The Development of the Frog 61 



this it passes into the second or auricular 

 chamber, thence to the ventricle, and from that 

 to the truncus arteriosus (Fig. 17)." As there 

 is, at first, no division into right and left sides, 

 the blood passes in succession through these 

 various chambers. 



" The truncus arteriosus divides distally into right and 

 left branches, from each of which four afferent branchial 

 vessels (Fig. 26, A F, 1-4) arise. The four vessels of 

 each side run outwards along the hinder borders of the 

 four branchial arches, giving off along their whole length 

 numerous branches to the gill-tufts on these arches 

 From the gills the blood, now aerated, passes into the 

 efferent branchial vessels (Figs. 23 and 26, EF, 1-4). 

 These lie alongside of the afferent branchial vessels, 

 and just in front of them, but do not communicate with 

 them except through the capillary loops of the gills. 

 The four efferent branchial vessels of each side unite in 

 the dorsal wall of the pharynx to form the dorsal aorta - 

 the two aortae are continued forwards to the head as the 

 carotid arteries, while posteriorly they unite to form the 

 single dorsal aorta, from which branches arise supplying 

 all parts of the body (Figs. 24, A, and 26, A). 



The lungs arise at a very early stage, but are for a long 

 time extremely small and of little functional importance. 

 Each lung receives blood from a branch of the fourth ef- 

 ferent branchial vessel (Fig. 26, AP), and returns it di- 

 rectly to the auricle by the pulmonary vein. As the tadpole 

 increases in size, and the lungs become of greater im- 

 portance, a septum appears, dividing the auricle into 



