THE LATER DEVELOPMENT OF THE FROG 177 



(Weber); these very early fuse together anteriorly forming a 

 median region, the future bulbus aortce (Fig. 62). From the 

 antero-lateral margins of the bulbus, two short strands of cells 

 extend forward in the floor of the pharynx as the rudiments 

 of the bifurcated truncus arteriosus or ventral aortce. Posteriorly 

 the two endothelial tubes are only incompletely fused and are 

 asymmetrically developed. That of the right side forms a 

 dilated flexed tube which is the rudiment of the ventricle and 

 the right vitelline vein, while that of the left side is more elon- 

 gated and is dilated posteriorly as the rudiment of the auricle, 

 continuing posteriorly as the left vitelline vein (Fig. 62). Both 

 vitelline veins pass directly into the liver and yolk-mass. These 

 two cardiac tubes gradually fuse more extensively and their 

 cavities become somewhat confluent, so that the ventricular 

 region is in a small degree formed of the left tube also; the 

 more posterior auricle similarly receives a small addition from 

 the end of the right tube with which the right vitelline vein 

 is continuous. 



The heart rudiment begins to elongate at once and is thrown, 

 by horizontal folds, into an S-form, whereupon the dorsal meso- 

 cardium disappears, leaving the heart tube attached to the peri- 

 cardial wall only at its ends. The posterior limb of the heart 

 lies toward the left side and abuts against the liver; this forms 

 the region of the sinus venosus and auricles. The anterior 

 section, and the right or middle section which is the region of 

 the ventricle, soon swing downward, becoming relatively 

 ventral in position, while the auricle then extends through nearly 

 the entire dorsal part of the pericardial cavity (Fig. 66). 



These limbs of the heart tube are very early separated from 

 one another by constrictions. Shortly after the opening of the 

 mouth the auricle becomes divided into right and left auricles by 

 the downgrowth of the interauricular septum from its dorsal wall. 

 The sinus venosus remains connected with the right auricle. 

 The left auricle receives the pulmonary veins, but these are 

 only slightly represented during the tadpole stage. The wall 

 of the ventricle becomes much thickened by the ingrowth of 

 a muscular network from its inner surface. A few days after 



