The pronephros has only been observed in Birds, and is very possibly not developed in other Amniota. The Müllerian duct is also usually stated to develop as a groove of the peritoneal epithelium, shewn in the Lizard in [fig. 354], 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 Müllerian 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 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 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 themselves 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.
According to the view of Remak and Kölliker 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 accordance with the account given above, that the outgrowths of the ureter form only the collecting tubes, and that the secreting tubuli, etc. are formed in situ in the adjacent mesoblast.
Braun (No. [542]) has arrived at the conclusion that in the Lacertilia the tissue, out of which the tubuli of the metanephros are formed, is derived from irregular solid ingrowths of the peritoneal epithelium, in a region behind the Wolffian body, but in a position corresponding to that in which the segmental tubes take their origin. These ingrowths, after separating from the peritoneal epithelium, unite together to form a cord into which the ureter sends the lateral outgrowths already described. These outgrowths unite with secreting tubuli and Malpighian bodies, formed in situ. In Lacertilia the blastema of the kidney extends into a postanal region. Braun’s account of the origin of the metanephric blastema does not appear to me to be satisfactorily demonstrated.
The ureter does not long remain attached to the Wolffian duct, but its opening is gradually carried back, till (in the Chick between the 6th and 8th day) it opens independently into the cloaca.