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. 



Appnvrlmir tn +>IA 

 Observations of SEDG- 



WICK, therefore, the 



anterior end of the 



Mullerian duct would 



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

 off of cells from the mesonephric duct. Thus an agreement with 

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



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

 P* 1 "* of the Mlill erian duct with the mesonephric duct in 

 the Chick, after BALFOUR AND SEDGWICK. 



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



* " ha * with the waU ' 



, Mullerian duct ; Wd, Wolffian duct. 



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



NAGEL. 



A A female embryo 21 mm. long. 

 , A male embryo 22 mm. long. 

 W.g., Wolffian duct ; M.g., end of the Mullerian 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 



