STRUCTURE AND DEVELOPMENT OF I-EPIDOSTEUS. 827 



and naturalists who have the opportunity ought also to look out 

 for such openings. 



The mode of origin of the anterior part of the genital duct 

 of Lepidosteus appears to us to tell strongly in favour of the 

 view, already regarded as probable by one of .us 1 , that the 

 Teleostean genital ducts are derived from those of Ganoids ; 

 and if, as appears to us indubitable, the most primitive type of 

 Ganoid genital ducts is found in the Chondrostei, it is interesting 

 to notice that the remaining Ganoids present in various ways 

 approximations to the arrangement typically found in Teleostei. 

 Lepidosteus obviously approaches Teleostei in the fact of the 

 ovarian ridge forming part of the wall of the oviduct, but differs 

 from the Teleostei in the fact of the oviduct opening into the 

 kidney ducts, instead of each pair of du^ts having an independ- 

 ent opening in the cloaca, and in the fact that the male genital 

 products are not carried to the exterior by a duct homologous 

 with the oviduct. Amia is closer to the Teleostei in the arrange- 

 ment of the posterior part of the genital ducts, in that the two 

 genital ducts coalesce posteriorly ; while Polypterus approaches 

 still nearer to the Teleostei in the fact that the two genital ducts 

 and the two kidney ducts unite with each other before they 

 join ; and in order to convert this arrangement into that charac- 

 teristic of the Teleostei we have only to conceive the coalesced 

 ducts of the kidneys acquiring an independent opening into the 

 cloaca behind the genital opening. 



The male genital ducts. The discovery of the vasa efferentia 

 in Lepidosteus, carrying off the semen from the testis, and trans- 

 porting it to the mesonephros, and thence through the mesone- 

 phric tubes to the segmental duct, must be regarded as the most 

 important of our results on the excretory system. 



It proves in the first place that the transportation outwards 

 of the genital products of both sexes by homologous ducts, 

 which has been hitherto held to be universal in Ganoids, and 

 which, in the absence of evidence to the contrary, must still 

 be assumed to be true for all Ganoids except Lepidosteus, is 

 a secondary arrangement. This conclusion follows from the 

 fact that in Elasmobranchs, &c., which are not descendants of 



1 F. M. Balfour, Comparative Embryology, Vol. II., p. 605 [the original edition]. 



532 



