ORGANS OF CIRCULATION. 



179 



the connecting semicircular vessels (aortic arches), and thence 

 back through the dorsal tube (dorsal aorta) to again enter the 

 posterior (venous) portion of the ventral tube, and thence back 

 to the heart. 



In development much of this probable ancestral history has 

 been masked. Many of the vessels which theoretically should 

 appear as spaces between the myotomes are formed as solid cords 

 of cells (often as a single row of cells), which later become 

 canalized and converted into tubes. We may first describe this 

 system of circulatory vessels as they become developed in the 

 lower vertebrates, taking them up in the order heart, arteries, 

 and veins, and then trace the modifications which occur in the 

 higher groups. This 

 method has the advantage, 

 as it traces the ontogenetic 

 steps by which the amniote 

 circulation arises. 



In the development of 

 the heart three parts are to 

 be considered, the epi- 

 thelium lining it, its mus- 

 cular walls, and the cavity 

 (pericardium) in which it 

 is suspended. 



Just behind the place 

 where the first (hyoman- 

 dibular) gill slit is to ap- 

 pear, the descending edges 



FIG. 193. Section through the throat region 

 of an embryonic Amblystoma, illustrating the 



Of the lateral plates, COr- early formation of the heart, e, endothelium of 

 responding in length tO heart; , ectoderm;/, fusion of ectoderm and 



several somites, meet just to f rm thr ugh ^ the jf 11 cleft wil1 



/ develop later from the gill pouch, g; w, myo- 



above the ventral epider- tome; ms, remains of ventral mesocardium, 

 mis, while more dor Sally the dorsal mesocardium has not yet formed; 



they enclose a groove-like n > notochord ; A pericardial wall; p c , peri- 



cardial cavity ; s, spinal cord. 



space open to the yolk 



above. In this groove appear cells which ultimately develop 

 into the epithelium (endothelium) of the heart ; but the origin 

 of these cells is not certainly known. The evidence tends to 



