414 



THE EVOLUTION OF MAN. 



a high physiological significance, as they modify into 

 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 kidney 

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

 Johannes Muller, " Miiller's duct " (ductus Mulleri), while 

 the earlier, primitive kidney duct is distinguished as the 

 "Wolffian duct" {ductus Wolfii). The actual origin of 

 Miiller'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 

 V kidney duct breaks up by difierentiation (or fission) into 

 two secondary, similar ducts; these are the Wolffian and 



Jhtk. 321. — Primitive kidneya and rudiments of the sexnal orgam. A and 

 B, of Amphibia (Frog larvae) ; A, earlier, B, later condition. 0, 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 

 Miillerian duct (») and the Wolffian duct (u9')> which unite behind into a 

 genital oord (gr) ; I, groin-oord of the primitive kidneys. (Aftar Qegenbaor.) 



