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familiar from our previous studies. About the fifth segment a 

 thickening in the mesoblast occurs, which grows backwards as a solid 

 column of cells ; this becomes hollow, and is seen in transverse section 

 at A'] later on the hollow extends at one part into the pleuro- 

 peritoneal cavity by a trumpet-shaped opening, 

 and this is seen cut through at A". 



This duct is termed the archinephros. The 

 prominence created by this duct grows into the 

 pleuro-peritoneal cavity; and a number of con- 

 voluted tubes, one in each segment, open into the 

 duct, which soon splits into two longitudinally; 

 one division, the pronephros or Milllerian duct 

 (fig. 672, M), has the original opening into the 

 body cavity; the other convoluted tubes open 

 into the other division; they become united 

 together by connective tissue, and form a solid 

 organ called the Wolffian body, or mesonepkros. 

 The duct is called the mesonephric, or Wolffian 

 duct (fig. 672, W). The two ducts open into the 

 cloaca, which also receives the hinder opening of 

 the alimentary canal. 



The tubules of the Wolffian body become 

 more convoluted, and form the tubules of the 

 primitive kidney; some of their original openings 

 into the peritoneal cavity can be traced, even in 

 the adult. 



From the lower end of the Wolffian duct a 

 protrusion or growth takes place, and this also 

 becomes hollow, and a number of segmental 

 tubes develop and form with it an organ similar to the Wolffian 

 body ; this is called the metanephros, and it forms the hind kidney, 

 which represents the true kidney of the higher vertebrates (K, fig. 

 673) ; the metanephric duct becomes the ureter. 



In the female the Miillerian ducts become the oviducts, and, 

 where they join, the uterus. In the male they disappear. The 

 head or Wolffian kidney, and the hind or true kidney both execute 

 renal functions in both sexes ; but in the male, the Wolffian tubules 

 apply themselves to the testis and constitute its efferent ducts; the 

 main Wolffian duct becomes the vas deferens. Thus in fishes and 

 amphibians, the semen passes through tubules which are also renal 

 in function. 



In the higher vertebrates the duct of the archinephros becomes 

 the Wolffian duct, and the segmental tubules, which are rather more 

 numerous than one to each segment, get bound into the Wolffian 

 body. The Miillerian duct is not split off from this, but is formed 



M W. 



FIG. C7'2. Diagram re- 

 presenting the splitting 

 of the archinephros 

 into Miillerian (M) and 

 Wolffian (W) ducts. 



