DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG.' 227 



backwards and outwards, divides into the afferent vessels for the 

 second and third branchial arches and their external gills (Fig. 12). 



The blood corpuscles have by this time lost almost all their yolk 

 granules, and have the flattened oval shape characteristic of those 

 of the adult frog. 



4. The Blood Vessels of the Visceral Arches. 



a. The Mandibular Arch. — The only vessel that can be referred to 

 this arch is what we have called the mandibular artery (Fig. 11, E M), 

 which has relations much the same as in the last stage. It arises 

 from the dorsal aorta, at a level a little posterior to the infundibulum 

 and to the anterior end of the notochord, and runs at first outwards, 

 then downwards and slightly forwards. It lies distinctly in the 

 mandibular arch, close to the anterior surface of the hyomandibular 

 gill pouch. At a level about midway between the dorsal and ventral 

 Avails of the pharynx the mandibular artery suddenly enlarges, or 

 rather opens into a wide vein (Fig. 11, V M), which continues its 

 course to the ventral wall of the pharynx, where it communicates 

 by two or more branches with the venous spaces of the floor of the 

 mouth in front of the sucker. A comparison of Figs. 7 and 11 will 

 show that the mandibular vessels have practically identical relations 

 in the two stages. 



b. The Hyoid Arch. — The diverticulum from the dorsal aorta, 

 which represents the efferent vessel of the hyoid arch at 6 J mm. (Fig. 

 7, E H), gradually diminishes in size as the tadpole gets older ; at 

 8£ mm. it can still be recognised as an extremely small bud on the 

 aorta, opposite the middle of the hyoid arch; at 9 mm. it has entirely 

 disappeared (Fig. 11). 



At this stage the sole vessel which can be said to belong to the 

 hyoid arch, and this only by comparison with the earlier stages, is 

 the hyoidean vein (Fig. 11, V Y), which lies in the ventral part of 

 the hyoid arch, and communicates below with the venous spaces 

 of the floor of the mouth. Its general relations are the same as at 

 6£ mm., but it is rather smaller, and more irregular in shape. 



c. The First Branchial Arch. — As might be expected from the size 

 of the gill, the vessels of this arch are well developed. 



The afferent vessel, or branch oi the truncus arteriosus (Figs. II 

 and 12, A F.I), is larger than before. On entering the gill it dilates 



