This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(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 considerable size before they are born: in such cases impregnation must take place internally; and the males in these species have, in fact, a nipplelike 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 advanced 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 dividing and subdividing to some extent, as they diverge from the common duct, they become converted into innumerable anastomosing ramifications; 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 considerably-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 attention of the student to the different gradations of structure met with in this part of the animal economy.
* Muller, De Glandularum Structura penitiori. Lipsiae, fol. 1830.
(1846). 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. 325.) 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.
(1847). 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 in the interior of the peritoneal 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.
(1848). The male organs of the Lamprey and Eel, together with the ovaria of the female, and the kidneys and ureters, were accurately described by Hunter in the Catalogue of his Collection, and their form and structure are illustrated by the preparations and drawings still preserved in the College of Surgeons *; but in such fishes the testis of the male so exactly resembles the female ovary, that it was imagined even by Sir E. Home that no males existed, or that the females were themselves hermaphrodite. According to Rathke 1, however, the testes of the male are composed of solid granules, precisely like the female ova; and the secretion derived from them is in like manner allowed to escape into the abdomen, from which it is expelled through similar openings in the peritoneum.
(1849). In the Sharks and Rays we meet with a very important addition to the female sexual apparatus, namely, an oviduct, by which the germ is seized on its escape from the ovarium, and furnished with the additional covering necessary in such fishes for the security of the fetus.
Fig. 325. Reproductive organs of the Lamprey (Petromyzon marinus).
* See Physiological Catalogue, vol. iv. pp. 48, 129, pis. 59 & 60.
1 Neueste Schriften der naturforschenden Gesellschaft zu Danzig. Halle, 1824.
(1850). In these genera the folds of the ovarian membrane become less extensively spread out, and, from the size of the yelks of the eggs formed therein, the organ assumes a racemose appearance. The ovaries now form two large bunches placed on each side of the spine; and the ova when mature would necessarily escape into the abdominal cavity, as those of the Lamprey and Eel do, were they not seized by the patulous orifices of the two long and membranous oviducts whereby they are conveyed out of the body.
(1851). There is, moreover, in the Chondropterygian Fishes, a necessity for defending the young, during the earlier stages of their growth, by means which it would have been quite foreign to the purposes of Nature to have adopted in the other division of this extensive class. The earth is peopled only at its surface, and the vegetable banquet there spread is abundantly sufficient for the support of terrestrial beings. The ocean, however, being densely populated at every assignable depth, could never have supplied vegetable food to anything like the extent required to satisfy her progeny; hence therefore the necessity for that astonishing fertility so remarkable in the osseous fishes. Nine millions of ova have been calculated to be spawned at a birth by a single Cod-fish: such spawn, being naked and unprotected, is eagerly devoured by thousands of hungry mouths, or the feeble young soon fall a prey to countless voracious persecutors. If, however, it was obviously requisite that the progeny of osseous fishes should be thus multitudinous in order to provide a sufficiency of needful food, it is equally clear that it would have been incompatible with the design of the Creator that the ravenous Sharks should be endowed with equal fecundity: their eggs are consequently few in number; and, in proportion to their scarcity, jealous precaution must be taken to ensure the safety of the included young, in order to prevent the complete extinction of the race.