46 Whitney, on the Metamorphosis of the Tadpole. 
skin sufficiently clear of pigment to afford us a glimpse of the 
heart and other internal organs. The motion of the heart is 
detectable, but its shape , at present, is indefinite, and the 
blood-vessels are imperceptible. Hence the difficulty of 
tracing the vascular connection of the outer gills with the 
heart and inner gills ; for upon this “ the change or conver- 
sion from external to internal gills ” evidently depends. 
Hitherto I have not succeeded in obtaining, either by 
artifice or chance, a tadpole at this stage sufficiently trans- 
parent to exhibit to the eye this vascular connection. 
Nevertheless, by a demonstration of the vascular system of 
the internal gills, effected at a little later period, we may 
obtain, I think, a satisfactory key to the comprehension of 
that which exists, at the stage we are speaking of, in refer- 
ence to the outer gills. But the reader will presently judge 
for himself of the merits of this conclusion. The right external 
gill shortens, is retracted, and disappears, sooner than the 
left one. The gill is not simply retracted within the fissure, 
but there is an absolute shrinking, withering, and ultimate 
absorption of its substance. When this right gill has shrunk 
within the fissure the latter is closed completely, so that the 
gill chamber on that side is shut up. The remnant of the 
gill, tucked in, as it were, is still perceptible, as represented 
in fig. 6, c. In a few days this remnant is reduced to a bit of 
shapeless substance, as seen in fig. 12, PI. IV, d , which, in a few 
hours, is entirely removed. On the left side the process of re- 
traction is slower. A small protrusion of left gill may be seen 
for a day or two after the right one has disappeared ; and 
when the left is absorbed the operculum on that side is found 
to remain open as the aperture of exit for the water that 
bathes the internal gills. The permanent form of the oper- 
culum is seen in fig. 6. I have alluded to the changes of 
form which the body of the tadpole undergoes during the 
period in which the external gills shrink and disappear 
(figs. 3 & 5). This change accompanies the first stage of 
development in the second set of respiratory organs — the 
internal gills. Fig. 4 represents the tufts of digit-like pro- 
cesses composing the inner gills in their first stage, while the 
outer gills are yet in full development. This figure is com- 
posed chiefly from some dissected preparations made by my 
friend Mr. Archer, after immersing the tadpole for a few days 
in a weak solution of chromic acid. In this very early stage 
the opacity of the skin renders it impossible to see the inci- 
pient tufts composing the inner gills while the tadpole is alive; 
hence the value of Mr. Archer’s preparations as exhibiting 
th e form and position of the tufts at this period; but they 
