DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 105 



The same idea of the shifting of the arteries caudalward was further developed 

 by Tandler in two papers in 1904 and by Broman in 1908. These workers ex- 

 tended their observations over a long series of embryos, Broman giving a study 

 of 41 specimens. In one of the youngest specimens in his series, an embryo 

 measuring 3.4 mm., the upper ventral branch was between the sixth and seventh 

 interspaces. He described the branches as tending to occur between the inter- 

 spaces, there being two or three to a somite. He found that the cceliac axis and 

 the superior mesenteric artery are not segmental vessels (that is, opposite the 

 interspaces), while the inferior mesenteric artery is sometimes opposite and some- 

 times between the somites. Broman gives an analysis of the literature and an 

 extensive discussion of the methods by which the shifting of the ventral arteries 

 may take place. 



In the human embryo ventral branches of the aorta have been described 

 from about the seventh segment caudalward. In the pig these ventral branches 

 are very numerous approximately one to a segment and one to an interspace. 

 They are originally of uniform size and about equidistant apart. They unite 

 into an extensive plexus of larger vessels in the more cephalic region and into a 

 long artery in the caudal region. It is easy to follow the method of the shifting 

 of arteries from such a primitive pattern; that is, anj^ of the vessels of the original 

 system could easily enlarge and the blood-stream be increased or decreased accord- 

 ing to the development of the region of the organ supplied. The entire wandering 

 of the arteries can be understood without presupposing the development of any 

 new vessels, but rather through the shunting of the blood through different 

 channels already present in response to the varying development of the parts 

 supplied by these arteries. Moreover, it is plain that the point brought out by 

 Evans is of importance namely, that the so-called wandering of arteries takes 

 place while the vessels have the structure of capillaries; that is, while their wall 

 consists of endothelium alone. From the position of the primitive ventral arteries 

 it is also easily seen that there might be variations as to whether the ultimate 

 ventral arteries of the older embryo came opposite an interspace on the same level 

 as the dorsal arteries or opposite a somite. 



THE UMBILICAL VESSELS. 



My series is not very complete in regard to the umbilical veins, but it shows 

 a few interesting points. In plate 4, figure 3, the relation of the somatopleure to 

 the fold of the amnion is very plain. In the somatopleure is the beginning of a 

 capillary plexus representing the umbilical veins. In plate 5, figure 1, the umbilical 

 veins are not injected, but they are well shown in plate 1, figure 1, in which it is 

 clear that the return flow of the blood from the caudal end of the embryo is in 

 part through the subintestinal artery in the splanchnopleure and in part through 

 the umbilical veins in the somatopleure. At the stage of plate 1, figure 1, the 

 umbilical veins have established their connections with the liver, though they 

 still connect with the sinus venosus. In figure 1 of plate 1 and figure 2 of plate 5 

 it is clear that cephalic to the duct of Cuvier there is also a capillary plexus in 



