CIRCULATORY ORGANS. 2 69 
arranged. Hence these parts are first described, the additions and 
modifications being taken up later. 
Tue HEarr. 
The heart, the central organ for the propulsion of the blood, lies in a 
sac, the pericardium, a part of the coelom, which is ventral to the 
pharynx or cesophagus and is partially filled with a serum, the per- 
Fic. 276. Fic. 277. 
Fic. 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. 
Fic. 277.—Early stage of the heart; the descending plates of fig. 276 have met, forming 
the heart and ventral aorta. c, peritoneal ccelom; ~, pericardial coelom; 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 
