EXCRETORY OROAXS. 595 



pi'onephros, which in the Ichthyopsida develop as a single formation, 

 develop in tlie Bird as two independent structures — a far from extravagant 

 supposition, considering that the pronephros in the Bird is undoubtedly 

 quite functionless. 



With reference to the posterior position of the pronephros it is only 

 necessary to remark that a change in position might easily take place after 

 the acquirement of an independent development, and that the shifting is pro- 

 bably correlated with a shifting of the abdominal opening of the Miillerian 

 duct. 



The pronephros has only been observed in Birds, and is very possibly 

 not developed in other Amniota. The Miillerian duct is also usually 

 stated to develop as a groove of the peritoneal epithelium, shewn in the 

 Lizard in fig, .S54, md., which is continued backward as a primitively 

 solid rod in the space between the Wolffian duct and peritoneal 

 epithelium, without becoming attached to the Wolffian duct. 



On the formation of the Miillerian duct, the duct of the meso- 

 nephros 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 metanephros 

 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 metanephric 

 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 continuous 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 dorsal- 

 wards, 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 Amniota is a specially differentiated posterior section of the 

 primitive mesonephros. 



88—2 



