iv OPISTHONEPHKOS 259 



toneal canal. The earlier developmental material does not suffice to 

 show definitely whether or not, as is probable, this peritoneal canal 

 is a secondary connexion with the peritoneal epithelium. The 

 peritoneal funnels exist only for a time during larval life : in specimens 

 90 mm. in length they had disappeared. In Calamichthys (Lebedin- 

 sky, 1895) the peritoneal canals have been found still persisting in a 

 larva of 15 cm. 



Actinopterygian Ganoids. — Here again the definitive nephro- 

 tomies appear as solid masses of cells arranged segmentally. The 

 gap separating them from the pronephros is in the more primitive 

 Sturgeons about 3 or 4 segments, in the more highly evolved Amia 

 16 or 17 (Jungersen, 1893-4). ■ The rudiment grows in length, 

 develops a lumen secondarily, joins on to the duct by its lateral 

 end while its mesial end dilates to form the Malpighian body — all 

 in the usual fashion. At a late period — after the Malpighian bodies 

 have already assumed their characteristic features — they develop 

 peritoneal canals as outgrowths from their walls which meet and fuse 

 with the peritoneal epithelium secondarily. Later on the peritoneal 

 canals become again obliterated and appear to be absent in the adult 

 except in the case of Amia. 



Teleostei. — In the Teleostean fishes, as is indeed the case to a 

 certain extent in all the members of the Teleostomi, the opistho- 

 nephros is delayed in development in correlation with the prolonged 

 functioning of the pronephros. According to Felix (1897) in the 

 Trout the first opisthonephric units or definitive nephrotomes 

 begin to make their appearance about 70 days after fertilization as 

 rounded clumps of cells, in the centre of which a small lumen 

 appears. These lie immediately dorsal to the duct, in the connective 

 tissue trabeculae which at this stage of development traverse the' 

 cavity of the interrenal vein. These rudiments appear first about 

 the middle third of the duct and gradually spread backwards, those 

 in front being segmental in position while those farther back are no 

 longer segmental and fuse together into irregular masses. Each 

 rudiment grows actively in length and goes through the usual series 

 of changes before joining up to the duct. 



To the primary units just described are added secondary and 

 tertiary units. These develop exactly as do the primary except that 

 in the case of the tertiary set the tubule may fuse either with the 

 duct directly or with an already developed tubule. 



As the tubules increase enormously in length they become inex- 

 tricably entangled together extending even across the median plane 

 so that the substance of the two kidneys becomes continuous through 

 the interrenal trabeculae. It is further characteristic of the tele- 

 ostean kidney that there takes place in it a great development of 

 round-celled pseudolymphoid (Felix) tissue. This forms a packing 

 tissue between the tubules and appears to be formed by proliferation 

 from the walls of the interrenal venous spaces. 



The opisthonephros extends back for a short distance behind the 



