DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 211 



arch is very instructive. From the lateral branch of the truncus 

 arteriosus, just before its division into the afferent trunks for the 

 first and second branchial arches, a short, anteriorly directed diver- 

 ticulum arises (Fig. 6, A H). Its length is about equal to the 

 diameter of the trunk for the first branchial arch, and it ends blindly. 

 Its position and relations inevitably suggest that it is a true afferent 

 trunk ; and inasmuch as it lies just anterior to the trunk for the 

 first branchial arch, it seems justifiable to refer it to the hyoid arch, 

 especially as the other elements of a branchial vessel are present in 

 this arch. 



These other elements consist, firstly, of the lacunar vessel, which 

 at 4| mm. extended along the whole length of the arch (Fig. 4, 

 E H'). At the present stage, 5 mm., the middle portion of this 

 vessel has become obliterated, so that the vessel is divided into two 

 separate portions, of which one (Fig. 6, E H') lies in the dorsal 

 part of the arch, not far from the dorsal aorta, while the other 

 (Fig. 6, V Y) lies in the ventral part of the arch. This ventral 

 portion, V Y, which may be conveniently spoken of as the hyoid 

 vein, opens below, as shown in Fig. 6, into the longitudinal sinus 

 above the sucker, into which, as described above, the mandibular 

 vein, V, M, opens further forwards. 



Besides this lacunar vessel, which clearly corresponds to the 

 efferent vessel of a branchial arch, there is also present, in the dorsal 

 part of the arch, a diverticulum from the dorsal aorta (Fig. 6, E H). 

 Though still a short vessel, this diverticulum is considerably larger 

 than at 4| mm. (cf. Fig. 6, E H). Its distal or ventral end is blind, 

 and lies very close to the dorsal end of the hyoid efferent vessel, E H'. 

 In some, but not all specimens, at a slightly later stage, we have 

 found the two vessels meeting and opening into each other. 



The hyoid arch, therefore, possesses at 5 mm. all the vascular 

 elements found in the branchial arches excepting the afferent 

 branchial vessel. These elements show unmistakeable signs of 

 degeneration. The vascular arch is never completed j the efferent 

 lacunar vessel has already become obliterated in the middle part of 

 its course j the diverticulum from the truncus arteriosus is very small, 

 it never becomes connected with the other vessels of the arch, and it 

 disappears altogether at a slightly later stage; and the diverticulum 

 from the aorta, though rather better developed than that from tho 



