development of the blood vessels in the frog. 217 



3. The Heart and Pericardial Cavity. 



The pericardial cavity has much the same relations as before, but 

 is of smaller size, owing partly to the increased dimensions of the 

 heart, and partly to extension forwards of the liver. The cavity 

 communicates with the coelom behind, and also dorsally ; the dorsal 

 part of the co?lom on either side, into which the nephrostomes open, 

 being prolonged forwards as a narrow vertical channel, which com- 

 municates just in front of the Cuvierian veins with the dorsal and 

 posterior end of the pericardial cavity. 



The heart has the same general relations as before, but the vertical 

 twist is more marked (Fig. 7), the auricular portion lying distinctly 

 dorsal to the ventricular. The distinction between auricle and 

 ventricle is still very slightly marked. The walls of the truncus 

 arteriosus are rather thicker than before. 



The truncus arteriosus leaves the pericardial cavity in the median 

 plane in front, and immediately divides into right and left branches, 

 which run horizontally outwards (Fig, 10). Each branch divides 

 about the level of the side wall of the pericardial cavity, and about 

 midway between the median plane and the surface, into two vessels, 

 the anterior of which (Fig. 10, A F.I) runs directly outwards to the 

 first branchial arch, while the posterior runs backwards and out- 

 wards, and divides to supply the second and third branchial arches 

 (Fig. 10, A F.2, A F.3). 



4. The Blood Vessels of the Visceral Arches (Figs. 7-10). 

 a. The Mandibular Arch. — The diverticulum of the dorsal aorta 



that was described as a very minute structure in the 5 mm. tadpole, 

 is present, and has increased considerably in length (Fig. 7, E M). 

 It runs downwards, outwards, and slightly forwards, just in front of 

 the hyomandibular cleft ; and after a short course dilates suddenly, 

 or rather opens into a large vessel with sacculated walls, the 

 mandibular vein (Fig. 7, V M), which runs downwards to the floor 

 of the pharynx, where it opens into an irregular mesh-work of large 

 lacunar channels, lying just above the sucker, and at the sides of 

 the thyroid body. 



The vessels of the mandibular arch are peculiar in several respects. 

 Still wc believe that they may be compared with tolerable certainty 

 with those of the hyoidean and branchial arches. The diverticulum 



