DEVELOPMENT OF THE FROG. 553 



The lungs are developed as outgrowths from the oeso- 

 phagus, even before hatching, but increase in size very 

 slowly. After the appearance of the hind-legs, the larvae 

 come to the surface of the water to breathe, showing that 

 the lungs are now to some extent functional. At this stage 

 the tadpoles, now about two months old, are at the level of 

 Dipnoi. 



The changes in the relations of the blood vessels, which 

 accompany the successive changes in the methods of 

 respiration, and render these possible, are somewhat com- 

 plicated. 



When respiration is by the gills only, the circulation 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 the two aorta?. The aortas 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 

 aorta?. When the external gills are replaced by the internal, 

 a new set of gill-capillaries are developed, but otherwise the 

 circulation remains the same. As in Ceratodus, a pul- 

 monary artery arises from the fourth efferent branchial. At 

 the time when the hind-legs begin to be developed, a direct 

 communication 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 connection with the rest of the body, 

 soon atrophy, while the lungs become the important 

 respiratory organs. The fate of the various branchial 

 arteries may be gathered from the table on the following 

 page. 



Before, however, all these internal changes have taken 

 place, the external form undergoes a striking metamorphosis. 

 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 



