856 



DEVELOPMENT. 



[CH. LVIII. 



In the preceding diagram (fig. 675) we have a transverse section 

 through the embryo in which the structures represented will be 

 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 may be termed the archinephros. 

 The prominence created by this duct grows 

 into the pleuro-peritoneal cavity ; and a 

 number of convoluted tubes, one in each 

 segment, open into the duct, which soon 

 splits into two longitudinally ; one division, 

 the pronephros or Mullerian duct (fig. 676, M), 

 has the original opening into the body 

 cavity ; the other convoluted tubes open 

 into the other division of the tube ; they 

 become united together by connective tissue, 

 and form a solid organ called the Wolffian 

 body, or mesonephros. The duct is called the 

 mesonephric, or Wolffian duct (fig. 676, W). 

 The two ducts open into the cloaca which 

 also receives the hinder opening of the ali- 

 mentary canal. 



The tubules of the Wolffian body become 

 more convoluted and form the tubules of 

 the head-kidney ; some of their original 

 openings into the peritoneal cavity can be traced, however, even 

 in the adult. 



From the lower end of the Wolifian duct a protrusion or growth 

 takes place, and this also becomes hollow, and a pumber 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 ; the metanephric duct becomes the ureter. It is 

 represented at K, in fig. 677. 



In the female the Mullerian 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 defereus. 



7 



M W. 



Fig. 676. Diagram 

 representing the 

 splitting of the 

 archinephros into 

 Mullerian (M) and 

 Wolffian (W) ducts. 



