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 ccelomic cavity can be 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 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. 
elofo[eTslelsleleal TIS 
Wr 
sesso 
Fic. 278.—Diagrammatic cross sections of developing heart. Compare with figs. 
276 and 277. In A the descending mesothelial plates have nearly met, a numbez 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. InC the plates 
have met dorsally, with the resulting dorsal mesocardium; the ventral mesocardium has 
disappeared, placing the two ccelomic cavities, now the pericardium, in communication. 
c, coelom; ec, ectoderm; en, entoderm; end, endocardium; m, edges of descending meso- 
thelium; ~, 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.’ 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 ccelomic spaces on 
the two sides communicate with each other so that the myoepicardial 
mantle lies free on all sides in a ccelomic 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. 
