218 PROFESSOR MARSHALL AND EDWARD J. BLES. 



from the dorsal aorta (Figs. 6, 7, and 9, E M) must certainly be 

 regarded as homologous with the similar diverticula in the hinder 

 arches, as in relations and mode of development it is identical with 

 these. The lacunar vessel (Figs. 6 and 7, V M), which we have 

 termed mandibular vein, differs markedly from the efferent lacunar 

 vessels of the branchial arches in appearing first in the ventral part of 

 the arch (Fig. 6), instead of at the level of the external gills ; it differs 

 further in being from the first in connection at its ventral end with 

 the lacunar sinuses above the sucker. The condition of the hyoid 

 arch appears, however, to afford the clue. At 5 mm. the vessels, V Y 

 and V M (Fig. 6), lying in the ventral parts of the hyoid and man- 

 dibular arches respectively, have precisely similar relations. The 

 vessel, V Y, is shown by the 4 \ mm. stage (Fig. 4) to be the ventral 

 end of the hyoid efferent vessel. It seems reasonable, therefore, to 

 regard the mandibular vein, V M, as the efferent vessel of the man- 

 dibular arch. The fact that at 6-| mm. (Fig. 7) the mandibular vein has 

 extended dorsalwards, and is directly and unmistakeably continuous 

 with the aortic diverticulum, E M, affords evidence of the strongest 

 kind in support of this determination. 



On this view, the chief peculiarity of the vessels of the mandibular 

 arch is that, instead of the dorsal part of the efferent vessel being 

 the first to form, it is the ventral end that developes earliest ; and 

 this peculiarity may fairly be ascribed to the absence of a gill, and 

 to the presence and physiological importance of the sucker, with 

 which the mandibular vein is in close relation. 



It may be further noted that there is no room for doubt as to 

 these vessels belonging to the mandibular arch, inasmuch as they lie 

 clearly and distinctly in front of the hyomandibular cleft (c/. Fig. 9, 

 E M). 



From the relations of the vessels, it follows that the flow of blood 

 must be from the dorsal aorta ventralwards towards the sucker. 



b. The Hyoid Arch. — The small diverticulum of the truncus 

 arteriosus that was present at 5 mm. has disappeared entirely, 

 no vestige of it remaining. The diverticulum from the dorsal aorta 

 is still present, but is much shorter than before, and ends blindly, 

 (Fig. 7, E H). 



The only other vessel that belongs to the arch is a large sacculated 

 vein in the ventral half of the arch (Fig, 7, V Y), which commences 



