228 RROFESSOR MARSHALL AND EDWARD J. BLES. 



suddenly, and then divides, sending afferent branches to each of the 

 lobes of the gill ; these run, as already noticed, along the posterior 

 borders of the several lobes, give smaller branches to the minor lobes, 

 and communicate at the ends of the lobes directly with the efferent 

 vessels (Fig. 12). 



The proximal part of the afferent vessel, just before it reaches 

 the external gill, bears along its ventral border a series of small 

 sacculations, usually three or four in number (Fig. 11, A F.I) : these 

 lie very close to the surface epithelium, which is slightly raised 

 opposite them, and form the first indication of the internal gills. 

 The earliest age at which we have seen these is in tadpoles of 8 J mm. 

 length. The internal gills are situated ventrally to the external gills 

 and in a line with them, external and internal gills forming apparently 

 one continuous series of structures. 



The efferent branchial vessels are in direct continuity, as noticed 

 above, with the afferent vessels at the extremities of the lobes 

 of the gill. At the base of the gill they join the main efferent 

 vessel of the arch (Fig. 11, E F.I), which runs upwards to the dorsal 

 aorta. 



Ventral to the gill, this efferent vessel is prolonged downwards, 

 lying close to the anterior border of the afferent vessel. It narrows 

 rapidly and soon loses its lumen, but is continued as a solid cord of 

 cells to the ventral surface of the pharynx, not extending, however, 

 to the bifurcation of the truncus arteriosus (c/. Figs. 11 and 12, 

 E F.I). Along this part of their course the afferent and efferent vessels 

 lie very close together, but do not communicate. 



The lingual artery is first recognisable about this stage. Though 

 it has as yet no communication with the vessels of the first branchial 

 arch, it may conveniently be described here, as a connection with the 

 first branchial efferent vessel is established very shortly afterwards. 

 At 9 mm. the condition of the lingual artery is shown in Figs. 11 

 and 12, A L). It consists of a swollen part, which lies immediately 

 in front of the bifurcation of the truncus arteriosus, and from which 

 two vessels arise. Of these, one, the lingual artery proper, runs 

 directly forwards in the floor of the mouth for a short distance, 

 giving off near its base a ventrally directed thyroid artery to the 

 thyroid body (Fig. 11, A R). The other branch runs outwards and 

 backwards, parallel to the afferent vessel of the first branchial arch, 



