_coelom_ 



pronephric 

 funnel- 



nite 7 



somite 12 

 nephric duct 



nephric duct 



^ 



lateral vesicle 



capsule of primary 

 mesonephric tubule 



collecting tubule 



B 



lateral vesicle 



r 



secondary 

 tubule 



secondary tubule 

 tertiary tubule 



^ 



secondary 

 collecting tubule 



nephrostome^^'^P^ 



tertiary tubule 



secondary tubule 



primary mesonephric tubule 



Figure 10-24. Stages in the development of tubules in Squalus. A, pronephric area of 5-mm 

 embryo of 25 to 27 somites; B, beginning of tubule formation in mesonephric region; C-F, progres- 

 sively later stages. (After Borcea, 1906) 



the formation of secondary and tertiary tubules which are 

 budded off in pairs from the lateral vesicle. The new gen- 

 erations of tubules enter secondary collecting tubules evagi- 

 nated from the region of junction of the primary collecting 

 duct and the primary nephron, the segmental collecting 

 duct. The more posterior collecting tubules of some sharks, 

 including Squalus, tend to move their point of attachment 

 on the nephric duct more and more posteriorly. Concur- 

 rently the Mijllerian duct is separating from the anterior 

 end of the nephric duct. In the female this separation con- 

 tinues in a posterior direction until the Mijllerian duct opens 

 separately into the cloaca. 



In Pnstiurus the segmental tubules form individual units; 

 the expansion becomes a Bowman's capsule enclosing a 



glomerulus (Figure 10-25). Each segmental tube gives rise 

 to several generations of tubules which have a common col- 

 lecting tubule. The bunches of segmental tubules form the 

 segments of the definitive kidney. In the adult, connection 

 between tubule and funnel is probably lost. 



In the later stages of development of the male, the most 

 anterior part of the mesonephros is modified into the epididy- 

 mis. About six segmental tubules are involved in the transport 

 of sperm from the testis to the nephric duct. Their glomeruli 

 and peritoneal funnels disappear. The way the connection be- 

 tween testis and kidney develops is not known, but it involves 

 either the conversion of the peritoneal funnels into the ductuli 

 efferentia (certainly the funnels disappear) or more probably 

 it involves independent tubular extensions from the glomer- 



THE EXCRETORY SYSTEM • 31 1 



