246 



AN INTRODUCTION TO ZOOLOGY 



and their appearance causes the mesonephros to increase markedly 

 in size and they obscure its primitive segment al arrangement. 



In the dogfish the secondary tubules are more numerous in the 

 posterior half of the body, and both they and the primary tubules 

 become much longer and more coiled, and in structure they are 

 essentially like the urinary tubules in Rana. Then, too, they lose 

 their direct connection with the mesonephric duct, and the collecting 

 parts of certain tubules enlarge and form ducts that constitute 

 secondary ureters. These open into the posterior part of the meso- 

 nephric duct in the female, or into one larger tube which only joins 



FIG. 80. Diagram of development of kidneys of a female Scyllium. 



C., cloaca; Co. M., caudal mesonephros ; C.M., cranial mesonephros ; M.D., mesonephric duct ; 

 N., nephrostome ; P., pronephros ; P,D., pronephric duct ; S.U., secondary ureter. 



the extreme end of the mesonephric duct in the male. Thus it is 

 that the mesonephros in Scyllium becomes divided into two distinct 

 parts, a head kidney or cranial mesophros and a much more sub- 

 stantial tail kidney or caudal mesonephros, often, but quite erro- 

 neously, called the metanephros, an entirely separate body developed 

 only in the higher Craniata, the Reptiles, the Birds and the Mammals. 

 The kidneys of the adult, as we have seen, represent the 

 persistent functional mesonephros of the embryo, and are divided 

 into anterior and posterior moieties. They lie between the dorsal 

 peritoneum and the muscles of the back, and so are outside the 

 ccelom as in Rana. They lie partly embedded in hollows in the 



