370 EMBRYOLOGY. 



tion of the anterior end, which bears the orifices leading into the body-cavity 

 A small territory of the epithelium oE the body-cavity immediately adjacent to 

 the pronephros serves for the construction of this portion. The epithelium 

 becomes thickened, owing to the fact that its cells take on a cylindrical shape ; 

 it sinks in to constitute a groove, and then becomes constricted off from the 

 surrounding tissue in the form of a short funnel, which in front remains in 

 connection with the body-cavity by means of a broad opening, but posteriorly 

 becomes continuous with the part of the Miillerian duct that is produced by 

 fission. The pronephric tubules and the glomerulus degenerate. 



The fission of the single mesonephric duct into two canals lying 

 close together is a peculiar process, which is intelligible only upon 

 the assumption that the mesonephric duct has possessed a double 

 function. Probably it originally served as an outlet for the secre- 

 tions of the mesonephric tubules, and also by means of its pronephric 

 funnel took up out of the body-cavity the sexual products (eggs or 

 seminal filaments) eliminated into it at their maturity, and con- 

 ducted them to the outside. Similar conditions are often observed 

 in Invertebrates, e.g., in various divisions of the Worms, in which 

 also the segmental canals, which break through the body-wall, 

 transmit to the outside both secretions from the body and sexual 

 products. In Vertebrates each of the two functions is assigned to a 

 special canal, one of which loses its communication with the body- 

 cavity, but remains in connection with the transverse mesonephric 

 tubules, while the other retains as its part the ciliate funnel of the 

 pronephros, and thus is adapted to conducting away the sexual pro- 

 ducts (eggs). 



In Reptiles, Birds, and Mammals the manner of the development 

 of the Miillerian duct is still a subject of scientific controversy. 

 Most observers (WALDEYER, BRAUN, GASSER, JANOSIK, and others) 

 state that at no time was a process of fission observed. According 

 to their representation the Miillerian duct arises in Birds and 

 Mammals quite independently as a new structure, at a time when the 

 mesonephros is already well developed and has the form of a band- 

 like body (the mesonephric fold) projecting into the body-cavity 

 (fig. 210). One then sees on the lateral face of the anterior region of 

 this body that the epithelium of the body-cavity over a limited area 

 (a 1 ) is thickened in a remarkable manner and composed of cylindrical 

 cells, whereas elsewhere the cells are flattened. The thickened 

 portion of the epithelium sinks down in the form of a funnel and 

 applies itself closely to the mesonephric duct (?/), which is near at 

 hand. The blind end of the funnel grows from this point backwards 

 independently, as is usually asserted, by means of the proliferation 



