268 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
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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, 
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. 275). A dorsal tube carries the blood toward 
the tail. From this transverse vessels—right and left, somatic and 
splanchnic—arise, which connect with two ventral longitudinal tubes, 
Fic. 275.—Diagram of the primitive vertebrate circulation. @, 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; sz, subintestinal 
vein; va, ventral aorta. 
one in the wall of the alimentary tract and extending forward to its junc- 
tion 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 the pharynx and anterior 
to the junction of the two tubes. It marks the line of division of the 
transverse tubes into ascending and descending, those in front of the 
heart carrying the blood upward while those behind return it to the 
ventral vessels which carry it forward. The transverse vessels are not 
continuous, but capillaries intervene between their dorsal and ventral 
moieties. 
The Embryonic Circulation. 
In all vertebrates a series of blood-vessels is laid down in the early 
stages, forming a framework around which the rest of the circulation is 
