DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 201 



vessel by lacunar channels. Above the gill the efferent vessel narrows 

 rather suddenly, and then continues upwards as a definite cylindrical 

 vessel to the dorsal aorta, into which it opens (Fig. 4, A). 



The efferent vessel has a well-developed wall of flattened epithelial 

 cells ; its cavity contains a few blood corpuscles, which also occur, 

 though in smaller numbers, in the afferent vessel. 



d. The Second Branchial Arch. — The vessels of this arch are very 

 similar to those of the first branchial arch. 



The afferent vessel (Fig. 4, A F") is lacunar in character, and of 

 very small size. It lies opposite the base of the second external gill, 

 and communicates through one or more narrow intercellular passages 

 with the efferent vessel ; below the gill it extends a very short 

 distance ventrally. 



The efferent vessel (Fig. 4, E F") is about the same size as that of 

 the first branchial arch, and has very similar relations. It is widest 

 opposite the external gill ; below this point it narrows rather suddenly, 

 and runs downwards in the arch to the level of the floor of the pharynx, 

 where it ends blindly some little distance from the truncus. Above 

 the gill it also narrows suddenly, and then continues upwards 

 and inwards as a cylindrical vessel with more definite walls than 

 before to the dorsal aorta, A, into which it opens. It contains blood 

 corpuscles. 



e. The Third Branchial Arch. — A small lacunar space or cleft 

 (Fig. 4, E F'"), about the level of the external gills, is the only vessel 

 present in this arch. It apparently represents the commencement of 

 the efferent vessel of later stages. 



/. The Fourth Branchial Arch. — In this there are as yet no vessels. 



5. The Dorsal Aort/e. 



Lying along the roof of the pharynx there are two aortas, one on 

 each side (Fig. 4, A). Opposite the hinder end of the pharynx they 

 are almost parallel to each other, and lie just above the pharynx and 

 a little ventral to the notochord. Passing forwards, the two aortas 

 diverge from each other very markedly; they are furthest apart 

 opposite the first branchial arch, in front of which they converge 

 again. Opposite the hyoid arch each divides into a dorsal and a ventral 

 branch. The dorsal branches of the two sides (Fig. 4, A M') run 

 upwards and inwards, and meeting each other in the median plane, 



