20 THE SKATE 



amphibians when it is probably functional as an excretory 

 organ. They persist in adult lampreys and a few fishes, 

 while in reptiles, birds and mammals they appear during 

 embryonic stages as transient structures without function. 

 (2) As the pronephroi degenerate a second excretory organ 

 is developed, the mesonephros (pi. mesonephroi) or Wolf- 

 fian body. The mesonephros is the kidney of adult fishes 

 and amphibians and appears in the embryos of reptiles, 

 birds and mammals when it is functional until the forma- 

 tion of the (3) metanephros or true kidneys of the adult 

 reptiles, birds and mammals. 



A. The Female, 



The main portion of the kidney of the female skate con- 

 sists of rounded lobes (masses about 30 x10 mm.) which 

 are located against the dorsal wall of the body cavity at 

 the extreme posterior end and on either side of the median 

 axis. Carefully remove the pleuroperitoneum which covers 

 the ventral surface of the kidney on the left side 

 starting at its lateral margin and proceed towards the 

 midline. This reveals the caudal mesonephros. An an- 

 terior extension, the cranial mesonephros is nearly degen- 

 erate in the female, but may be noticed as a brownish diffuse 

 tissue extending from the anterior end of the caudal 

 mesonephros. 



In the male skate it is much better developed and extends 

 forward as a light-colored flat body on either side of the 

 median axis. 



On the median surface of the caudal mesonephros are 

 several ducts, the accessory mesonephrio ducts (to avoid 

 later difficulty the term ureter should be applied only to the 

 excretory duct of the metanephros). These ducts may best 

 be located by dissecting away, with blunt end of probe or 

 scalpel, the connective tissue which extends from the 



