722 AMNIOTA. 



the Wolffian duct and peritoneal epithelium, without becoming 

 attached to the Wolffian duct. 



On the formation of the Miillerian duct, the duct of the 

 mesonephros becomes the true mesonephric or Wolffian duct. 



After these changes have taken place a new organ of great 

 importance makes its appearance. This organ is the permanent 

 kidney, or metanephros. 



Metanephros. The mode of development of the metane- 

 phros has as yet only been satisfactorily elucidated in the Chick 

 (Sedgwick, No. 549). The ureter and the collecting tubes of 

 the kidney are developed from a dorsal outgrowth of the hinder 

 part of the Wolffian duct. The outgrowth from the Wolffian 

 duct grows forwards, and extends along the outer side of a mass 

 of mesoblastic tissue which lies mainly behind, but somewhat 

 overlaps the dorsal aspect of the Wolffian body. 



This mass of mesoblastic cells may be called the meta- 

 nephric blastema. Sedgwick, of the accuracy of whose 

 account I have satisfied myself, has shewn that in the Chick it is 

 derived from the intermediate cell mass of the region of about 

 the thirty-first to the thirty-fourth somite. It is at first con- 

 tinuous with, and indistinguishable in structure from, the portion 

 of the intermediate cell mass of the region immediately in front 

 of it, which breaks up into Wolffian tubules. The metanephric 

 blastema remains however quite passive during the formation of 

 the Wolffian tubules in the adjoining blastema ; and on the 

 formation of the ureter breaks off from the Wolffian body in 

 front, and, growing forwards and dorsalwards, places itself on 

 the inner side of the ureter in the position just described. 



In the subsequent development of the kidney collecting tubes 

 grow out from the ureter, and become continuous with masses of 

 cells of the metanephric blastema, which then differentiate them- 

 selves into the kidney tubules. 



The process just described appears to me to prove that the 

 kidney of the A mniota is a specially differentiated posterior section 

 of the primitive mesonephros. 



According to the view of Remak and Kolliker the outgrowths from the 

 ureter give rise to the whole of the tubuli uriniferi and the capsules of the 

 Malpighian bodies, the mesoblast around them forming blood-vessels, etc. 

 On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in 



