730 THE CIRCULATORY SYSTEM 



represent the initial stages in the development of the various coelomic divisions 

 of the primitive coelomic space (Chap. 20). 



As the cardinal and intestinal systems of the primitive vascular system be- 

 come joined together centrally via the common cardinal veins, the two systems 

 become joined peripherally by means of a series of intersegmental blood 

 vessels. The latter arise from the dorsal aortae and travel dorsally between 

 the somites and myotomes to the central nerve tube (fig. 232E). In the 

 nerve-tube area, the primitive intersegmental arteries become continuous with 

 the rudiments of the intersegmental veins which course laterad to join the 

 anterior and posterior cardinal veins. When the above vascular channels are 

 well established, another set of veins is formed between the somatopleural 

 mesoderm of the hypomere and the developing integument (figs. 332D; 336C, 

 D). The last veins course along the lateral body wall, arising in the pelvic area 

 and emptying into the sinus venosus of the heart. In fishes and amphibia, 

 these veins are called lateral veins, but in reptiles, birds, and mammals, 

 they are denominated the allantoic or umbilical veins as they drain principally 

 the allantoic area of the embryo. 



4. Regions of the Primitive Vascular System 



The primitive morphological plan of the vascular system, as outlined above, 

 is a basic condition strikingly comparable in all vertebrate embryos. In view 

 of the later changes of this fundamental vascular plan necessitated by the 

 adaptation of the vascular system to the environmental conditions existing 

 within the various habitats of the adult, it is well to demarcate, for the 

 purposes of later discussion, certain definite regions of the primitive arterio- 

 venous system. These regions are (fig. 332D): 



( 1 ) the converging veins of the heart, composed of the lateral, common 

 cardinal, anterior and posterior cardinal, and vitelline veins, 



(2) the primitive heart, made up of the primitive sinus venosus, atrium, 

 ventricle, and bulbus cordis, 



(3) the branchial area, composed of the ventral aortae, aortal arches, and 

 adjacent dorsal aortae, and, 



(4) the dorsal aortae (later aorta) and efferent branches. 



C. Histogenesis of the Circulatory System 



1. The Heart 

 Consult Chap. 16. 



2. Formation of the Primitive Vascular Channels and 



Capillaries 



Two principal theories have emerged to account for the origin of the 

 primitive blood vessels in the embryo. These theories are the angioblast theory 

 and the local origin theory. 



