582 AMPHIBIA, 



the larva escapes from the surrounding jelly and swims 

 in the water. At this stage and for some time the 

 ectoderm is ciliated. There is a cloacal opening, but 

 the mouth is not yet more than a dimple. A glandular 

 crescent, often misnamed a sucker, lies on the under sur- 

 face of the head, and secretes a sticky slime, by means of 

 which the tadpole attaches itself to foreign objects. The 

 external gills soon become branched. . There are three 

 of them on each side, the first the largest. The mouth, 

 which has previously 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 four branchial arches. 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 mouth 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 a later stage ; 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 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 



