588 AMNIOTA. 



into it close to its cloacal aperture, after uniting to form one or two 

 p imai-y tubes (ureters) 



(4) 'Jlie testicular network {ve) consisting of (1) transverse ducts 

 from the testes, falling into (2) the longitudinal canal of the Wolffian 

 body, from which (3) transverse canals are again given off to the 

 Malpighian bodies. 



Amuiota. The amniotic Vertebrata agree, so far as is known, 

 very closely amongst themselves in the formation of the urinogenitai 

 system. 



The most characteristic feature of the system is the full de- 

 vel'tpmentof a metanephros, which constitutes the functional kidney 

 on the atrophy of the mesonephros or Wolffian body, which is a 

 purely embryonic organ. The first pa-t of the system to develop is 

 a duct, which is usually spoken of jis the Wolffian duct, but which is 

 really the homologue of the segmental duct. It apparently develops 

 in all the Amhiota nearly on the Elasmobranch type, as a solid rod, 

 primarily derived from the somatic mesoblast of the intermediate 

 cell mass (fig. 401 W.d)^. 



The first trace of it is visible in an embryo Chick with eight 

 somites, as a ridge projecting from the intermediate cell mass to- 

 wards the epiblast in the region of the seventh somite. In the 

 course of further development it continues to constitute such a ridge 

 as far as the eleventh somite (Sedgwick), but from this point it grows 

 backwards in the space between the epiblast and mesoblast. In an 

 embryo with fourteen somites a small lumen has appeareii in its 

 middle part and in front it is connected with rudimentary Wolffian 

 tubules, which develop in continuity with it (Sedgwick). In the 

 succeeding stages the himen of the duct gradually extends backwards 

 and forwards, and the duct itself also passes inwards relatively to the 

 epiblast (fig. 4U2 . Its hind end elongates till it comes into con- 

 nection with, and opens into, the cloacal section of the hind-gut^ 



It might have been anticipated that, as in the lower types, the 

 anterior end of the segmented duct would either open into the body 

 cavity, or come into connection with a pronephros. Neither of these 

 occurrences take place, though in some types (the Fowl) a structure, 

 which is probably the rudiment of a pronephros, is developed ; it 

 d(jes not however appear till a later stage, and is then unconnected 

 with the segmental duct. The next part of the system to appear is 

 the mesonephros or Wolffian body. 



This is formed in all Anmiota as a series of segmental tubes, 

 which in Lacertilia(Braun) correspond with the myotomes, but in Birds 

 and Mammalia are more numenjus. 



' Daiiskj' and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as 

 developing from a groove opening to the peritoneal cavity, which subsequently becomes 

 constricted into a duct. I have never met with specimens such as those figured by 

 these authors. 



- The foremost extremity of the segmental duct presents, according to Gasser, 

 curious irregularities and an anterior completely isolated portion is often present. 



