270 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



iver the hypomeral portions (lateral plates) of the coelomic walls grow 

 ventrally beneath the alimentary canal, in much the same way as 

 farther back (p. 121). In these descending plates splanchnic, mesen- 

 terial and somatic walls, as well as the coelomic cavity can be recognized. 

 As they descend, cells which have received the name of vascular 

 cells appear between the coelomic walls and the entoderm. The 

 origin of these has been in dispute, but the present evidence favors 

 their origin from the mesothelium. Some of these vascular cells are 

 more dorsal and aid in the formation of the dorsal blood-vessels, while 

 the ventral (fig. 278, A) contribute to the heart and the ventral trunks. 



jS&HEaSSr 



FIG. 278. Diagrammatic cross sections of developing heart. Compare with figs. 

 276 and 277. In A the descending mesothelial plates have nearly met, a number of 

 vascular cells between them. In B the plates have met ventrally, forming the ventral 

 mesocardium ; most of vascular cells utilized in forming the endocardium. In C the plates 

 have met dorsally, with the resulting dorsal mesocardium; the ventral mesocardium has 

 disappeared, placing the two coelomic cavities, now the pericardium, in communication. 

 c, coelom; ec, ectoderm; en, entoderm; end, endocardium; m, edges of descending meso- 

 thelium; p, pericardium; v, vascular cells. 



The descent of the lateral plates continues until their lower edges 

 meet just dorsal to the ventral ectoderm and the ventral parts of the 

 mesenterial regions of the two sides fuse to a vertical plate, the ventral 

 mesocardium (fig. 278, B), above which is a groove in which the 

 ventral vascular cells lie. Next, the edges of the plates crowd in above 

 the groove and meet to form a dorsal mesocardium, the result being 

 that groove is converted into a tube. The mesocardia disappear early, 

 the ventral usually being lost before the dorsal is formed (fig. 278, 

 C) . The walls of the tube, which are to form the muscular and epicar- 

 dial walls of the heart, are called the myoepicardial mantle. 1 The 

 vascular cells, which are enclosed in this mantle, gradually arrange 

 themselves as a continuous sheet, the endocardium, which lines the 

 future heart. 



With the disappearance of the mesocardia the coelomic spaces on 

 the two sides communicate with each other so that the myoepicardial 

 mantle lies free on all sides in a coelomic sac, being bound to the walls 

 only at the two ends. This cavity or sac is the pericardial cavity, 



1 The fact that the heart muscles arise from this layer mesothelial and yet not myotomic 

 partly explains the differences between cardiac and other muscle. 



