THE 01U5AXS OF THE MIDDLE GERM-LAYER. 353 



II. The Development of the Urinary and Sexual Organs. 



The development of the urinary and sexual organs cannot be discussed 

 separately in two chapters, because these systems of organs are most in- 

 timately connected with each other, both anatomically and genetically. 



First, both take their origin at one and the same place on the epi- 

 thelial investment of the body- cavity ; secondly, parts of the urinary 

 system subsequently enter into the service of the sexual apparatus, 

 for they furnish the passages or canals which are entrusted with the 

 evacuation of the eggs and semen. In anatomy also one therefore 

 properly embraces the two genetically united systems of organs under 

 the common name of urogenital system or apparatus. 



Again in this subject we turn to one of the most interesting 

 portions of embryology. The urogenital system claims an interest 

 particularly from a morphological point of view, because a great 

 number of important metamorphoses are effected in it during 

 embryonic life. In the higher Vertebrates the pronephros and the 

 mesonephros are formed first; they are organs of an evanescent 

 nature, which in some cases disappear and are replaced by the 

 permanent kidney, in other cases their ducts alone are preserved. 

 But these transitory structures correspond to organs which are 

 permanently functional in the lower Vertebrates. 



In late years, the attention of investigators having been directed to 

 a series of entirely new and unexpected phenomena, by the excellent 

 i-OM-arches of WALDEYER and SEMPER, the topic " urogenital 

 organs" has been carefully worked out by very many different 

 observers through the investigation of each separate class of Verte- 

 brates. There has arisen a voluminous literature, and many im- 

 portant facts have been brought to light. Nevertheless it is not to 

 be denied that conceptions concerning many fundamental questions 

 are still very divergent. 



As in several previous chapters, I shall also here give to the 

 discussion a broader foundation by treating somewhat more ex- 

 haustively of the lower Vertebrates in certain questions. 



(a) The Proucphros and the Mesonephric Duct. 

 The first thing that becomes noticeable in the origin of the uro- 

 pMiital apparatus is the fundament of the pronephros [head-kidney]. 

 This is a structure which has now been demonstrated in the embryos 

 of all Vertebrates, but which plays in some a greater part, in others 

 a lesser one. In some Vertebrates (Myxino, Bdellostoma, Bony 

 Fishes) it is retained permanently; in others, as the Amphibia, it 



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