DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 213 



d. The Second Branchial Arch is very similar to the first as regards 

 its vessels, though these are of slightly smaller size, and less fully 

 developed. 



The afferent branch from the truncus arteriosus (Fig. 6, A F.2) 

 is posterior and slightly ventral to that for the first arch. The 

 efferent branchial vessel (E F") does not extend so far down the 

 arch as does that of the first arch ; opposite the gill it gives off a 

 diverticulum which enters the gill, but ends blindly close to the 

 afferent branchial vessel, but without reaching this. The afferent 

 branchial vessel (A F") is still a small irregular lacunar space, which 

 in some specimens communicates with the efferent vessel opposite the 

 gill, but in others ends blindly; its ventral portion extends down- 

 wards towards the truncus arteriosus, but ends blindly some distance 

 from it. 



e. The Third Branchial Arch has vessels which agree almost exactly 

 with those of the hyoid arch. From the posterior border of the 

 branch of the truncus arteriosus to the second branchial arch, just 

 after this latter has separated from the branch for the first arch, a 

 small backwardly-directed diverticulum arises (Fig. 6, A F.3), which 

 is shown by its later development to be the commencement of the 

 afferent branch for the third arch ; it is at this stage extremely short, 

 and ends blindly. 



The efferent branchial vessel (E F'") is represented as before by a 

 small irregular lacunar space in the third branchial arch at the level 

 of the external gills, which at present has no communication with 

 any other vessel. 



From the dorsal aorta a small and short blind diverticulum (E F.3) 

 extends outwards and downwards in the arch. It is directed straight 

 towards the lacunar efferent vessel, but stops short some distance 

 from it. 



/. The Fourth Brcmchial Arch has as yet no vessels. 



(j. The JJorml Aorta. — The dorsal aorta) in the roof of the pharynx 

 have much the same relations as before ; they are furthest apart 

 opposite the gills, in front of which level they bend in towards each 

 other rather sharply, at the same time narrowing considerably. They 

 nearly meet just behind the infundibulum, where they arc connected 

 by the posterior infundibular commissural vessel (Fig. G, A M'), and 

 then continue forwards along the sides of the brain, between it and 



