DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 261 



the efferent lacunar vessel by capillary loops in the gill, 

 while ventrally it grows downwards in the arch, and meets 

 and opens into an afferent diverticulum from the truncus 

 arteriosus. 



17. The course of the circulation is therefore from the truncus 



arteriosus to the afferent branchial vessel, then through 

 the capillary loops of the gills to the efferent branchial 

 vessel, and then on to the dorsal aorta. 



18. In Rana esculenta, according to Maurer, the condition is 



very different : the efferent vessel is at a very early stage 

 connected with the truncus arteriosus, and affords a direct 

 passage to the aorta. In one single specimen of Bana 

 temporaries we have found a similar condition. 



19. In the hyoid arch vessels appear at an early stage, which 



admit of close comparison with the vessels of a branchial 

 arch. They soon, however, undergo degenerative changes, 

 and disappear entirely at an early stage. 



20. In the mandibular arch vessels are formed which depart 



widely from the arrangement in a branchial arch, but yet 

 admit of comparison with this. Of these vessels, the 

 only one that persists is the diverticulum of the dorsal 

 aorta, which becomes the pharyngeal artery of the adult 

 frog. 



21. The internal carotid artery is formed as an extension for- 



wards of the dorsal aorta of each side. 



22. The lingual artery arises independently in the floor of the 



mouth : it soon becomes connected with the ventral end 

 of the efferent vessel of the first branchial arch. 



23. Close to the ventral ends of the efferent branchial vessels 



direct connections become established between the afferent 

 and efferent branchial vessels. These direct connections 

 are formed in all four branchial arches : they are ventral 

 to the gills, and afford a direct, but at first very narrow 

 passage, by which the blood can pass from the truncus 

 arteriosus to the efferent vessel, and so to the dorsal 

 aorta, without passing through the vessels of the gills. 

 They appear first in tadpoles of about 12 mm. length. 



