or THE LARVAL POLTPTEEUS. 337 



nephrostomes opening into them, makes very probable, then we may regard the male 

 and- female ducts of Cyclostomata, Crossopterygii, Ganoidei, andTeleostei as homologous, 

 and the female ducts of other Gnathostomata as independent structures, ^. e. Miilleriaa 

 ducts. It is now known that these Miillerian ducts arise as a splitting off from the 

 Archinephric duct only in Elasmobranchs, but this difference in the mode of origin of 

 the oviducts is not considered by many authors to constitute a proof that they are not 

 homologous. The peculiarity of the oviducts of these groups (Elasmobranchii, Dipnoi, 

 Amphibia, and Amniota) in tending to appear in the males as more or less rudimentary 

 structures, seems to me to be a strong argument in favour of their being homologous in 

 these groups ; and it is very noticeable that these rudiments are usually, if not invariably, 

 absent in the Crossopterygii, Ganoidei, andTeleostei, those groups, namely, in which the 

 oviduct in the female appears not to be homologous with the Miillerian ducts but 

 rather with genital ducts of the male. 



Summary of the Stnictm'e and Development of the JJrinogenital Organs. 



The pronephros oi Pohjpterus is beginning to degenerate in the larva 30 mm. in 

 length. At this stage it consists of a small pronephric chamber containing the glomus 

 and of a duct leading from the chamber to pass to and become the duct of the 

 mesonephros, and consisting of three portions — a very narrow portion leading outwards, 

 a dilated and much convoluted portion lying close under the skin, and a very narrow 

 portion leading inwards and backwards to become the segmental duct of the 

 mesonephros. 



The pronephros of FohjpterKS resembles very much that of Amia. 



The mesonephros anteriorly is at this stage becoming complicated. Posteriorly it 

 has still a segmental arrangement, each segment being provided with one Malpighian 

 corpuscle with its tubule leading to the segmental duct and its peritoneal funnel. The 

 peritoneal funnels open upon the rudiment of the oviduct, which is not yet folded off 

 from the coelom. The genital pores do not yet penetrate the walls of the urinary 

 sinus. 



From examination of somewhat older specimens, the male and female ducts appear to 

 arise in a somewhat similar manner, and it is regarded as probable that in the youno- 

 male larva the peritoneal funnels open upon the rudiment of the male duct. It is 

 suggested that these peritoneal funnels, if they persisted, would then answer to thevasa 

 efi'erentia passing from the longitudinal carial to the Malpighian corpuscles of 

 Lepidosteus. 



The male and female ducts of Crossopterygii and Teleostei are regarded as homolo- 

 gous structures, which in the males of other Gnathostomata have been gradually 

 absorbed in the testicular network, and in the females of other Gnathostomata except 

 the Ganoidei are replaced by Miillerian ducts. 



