THE AETERIES OF THE HEAD. 181 



vessel of the first branchial arch, ef.i, but does not quite meet 

 this. At this stage, therefore, the lingual artery is a closed 

 vessel, having no communication with any other blood-vessel. 



Slightly later, in tadpoles of about 12 mm. length, the pos- 

 terior end of the lingual artery and the ventral end of the first 

 efferent branchial vessel become directly continuous with each 

 other (Fig. 80). The bulb-like swelling is still present, and it 

 is immediately dorsal to this that the direct connection between 

 the afferent and efferent branchial vessels is effected, as described 

 above. The anterior end of the lingual artery has by this time 

 grown forwards considerably (Fig. 80), extending to the lower 

 jaw and lower lip. 



The lingual artery thus arises in the floor of the mouth in- 

 dependently of any other vessel, but soon acquires connection 

 with the ventral end of the first efferent branchial vessel, of 

 which in the later stages it appears to be a direct continuation. 



b. The carotid glan d of the frog is formed by elaboration of 

 the direct passage between the afferent and efferent vessels of 

 the first br anc hial ar ch. At 12 mm., as just noticed, the lingual 

 artery and the first efferent branchial vessel are continuous ; the 

 lingual artery has at its base (Fig. 80) a small bulb-like swelling, 

 CG, and immediately dorsal to this swelling the afferent and 

 efferent branchial vessels are in direct communication with each 

 other. This direct passage is at present a single, and a narrow 

 one : it traverses a small plate of epithelial cells, which is budded 

 off from the epithelium of the first branchial cleft, and wedged 

 in between the afferent and efferent vessels. 



In the later stages of development this passage becomes 

 plexiform, there being now three or four openings into the 

 afferent vessel, and about the same number into the efferent 

 vessel, one or more of the latter leading directly into the bulb- 

 like swelling at the base of the lingual artery. 



Thi s plexiform communication forms the carotid gla nd ; the 

 history of its formation shows that it is not to be regarded as 

 a persistent or modified part of a gill, as was formerly held to 

 be the case, but that it is a specially acquired structure, formed 

 by elaboration of the direct passage between the afferent and 

 efferent branchial vessels, a passage which is present in a simpler 

 form in the hinder arches as well. 



Solid epithelial plates, of a similar character to that in which 



