242 PROFESSOR MARSHALL AND EDWARD J. BLES. 



the efferent vessel. (2) An outgrowth from the dorsal aorta. (3) 

 'An outgrowth from the truncus arteriosus. 



These three factors develope in the order here given ; they arise 

 quite independently of each other, and have precisely similar relations 

 in all four branchial arches. Later on, the outgrowth from the 

 aorta joins with the dorsal end of the efferent gill vessel, and the 

 outgrowth from the truncus arteriosus joins with the ventral end 

 of the afferent gill vessel, and in this way the gill circulation is 

 definitely established. 



In the hyoid arch the same three factors are present, arising in 

 the same order and in the same manner. 



At 4J mm. (Fig. 4) there is in the hyoid arch an elongated lacunar 

 vessel extending almost the entire length of the arch ; there is also 

 a small diverticulum of the dorsal aorta (Fig. 4, E H) lying opposite 

 and close to the dorsal end of the venous lacuna, but not meeting it ; 

 and there is as yet no outgrowth from the truncus arteriosus. 



At 5 mm. (Fig. 6) the lacunar vessel has divided into dorsal and 

 ventral portions, of which the dorsal is blind, while the ventral 

 (Fig. 6, V Y) opens into a lacunar space above the sucker. The 

 diverticulum from the aorta, E H, is larger than before, but is still 

 blind ; and there is a small blind diverticulum from the truncus 

 arteriosus, A H. 



At 6J mm. (Fig. 7) the dorsal portion of the lacunar vessel has 

 disappeared ; the ventral part, V Y, has the same relations as before ; 

 the diverticulum from the aorta, E H, is present, but very small ; 

 and the diverticulum from the truncus arteriosus has disappeared. 



At 9 mm. (Fig. 11) the diverticulum of the aorta has disappeared, 

 and the only vessel remaining in the arch is the ventral portion of 

 the lacunar vessel, V Y, which has the same relations as before. 



At 12 mm. (Fig. 13) the lacunar vessels of the floor of the mouth 

 have become modified into definitely arranged veins, and it is no 

 longer possible to refer any of these distinctly to the hyoidean vessels 

 of the earlier stages. 



These stages appear to us to form a consistent history of the 

 changes undergone by the hyoidean vessels, and to enable a satis- 

 factory comparison to be made between them and the vessels of 

 the branchial arches. The only vessel of the typical branchial set 

 not represented in the hyoid arch is the afferent branchial vessel. 



