192 THE FEOG. 



T he Wolffian tubules are, at first, solid masses of spheric al 

 c ells; th ese arise in the mesoblast of the body wall, and are for 

 a time independent of bot h the peritoneum and the segment al 

 ducts. T he cellular masses soon become elongated into soli d 

 rods ; by separation of the component cells along their axes, the 

 rods become tubes; and these tubes, growing outwards and 

 towards the dorsal surface, meet with, the segme ntal ducts and 



nppn jnfr. t.TiPgQ 



A t the opposite end of each tubule, a Malpighian body is 

 formed, t he end of the tubule being dilated into a bulb-like 

 e nlargeme nt, wh ich becomes doubled up, to for m the Malpighian 

 capsule,^ by the ingrowth of a knot of blood-vessels derivedf 'rom 

 a branc h of the dorsal aort a (cf. Fig. 87, a). 



From the neck, or part of the tubule immediately beyond 

 the Malpighian body, a short solid rod of epithelial cells arises, 

 which grows towards the peritoneum and fuses with this. By 

 separation of the cells the rod becomes tubular, and^p ens at i ts 

 nntpvjTr^pp.Titnp eal end into the body cav ity. T he peritone al 

 opening is a nephrostom e (Fig. 87, Ks), an d the tube into w hich 

 it leads may be called the ne phrostomial_tu bule . 



Although the walls of the nejDhrostomial tubules are at first 

 continuous with the necks of the Malpighian bodies, it is ve ry 

 dou btful whether the nep hrostom ial tubule s e ver open i nto 

 the Wolfiian tubule s ; and, shortly after their first appearance, 

 in tadpoles of 18 to 20 mm. length, the ne phrostomial tubules 

 brea k awa y co mpletely fr om the Wolfiiar tnhnlpa and acquire 

 openin gs at their i nne r ends into the renal veins , on the ven tral 

 surface of the kidne y. This curious arrangement persists through- 

 out life. On the ventral surface of the kidney of an adult frog 

 there are as many as two hundred or more of these nephrostomes 

 present, as funnel-like depressions or mouths : these lead into 

 short tubes, lined by flagellated epithelial cells, and running 

 obliquely inwards into the substance of the kidney, where they 

 end by opening into the renal veins. 



The anterior three or four pairs of Wolfiian tubules never 

 complete their development, but early undergo fatty degenera- 

 tion. The hinder tubules are at first segmentally arranged, but 

 owing to the formation of new tubules, soon become much 

 more numerous than the segments in which they lie. They in- 

 crease greatly in length, become markedly convoluted, and soon 



