212 PROFESSOR MARSHALL AND EDWARD J. ELES. 



truncus arteriosus, also persists for but a short time, and at the next 

 stage (6| mm.) has almost disappeared. These degenerative changes 

 may reasonably be associated with the absence of a gill on the hyoid 

 arch, which will also explain the failure to develope an afferent 

 branchial vessel for this arch. 



From the accounts given by Maurer and others, there seem to be 

 very great differences in the condition of the vessels of the hyoid and 

 mandibular arches met with amongst Amphibia themselves, and it. 

 is hardly possible to determine which of these conditions is to be 

 regarded as the most primitive for the group. It is instructive, 

 however, to note that in the frog the vessels of these arches are in a 

 more degenerate condition than in the Amniote Vertebrates, in which 

 these vessels form continuous arches between the heart and aorta in 

 the early stages of development. 



c. The First Branchial Arch. — The afferent branch from the truncus 

 arteriosus (Fig. 6, A F.I) has already been described. It is a vessel 

 of fair size and with well-formed walls, which runs transversely out- 

 wards beneath the pharynx, and ends blindly about the junction of 

 its floor and sides. The efferent branchial vessel, E F', is very large 

 opposite the external gill, where it lies in the middle of the arch. 

 From this point it extends downwards in the anterior part of the 

 arch towards the ventral surface, narrowing as it does so, and ending 

 blindly at the lower part of the side of the pharynx, in a line with, 

 but a little distance from, the blind end of the afferent branch from 

 the truncus arteriosus. Above the gill the efferent vessel, as in the 

 earlier stage, narrows considerably, becomes a tubular vessel with 

 well-formed epithelial walls, and opens into the dorsal aorta opposite 

 the widest part of the pharynx. 



The afferent branchial vessel (Fig. 6, A F') has much the same 

 relations as in the earlier stage ; it is still lacunar, and extends 

 downwards towards the truncus arteriosus somewhat further than 

 before, but does not yet meet it. The dorsal end of the afferent 

 vessel communicates with the efferent vessel by one well-developed 

 capillary loop in the external gill; and ventral to this loop the afferent 

 and efferent vessels are in direct communication by very narrow and 

 irregular lacunar passages. 



The efferent branchial vessel contains numerous blood corpuscles, 

 apparently budded off from its walls. 



