THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



389 



is attached to the dorsal body wall by a distinct mesentery which, at its cephalic 

 end, also sends off a band to the diaphragm the diaphragmatic ligament of 

 the mesonephros. The peritoneum is reflected over the surface of the meso- 

 nephros, and on the ventro-medial side the mesothelium becomes thickened to 

 form the genital ridge (p. 404; Figs. 314 and 346). The mesonephric ducts are 

 embedded in the lateral parts of the organs and extend throughout practically 

 their entire length. Since the ducts are identical with the pronephric ducts, 

 they open at first into the caudal end of the gut, or cloaca (p. 385; Fig. 360). 

 At a little later period, when the urogenital sinus is formed, they open at the 

 junction of the latter with the bladder (Fig. 363). Still later they open into the 



Appendage t * 

 oftes ' 



Testicle 



Appendage of epididymis 



Mesonephric duct 

 '(duct of epididymis) 



-Paradidymis 



._ Aberrant ductule 



_ Mullerian duct 



_ Urogenital sinus 



FIG. 347. Diagram representing certain persistent portions of the mesonephros 

 in the male (see table). Kollmann. 



sinus itself (p. 400) . A description of their further development is best deferred 

 to the section on the male genital organs, since they become the genital ducts 

 (p. 416). 



The mesonephroi function as urinary organs during the period of their 

 existence in the embryos of all higher Vertebrates. Excretory products are con- 

 veyed directly to the tubules by means of the glomeruli instead of being de- 

 posited in the ccelom and then taken up by the tubules, as is the case in func- 

 tional pronephroi (p. 386). The main excretory ducts are the same as in the 

 pronephroi. Aside from the vessels in the glomeruli the mesonephroi are ex- 

 ceedingly vascular organs. Large and small branches of the posterior cardinal 

 veins ramify among the tubules (Figs. 314 and 232). The blood undergoes 



