GENERATIVE ORGANS. 311 



The generative gland of Cyclostomes consists of a long un- 

 paired organ suspended to the dorsal wall of the body-cavity by 

 a fold of peritoneum, the mesoarium or mesorchium, as the 

 case may be. In other Fishes, the gonads are only exceptionally 

 unpaired, and even then, this is only a secondary condition : in 

 all other Vertebrates they are also originally paired. There is 

 usually a want of symmetry observable between the organ of the 

 right and left sides, and that of one side may even entirely disappear, 

 as in Ammodytes tobianus, Cobitis barbatula, and others. 

 Secondary fusions may also take place. 



The ovaries and testes of Teleostei closely correspond with 

 one another as regards position and the arrangement of their 

 ducts. The ovary usually forms an elongated sac, which is blind an- 

 teriorly, and on the inner walls of which the ova arise ; this sac is 

 continued backwards to form the oviduct. The latter, which is 

 generally short, as a rule fuses with its fellow to form an unpaired 

 canal ; this opens either on a level with the integument or on 

 a papilla, which may become elongated to form a tube or 

 " ovipositor." l 



The testis of Teleosteans is elongated, and often lobulated in 

 form. Its duct, which is often intensely white, opens between the 

 rectum and the urinary aperture, after uniting with its fellow to 

 form an unpaired canal. 



In cases where traces of copulatory organs or other accessory structures 

 which are spoken of as " seminal vesicles," or " prostates," are present, they, 

 like the so-called urinary bladder, have nothing whatever to do with the 

 similarly-named structures of higher Vertebrates. 



In the above description the terms " Miillerian " and " Wolffian 

 ducts" have been purposely avoided, for it is very doubtful 

 whether the generative ducts of Teleostei are in any way homo- 

 logous with them, and further investigations are necessary to decide 

 the question (comp. note on p. 313). 



In by far the greater number of Elasmobranchs the ovaries 

 are paired, and this is always the case as regards the oviducts, 

 which, unlike those of Teleosteans, are always separate from the 

 ovaries, and correspond to the Miillerian ducts. Their anterior 

 portion has a common opening into the body-cavity, and further 

 back each is provided with a so-called "oviducal gland." The 

 anterior part of the oviduct is always narrower and more delicate 

 than the posterior, which dilates to form a kind of uterus, in 

 which (when the shark is viviparous) the embryo undergoes 



1 In the genus Girardinus, which, like many other Fishes (numerous Elasmo- 

 branchs for instance), brings forth its young alive, the body-cavity of the female 

 is much longer than that of the male, and the ovary serves also for a uterus. 

 Contrary to the general rule, the ova do not become free from their place 

 of origin before fertilisation, so that the spermatozoa must bore their way 

 through the germinal and follicular epithelium, and thus even pass into 

 the parenchyma of the ovary. 



