GENEEATIVE OBGANS OF OSSEOUS FISHES. 665 



are expelled in countless thousands into the surrounding element through 

 the orifice of the ovarian sac (fig. 317, r), which is situated immediately 

 behind the anus (fc) and in front of the urinary canal (s). 



(1842.) Generally, as has been already stated, the ova of fishes are 

 fecundated after their expulsion ; but there are a few instances, as for 

 example the Viviparous Blenny (Zoarcus viviparus) of our own shores, 

 in which the young are hatched in the ovary, and grow to a consider- 

 able size before they are born : in such cases impregnation must take 

 place internally ; and the males in these species have, in fact, a nipple- 

 like prolongation of the orifice of the duct through which the semen 

 escapes, probably for the purpose of introducing the seminal fluid into 

 the interior of the ovary of the females. Nevertheless even in these 

 the ovaria present the same structure as in ordinary fishes, the only 

 difference being that their eggs are retained until the embryo is far ad- 

 vanced in its development, instead of being prematurely extruded. 



(1843.) The testicle in the males of osseous fishes, generally named 

 " the milt," equals in bulk the ovary of the other sex ; and the quantity 

 of the secretion furnished by it must be exceedingly great. The entire 

 organ is composed of slender and very delicate convoluted caeca, in which 

 the semen is elaborated. These tubes towards the circumference of the 

 testis all terminate in blind extremities, but by their opposite ends they 

 communicate with the general excretory duct ; so that, by blowing air 

 into the latter, the entire organ becomes amazingly distended. In some 

 cases the seminiferous tubules run parallel to each other, and become 

 furcate as they approach the exterior of the testis : in others, after di- 

 viding and subdividing to some extent, as they diverge from the common 

 duct, they become converted into innumerable anastomosing ramifica- 

 tions ; so that the whole substance of the testis appears to be made up 

 of reticulate tubes, which during the spawning season, when they are 

 filled with the creamy fluid that they secrete, are visible even with the 

 naked eye*. 



(1844.) It will be observed by the anatomical reader, that while in 

 the OSSEOUS FISHES the ova escape into the interior of the ovary, and 

 are expelled through an excretory orifice resembling the duct of an 

 ordinary gland, in the CARTILAGINOUS FISHES and in all other VERTE- 

 BRATA the germs burst from the exterior of the ovarium, where they are 

 generally seized by Fallopian tubes, and either conveyed out of the 

 body as eggs, or, being hatched internally, the offspring are nourished 

 in receptacles provided for the purpose, until they arrive at a consider- 

 ably-advanced state of development. 



(1845.) But it is only by degrees that these more perfect ovigerous 

 organs make their appearance, and we would particularly solicit the at- 

 tention of the student to the different gradations of structure met with 

 in this part of the animal economy. 



* Muller, De Glandularum Structura penitiori. Lipsia?, fol. 1830. 



