44 Whitney, on the Metamorphosis of the Tadpole. 
ment, it must be confessed, appears to be applicable even to 
that accomplished and excellent authority, the late Dr. 
Thomas Williams, author of the article “ Respiration ” in 
the supplementary volume of ‘ Todd’s Encyclopaedia.’ 
The tadpole furnishes a remarkably interesting example of 
gill structure and function, because the tadpole gills in tran- 
situ exemplify in turn the simplest and the most elaborate 
forms which these organs of aquatic respiration ever present. 
As a general definition, a gill may be said to be a prolonga- 
tion of surface externally , supplied with blood-vessels, and 
adapted by its form and position for exposure to an aqueous 
medium. On the other hand, a lung is a prolongation of 
surface internally, forming a pulmonary cavity or sac adapted 
to receive air. 
The tadpole is furnished with two sets of gills, the external 
and the internal, and these are intimately connected. The 
vascular anatomy of this connection, upon which depends 
the change whereby the respiratory function is transferred 
from the outer to the inner gills, consequent on the atrophy 
of the former, simultaneously with the development of the 
latter organs, does not, as I have said, appear to be satisfac- 
torily explained. 
The first indication of the external gills is seen in the stage of 
development represented in fig. 1,P1. Ill, in which a slight pro- 
minence, seen just at the junction of the head with the body, 
denotes the point at which the developing gill will protrude. 
In a few hours more the prominence grows into a tubercular 
like swelling, and we may even now begin to trace the motion 
of the blood in these incipient gills. M. Milne Edwards, in 
his large work on c Physiology,’ vol. ii, p. 206, quoting 
Rusconi, says that it is only after birth that the blood begins 
to circulate in the outer gills. But the writer has seen 
examples in which these gills were protruded, with a visible 
vigorous circulation in them, while the tadpole was still 
within the capsule of the egg. When hatched, the form of 
the gill is still more conspicuous, and in twenty-four hours 
after birth (fig. 2) the comb shape of the organ and the 
circulation within it are clearly exhibited. The outer or 
rather outermost skin at this period is a thin but opaque 
lamellated covering, which is gradually removed, and partly 
by the mechanical action of the water, which seems to wash 
away the thin lamellae of scurf-like epidermic scales from the 
true skin beneath. Thus are revealed successively the trans- 
parent coat of the eye, of the external gills, and, by degrees, 
of the rest of the body. And thus the internal organs become 
hourly more perceptible, until in the course of two or three 
