602 AMPHIBIA. 



three branchial arches. The mouth, which has pre- 

 viously been merely a blind pit, opens into the gut, 

 the gut itself lengthens rapidly, and becomes coiled 

 like a watch-spring ; the larvae feed eagerly on vegetable 

 matter and increase in size. The glandular crescent forms 

 two small discs, which gradually disappear as the power 

 of locomotion increases. About the time when the mouth 

 is opened, four gill clefts open from the pharynx to the 

 exterior. 



A second period, the true tadpole stage, now begins. 

 A skin-fold or operculum covers the external gills, 

 which then atrophy, and are replaced by " internal " 

 gills developed on the ventral halves of four branchial 

 arches. These gills, though called internal, are covered 

 with ectoderm like their predecessors, and are com- 

 parable not to ordinary fish-gills, but to the external 

 gills of Polypterus, Protopterus, and Lepidosiren. The 

 mouth acquires horny jaws, and the fleshy lips bear 

 horny papillae. By the continued growth of the opercular 

 fold the gill-chambers are closed, with the exception of 

 a single exhalant aperture on the left side. Through 

 this opening, the water which is taken in by the moutTi 

 in respiration passes outwards, having washed the gills 

 on its way. 



In the third period the rudiments of the limbs appear. 

 The fore-limbs are concealed within the gill-chambers, and 

 so are not obvious until later; but the hind-legs may be 

 watched in the progress of development from small papillae 

 to the complete limb. 



The lungs are developed as outgrowths from the oeso- 

 phagus, even before hatching, but grow 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 



