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 aortas. The aortae 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 aortse. 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 Ceratodus, 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). 



