DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 249 



Owing to the greatly developed lymph spaces, the body of the 

 tadpole, when viewed by transmitted light, appears surrounded by a 

 semi-transparent margin or fringe. 



2. Internal Anatomy. 



The mouth is still on the ventral surface, but is more anteriorly 

 placed than before. The buccal region has grown forward con- 

 siderably, so that the anterior end of the brain is now a long way 

 behind the front of the head. The pharynx has much the same 

 shape as before ; on its floor are two rows of papillae, converging 

 posteriorly. At the hinder and lateral part of the pharynx are the 

 gill clefts, overhung in front by the velar folds. There is as yet 

 no tongue. 



The first branchial cleft opens into the side of the pharynx, just 

 in front of the outer end of the velar fold, and at the hinder end of 

 a groove (cf. Fig. 14, C G), which runs obliquely backwards and 

 outwards across the floor of the mouth. 



The filters on the inner borders of the branchial arches are greatly 

 developed : the inner border of each arch is raised into a prominent 

 longitudinal fold, which bears on both its faces closely set transverse 

 ridges, notched along their free edges in a complicated manner into 

 series of tooth-shaped processes (cf. Fig. 15, F.I, 2,3,4). These filters 

 are extremely vascular, but as the blood is returned from them 

 to the somatic veins, it is probable that they are not actively 

 respiratory. 



The external gills are absent, and the internal gills (Fig. 15, 

 G L. 1-4) are arranged as before, but are rather better developed. 



The fore limbs project into the upper angles of the opercular cavity 

 as solid blunt processes, about twice as long as they are broad. 



The alimentary canal has greatly increased in length, the intestine 

 being much convoluted. The head kidneys are greatly reduced in 

 size, and their tubules are in process of rapid degeneration. The 

 Wolffian bodies are now well developed ; their anterior ends are 

 situated some distance behind the head kidneys, while their posterior 

 and larger ends extend into the hinder part of the body cavity. 



The lungs are large, thin walled, and filled with air; they lie in 

 the abdomen close to its dorsal wall, above all the other abdominal 

 viscera and about the level of the notochord. 



