of Dipnoan Fishes. 333 



Finally, in Discoglossus the transverse canals are reduced to 

 a single one lying anteriorly and connecting testis with kidney 

 duct directly. I am not aware that any raorphologist would derive 

 the complex testicular network so characteristic of most frogs from 

 a condition such as that of Alytes : on the contrary it would be 

 probably universally admitted that in Alytes we have a con- 

 dition derived from that which is more usual by a reduction of 

 the testicular network to a vestige at its anterior end. 



Passing then to a consideration of the conditions described in 

 the two-lunged Dipnoans we find that they fall in perfectly with 

 the belief that they may have come about by a similar process to 

 that which has taken place amongst the Anura — that the single 

 connexion between testis and kidney apparatus in Protopterus 

 represents the posterior one of the metamerically repeated con- 

 nexions of Lepidosiren, and that the little series of such connexions 

 in Lepidosiren represents in turn the posterior members of a once 

 much more extensive series, stretching through the greater part 

 of the length of testis and kidney. 



I have pointed out in my paper in Proc. Zool. Soc. London, 

 1901, vol. II., that it is but a step from the condition found in 

 Protopterus to that found by Budgett and Jungersen in Poly- 

 pterus : that the step is a very short one is indicated by what 

 has taken place in the Anura. Jungersen's work on Teleostean 

 development also shows that he is justified in saying that the 

 relations of testis and duct in Polypterus lead up directly to the 

 relations of those structures in Teleosts. 



I believe then that the conditions shown to be present in 

 Protopterus and Lepidosiren afford the strongest evidence for 

 accepting in the meantime as our working hypothesis the view 

 that the common ancestral forms of the various groups of gnatho- 

 stomatous vertebrates possessed a well-developed testicular net- 

 work, and that the cases where this is now absent have arisen 

 through the enlargement of the posterior element of the network 

 and the atrophy of the remainder. 



