THE YOUNG SCIENTIST. 
35 
closed by the skin, which has been formed 
for some time ; and it is at the expense of 
this alimentary matter that development 
proceeds. The external characters are in 
keeping with the imperfect condition of 
the animal at this period. The head is 
large, and appears to be divided in two 
on the under surface, each half being pro- 
longed as a sort of process by which the 
animal attaches itself to surrounding ob- 
jects ; as yet there are no traces of either 
eyes, nostrils, respiratory or auditory or- 
gans ; and the belly, of an oblong form, 
is continued posteriorly as a short tail, 
bordered with a riband-like membrane. 
About the fourth day after birth, the head, 
which is now as long as the body, has 
somewhat the appearance of a thimble; 
the mouth is provided with a pair of soft 
lips ; the nostrils, eyes, and auditory ap- 
paratus have made their appearance ; the 
head is separated by a' deep groove from 
the belly, which has assumed a spherical 
form, and from which spring a pair of 
opercula, clothed with little branching 
gills. And the tail has grown so much 
that it is now quite as large as the bodj^ 
The mouth is very soon armed with a 
horny beak, capable of dividing the vege- 
table food. The intestine, which is ^now 
very large, becomes more fully formed, 
and assumes a spiral arrangement. The 
tail is elongated and widened, and the little 
creature is then called a tadpole. At this 
period, one of those alterations occurs 
which are so intimately associated with 
the ideas we are endeavoring to convey, 
that we must not pass them by in silence. 
Our larva first breathed by its skin alone, 
and afterwards by a pair of little T3ranch- 
ing gills attached to the opercula. About 
the seventh or eighth day the opercula 
are gradually soldered to the abdomen, 
and the gills fade away and disappear. 
At the same time a set of new and more 
complex branchia are developed in the 
chambers situate on either side of the 
neck. The new gills are arranged in tufts 
attached to a solid framework of four car- 
tilaginous arches, and are about one hun- 
dred and twelve in number for each side 
of the body. Before the tadpole can be- 
come a frog, it must do away with these 
second gills and replace them by lungs ; 
and at the necessary time a set of changes 
takes place analogous to those we have 
described. The vascular tufts are atro- 
phied, and the lungs, which till now were 
solid and rudimentary, open up and in- 
crease in size. The circulatory organs are 
correspondingly modified. The calibre of 
the large bronchial organs is diminished, 
and the pulmonary trunks increase in 
number and diameter. Later on the solid 
parts of the branchial apparatus disap- 
pear also, the bones and cartilages being 
gradually absorbed. Eventually the alter- 
ation is fully accomplished, and there re- 
mains not the slightest trace of the former 
branchial apparatus. In this instance, 
not only has there been transformation 
and substitution, but an actual metamor- 
phosis has occurred ; for the respiration, 
which was aquatic before, has become at- 
mospheric, and, strictly speaking, the 
animal from having been a fish has been 
converted into a batrachian. If we ex- 
amine any particular apparatus, we shall 
find it presenting many curious phe- 
nomena in the course of its development. 
We shall find that as the herbivorous 
habits give place to carnivorous ones, the 
digestive apparatus undergoes a change 
adapting it to the new form of diet. The 
mouth increases in size and gape; the 
little beak organs, or, more correctly, the 
horny lips, are replaced by teeth. The 
tadpole at first exhibits no trace of either 
external or internal limbs. It swims 
about like a fish by the action of its tail, 
which is an extensive organ, longer and 
wider than the body, supported by a pro- 
longation of the vertebral column, moved 
by powerful muscles, and supplied with 
large blood-vessels and numerous nervous 
branches. Beneath the skin and muscles 
of the anterior and posterior regions of 
the body, two little projections appear at 
a certain period. These are the limbs, 
and are at first attached to the adjacent 
structures by the nerves and blood-vessels 
which are supplied to them. These projec- 
tions increase in size, their appendages ap- 
pear in due course, and eventually the hip 
and shoulder bones are developed. As 
soon as these locomotive organs enter 
upon the discharge of their functions, the 
tail begins to disappear. Its skin, mus- 
