Excretory and Reproductive Systems 87 



Origin of Mullerian Duct 



The Miillerian duct presents problems. In adult females its an- 

 terior end opens into the abdominal coelom. Ripe eggs are discharged 

 from the external surface of the ovary (exceptional conditions existing 

 in bony fishes) into the coelom, whence they pass, via the Miillerian 

 oviduct and cloaca, to the exterior. The funnel-like expansion of the 

 coelomic end of the oviduct may be so closely wrapped about the 

 ovary that the discharged eggs do not become free in the body-cavity, 

 but they are at least potentially temporarily in the coelom. 



Just as the oviduct is a passage from coelom to exterior, so do the 

 nephrostomes of renal tubules put the coelom into communication with 



Fig. 86. Diagram of renal (Malpighian) cor- 

 puscle, (a) Artery; (b) Bowman's capsule; (gl) 

 glomerulus; (n) nephrostome; (/) excretory 

 tubule; (») vein. (Courtesy, Kingsley: "Compara- 

 tive Anatomy of Vertebrates," Philadelphia, The 

 Blakiston Company.) 



the exterior via the urinary ducts. The relatively few tubules of the 

 usually briefly transitory pronephros possess nephrostomes. Very satis- 

 factory evidence exists that, in sharks, the premesonephric region of the 

 pronephric duct persists as the anterior region of the Miillerian duct, 

 and that the ostium (mouth) of the oviduct is an enlarged pro- 

 nephric nephrostome, or possibly results from fusion and enlarge- 

 ment of two or more nephrostomes. It has been shown, further, that 

 during the development of the shark's mesonephros, the prospective 

 mesonephric part of the pronephric duct splits throughout its entire 

 length. Its more median derivative loses all connection with the pro- 

 nephric duct and becomes the definitive Wolffian duct, while the more 

 lateral derivative retains its original continuity with the premeso- 

 nephric part of the pronephric duct, thus preserving a continuous pas- 

 sage from the coelom to the cloaca — a passage (Miillerian duct) des- 

 tined to be used for transmission of eggs from the coelom to the 

 exterior. 



