54 Whitney, on the Metamorphosis of the Tadpole. 
by the several very interesting preparations which my friend 
Mr. Archer has successfully made. The chromic acid renders 
the tissues friable, so that they can be readily peeled away. 
During the period of the tadpole’s transparency a part of 
each lung is usually perceptible, and may be seen, on looking 
through either side of the abdomen, as a transparent, saccu- 
lated bladder of air. On one occasion I met with an example 
in which the tadpole exhibited the lungs, through the back, 
in a very beautiful and unusual manner. They were lying 
side by side, close to the spine, and, to the naked eye, looked 
like a double row of brilliant globules, glistening like quick- 
silver. (Fig. 11 represents them as seen under the micro- 
scope.) This singular display was due to a temporary shifting 
of the bowels, whereby the lungs were distinctly brought 
into view ; for on looking at the same tadpole on the follow- 
ing day I found that the bowels had changed their position, 
and then the lungs were concealed. Fig. 7 represents the 
lungs in a tadj^ole at about the middle period of the meta- 
morphosis, showing the progressive development and elonga- 
tion of the tubular sacs. 
When the last vestiges of the tadpole form — the remnants 
of the tail and gills — have disappeared, the lungs, though 
small and extremely delicate, are found to present a perfect 
miniature of what they afterwards become in the full-grown 
frog. Of all the sights to be seen in tracing the metamor- 
phosis of the tadpole, the most splendid is that of the crimson 
gill with its vital current in vigorous circulation. Next to 
that, as something beautiful to see, is the appearance presented 
by the lung of the young frog with its circulation in vigorous 
movement. To obtain this sight the lungs should be full of 
air, and the heart vigorously beating. These conditions, 
however, are seldom easy to attain. Put the young frog into 
a wine-glass, and drop upon him a single drop of chloroform. 
This suffices to extinguish sensibility. Then lay him on the 
back on a piece of cork, and fix him with small pins passed 
through the web of each foot. Remove the skin of the 
abdomen with a fine pair of sharp scissors and forceps. Turn 
aside the intestines from the left side, and thus expose the left 
lung, which may now be seen as a glistening, transparent 
sac, containing air-bubbles. With a fine camel-hair pencil 
the lung may now be turned out, so as to enable the operator 
to see a large part of it by transmitted light. Unpin the frog 
and place him on a slip of glass, and then transmit the light 
through the everted portion of lung. Remember that the 
lung is very elastic, and is emptied and collapsed by very 
slight pressure. Therefore, to succeed with this experiment. 
