THE ORGANS OF THE MIDDLE GERM-LAYER. 



369 



of the fundament of the mesonephros at the same time. These have 

 to do with the evolution of the Mlillerian duct and the sexual organs. 



(d) The Miillerian Duct. 



The Miillerian duct is a canal which is found lying at first parallel 

 and close to the mesonephric duct in the embryos of most Vertebrates 

 (Selachians, Amphibia, Reptiles, Birds, Mammals). It is a canal that 

 is established in both sexes in the same 

 manner, but subsequently acquires in each 

 a different function. It takes its origin 

 in the lower Vertebrates from the mesone- 

 phric duct, as can be most easily followed 

 in the Selachians (SEMPER, BALFOUR, 

 HOFFMANN). In this case the mesonephric 

 duct becomes enlarged, acquires in cross 

 section (fig. 209 4 ) an oval form, and pre- 

 sents a different condition in its dorsal 

 (sd) and ventral (od) halves, the latter 

 being at the same time in immediate con- 

 tact with the peritoneal epithelium. The 

 mesonephric tubules open into the dorsal 

 half, while ventrally the wall is consider- 

 ably thickened. Then a separation of the 

 two parts takes place, which begins at a 

 little distance from the anterior end (cross 

 sections 3-1) and pi-oceeds backward to 

 the point of opening into the hind gut. 

 Of the parts which result from the fission, 

 that which lies dorsally is the permanent 

 mesonephric duct (wd) ; it exhibits at first 

 a broad lumen and receives the urinary 

 tubules (fig. 207 st). Ventrally, between it and the epithelium of 

 the body-cavity, lies the Miillerian duct (fig. 209 od and fig. 207), 

 which is at first only a narrow passage, but later a much enlarged 

 one. In the process of fission the anterior initial part of the primary 

 canal (fig. 207 pd), which was described at p. 353 as pronephros and 

 which opens into the body-cavity by means of a cilia te funnel (o), 

 becomes a part of the latter duct, and the ciliate funnel becomes the 

 ostium abdominale tubse. 



Also in the case of the Amphibia the Miillerian duct ia developed by being 

 split off (FtiRBRiNGER, HOFFMANN) from the mesonephric duct, with the excep- 



24 



Fig. 209. Four cross sections 

 through the anterior region 

 of the mesonephric duct of a 

 female embryo of Scyllium 

 canicula, after BALFOUR. 



The figure shows how the Miil- 

 lerian duct (od) is split off 

 from the meaonephric duct 

 (sd and wd). 



