292 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



work. For convenience these will be taken up under the heads of 

 the heart, the arteries and the veins. 



It is impossible here to follow in detail the development of all 

 parts of the circulatory system, or even to mention all of the vessels 

 in all of the groups. All that can be attempted is an account of the 

 more important parts and their modifications, with here and there 

 references to their history which will render their peculiarities more 

 intelligible. Most of the major trunks are now known to appear at 

 first as lines of vascular cells, similar to and arising in the same way 

 as those described in connexion with the heart (below) , and it seems 

 possible that the intima of all of the blood-vessels is in genetic rela- 

 tions to such lines of cells. It should be remembered that the vas- 

 cular system is extremely variable, even within the limits of the 

 species. 



THE HEART 



The heart, the central organ for the propulsion of the blood, lies 

 in the pericardial sac, the walls of which constitute the pericardium, 

 the cavity being partially filled with a serous pericardial fluid. The 

 walls of the heart are largely muscular, the muscles forming the 

 myocardial layer, which is covered on its inner and outer surfaces 

 by epithelium (endothelium). The epithelial layer turned toward 

 the pericardial cavity is the epicardium (really pericardial in nature), 

 while that on the side of the blood cavity of the heart is the 

 endocardium. 



The development of the heart is simplest in the vertebrates with 

 relatively small yolk. It is more modified in the elasmobranchs, 

 and is most complicated in the large yolked eggs of the sauropsida 

 and in the mammals where the yolk sac is large, though the yolk is 

 small. The following account is based upon the development in 

 the amphibia: 



From just behind the point where the first or spiracular gill cleft 

 is to form, backward to the region just in front of the anlage of the 

 liver the hypomeral portions (lateral plates) of the ccelomic walls grow 

 ventrally beneath the alimentary canal (figs. 312-314) in much 

 the same way as farther back (p. 16). In these descending plates 

 splanchnic, mesenterial and somatic walls, as well as the ccelomic 

 cavity are recognized. As they descend, cells which have received 

 the name of vascular cells appear between the ccelomic walls and the 

 entoderm. The origin of these has been in dispute, but the present 



