446 



COMPARATIVE ANATOMY 



Metanephros. 



All the tubules of the mesonephros, as well as the coiled 

 portions of the tubules and the glomeruli of the metanephros, 

 arise essentially from the same matrix (the " nephrogenetic 

 blastema "), the latter organ becoming later differentiated than the 

 former: the straight, collecting portions of the metanephric tubules, 

 on the other hand, originate as outgrowths from the mesonephric 



ducts, as do also the meta- 

 nephric ducts (ureters), and, 

 in Mammals, the pelvis of the 

 kidney. The definitive kidney 

 of the Amniota is therefore not 

 a new organ, but corresponds 

 to a well-developed and special- 

 ised posterior portion of the 

 mesonephros, this specialisa- 

 tion having come about owing 

 to a still further separation of 

 that part of the kidney duct 

 which retains its original func- 

 tion from that part which is 

 taken into the service of the 

 generative organs. Nephro- 

 stomes are wanting in the 

 The posterior 

 ureter loses its 

 connection with the meso- 

 nephric duct and opens inde- 

 pendently either into the 

 cloaca or into a urinary bladder 

 (Figs. 338, 339, and 357-364). 

 Thus it will be seen that 

 the distinction between the 

 pronephros and mesonephros is 



much greater than that between the latter and the metanephros ; 

 all three organs, however, are derived from the same matrix, and 

 merely represent three generations of the same ancestral organ. 



Kl 



hind-kidney. 

 FIG. 338. DIAGRAM ILLUSTRATING THE ^ -? i*' 



GENETIC RELATIONS BETWEEN THE 

 MESONEPHROS AND METANEPHROS. 

 (Modified from Schreiner. ) 



AG, allantoic duct; D, intestine; HK, 

 urinary tubules arising in the nephro- 

 genetic blastema ; Kl, cloaca ; NG, 

 metanephric duct ; **, outgrowths 

 from metanephric duct ; tf, outgrowths 

 from mesonephric duct ( UO). 



THE MALE AND FEMALE GENITAL DUCTS. 



In certain lower Vertebrates (Elasmobranchs) a second duct, 

 known as the Miillerian duct, becomes differentiated from the 

 primary mesonephric duct, and takes on an important relation to 

 the female generative organs, serving as an oviduct for conveying 

 the sexual products to the exterior, and remaining in connection 



