ISO MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



show that they are derived from the yolk (entoderm) ; but the 

 investigation is difficult, and they may arise from the mesothe- 

 lium, or, less probably, they may be mesenchymatous in origin. 

 These cells arrange themselves into a tube which is to form the 

 lining (epithelium) of the heart and ventral aorta, while behind 

 the heart region they extend backwards as a pair of tubes, the 

 omphalomesaraic veins to be described later, on either side of 



the yolk. 



In the heart region the 

 edges of the lateral plates 

 now fuse in the median 

 line above and below the 

 endothelial tube, thus giv- 

 ing rise to two longitu- 

 dinal folds, a dorsal and a 

 ventral mesocardium ; 

 while that part of the lat- 

 eral plates surrounding 

 the endothelium later de- 

 velops the muscular wall 

 (myocardium) of the heart 

 and the ventral aorta. 

 The downward growth of 

 the lateral plates brings 

 the ccelom just outside the 

 myocardium, and this part 

 of the coelom becomes cut 

 off from the rest, and 

 gives rise to a space, the 



FIG. 194. Early heart of Amblystoma, after 

 a reconstruction by Dr. F. D. Lambert, a, 

 auricle ; b *, b &, branchial arches, 1-4 ; dm, 

 dorsal mesocardium; /*, hyoid arch; m, man- 

 dibular head cavity ; o , omphalomesaraic veins ; 

 /, pericardial chamber ; j, sinus venosus ; /, 

 truncus arteriosus ; v, ventricle; I, first aortic 



arch. pericardial cavity, sur- 



rounding the heart. 



At first the dorsal and ventral mesocardia are entire, and 

 while dividing the pericardial space into right and left halves, 

 suspend the tube in this chamber in the same way that the in- 

 testine is supported by the mesenteries farther back. Soon the 

 ventral mesocardium breaks down, while a little later the dorsal 

 membrane becomes reduced to a small support for the posterior 

 portion of the heart. At first the tube is straight, and equal in 





