100 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



teenth or fifteenth somite these tiny branches from the two aortse unite in a 

 plexus of large arteries on either side of the stalk of the yolk-sac, which join and 

 give rise to the omphalo-mesenteric arteries on the yolk-sac. The large arteries 

 are seen only on one side in plate 5, figure 1, and plate 1, figure 1, but are shown 

 on both sides in figure 2 of plate 5. From the fourteenth somite caudalward the 

 ventral branches of the aorta are uninjected in this specimen (plate 5, fig. 1), but 

 show in other specimens leading to a single artery which arises in the caudal end 

 of the embryo. Opposite the caudal end of the embryo the ventral branches of 

 the aortse form a sheet of capillaries on either side of the alimentary canal, which 

 deserves careful consideration. These two sheets of capillaries form a plexus which 

 completely surrounds the entire caudal end of the gut cephalic to the allantois, the 

 stalk of the allantois, and the blind end of the gut, caudal to the allantois. This 

 capillary plexus gives rise to two arteries, the paired allantoic arteries and the single 

 subintestinal artery. Thus, we have here examples of arteries in the embryo which 

 arise in a capillary plexus and end in a capillary plexus. The primitive allantoic 

 arteries arise in a plexus around the stalk of the allantois and pass to the capillaries 

 of the body of the allantois; the subintestinal artery arises in a capillary plexus 

 around the gut and runs to the capillaries of the yolk-sac. 



The allantoic arteries, as seen in plate 1, figure 1, extend into a plexus on the 

 ventral or cephalic surface of the allantois; this plexus arches around the dome of 

 the allantois, though not completely shown in the drawing, and reaches the veins 

 on the caudal surface. The two allantoic veins join the umbilical veins at the 

 point where the stalk of the allantois is fused with the body-wall. A section 

 through the allantoic arteries from an injected embryo of the same litter as the 

 specimen of plate 1, figure 1, is shown in text-figure 6, and shows the allantoic 

 arteries following the wall of the gut into the allantois. In the series from which 

 text-figure 6 is taken there are a few tiny capillaries extending dorsalward from 

 the allantoic arteries just at the point where these arteries pass ventral to the ccelom. 

 These capillaries grow lateral to the coelom, and when the posterior limb-buds 

 begin they will anastomose with the iliac arteries. These capillaries will become 

 the umbilical arteries in the somatopleure. 



These observations on the pig agree with the findings of Hochstetter in the 

 rabbit (1890) and show that in these forms the primary allantoic arteries are vitel- 

 line vessels, while the central ends of the umbilical arteries are vessels of the soma- 

 topleure, which appear later and anastomose with the primitive allantoic arteries. 



In the study of the R. Meyer human embryo 300, Felix (1910) gives an 

 exceedingly interesting reconstruction of the vascular system of a human embryo 

 which is of the same stage as my figure 1 of plate 1. This reconstruction (fig. 7, 

 Morph. Jahrb, 1910, XLI, p. 590) shows that the primitive artery of the fetal mem- 

 branes at the caudal end of the embryo arises in a capillary plexus around the 

 gut, just as is shown in my figure 1 of plate 5 and figure 1 of plate 1. The posi- 

 tion of this plexus in the wall of the gut is shown in section in Felix's figure 9, 

 which is to be compared with my text-figure 6. The same relations are shown for 

 the chick in Duval's Atlas, plate xxni, figure 372. In the human embryo this artery 



