58 
Whitney, on the Metamorphosis of the Tadpole. 
placed on the water, whereon the tadpole can mount and 
breathe the air. 
Curious and beautiful is the final stage of the metamor- 
phosis, when the waning tadpole and incipient frog coexist, 
and are actually seen together in the same subject (fig. 10). 
The dwindling gills and the shrinking tail — the last remnants 
of the tadpole form — are yet seen, in company with the 
coloured, spotted skin, the newly formed and slender legs, 
the flat head, the wide and toothless mouth, and the crouching 
attitude of the all but perfect reptile.* 
By the process now described, the three systemic arteries 
(fig. 13) become continuous with the corresponding efferent 
trunks that convey the blood for distribution through the 
body, while, simultaneously, the vital fluid is being abstracted 
from the special trunks belonging to the gill and its vascular 
crests. These, with the gill structure connected with and 
dependent upon them, being thus deprived of their blood, 
shrink, become absorbed, and so disappear. Such appears 
to be the beautifully simple mechanism by which the transi- 
tion in the type of the respiratory function from fish to 
reptile is accomplished. 
We have now to consider the third set of respiratory 
organs, the lungs. When examined in the young, newly 
formed frog, they are found to have arrived at organic and 
functional completeness ; but they coexist (in an incipient 
form) with the gills of the tadpole , and pass through their 
gradations of development simultaneously with those phases 
of maturity, decline, and decay, in the gill organs, whicli have 
been described. If we take a tadpole a few days old, when 
the outer gills are fully developed, and immerse it for another 
few days in a weak solution of chromic acid (Mr. Archer’s 
method), we may, by placing the tadpole under a dissecting 
microscope, and with the aid of a needle and camel-hair 
brush, then remove the integuments, disclose the tufts of the 
inner gills, and by carefully getting rid of a prominent roll of 
intestine that occupies the upper part of the abdomen, succeed 
in revealing the incipient lungs. These are situated behind 
the gut and close to the spine, and appear as a pair of minute 
tubular sacs, united at their upper and open extremities 
(fig. 4). It is much easier to describe than to perform this 
little operation ; but that it has been achieved is amply proved 
* With the loss of the inner gill, the teeth and fringed lip possessed by the 
tadpole also disappear, because the labial artery, whicli supplies these organs 
with blood, has its origin in the gill, and proceeds directly upwards to the 
mouth. Simultaneously, therefore, with the loss of the gill, the oral ap- 
pendages proper to the fish are also removed. 
