DEVELOPMENT OE THE BLOOD VESSELS IN THE FROG. 23? 



though its relations have altered considerably (Fig. 13, AY). The 

 proximal part, in connection with the aorta, remains the same as 

 before; but while at 9 mm. the mandibular artery opened distally into 

 the large mandibular vein, and so communicated with the system of 

 sinuses in the floor of the mouth above the sucker (Fig. 11, VM), 

 at the stage we are now considering, 12 mm., the mandibular artery 

 has no connection with lacunar spaces, but continues its course as a 

 vessel of small size with distinct walls, forwards, outwards, and 

 downwards in the roof and sides of the pharynx, to the articulation 

 between the cartilage of the hyoid arch and the skull, where it ends 

 in muscles in the neighbourhood of the joint. Before reaching the 

 articulation it gives off an anteriorly directed branch (Fig. 13, AS), 

 which runs forwards and outwards in the roof of the pharynx. 



The artery, AY, Fig. 13, has the course and distribution of the 

 pharyngeal artery of the adult, to which we believe it to correspond. 

 The anterior branch, A S, is probably the posterior palatine artery* 

 of the adult. These arteries acquire new interest if our determina- 

 tion of the pharyngeal artery as formed, at any rate in part, from the 

 efferent vessel of the mandibular arch, proves correct. 



The lacunar system of veins in the floor of the mouth, to which 

 we have referred at the earlier stages, diminishes markedly on the 

 degeneration of the sucker, which is already well advanced at 12 mm. 



b. The Hyoid Arch. — At the stage now being considered (12 mm.) 

 there is no trace left of the vessels belonging to the hyoid arch. 



e. The First Branchial Arch. — The afferent vessel of the first 

 branchial arch (Figs. 13 and 14, A F.I) runs at first almost horizontally 

 outwards and slightly downwards along the ventral border of the 

 arch, lying ventral to the cartilaginous bar of the arch, and behind 

 the efferent vessel ; it diminishes in size as it passes outwards and 

 upwards, and ends on reaching the middle of the side wall of the 

 pharynx ; it gives numerous branches to the gill tufts, and before 

 entering the arch a small branch to the velar fold on the floor of 

 the pharynx. The afferent vessel of the left side is distinctly larger 

 than that of the right side, apparently in connection with the larger 

 size of the left external gill. 



The efferent vessel (Figs. 13 and 14, E F.I) lies just in front of the 



*cf. Ecker, "The Anatomy of the Frog," edited by G. Haslain. Oxford, 1881) ; 

 1-. 224, 



