532 PISCES FISHES. 



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 fur- 

 cate as they approach the exterior of the testis : in others, after 

 dividing and subdividing to some extent, as they diverge from the 

 common duct, they become converted into innumerable anastomos- 

 ing ramifications ; so that the whole substance of the testis appears 

 to be made up of reticulate tubes, which during the spawning sea- 

 son, when they are filled with the creamy fluid that they secrete, 

 are visible even with the naked eye.* 



(580.) 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 VERTEBRATA 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 considerably advanced state of 

 developement. 



But it is only by degrees that these more perfect ovigerous 

 organs make their appearance, and we would particularly solicit the 

 attention of the student to the different gradations of structure met 

 with in this part of the animal economy. 



In the Eel and the Lamprey we have the first appearance of 

 an ovary, such as is common to the higher Vertebrata. It consists 

 of a very extensive vascular membrane covered by the peritoneum, 

 and attached in broad folds beneath the spine, extending nearly 

 from one end of the abdomen to the other (fig- 235). This viscus 

 is not hollow, neither has it any excretory duct, so that naturalists 

 were long at a loss to explain how the ova of these creatures were 

 expelled. 



The extensive membrane above alluded to, as is now sufficiently 

 well determined, produces in its substance the germs of the future 

 progeny ; and these, as they become mature, break loose from the 

 nidus wherein they were generated into the interior of the peri- 

 toneal cavity of the Eel, and float loosely in the abdomen : there is 

 no Fallopian tube as yet developed ; but two simple orifices, placed 

 on each side of the anal opening, serve to give exit to the countless 

 eggs, which thus escape into the surrounding water. 



The male organs of the Lamprey and Eel, together with the 



* Miiller, de Glandularum structura penitiori. Lipsiae, fol. 1830. 



