470 On Progressive Development (Snr. 
having a free margin, are fixed, being connected with the integument 
by their external border. The consequence of this conformation is, 
that the water which passes over the branchiz makes its exit through 
distinct canals opening on the surface, whose number varies from 
four to seven in different genera of the order. 
In all this may be observed an evident tendency to a higher degree 
of development, an attempt on the part of nature to cause the respi- 
ratory apparatus of the most perfect of the class of fishes to assume 
the appearance of that possessed by the most inferior among reptiles, 
and the next step will be to inquire whether there is not to be found 
some intermediate state between the two. 
The larva of the common frog is, during its larva condition, bond 
fide, a fish ; its respiration is aquatic ; its circulation double; it possess- 
es four branchiz on each side, suspended from branchial arches, not 
enclosed however by an operculum as in fish, but hanging free from 
each side of the neck. The heart consists of two cavities, an auricle 
and a ventricle ; the whole of the blood passes through the branchiz by 
four branchial arteries on each side; it is returned by as many branchial 
veins, which afterwards unite to form the abdominal aorta. This cir- 
culation is strictly branchial not systemic, and is in every respect the 
circulation of a fish. During this fish-like condition of the larva, the 
spinal cord presents no enlargements in its course, and extends down 
through a number of coccygeal vertebre ; at this period also the optic 
lobes are larger than the hemispheres of the cerebrum, as in fish. 
This then may be considered to be the intermediate point of deve- 
Jopment between two series of forms of animal life, and here is the 
stage from whence to set out in marking the changes which are re- 
quired to render, not only the same type, but the same individual capa- 
ble of exercising its functions in a medium very different from that in 
which it originally existed. 
After remaining in its icthyoid condition for an indefinite period of - 
time, the duration of which is influenced by a variety of circumstances 
immediately affecting the development of the animal, as temperature, 
the action of light, the abundance or scarcity of food, &c. the tadpole 
begins to undergo certain changes, which are the prelude to a complete 
metamorphosis; changes which are to give it the organs and habitudes 
of a land animal, and enable it to act a part in a situation totally foreign 
to that to which it has been accustomed. 
This first of this series of changes takes place in the nervous system. 
The direction of development, which has hitherto been longitudinal, 
becomes lateral ; the spinal cord shrinks up, and the coccygeal verte- 
