178 



UROGENITAL SYSTEM 



The segmental or Wolffian duct appears as a solid cord of cells, which extends 

 backwards close under the ectoderm external to the nephrogenetic cord as fai 

 as the cloaca. It soon acquires a lumen (embryo of 3 mm.), and opens into thai 

 chamber (embryo of 4 -2 mm. ; Keibel). 



In the rabbit (Rabl) and marmot (Janosik) the head end of the duct arises by the fusion 

 of cords of cells which represent rudimentary pronephric tubules, so that the mode of origin 

 characteristic of the Anamnia is repeated, though in a very abbreviated form. It is not 

 improbable that the head end of the duct arises in the same fashion in the human embryo. 

 Graf Spee, Kollmann, Flemming, and others have, however, derived the duct from the ectoderm, 

 which certainly dips in along the line of the rudiment and lies in contact with it. It is as 



neural canal 



segment 



sderostome 



Wolffian duct 



FIG. 223. TRANSVERSE SECTION THROUGH THE TRUNK AND HIND-LIMB BUDS op A RABBIT-EMBRYO OF 



THE TENTH DAY. (T. H. Bryce.) ON THE MESIAL ASPECT OF EACH WOLFFIAN DUCT THE NEPHRO- 

 GENETIC CORD. 



yet uncertain whether the ectoderm actually shares in the backward extension of the cellular 

 cord, or whether the duct grows independently. An ectodermic origin would be explained in 

 terms of Riickert's theory of the phylogeny of the duct viz. that it is formed by a union 

 of the outer ends of the nephridia which^opened primitively on the surface. 



IVIesonephros or Wolffian body. The mesonephros appears first as a ridge 

 (Wolffian ridge] on each side of the attachment of the primitive mesentery, which 

 extends from the fifth cervical to the fourth lumbar segment. It is formed by the 

 enlargement of the intermediate cell-mass, as the tubules and their Malpighian 

 corpuscles develop in its tissue. As the tubules increase in number the ridges 





