171 THE PROG. 



afferent and efferent vessels, close to the truncus arteriosus; 

 (Fig. 80). The precise mode in which the communication is 

 established will be described in the section dealing with the 

 development of the carotid gland (p. 181). As the communi- 

 cation is ventral to the gills, both external and internal, any 

 blood which passes across it will get from the heart direct to the 

 aorta, without passing through any part of the gill circu- 

 lation, i.e. without being aerated ; and the efficiency of the 

 gill respiration will consequently be impaired in direct propor- 

 tion to the amount of blood which takes this short cut in 

 preference to the circuitous route through the gill capillaries. 



The subsequent changes in the vessels of the first branchial 

 arch may conveniently be considered after the vessels of the 

 other arches have been described. 



b. The vessels of the second branchial arch (Figs. 77, 78, 

 and 80, af.2, EF.2) develop in exactly the same way as those of 

 the first branchial arch, and almost simultaneously with these. 



c. The vessels of the third branchial arch (Figs. 77, 78, and 

 80, AF.3, EF.3) are formed rather later. They are of smaller size 

 than those of the first and second branchial arches, but in other 

 respects are similar to these. The external gill, and the vessels 

 supplying it, are considerably smaller than those of the two 

 arches in front. 



d. The vessels of the fourth branchial arch (Figs. 77, 78, 

 and 80, af.4, EF.4) arise still later, but in essentially the same 

 manner, except that no external gill is formed *on this arch. 

 The vessels are well established before the mouth opening 

 appears. From the dorsal end of the efferent vessel of this arch 

 the pulmonary artery, ap, arises as a diverticulum which grows 

 backwards along the outer side of the lung (cf. Fig. 76, p. 166). 



It should be noticed that the pulmonary artery arises, and 

 acquires its relations with the lung, before the afferent branchial 

 vessel of the arch has joined the diverticulum from the truncus 

 arteriosus ; indeed, before this latter has commenced to develop 

 (cf Fig. 78). Consequently, the only blood that can at this 

 stage reach the lung is blood from the dorsal aorta, and not blood 

 from the heart ; in other words, the lung in the early stages of 

 its development receives arterial and not venous blood. 



The afferent vessels of the fourth branchial arch develop 

 very late ; and the afferent diverticulum from the truncus 



