THE HEART 293 



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. 312, -4) contribute to the heart 

 and the ventral trunks. 



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 

 mesocardiuin (fig. 312, 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 mesccardium, the re- 

 sult being that groove is converted into a tube. The mesocardia 

 disappear early, the ventral usually being lost before the dorsal is 

 formed (fig. 31 2, C). (A portion of the ventral mesocardium persists 



Fig. 312. — Diagrammatic cross sections of developing heart. Compare with figures 

 313 and 314. 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 meso- 

 cardium has disappeared, placing the two coelomic cavities, now the pericardium, in 

 communication, c, ccelom; ec, ectoderm; en, entoderm; end, endocardium; m, edges of 

 descending mesothelium; p, pericardium; v, vascular cells. 



in turtles and crocodiles, connecting the apex of the heart to the 

 pericardial wall). The walls of the tube, which are to form the 

 muscular and epicardial walls of the heart, are called the myoepicar- 

 dial mantle.^ 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, 

 the extent of which is decreased by the fusion laterally of the somatic 

 and splanchnic walls (fig. 314). 



In front of and behind this tube the descending lateral plates are 

 kept from meeting in the middle line by the projections for the mouth 



* The fact that the heart muscles arise from this layer — mesothelial and yet not 

 myotomic — partly explains the differences between cardiac and other muscles. 



