256 PROFESSOR MARSHALL AND EDWARD J. BLES. 



The changes required to convert this condition of the truncus 

 arteriosus into that of the adult are very slight. We have not 

 ourselves followed out these changes, and would refer for details 

 of the mode in which they are effected to the paper by Boas quoted 

 above. 



4. The Blood Vessels of the Visceral Arches. 



a. and b. The Mandibular and Hyoid Arches. — These have under- 

 gone no changes of importance since the last stage : the vessels 

 of these arches have, in fact, already acquired their adult arrange- 

 ment. 



c. The First Branchial Arch. — Both afferent and efferent vessels 

 are present, and the relations are almost exactly those of the last 

 stage, the only change of importance concerning the communication 

 between the afferent and efferent vessels at the base of the gill. 



In the former stage this communication was effected by a single 

 short passage of capillary size, while at the present stage it takes 

 place through a well-developed plexiform carotid gland. From this 

 plexus the lingual artery arises by two roots, which embrace the 

 gland, one lying above it and one below. 



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

 larger than those of the first branchial arch ; they are also larger on 

 the left side than the right. We have found extreme difficulty in 

 determining whether a direct connection between the afferent and 

 efferent vessels of this arch is or is not present at this stage. In 

 some specimens such a connection appears to exist, while in others 

 we have entirely failed to find it. If it is constantly present, it is 

 certainly far less obvious in this than in the other arches. 



e. The Third Branchial Arch. — The- vessels of this arch are 

 decidedly smaller than those of the first or second arches. A direct 

 connection between the afferent and efferent vessels is constantly 

 present at the base of the gills. 



/. The Fourth Branchial Arch. — Both afferent and efferent vessels 

 are present ; they are smaller than those of the other arches, and lie 

 very close together. Direct communication between the two is 

 present in some cases, but we have not been able to detect it in all 

 specimens. The dorsal end of the efferent vessel opens into the 

 aorta immediately behind the third efferent vessel, but independently 



