URINARY ORGANS 455 



The urinary duct in both groups probably represents the prone- 

 phric duct, and may lie more or less freely, or be embedded in the 

 substance of the kidney. Posteriorly the two ducts usually fuse 

 together and become expanded to form a kind of urinary bladder, 

 as is also the case in Ganoids (cf. Figs. 350 and 351); in its forma- 

 tion, however, the ectoderm of the cloaca also takes part. The 

 bladder usually opens behind the anus either independently, 

 or together with the genital ducts by a simple pore, or on the 

 summit of a urinogenital papilla. Thus a differentiation of the 

 pronephric duct into a Wolffian and a Miillerian duct is not 

 known to occur in Teleostei, nor does the mesonephros come into 

 connection with the gonads : the early development both of the 

 pronephros and mesonephros exhibits special peculiarities. 



A close examination of the organ, which appears to the naked 

 eye as the kidney in Teleosts and Ganoids, shows that a larger or 

 smaller portion of it more particularly the anterior part consists 

 of an adenoid or lymphoid tissue, comparable to the hcemolymph 

 glands described on p. 435. 



Dipnoans. The pronephros of the larva consists of several 

 tubules, and possesses two nephrostomes. The mesonephric 

 tubules have at first a strictly segmental arrangement, but later 

 are more numerous than the corresponding myotomes : they 

 possess no nephrostomes. The mesonephros forms the definitive 

 kidney ; it is relatively longer in Protopterus than in Ceratodus, 

 extending through a considerable portion of the body-cavity, 

 being narrow anteriorly, and gradually broadening out further back 

 (Fig. 352). In Lepidosiren and Protopterus the posterior instead of 

 the anterior end of the kidney comes into relation with the testis 

 and so may be spoken of as a " posterior epididymis." In 

 Protopterus this is fused with its fellow and covered with black 

 pigment. The kidneys are invested by lymphoid and adipose 

 tissue, especially on their lateral and posterior borders. In the 

 female, the mesonephric ducts communicate with the cloaca 

 independently, behind the genital papilla : in the male they open 

 dorsally into the base of the cloacal csecum (Fig. 247) on a single 

 (Protopterus) or paired (Lepidosiren) papilla. The cloacal caecum 

 is possibly comparable to the sperm-sac of Elasmobranchs, and to 

 the urinary bladder of Teleostomes, 



Amphibians. Although the primary pronephric rudiments 

 extend over a large number of segments, the number of actual 

 pronephric canals is usually limited to 12 or 13 in the Gymno- 

 phiona, three in the Anura, and two in the Urodela. 1 



Thus the most typical condition of the pronephros amongst 

 Vertebrates is met with in the embryos of Gymnophiona, and this 



1 The pronephros undergoes degeneration at the beginning of metamorphosis 

 in Urodeles and Anurans, and at a relatively earlier stage in the Gymnophiona. 



