192 THE FROG. 



The Wolffian tubules are. at first, solid masses of spherical 

 cells ; these arise in the mesoblast of the body wall, and are for 

 a time independent of both the peritoneum and the segmental 

 ducts. The cellular masses soon become elongated into solid 

 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 segmental ducts and 

 open into these. 



At the opposite end of each tubule, a Malpighian body is 

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

 enlargement, which becomes doubled up, to form the Malpighian 

 capsule, by the ingrowth of a knot of blood-vessels derived from 

 a branch of the dorsal aorta (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 opens at its 

 outer or peritoneal end into the body cavity. The peritoneal 

 opening is a nephrostome (Fig. 87, KS), and the tube into which 

 it leads may be called the iiephrostomial tubule. 



Although the walls of the iiephrostomial tubules are at first 

 continuous with the necks of the Malpighian bodies, it is very 

 doubtful whether the iiephrostomial tubules ever open into 

 the Wolffian tubules ; and, shortly after their first appearance, 

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

 break away completely from the Wolffian tubules, and acquire 

 openings at their inner ends into the renal veins, on the ventral 

 surface of the kidney. 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 iiephrostomes 

 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 Wolffian 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 



