58 Vertebrate Embryology 



" The heart appears at the time when the medullary 

 folds have rolled in, and have met along the mid-dorsal 

 line; it lies below the pharynx, and anterior to the liver 

 (Fig. 12). The mesoderm in this region shows a tendency 

 to split into two sheets, and, where the heart is about to 

 develop, a cavity, a part of the coelom, appears between 

 the sheets. A cross-section of the larva (Fig. 25, A) shows 

 on each side of the mid-ventral line in the region of the 

 heart the somatic and splanchnic layers widely separated 

 from each other. The coelomic cavities of the right and 

 left sides are not continuous across the middle line, but 

 anterior and posterior to this section the coelomic cavity 

 is found to be continuous before and behind with the 

 general coelomic space on each side. A few scattered 

 cells lie in the middle line between the splanchnic layer 

 and the wall of the pharynx (Fig. 25, £). These cells have 

 been described as originating from the ventral wall of 

 the archenteron, and, if so, have had a different origin 

 from the other cells of the heart. 



"At a somewhat later stage of development the walls 

 of the coelomic cavities of the right and left sides sepa- 

 rate further (Fig. 25, £). The splanchnic layer thickens, 

 and begins to surround the proliferation of scattered 

 'endodermal cells.' These endodermal cells arrange 

 themselves in a thin-walled tube stretching throughout 

 the heart region (Fig. 25). Subsequent development 

 shows that this tube becomes the endothelial lining of 

 the heart. Around this endothelial tube the thickened 

 splanchnic layers now begin to push in from the sides 

 between the tube and the lower wall of the pharynx. 

 The tube becomes finally entirely surrounded by meso- 

 derm (Fig. 25). The mesoderm from the sides that has 



