z>4 THE BIOLOGY OF THE FROG CHAP. 



adult condition, they arise from cells of a very similar, 

 although perhaps not identical, structure. 



The Structure of the Heart. - The heart of the frog is 

 situated in the anterior part of the body cavity, ventral to the 

 liver. It lies within a sac, the pericardium, whose cavity 

 is completely cut off. from the ccelom, although originally 

 continuous with it in early development. The pericardium 

 is composed of two layers, the parietal, forming the outer 

 wall of the sac, and the visceral, which closely invests the 

 heart. The two layers are continuous, passing into each 

 other in the region of the truncus arteriosus below and the 

 anterior venae cavae above. The relation is such as would be 

 produced if the heart were pushed into the hollow pericar- 

 dial sac from in front, although as a matter of fact it is not 

 brought about in this way. 



When the pericardium is opened on the ventral side, the 

 following parts corne into view; (i) The conical ventricle, 

 with its apex pointing backward ; this part of the heart has 

 very thick muscular walls and appears paler than the rest. 



(2) The auricles lie immediately in front of the ventricle. 

 The auricles are thin-walled and are separated from each 

 other internally by a septum, but from the outside they 

 present only a faint indication of this division ; they are 

 clearly separated from the ventricles by the coronary sulcus. 



(3) The bulbus cordis, lying in front of the right side of the 

 ventricle ; it is a thickened muscular tube, extending ob- 

 liquely across the right auricle ; anteriorly it is continued 

 into the thinner-walled truncus arteriosus from which it is 

 demarcated by a sulcus. (4) The truncus arteriosus is 

 somewhat narrower than the bulbus ; it soon divides into 

 two diverging trunks, which give rise to three arteries, the 

 common carotid, the aorta, and the pulmo-cutaneous. 



On the dorsal side of the heart is the triangular, thin- 



