CIRCULATORY ORGANS. 



269 



arranged. Hence these parts are first described, the additions and 

 modifications being taken up later. 



THE HEART. 



The heart, the central organ for the propulsion of the blood, lies in a 

 sac, the pericardium, a part of the ccelom, which is ventral to the 

 pharynx or oesophagus and is partially filled with a serum, the per- 



FIG. 276. FIG. 277. 



FIG. 276.- -Diagram of the formation of the heart tube, showing the descending meso- 

 thelial plates from above, c, ccelom; cd, first appearance of the Cuvierian ducts; h, grooves 

 to form heart and ventral aorta; /, liver; m, mouth; ma, mandibular artery; om, omphalo- 

 mesenteric veins; so, sp, somatic and splanchnic walls of ccelom. 



FIG. 277. Early stage of the heart; the descending plates of fig. 276 have met, forming 

 the heart and ventral aorta, c, peritoneal ccelom; p, pericardial ccelom; ppc, pericardio- 

 peritoneal canals; other letters as in fig. 276. 



icardial fluid. In the heart we have to consider its epithelial lining 

 (endocardium), its muscular walls (myocardium) and its covering 

 epithelium and connective tissue (epicardium). 



The development of the heart is simplest in the vertebrates with 

 relatively small yolk. It is more modified in the elasmobranchs, 

 where the head is early completed below, and is most modified in the 

 large yolked eggs of the sauropsida and in the mammals where the yolk 

 sac is large, though the yolk is small. The following account is based 

 upon the development in the amphibia: 



From just behind the point where the first or spiracular gill cleft 

 is to form, backward to the region just in front of the anlage of the 



