TADPOLE, CIRCULATION IN. 687 



crest, whence a returning vessel may be traced carrying 

 back the blood across the branchial artery, and to a vessel 

 lying close to and taking the same course as the artery 

 itself. Carrying the eye along the latter vessel we find, 

 at a short distance from the first of these crest branches, a 

 second, which leaves the main trunk and enters the crest, 

 when a corresponding returning vessel conveys the blood 

 across the arterial trunk into the vessel lying beside it, as 

 in the former instance. A succession of these branches 

 (each taking a similar course) may be traced from one end 

 of the crest to the other. But it is now to be observed 

 that the trunk from which these arterial branches spring 

 diminishes in size as it proceeds in its course (like the gill 

 artery in fishes), while the vessel running parallel to it and 

 receiving the stream as it returns from the crest enlarges in 

 the same degree. Thus, the artery or afferent vessel which 

 brings the blood to the gill is large at its entrance, but 

 gradually diminishes and dwindles to a point at the op- 

 posite end of the crest ; while the venous or efferent vessel, 

 beginning as a mere radical, gradually enlarges, and thus 

 becomes the trunk that conveys the blood out of the gill 

 to its ultimate destination. Calling this vessel the upper 

 branchial vein as long as it remains in contact with the 

 gill, we subsequently change its name when it leaves the 

 gill and winds upwards for distribution to the head, and 

 then designate it the cephalic artery. The middle branchial 

 artery and vein proceed in like manner in connexion witli 

 the middle crest, and the lower artery and vein in connexion 

 with the lower crest. The middle and lower venous trunks, 

 having reached the extremity of the crests, curve down- 

 wards and inwards, and then leave the gill. The former 

 trunk, converging towards the spine, meets its fellow, and 

 with it forms the ventral aorta. The latter gives origin to 

 the pulmonary artery, and supplies also the integuments 

 of the neck. Curious and beautiful is the final stage of 

 the metamorphosis, when the waning tadpole and incipient 

 frog coexist, and are actually seen together in the same 

 subject. The dwindling gills and the shrinking tail — the 

 last remnants of the tadpole form — are yet seen, in com- 

 pany with the coloured, spotted skin, the newly-formed 

 and slender legs, the flat head, the wide and toothless 



