DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 215 



The condition of the heart is worthy of notice, especially if it 

 be remembered that as yet the truncus arteriosus ends blindly in 

 front. 



IV. THE CONDITION OF THE HEART AND BLOOD VESSELS 

 IN 6|mm. TADPOLES. 



1. External Characters. 



As in the previous stage, the increase in length is due almost 

 entirely to growth of the tail, which now forms nearly half the 

 entire length of the tadpole. Apart from this, the chief changes 

 are as follows : — The body is much more slender, and the ventral 

 surface less protuberant, owing to continued absorption of the yolk. 

 The dorsal fin is much more prominent, and extends forwards to the 

 hinder end of the head; the ventral fin also is better developed 

 than before, beginning immediately behind the yolk mass, and having 

 the cloacal aperture on its ventral edge. 



In the head, the sucker is of large size, as before; the stomatodseal 

 depression is deeper and better defined than at the earlier stages, 

 and is overhung by a distinct upper lip. The external gills of the 

 first and second branchial arches are much larger than before ; they 

 are directed outwards and backwards, the first partially overlapping 

 the second, which is placed rather more dorsally. Each gill is deeply 

 notched along its posterior border into four to five lobes, which decrease 

 in size from above downwards ; the uppermost lobe is much the 

 largest, and gives off minor lobes along its posterior or inner surface. 

 External gills are present as small unbranched processes on the third 

 branchial arch as well, but are overlapped and concealed by the gills 

 of the first and second arches when the animal is viewed from the side. 



The head is separated from the body by a distinct constriction, 

 immediately behind which a rounded swelling is formed by the 

 head kidney on each side. 



Though the mouth is not yet open, the tadpole shows a distinct 

 increase in bulk as compared with the 5 mm. stage. It has occurred 

 to us as possible that the suckers may be used for absorbing food, and 

 that in this way the increase may be explained. Tadpoles up to about 

 this age adhere by their suckers to the gelatinous mass of the spawn 



