60 WILLIAM MORTON WHEELER, 



other cases the tubules are said to grow in from the peritoneal epi- 

 thelium as hollow or solid cords of cells, 3) In still other cases they 

 are said to be evaginated from the pronephric duct. These different 

 variations may be considered seriatim for the purpose of reducing 

 them to the schema exhibited by the Selachians. 



1) Among recent investigators who claim that the retroperitoneal 

 meseuchyraa gives risq to the mesonephric tubules, I may mention 

 JuNGERSEN (1893, p. 467, Acipenser; and 1894, p. 249, Amia), Bujor, 

 quoted above (1891, Petromyzon), and Felix (1897b) who gives this 

 method of origin for the Salmonid "Nachniere", an organ which somewhat 

 resembles the Amniote metanephros. 2) The origin of the mesonephric 

 tubules from hollow or solid evaginations of the peritoneum into the 

 retroperitoneal tissue is claimed to occur in Fetromyzon by Für- 

 bringer (1878), ViALLETON (1890) and myself. Field (1891, p. 262) 

 inclines to the belief that the mesonephric tubules of Amblystoma are 

 formed in this manner, and Goette (1875), Spengel (1876) and 

 Fürbringer (1877) long since claimed the same for the origin of the 

 mesouephros in other Amphibians. Goette and Fürbringer placed 

 the Teleostei, and Braun (1877) the Reptilia in the same category. 

 3) The derivation of the mesonephric tubules from evaginations of the 

 pronephric duct, has been described by a few investigators of the 

 Teleostei, Nusbaum (1878) and Felix (1895 and 1897 b). The five 

 to nine mesonephric tubules of Salmonidœ, according to Felix, arise 

 as strictly metameric diverticula of the mesial dorsal wall of the pro- 

 nephric duct. They are completely constricted off and lie for some 

 time freely above the duct, but subsequently they open into it again. 

 Felix denies emphatically that the peritoneum takes part in forming 

 the tubules: "Das Pleuroperitonealepithel wuchert nicht in den retro- 

 peritonealen Raum vor, noch bildet es solide Einstülpungen." The 

 formation Felix describes is certainly very unusual, and if his paper 

 did not show signs of very careful workmanship one would certainly 

 feel inclined to doubt his interpretation. 



That all three of these modes of mesonephros formation may 

 occur in different Vertebrates may, I think, be conceded. That they 

 should depart so far from the typical condition shown in the sharks, 

 in which the mesonephros actually exists, as soon as the mesoderm 

 undergoes segmentation, in the form of metameric connections between 

 the myotomes and parietal mesoderm, is due to the great differences 

 in the behavior of the extra-myomeric mesoderm in different Verte- 

 brates, for on this depends the development of the mesonephros. 



