52 Whitney, on the Metamorphosis of the Tadpole. 
absorption of the externa t with the commencing development 
of the internal gills (fig. 5). 
With the disappearance of the outer gills, those vascular 
changes by which the inner gills attain their full growth and 
maturity are rapidly developed. These we have already 
traced. It now remains to follow, as closely as we can, the 
vascular arrangements by which the inner gills, having 
accomplished their function as the tadpole’s second set of 
respiratory organs, are, in their turn, to be removed and 
succeeded by the true reptilian organs of respiration — the 
lungs of the frog. 
Each branchial artery, on entering the inner gill, communi- 
cates directly with the corresponding branchial vein by a very 
fine twig, which is, in fact, the radical of the venous trunk 
(fig. 13, /). This trunk, it must be remembered, gradually 
enlarges as it receives into it the newly aerated blood conveyed 
from the crests by the cross branches. Now, if we examine 
the gill in a very transparent subject at the period when the; 
fore legs are about to protrude, or make a successful removal 
of the integument covering it, we shall find that the fine 
connecting twig has, in each instance, enlarged to a channel of 
the same calibre as the artery itself (fig. 9, PI. III). Hence, in 
the case of the upper pair of branchial vessels, more blood 
begins to flow continuously from the gill artery into the efferent, 
or cephalic trunk, without traversing the gill-crests ; in the 
second pair, into the trunk which forms the aorta ; while in 
the third a large part of the current passes at once into the 
developing pulmonary artery and growing lungs. All this 
time the blood is, of course, exposed to the aerating influence 
of the water with which the gill is always bathed, but cannot 
be so thoroughly aerated as when (in the early stage of tad- 
pole life) the entire current traverses the fine plexus of which 
the crests are composed. Thus, the blood begins to assume 
the mixed quality which distinguishes creatures of the rep- 
tilian type. 
At a yet later period (when the legs are protruded) we find 
an obvious diminution in the size of the inner gills, for the 
proportion of blood which then flows continuously into the 
systemic circulation and pulmonary arteries hourly increases, 
while that which yet finds its way into the crests diminishes 
in the same ratio. With the advent of the extremities the 
creature instinctively seeks to breathe in air rather than in 
water ; and so little of real gill function remains that, if tad- 
poles at this stage are confined in a bottle of water, they are 
very apt to die from drowning unless a piece of cork be 
