THE ORGANS OF THE MIDDLE GERM-LAYER. 



373 



\ 



of figure 212 exhibit this condition. Figure B shows the place 

 where the ventral wall of the mesonephric duct is thickened into 

 a ridge (md) by an increase of the epithelial cells; upon a cross 

 section (A) made farther forward the thickened part has become 

 detached as a cord 

 (md), which subse- 

 quently becomes still 

 more isolated and ac- 

 quires a cavity of its 

 own. The condition 

 recalls very clearly 

 the appearances 

 which the cross sec- 

 tions through embryo 

 Selachians (fig. 209) 

 gave. 



According to the 

 observations of SEDG- 

 WICK, therefore, the 

 anterior end of the 

 Miillerian duct would 



be derived from the pronephros, but the posterior end by a splitting 

 off of cells from the mesonephi-ic duct. Thus an agreement with 

 the conditions in the non-amniotic Vertebrates would be established. 



Fig. 212. Two S3ctions to show the union of the solid terminal 

 part of the Miillerian duct with the mesonephric duct in 

 the Chick, after BALFOUB AND SEDQWICK. 



In A the terminal part of the duct is still quite distinctly 

 separate ; in it has united with the wall of the mesone- 

 phric duct. 



md, Miillerian duct ; IVd, Wolfflan duct. 



M.g. 



Fig. 213. Cross sections through the Wolffian and Mullerian ducts of two human embryos, after 



NAGEL. 



A A female embryo 21 mm. long. 

 B, A male embryo 22 mm. long. 

 W.g., Wolffian duct ; M.g., end of the Miillerian duct in process of development. 



It still deserves to be especially mentioned that in human embryos 

 also the Mullerian ducts (fig. 213 A and B M.g.) during their 

 development have their posterior ends fused for a short distance with 

 the mesonephric duct (W.g.). NAGEL, to whom we are indebted for 

 this fine observation, expresses himself, it is true, against a splitting 



