DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 235 



acquires its characteristic spongy structure; and the formation of 

 definite valves in the truncus arteriosus. 



The sinus venosus has undergone little change except that the 

 posterior vena cava is now present, and joins the hepatic vein just as 

 this enters the sinus venosus. The opening from the sinus venosus 

 to the auricular portion of the heart, which before was in the median 

 plane, now lies distinctly to the right side, and leads into the right 

 auricle (Figs. 13 and 14). 



The auricle is, as already noticed, almost completely divided into 

 right and left cavities. This division is effected by the growth 

 downwards, from the dorsal wall, of a septum with a free ventral 

 edge. This septum appears first, in tadpoles of about 9| mm. length, 

 as an inwardly projecting ridge running obliquely across the dorsal 

 wall of the auricle from the left anterior to the right posterior wall ; 

 the ridge consisting of a band-like thickening of the outer or 

 muscular wall of the auricle, carrying a fold of endothelium before 

 it. The septum, which from the first is more prominent in front, 

 grows rapidly, hanging down into the auricular cavity as a fold with 

 a free ventral edge ; at 1 2 mm. it extends almost to the auriculo- 

 ventricular aperture. The two chambers into which the auricle is 

 thus divided are at first of very unequal size, the left being much 

 the smaller. Owing to the obliquity of the septum the left auricle 

 lies somewhat dorsally to the right, and does not extend so far 

 forward as this. 



The pulmonary veins, which are of small size (Fig. 14, V P), run 

 along the inner sides of the lungs, as in the adult. In front they 

 unite together, and the median vein so formed runs forwards over 

 the sinus venosus to open into the dorsal surface of the left auricle. 



The outer wall of the ventricle is no thicker than that of the 

 auricle, but the ventricular cavity is much subdivided by ingrowing 

 muscular trabecular, many of which are thicker than the ventricular 

 wall itself. These trabecules are arranged in a generally radiate 

 manner ; they branch and unite together so as to form a sponge-work, 

 in the meshes of which lie blood corpuscles. 



The opening from the ventricle to the truncus arteriosus is circular; 

 its margin is thickened, but there are as yet no distinct valves. 



The truncus arteriosus is divided internally into proximal and 

 distal portions by a pair of valve-like folds situated at the constricted 



