188 THE FROG. 



hinder end the duct grows backwards, and, shortly before the 

 hatching of the tadpole, acquires an opening into the cloaca, 



The condition at the time of hatching is shown in Figs. 74 

 and 82. The head kidney, KP, is a slightly convoluted tube 

 opening into the body cavity by the three nephrostomes, ks ; of 

 which the anterior one is at a level slightly ventral to tliat of 

 the other two. Opposite the nephrostomes, and projecting into 

 the dilated dorsal portion of the body cavity, is the glomerulus 

 (Fig. 82, gm), a sacculated diverticulum of the aorta, the develop- 

 ment of which has been described above. Behind the nephro- 

 stomes, the segmental duct is continued backwards in an almost 

 straight course to the hinder end of the body, where it opens 

 into the cloaca (Fig. 74, ka). 



After the tadpole has hatched, the head kidney increases 

 considerably in size (Figs. 76, 83, and 84). The tubules of which 

 it consists become more markedly convoluted; and further 

 complication is caused by the formation of lateral diverticula, of 

 which there are three principal ones, directed outwards, and 

 themselves branched. The three nephrostomial openings persist ; 

 their walls are formed of pigmented cells, bearing long flagella, 

 directed inwards. The head kidney reaches its full develop- 

 ment in tadpoles of about 12 mm. length (Figs. 83 and 84). 

 when it is almost spherical in shape, and formed of an intri- 

 cately coiled mass of tubes, imbedded in the greatly dilated 

 anterior cardinal sinus, vc. 



In tadpoles of about 20 mm. length, the head kidneys 

 commence to degenerate. The first step in the process consists 

 in irregular dilatation of the tubules, more especially of the 

 blind lateral diverticula. This dilatation may be so great that 

 a single tubule may nearly equal in diameter the entire head 

 kidney of the earlier stages ; but as the dilatation of . some 

 tubules is accompanied by compression of others, the size of the 

 head kidney as a whole remains practically unaltered. 



This irregular dilatation is accompanied by, and perhaps 

 due to, partial or complete obstruction of the segmental duct 

 behind the head kidney. The epithelium of the kidney tubules 

 soon shows degenerative changes, the cells becoming flattened 

 out in an irregular manner, their outlines indistinct, the cell con- 

 tents cloudy, and the inner surfaces of the cells ragged. From 

 this time degeneration proceeds steadily, and the whole organ 



