DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 219 



with a narrow blind end about the level of the middle of the 

 pharynx, and runs downwards, enlarging rapidly, and opening at its 

 ventral end into the lacunar spaces above the sucker, with which 

 we have just seen the mandibular vessel to be also connected. This 

 vessel, V Y, may be spoken of as the hyoidean vein, and clearly 

 corresponds to the vessel, V Y, of the 5 mm. stage (Fig. 6). The 

 degenerative changes in the vessels of the hyoid arch, that had 

 already commenced at 5 mm., have thus progressed considerably. 

 The dorsal part of the efferent lacunar vessel (Fig. 6, E H') has 

 disappeared completely, and the aortic diverticulum, E H, has 

 undergone reduction in size. 



c. The First Branchial Arch. — The afferent vessel of this arch is, 

 as noticed above, the anterior of the two vessels into which the 

 truncus arteriosus divides on each side (Figs. 7 and 10, A F.1). It 

 runs at first directly outwards, then outwards and upwards in the 

 first branchial arch ; it is at first small, but opposite the base of 

 the gill dilates rather suddenly. From this dilated part branches are 

 given off into each lobe of the gill, the branch of each lobe running 

 along its posterior surface to its tip, where it opens into the efferent 

 vessel (Fig. 10, A F.I, E F.l). The afferent and efferent vessels of 

 each lobe are also connected directly by small capillary loops which 

 lie in the minor processes arising from the lobe (Figs. 8, 9, 10). The 

 efferent vessel, E F', lies immediately in front of the afferent ; it is 

 wide opposite the gill, and widest of all opposite its uppermost or 

 dorsal lobe. Above this level it narrows very considerably, and 

 continues its course upwards and inwards until it reaches and opens 

 into the dorsal aorta. Ventrally to the gill the efferent vessel 

 narrows considerably, and is continued inwards in the floor of 

 the pharynx towards the median plane. Along this part of its 

 course it lies in close contact with the anterior wall of the afferent 

 vessel. The efferent vessel extends inwards almost to the level of 

 the division of the truncus into the afferent branches for the first and 

 second branchial arches (Figs. 8 and 10, E F.l) ; it ends blindly, and 

 has at this stage no connection with the afferent vessel at this point. 

 The relations of the afferent and efferent vessels of this arch are 

 shown somewhat diagrammatically in Fig. 8, where the greater part 

 of the afferent vessel, A F.l, has been removed on the left side ; the 

 figure shows the base only of the external gill, and the capillary 



