414 



THE EVOLUTION OF MAX. 



a high physiological significance, as they modify into ex- 

 cretory ducts of the sexual glands. In all Amphirhina or 

 Gnathostomi — therefore in all Vertebrates from Fishes up 

 to Man — at a very early period, a second similar canal 

 appears in the embryo at the side of each primitive kidnej 

 duct. This canal is commonly called, after its discoverer, 

 Johannes Miiller, " Mliller's duct " (ductus Millleri), while 

 the earlier, primitive kidney duct is distinguished as the 

 " Wolffian duct " (ductus Woljjfii). The actual origin of 

 Mliller's duct is still undetermined ; Comparative Anatomy 

 and Ontogeny seem, however, to indicate that it proceeds 

 by differentiation from the Wolffian duct. It is, probably, 

 most correct to say, that the original (primary) primitive 

 kidney duct breaks up by diiTerentiation (or fission) into 

 two secondary, similar ducts; these are the Wolffian and 



Fia. 321. — Primitive kidneys and rudiments of the sexual organs. A auJ 

 B, of Amphibia (Frog larvae) ; A, earlier, B, later condition. C, of a Mam- 

 mal (embryo of Ox) : u, primitive kidneys ; k, sexual glands (rudiments of 

 testes and ovaries). The primary primitive kidney duct (^ug in Fig. A) 

 separates (in B and C) into the two secondary primitive kidney ducts ; the 

 Alullerian dnct (m) and the Wolffian duct iug'), which unite behind into a 

 genital cord (g) ; I, groin-cord of the primitive kidneys. (After Gegenbaur.) 



