Whitney, on the Metamorphosis of the Tadpole. 45 
weeks from date of birth, the entire skin, freed from those 
pigmentary scales, attains its highest degree of transparency, 
and heart, and gills, liver, intestines, and blood-vessels, are 
seen in all the brilliancy of life and motion, and with as much 
clearness as if we were looking at them through an integu- 
ment of glass. 
To return to the external gills. On the fourth or fifth day 
after birth, and after the young tadpole is freed from the 
remnants of the gelatinous mass he has been feeding on, and 
has begun to live on vegetable diet, the external gills reach 
their highest point of development. In this condition they 
appear to the naked eye (fig. 3) on either side as a pair of 
depending, slender filaments, delicately fringed. The length 
of the filaments differs in different instances. The example 
here figured was drawn from one of a batch of tadpoles, all 
remarkable for the length to which these filaments had 
grown, and equally remarkable, therefore, for their grace and 
beauty. On examining with the microscope we find these 
gills projected through an opening or fissure, situated on 
either side below the head, corresponding to the operculum in 
fishes. With the aid of the glass we resolve each filament, 
with its fringe, into a transparent, comb-shaped case (fig. 4, a), 
containing within it an arterial and venous system, to be 
presently described. The fringe (to follow our simile) corre- 
sponds exactly with the teeth of a comb, and, like them, 
consists of processes (about six), all proceeding at right 
angles from one side of the filament or back of the comb. 
The filaments on either side are traceable inwards through 
the fissure, where they become immediately and closely con- 
nected with a bed or cluster of small digital-like processes, 
some of which may be seen (at an early stage) just projecting 
through the operculum fissure. These beds or tufts are, in 
fact, the internal gills in their incipient, undeveloped state 
(fig. 4, b). 
The next perceptible change is a shortening of the 
filaments, with a corresponding retraction of the processes, 
which appear to be drawing inwards through the operculum. 
At the end of the first week, under favorable circumstances 
of temperature and light, the external gills are found to be 
fast disappearing, while just prior to their complete removal 
we may note a change in th e form of the tadpole. The con- 
traction below the head (fig. 3) is obliterated by a filling out 
in this direction, so that the tapering shape is exchanged for 
a square or rather rounded form (fig. 5). At this stage 
the body of the tadpole is daily becoming more transparent ; 
but not until th e external gills have nearly disappeared is the 
