404 FISHES CHAP. 



The nature and homologies of the genital ducts in the 

 different groups of Fishes are amongst the most puzzling of the 

 many problems which vex the soul of the Vertebrate morpho- 

 logist, and although there is a fairly general agreement on some 

 points, there are others of great importance of which it may be 

 said quot homines, tot sentential-. 



Broadly speaking, there are two types of genital ducts in 

 Fishes : ( 1 ) those which are obviously derived from some part 

 of the kidney system ; and (2) those which are special ducts and 

 appear to have no connexion with kidney-ducts. 



The Elasmobranchs offer a typical example of gonoducts of 

 the first kind. At an early embryonic period in both sexes 

 each archinephric duct becomes longitudinally split into two 

 ducts, of which one continues to receive the openings of the 

 mesoriephric tubules and remains as a mesonephric duct (Fig. 



229, B). 1 The other, which has no connexion with the meso- 

 nephros, opens anteriorly into the coelom by means of the 

 united nephrostomes of the pronephros, and is known as the 

 "Miillerian duct" (Fig. 230, C and D). In the adult male 

 the Miillerian ducts are useless vestiges, but in the female they 

 persist and act as oviducts, receiving the eggs set free from 

 the ovarian ovisacs through their coelomic apertures, and thence 

 conveying them to the cloaca. In the male, certain of the 

 anterior mesonephric tubules become connected with the testi- 

 cular ampullae by means of a network of slender tubules, the 

 " vasa efferentia " or testicular network, and through the latter the 

 spermatozoa pass from the testes to the mesonephric duct (Fig. 



230, C). Consequently, the mesonephric duct conveys both 

 spermatozoa and the kidney excretion to the cloaca. It is 

 obvious, therefore, that both the male and female gonoducts are 

 derived from kidney-ducts. 



The Teleostei afford an equally typical illustration of the 

 second type. Each female gonoduct (oviduct) is formed by a 

 backward growth of the same two peritoneal folds which enclose 

 the ovary ; these are converted into a " peritoneal tube " or 

 canal by the union of their margins. The male gonoducts 

 are also formed in continuity with the testes, that is, as 

 backward prolongations from the latter. Each duct, male or 



1 Semper, CentralUatt f. Med. Wiss. 1875, No. 29 ; F. M. Balfour, Journ. Anat. 

 a-nd Phys. x. 1875, p. 17 ; Id. Comparative Embryology, London, 1881, ii. p. 568. 



