104 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



part of the gut by the veins is merely relatively later in the pig, and the veins are 

 thus much more transitory. 



Branches of the omphalo-mesenteric veins growing down the mesentery begin 

 early in the pig. These are shown in plate 4, figure 3. They are not seen in plate 

 1, figure 1, because uninjected in the specimen. In other specimens from the same 

 litter there is a vein in the mesentery underneath the umbilical vein, as seen from the 

 side, and joining the main omphalo-mesenteric vein at the lower margin of the 

 liver. These branches are shown in plate 5, figure 2. The veins in the root of 

 the mesentery anastomose with the mesial cardinal (subcardinal) vein as soon as 

 it develops. This anastomosis was described by Hochstetter. 



The ventral subintestinal artery here described was discovered by Ravn in 

 1894 in the rat and mouse. The vessel was also described by Evans in the pig 

 (Manual of Human Embryology, Keibel and Mall, foot-note 56 on page 656). 

 Ravn's description can be readily followed in my plate 5, figure 1, as he described 

 the main omphalo-mesenteric artery arising in the caudal end of the embryo. 

 Both Ravn and Evans describe this subintestinal artery as arising from the um- 

 bilical artery. My specimens, however, are from still earlier stages, and prove 

 that this vessel arises, as does all the rest of the omphalo-mesenteric system, in 

 the wall of the yolk-sac or gut; that it is a true vitelline vessel. Its anastomosis 

 with the umbilical arteries in the somatopleure occurs later. Thus the subintes- 

 tinal artery in the pig and the subintestinal vein in the chick are vitelline vessels. 

 They disappear as single channels and help in the formation of the primitive 

 plexus in the wall of the gut in connection with the changes by which the gut 

 receives its permanent blood-supply and in connection with the gradual reduction 

 of the yolk-sac. 



The study of the ventral branches of the aorta in the human embryo is based 

 on the work of Mall, who in 1891 published an account of a human embryo 7 mm. 

 long, in which he described two main ventral branches, a coeliac axis and an 

 omphalo-mesenteric artery, and a series of small ventral branches in the lumbar 

 region, making a capillary network in the mesentery. He noted that the position 

 of both the cceliac axis and the omphalo-mesenteric artery was farther forward 

 than in the adult, and analyzed all the available material in a study of the shifting 

 of the arteries caudalward along the aorta. In this study he recorded human 

 embryos with the coeliac axis opposite the first, second, fourth, and sixth dorsal 

 nerves, as compared with the position in the adult opposite the twelfth nerve. 

 In 1897 he made a further study of the ventral arteries, especially in a human 

 embryo 2.1 mm. long. In this specimen he showed a series of ventral branches 

 extending from the seventh somite to the caudal end of the aorta. These vessels 

 he grouped together as the omphalo-mesenteric arteries. In the reconstruction 

 he showed that the upper arteries tended to be opposite the middle of the somites 

 rather than between the somites, as are the dorsal intersegmental vessels. In a 

 second analysis of the ventral aortic branches he showed that there is a constant 

 shifting of the coeliac axis and omphalo-mesenteric arteries caudalward. A double 

 origin of the omphalo-mesenteric arteries in one embryo suggested the method of 

 the wandering of the vessels. 



