GENERATIVE ORGANS. 89 



sistent Miillerian duct which has fused with the testis or it may 

 be something else. In other Pisces, with the apparent exception 

 of the Ganoid Polypterus, the testis has come to consist of tubules 

 which are connected by means of a network of tubes, called the 

 testicular network, with some of the kidney tubules. The con- 

 nection may take place along the greater part of the length of the 

 kidney, as in Lepidosteus and Acipenser, or it may be confined to 

 the anterior region (mesonephros), as in Elasmobranchs, or 

 finally, as in Dipnoi, it may only occur through the hind end of 

 the kidney. The connection is usually through the malpighian 

 bodies of the renal tubules, but in Amia the tubes from the testis 

 join the renal tubes beyond the malpighian bodies. In Poly- 

 pterus alone is there no connection with the kidney, the testis duct 

 passing directly back from the testis to join the longitudinal duct 

 near th? cloaca. This condition in Polypterus is not understood 

 any more than is the condition of the male Teleostean, though 

 theories have been put forward to account for it. It may be 

 that in these forms the Miillerian duct has acquired a connection 

 with the male gonad and persisted ; or it may be that the con- 

 nection is really effected through a part of the kidney which has 

 lost all kidney structure, as has happened in some male Amphibia 

 and in the higher Vertebrata. A study of development can only 

 settle the question, and that has not yet successfully been 

 made. 



To return to the longitudinal duct. This, as explained above, 

 is called at first the pronephric duct, except in Elasmobranchs, 

 in which it is called the segmental duct, there being no functional 

 pronephros. Later, when the kidney is formed and the prone- 

 phros has atrophied, it becomes the kidney duct. In Elasmo- 

 branchs, in which the kidney differentiates into meso- and meta- 

 nephros distinguished, not by any break in continuity, but by size 

 and by the course of the so-called collecting tubes of the nephridia 

 (see below), it is called the mesonephric duct, because it appears to 

 be related more especially to the mesonephric portion of the 

 kidney. Inasmuch as in the male Elasraobranch this meso- 

 nephric duct is chiefly concerned with carrying off the spermatozoa 

 which pass, as has been described above, through a part of the 

 mesonephros, it is also called the vas deferens. In the higher 

 classes of Vertebrata the mesonephric duct is called the Wolffian 

 duct in the embryo, and persists in the male adult as the vas 



