IV 



METANEPHROS 



257 



ur 



ur 



XXXIII and XXXIV to the level of the opening of the duct into 

 the cloaca. The nephmtnmal tissue in the segments mentioned 

 remains for a time passive but eventually it gives rise to the definitive 

 nephrotomes of the nietanephros. The metanephros is thereinn; 

 ontogenetically as was indicated long ago by Sedgwick (1880) in its 

 origin simply a taihvard continuation of the 'mesonephros. In the 

 terminology used in this book it consists of the greatly enlarged 

 posterior segment or segments of the opisthonephros. The develop- 

 ment of the metanephros is inaugurated by the appearance of the 

 rudiment of the 

 ureter or meta- 

 nephric duct. This 

 arises as an out- 

 growth (Fig. 138, B, 

 ur) from the dorsal 

 wall of the meso- 

 nephric duct near 

 its posterior end. 

 The outgrowth ex- 

 tends in a dorsal 

 direction and then 

 spreads out at its 

 tip, projecting very 

 slightly tailwards 

 but growing much 

 more actively in a 

 headward direction 

 along the outer side 

 of the hinder or 



metanephriC por- FlG 133. Reconstructed outlines of hind end of mesonephric 



tion 01 the nephro- duct and ureter in Bird embryos as seen from the left side, 



tomal mesenchyme. (After Schreiner, 1902.) 



This latter becomes A, duck embryo with 48 segments ; B, duck embryo with 50 segments ; 



aonnnrlarilv fnVinnf C > <luck embryo, 10'75 mm.; D, fowl embryo, 13-5 mm. mn, meso- 

 ocCULK iclllly (d/UOUL 



, j X ,r. np.1 nephros ; ur, ureter. The Arabic numerals indicate the position of 



the end Ot tile tilth the mesodenn segment s. 



day) marked off by 



a distinct break from the mesonephric portion. About the same 

 time the dorsal wall of the actively growing ureter begins to 

 develop pocket -like outgrowths (Fig. 138, D). These increase 

 in length, branch repeatedly, especially the hinder ones, and 

 become collecting -tubes. As this takes place the nephrotomal 

 mesenchyme becomes condensed into small portions, one of which 

 ensheaths the growing tip of each branch of the collecting -tubes. 

 In these terminal caps of mesenchyme definitive nephrotomes 

 gradually come into view, similar to those of the mesonephros. In 

 other words the definitive nephrotome is at first a mere rounded 

 cellular mass. This develops a lumen and, as the latter dilates, 

 assumes a vesicular form, and finally the actual tubule makes its 



VOL. II S 



mn 



