igoS.J 



EXCRETORY ORGANS OF METAZOA. 597 



tubules and like them is at first solid. Some of the more impor- 

 tant papers on the development of these structures are the follow- 

 ing: for the Amphibia, Fiirbringer (1878), Mollier (1890), Field 

 (1891), Semon (1891), and Brauer (1902); for the Cyclostomes, 

 Wheeler (1899), Price (1897); ^o^ the Selachii, Balfour (1881), 

 Van Wyhe (1889), Riickert (1888), Rabl (1896) ; for the Teleostei, 

 Hoffmann (1886), Henneguy (1888), H. V. Wilson (1891), Swsen 

 and Brachet (1901) ; for the Ganoidei, Parker and Balfour (1882), 

 Beard (1889) ; for the Reptiles, Hoffmann (1889), Gregory (1900) ; 

 for Aves, Sedgwick (1881), Balfour (1881), Renson (1883), Felix 

 (1891); and for the Mammals, Spee (1884), Flemming (1886), 

 Kollman (1891), Martin (1888). 



Mesonephros. — These tubules develop usually in the segments 

 behind the pronephroi, but there are certain segments that may con- 

 tain both of them, and they are more numerous and more differ- 

 entiated that the pronephroi. To understand their origin it is neces- 

 sary to recall that the ccelom becomes divided into the dorsal 

 myoccels (cavities of the myotomes or somites), the middle neph- 

 rocoels, both of these being segmented and paired, and the large 

 unsegmented hypocoel that is imperfectly paired ; these relations 

 were established particularly by Van Wyhe. Very early the 

 myocoels pinch off from the nephrocoels, whereby the latter are left 

 as short tubes, the dorso-lateral end of each ending blindly while 

 the ventral opens into the hypocoel. These peritoneal nephrocoels 

 become the mesonephroi and grow laterad to join with and open 

 into the segmental duct, for they develop no duct of their own. 

 The arterial connection is segmental : From the aorta a vessel grows 

 towards each tubule and ends in a capillary glomerulus against the 

 wall of the latter above the nephrostome; the wall of the tubule 

 forms a partial sheath (capsule of Bowman) around the glomerulus, 

 In Petromyson there is a larval as well as a definitive set of these 

 tubules, and there ma}- be several in each segment (Wheeler). 



The principal studies on the mesonephros are these : For Selachii, 

 Riickert (1888), Van Wyhe (1889), Rabl (1896); for Teleostei, 

 Fehx (1897); for Cyclostomata, Wheeler (1899), Price (1897), 

 Maas (1897); for Amphibia, Semon (-1891), Brauer (1962), Hall 

 (1904) ; for Reptiles, Gregory (1900J, Mihalkovics (1885), Wieder- 



