I3O MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



nection with the testis. Its tubules branch and anastomose, 

 and also connect with the seminiferous canaliculi, forming a 

 system of ducts conducting the spermatozoa into the anterior 

 end of the Wolffian duct. This plexus of tubules nearest the 

 testis is "the rete, nearer the duct it forms the vasa efferentia. 

 The Wolffian duct, by this assumption of reproductive func- 

 tions, is converted into a vas deferens, the anterior part of 



which becomes greatly coiled, this por- 

 tion being known as the epididymis. 

 More distally the duct may develop 

 marked muscular walls and form an 

 ejaculatory structure. 



As was said above, the posterior 

 portion of the mesonephros retains its 

 excretory powers in the ichthyopsida, 

 and as it pours its secretions into the 

 Wolffian duct, this tube in the male is 

 at once excretory and reproductive in 

 character. In the male amniotes, when 

 the metanephros is developed, the pos- 

 terior part of the mesonephros degen- 

 erates into a small body close to the 

 epididymis known as the paradidymis 

 (organ of Giraldi), and occasionally 

 forms one or more blind tubes (vasa 

 aberrantia), opening into the vas 

 deferens. 



In some vertebrates the Wolffian 

 and Miillerian ducts open directly into 



the cloaca, but in most they unite into a urogenital sinus, which, 

 in turn, empties by a single opening into the cloaca. In the 

 mammals (monotremes excepted) the separation of the uro- 

 genital sinus from the hinder end of the alimentary canal is 

 complete, a muscular partition (perineum) separating the vent 

 from the urogenital opening. 



Connected with the urogenital structures outlined above are 

 many accessory parts, glands, external genitalia, copulatory 

 organs, etc., some of which will be described in connection 



FIG. 137. Urogenital or- 

 gans of male frog; the testis, 

 T, turned to one side to ex- 

 pose A, the vasa efferentia 

 passing from the testis to J/, 

 the mesonephros. F, poste- 

 rior end of fat body ; P, post- 

 cava; S, suprarenal; U, 

 ureter. 



