IV 



METANEPHEOS 



257 



\ 3 l\ 32 I *?/,?*/ 



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

 the cloaca. The nephrotomal tissue in the segments mentioned 

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

 nephrotoines of the metanephros. The metanephros is therefore 

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

 origin simply a tailward 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- 

 tion of the nephro- 

 tomal mesenchyme. 

 This latter becomes 

 secondarily (about 

 the end of the fifth 

 day) marked off by 



a distinct break from the mesonephric 

 time the dorsal wall of the actively 

 develop pocket -like outgrowths (Fig. 

 in length, branch repeatedly, 

 become collecting -tubes. As 



Fig. 138. — Keconstructed outlines of hind end of mesonephric 

 duct and ureter in Bird embryos as seen from the left side. 

 (After Schreiner, 1902.) 



A, duck embryo with 48 segments ; B, duck embryo with 50 segments ; 

 C, duck embryo, 1075 mm. ; D, fowl embryo, 13-5 mm. nrn, meso- 

 nephros ; ur, ureter. The Arabic numerals indicate the position of 

 the mesoderm segments. 



portion, 

 growing 

 138, D). 

 especially the 

 this takes place 



About the same 



ureter begins to 



These increase 



hinder ones, and 



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 



