THE HEART. 109 



auricular septum is being completed and the lungs are coming 

 into use. 



At the time of the metamorphosis the condition of the heart 

 is practically that of the adult. The proximal and distal parts 

 of the trillions arteriosus, or pylaiigium and synangium, are now 

 separated by three pocket valves in place of the two simple 

 valves originally present ; the spiral valve of the pylangium is 

 more strongly developed than before ; and the synangium is 

 divided internally into anterior and posterior portions, the 

 former communicating with the first and second pairs of aortic 

 arches, and the latter with the third and fourth pairs. 



The mode in which the thickening of the wall of the ventricle 

 is effected, by the formation of a muscular reticulum within the 

 cavity, and not by a simple increase in thickness of the wall 

 itself, is of interest, inasmuch as it explains why the ventricle of 

 the frog's heart has no nutrient blood-vessels. The blood within 

 the ventricle occupies the meshes of the muscular reticulum and 

 so comes in close contact with all parts of the ventricular walls. 

 a condition which renders special nutrient vessels unnecessary. 



The pericardial cavity, in its early stages, communicates freely 

 with the general body cavity, of which it is merely the anterior 

 portion. Owing to the great bulk of the mass of yolk cells, the 

 body cavity in the abdominal region of the embryo is at first 

 merely a narrow chink (Fig. 70, cs) ; while the pericardial 

 cavity is already a space of considerable size. 



Later on, and especially as the large veins opening into the 

 sinus venosus increase in size, the opening between the pericardial 

 cavity and the general body cavity becomes much reduced ; but 

 up to the time of the metamorphosis there is free communica- 

 tion between the two cavities, through a pair of apertures at the 

 dorsal and posterior border of the pericardial cavity, close to the 

 sides of the laryngeal chamber. 



3. The Branchial Blood-vessels. 



The blood-vessels of the pharynx have close relations with 

 the visceral arches, into which the side walls of the pharynx are 

 divided by the gill-clefts. 



The first vessels to appear in the body, with the exception 

 of the heart and the great veins opening into it, are the dorsal 

 aorta?. These arise on each side as a series of isolated lacunar 



