258 THE DO&-FISH. 



is abortive, and appears as an ill-defined tract 

 of soft yellowish-brown degenerate gland-tissue, 

 arranged in patches corresponding to the vertebra. 



The metanephros, which forms the posterior third 

 or so of the kidney, is a well-developed, compact, 

 laterally compressed, and lobulated mass of a brown- 

 ish colour, extending back as far as the level of the 

 cloaca. 

 Dissect off the j)eritoneum from the ventral surface of one 

 of the kidneys, so as to expose it along its whole length. At 

 the hinder end dissect the metanephros along its outer side 

 from the bo'dy-wall, so as to see its full size. 



4. The Wolffian ducts are a pair of straight narrow tubes 



lying on the ventral surface of the kidneys along 

 their whole length. Their posterior ends are en- 

 larged to form a pair of tubular sacs, the urinary 

 sinuses, which run back side by side, unite together 

 posteriorly, and open into the cloaca at the apex of 

 the small urinary papilla in the posterior margin of 

 the opening of the oviducts. 



Find the opening at the apex of the urinary papilla : pass 

 a seeker through the opening and forwards along the urinary 

 sinus of one side : slit up the ventral wall of the sinus along 

 its whole length. 



5. The metanephric ducts or ureters are slender tubes, 



usually four to six in number on each side, which 

 run from the metanephros to open into the dorsal 

 surface of the urinary sinus by four to six slit-like 

 apertures lying along the septum between the two 

 sinuses. The anterior ureter is much longer than 

 the others. 

 Find the ureters, lying on the ventral surface of the meta- 

 nephros, and trace them back to the urinary sinus. Slit up 

 the sinus, unless this has already been done, and examine its 

 dorsal wall to see the openings of the ureters. 



6. The cloaca is a shallow depression between the pelvic 



