DEVELOPMENT OF THE FROG. 603 
is essentially that of a fish. From the two-chambered 
heart the blood is driven by afferent branchials to the . 
gills ; from these it collects in efferent vessels which 
unite on each side to form two aorte. The aorta’*send 
arteries to the head, and passing backwards unite to 
form the single dorsal aorta which supplies the body. 
For a time there are two dorsal aorte. When the first 
set of gills is replaced by the second set, new gill- 
capillaries are developed, but the circulation remains 
the same. As in Cevatodus, a pulmonary artery arises 
from the fourth efferent branchial. At the time when 
the hind-legs begin to be developed, a direct com- 
munication is established between afferent and efferent 
branchial vessels, so that blood can pass from the heart 
to the dorsal aorta without going through the gills. 
As the pulmonary circulation becomes increasingly 
important, the single auricle of the heart becomes. 
divided into two by a septum, and the pulmonary veins 
are established. At the time of the metamorphosis an 
increasing quantity of blood avoids the gills in the manner 
indicated above, and these, being thrown out of con- 
nection with the rest of the body, soon atrophy, while 
the lungs become the important respiratory organs. The 
fate of the various branchial arteries is shown in the table 
on the following page. 
The tadpole has by this time grown large and strong, 
feeding in great part on water-weeds. Now it seems to 
fast, but the tail,.which begins to break up internally, 
furnishes, with the help of phagocytes, some nourishment 
to other parts of the body. The habit becomes less 
active, the structural adaptations to the aquatic life 
disappear. “The horny jaws are thrown off; the large 
frilled lips shrink up; the mouth loses its rounded 
suctorial form and becomes much wider; the tongue, 
previously small, increases considerably in size; the eyes, 
which as yet have been beneath the skin, become 
exposed ; the fore-limbs appear, the left one being 
pushed through the spout-like opening of the branchial 
chamber, and the right one forcing its way through 
the opercular fold, in which it leaves a ragged hole” 
(Marshall). 
