G. Carl lliil)cr 13 



from Si>(i,a:e\viok".s paper, because he is the first to give a clear account of the his- 

 togenesis i)f the tissue into which grows the renal evagination. Rihbert. in a recent 

 contribulion, also discusses the origin of tlie tissue (whicli he cliaracterizes as a dif- 

 ferentiated cell-layer), which surrounds the blind end of the epithelial renal anlage, 

 but gives no definite conclusions. - Certain it is, he states, that the ureters grow 

 into a dense cell-cord, which may be regarded as tlie distal end of the mesonephros, 

 as is shown in his Fig. 4, combined from three sections of a series of sections of a 

 Guinea-pig embryo. The histogenesis of the nephrogenic tissue (renal blastema) has 

 been very fully considered by Schreiner. as has been stated in a brief review of his work 

 given on page 4. For all types studied by him, he was able to show very conclu- 

 sively that the nephrogenic tissue of the metanephros in common with that of the 

 mesonephros had its origin in the intermediate cell-mass and extended as an unsegmented 

 cord of cells through the regions in which is developed the mesonephros to where 

 the W'olflian duct terminates in the cloaca. In the region of the mesonephros, this 

 tissue gives origin to the mesonephric tubules ; posterior to the mesonephros. it be- 

 comes associated with the development of the metanephros, forming the nephrogenic 

 tissue, which surrounds the blind end of the epithelial renal evagination, as was 

 shown for the chick by Sedgewick. 'I"he above quotations were selected with a view 

 of presenting the various views held concerning the histogenesis of the nephrogenic 

 tissue. 



We niav now resume a consideration of the anlage and early develop- 

 mental stages of the metanephros as observed in the rabbit. The renal 

 evagination, which, as stated, buds dorsally into the nephrogenic tissue, 

 in its further growth extends in a dorsal direction (the account here given 

 follows Schreiner) pushing with it the nephrogenic tissue. This now 

 assumes a dorso-modian position with reference to the Wohfian duet, 

 instead, as before, of lying in contact with it. In its growth, the renal 

 evagination develops an enlarged blind end, which may be known as the 

 primary renal pelvis and a stalk which may be designated as the ureter. 

 The nephrogenic tissue, which surrounds the primary renal pelvis 

 differentiates into an inner zone composed of epithelioid cells, which 

 immediately surround it and an outer zone which presents the appearance 

 of a condensed mesenchyme, which shows no distinct demarcation toward 

 the surrounding mesenchyme ; the two zones form the metanephrogenic 

 tissue which has become separated from the mesonephrogenic tissue. 

 As development proceeds, the primary renal pelvis grows in a dorsal and 

 cephalad direction and elongates in an antero-posterior direction; at the 

 same time, it begins to rotate on its axis in such a way as to make its 

 dorsal surface have a more lateral position. The ureter likewise elongates 

 and curves cephalad. The metanephrogenic tissue with its inner and 

 outer zones is clearly recognized and surrounds the primary renal pelvis 

 as a continuous layer. In slightly older embryos, the primary renal 

 pelvis reache.'^ a ])osition dorsal to the posterior end of the mesonephro.-J. 

 In shape it has become still more elongated and presents two slightly 

 enlarged ends with a narrower middle portion from which arises the 

 ureter; it shows tbickenings and l)U(ldings in its wall which in slightlv 

 older stages are clearly recognized as evaginations, the anlagen of the 

 primary branches of the primary renal pelvis. The primary renal 

 pelvis now consists of a narrower middle ]jortion, from which arises 

 the ureter and two enlarged ends and from it and more particularly from 



