BLOOD-VASCULAR SYSTEM 289 



tending to narrow the segmentation cavity in these regions into two longi- 

 tudinal tubes. The epimeral part of the mesothelium divides into somites, 

 and of course the segmentation cavity extends between these, and as these 

 somites grow downward, these lateral extensions of the segmentation cavity are 

 carried ventrally, so that at last they form a series of pairs of transverse vessels 

 connecting the longitudinal trunks, thus forming the vessels of the somatic 

 wall. Other tubes, connecting the dorsal and ventral trunks, would form 

 between the two walls of the mesentery and between the splanchnic mesoderm 

 and the entoderm, thus outlining the vessels of the alimentary tract. 



Even more speculative is the suggestion that the original circulation was 

 lymphoidal and that the blood circulation is a specialization of a part of this, 

 .he definitive lymph-vessels being the unmodified part of the primitive system 

 uf vessels. 



An appreciation of this probable ancestral condition makes the 

 actual structures more easily understood. In development much of 

 this phylogenetic history has been lost, while other parts have been 

 masked by the development of additional vessels. Many vessels, 



Fig. 310. — Diagram of the primitive vertebrate circulation, a, anus; al, alimentary 

 canal; av, abdominal vein; ca, cv, caudal artery and vein; da, dorsal aorta; h, heart; ic, 

 intercostal (somatic) transverse vessels; iv, intestinal vessels; m, mouth; si, subintestinal 

 vein; va, ventral aorta. The arrows indicate the direction of the flow of the blood. 



which theoretically should arise as spaces between other tissues, are 

 actually formed as solid cords of cells, which are later canalized and 

 converted into tubes. Again, separate vessels of the embryo may 

 fuse during development into a single vessel of the adult. 



The chief features of the theoretically primitive condition may 

 be summarized here (fig. 310). A dorsal tube carries the blood 

 toward the tail. From this transverse vessels — right and left, so- 

 matic and splanchnic — arise, which connect \vith two ventral longi- 

 tudinal tubes, one in the wall of the alimentary tract and extending 

 forward to its junction with the second which runs in the ventral body 

 wall, a single tube coursing from the point of union to the anterior 

 end of the body. In Amphioxus various parts of this system develop 

 muscular walls and act as pumping organs. In the vertebrates, so 

 far as the blood system is concerned, there is a single pumping organ, 

 the heart (the portal heart of the myxinoids may be ignored in this 

 general statement). The heart arises in the ventral tube beneath 



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