AMPHIBIA 233 



The metamorphosis is ushered in by the appearance of 

 the limbs and the development of lungs. The auricle becomes 

 divided into two chambers by the appearance of a septum 

 which springs from the dorsal wall, and at the same time the 

 conus develops the longitudinal valve, which may be regarded 

 as a septum which remains incomplete in the frog. A pul- 

 monary vein is established between the smaller left auricle 

 and the lung, and the fourth branchial arch yields a branch 

 which goes to the lung. It also subsequently sends a branch 

 to the skin. The fate of the four branchial arches may be 

 simply said to be : the first forms the carotid, the second the 

 systemic or aorta, the third disappears, and the fourth, losing its 

 connexion with the dorsal aorta, becomes the pulmo-cutaneous. 

 The gills cease to function, and the lungs are used, together 

 with the skin, for respiration. The intestine of the larva is 

 conspicuously long and folded in a spiral. Towards the end 

 of the tadpole life it gradually loses this condition, and with 

 the cessation of feeding at metamorphosis it actually becomes 

 shorter. The horny jaws are discarded, and the mouth is 

 enlarged, together with the tongue. The eyes are also expanded, 

 and with the appearance of the hind-limbs, and then of the 

 fore-limbs, and the atrophy of the tail, the conditions of the 

 adult are practically reached. The tail shrivels up by a break- 

 ing down of its elements, which are carried away in the blood, 

 the process beginning at the tip. When the metamorphosis 

 is completed the young frog migrates from the water. 



