344 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



in the amniotes the pronephric duct does not divide,- but remains 

 solely in the service of the mesonephros and forms the Wolfl5an 

 duct, while the oviduct arises in another matter, to be described in 

 connexion with the reproductive organs (p. 351). In the teleosts 

 also there is no division of the pronephric duct. 



Metanephros. — The mesonephros is functional in the embryos of 

 all vertebrates and throughout life in the ichthyopsida. It also func- 

 tions for a short time after birth in certain reptiles (lizards) and in the 

 lowest mammals {Echidna, opossum). It becomes replaced in the 

 adults of all amniotes by the metanephroi, the only structures to 



Tlith 



Fig. 369. — Profile reconstructions of lizard {Lacerta agilis) {A) 16 mm. long; {B) 20 

 mm. long; and (C) human embryo 115 mm. long, after Schreiner. a, allantois stalk; c, 

 cloaca; cc, cranial collecting tubule; cd, caudal collecting tubule; k, permanent kidney 

 (metanephros); met, median collecting tubule; ms, mesonephros; mt, metanephric 

 (nephrogenous) tissue; mtb, mesonephric tubules; pet, primary collecting tupule; 

 pu, WolflBan duct (primitive ureter); r, rectum; s, secondary collecting tubule; «, ure- 

 ter; cm, u and pu, common portion of primitive and permanent ureters. 



which the name kidneys is strictly applicable. Each metanephros 

 arises behind the mesonephros of the same side. From the dorsal 

 hinder end of the Wolffian duct, near its entrance into the cloaca, a 

 tube, the ureter (fig. 369, A, k) grows forward, parallel to the parent 

 duct, into the tissue posterior and dorsal to the mesonephros. This 

 nephrogenous tissue is apparently serially homologous with that 

 from which the mesonephric tubules have arisen, but all traces of 

 metamerism have disappeared from it and the metanephros is not 

 gesmented in any stage of development. In this nephrogenous tissue 

 the anterior end of the ureter gives oS a varying number of branches 



