THiS HEART 295 



the pericardio-peritoneal canals (fig. 314). Elsewhere the peri- 

 cardial and peritoneal cavities are entirely separate in the adult. 



In teleosts and amniotes, where the early embryo is closely appressed to the 

 very large yolk sac, the development of the heart is modified. At first the 

 pharynx is not complete below but communicates ventrally with the yolk. 

 Hence the two hypomeres are prevented, for a time, from meeting ventrally. 

 Each, however, is accompanied by its vascular cells; its edge becomes grooved 

 and the grooves are rolled into a pair of tubes, lined with endocardium, so that 

 for a time the anlage of the heart consists of two vessels (fig. 315), each con- 

 nected in front and behind with its own mandibular artery and omphalo- 

 mesenteric vein, and is surrounded with its pericardial sac. Later the two tubes 

 approach and fuse, with the formation of mesocardia as before: these latter 

 soon disappearing, leaving the whole much as in the small yolked forms. 



In the early stages the pericardium is relatively large, but it does 

 not keep pace with the growth of the other parts, until finally in the 

 adult it is only large enough to accommodate the changes in size and 



I'iG. 315. — Section through the developing heart of a rabbit, after His. c, peri- 

 cardial ccElom; e, endothelium of half of heart; m, mesoderm; p, pharyngeal region not 

 yet cut off from yolk sac. 



shape of the heart, due to its alternating enlargement (diastole) and 

 contraction (systole). 



As long as the mesocardia are present the cardiac tube is a straight 

 canal, lying in the pericardial sac and connected with its walls in 

 front and behind. With their disappearance the tube increases in 

 length more rapidly than does the pericardium, the result being the 

 flexure of the tube on itself, something like the letter S, the flexures 

 being largely in the vertical plane. At the middle point of the flexure 

 the tube remains small, forming the atrio-ventricular canal, but in 

 front of and behind this the walls become thickened and the lumen 

 enlarged. The posterior and dorsal of the chambers thus formed 

 becomes the atrium (auricle), the ventral and anterior the ventricle 

 of the heart. 



The atrium is bounded posteriorly by a constriction, behind 

 which the tube expands into another chamber, the sinus venosus, 

 which extends back to the posterior wall of the pericardium and re- 



