo- 

 mesenteric vein extends along the dorsal border of the gut. As this new blood- 

 supply for the lower half of the intestine develops, the ventral vein of the earlier 

 stages of the chick, or the ventral artery of the pig, becomes reduced to a part of 

 the capillary plexus in the wall of the gut. It is interesting to note that in a pig 

 of 9.5 mm. the ventral artery of the gut is also accompanied by a plexus of ventral 

 veins, which correspond to the single ventral vein in the chick. Thus the differ- 

 ence in the two forms becomes readily understandable, for the invasion of that 



