1st. The production of the germ, which is a constant circumstance ; 2dly, fecun dation, which belongs to only the sexual generations ; 3dly, copulation, which is confined to those sexual generations, in which fecundation is accomplished with in the body.
Lastly, uterogestation, which belongs exclusively to viviparous generation.
The testes, and sometimes the vesicu lx seminales and prostate, vary most re markably in their magnitude in such ani mals as have a regular rutting season. They are very diminutive at other peri ods of the year, but swell at that par ticular time to a comparatively vast mag nitude. This change is particularly ob servable in the testes of the mole, spar row, and frog.
We may mention here, in a cursory and general manner, the peculiar organs possessed by the moles of some species, for the purpose of holding the female during the act of copulation. Of this kind are, the spur on the hind feet of the male ornithorhynchus ; the rough black tubercle formed in the spring sea son on the thumb of the common frog ; the two members, formed of bones arti culated to each other, near the genitals of the male torpedo and other cartilagi nous fishes ; the forceps on the abdomen of the male dragon-fly, &c.
A scrotum belongs to the mammalia on ly ; and is not found in all these. The aquatic genera, those which live under grotind, and others, want it.
The testes remain constantly in the ab domen in the ornithorhynchus, the ele phant, the amphibious mammalia, and the cetacea. Some animals have the power of- protruding them from the ab domen, and retracing them again into the cavity ; as the bats, mole, hedge hog, and shrew, besides several of the rodentia. They are thrust out of the ca vity, particularly at the rutting season.
The tunics vaginalis exists constantly in the mammalia. As the horizontal po sition of the ,body.obviates the danger of .herniae, the cavity of this membrane communicates by means of a narrow ca nal with the abdomen, in such animals as have the testes remaining constantly in the scrotum.
In some species, where the act of copu lation requires a longer portion of time, as in the dog, badger, &c. the corpus spongiosum of the glans, and of the pos terior part of the penis, swells during the act much more considerably than the rest of the organ, and thus the male and female are held together during a suffi cient space of time for the discharge of the seminal fluid.
Several species of mammalia, both among those which possess no vesiculz seminales, and thereby require a longer time for completing the act of copulation, and such as are not distinguished by this peculiarity, possess a peculiar bone in the penis, generally of a cylindrical form, but sometimes grooved. This is the case with some of the simia, most of the bat-kind, the hamster, and several others of the mouse-kind, the dog, bear, badger, weasel, seal, walrus, &c.
In most of the mammalia the urethra runs on the end of the glands, and forms a common passage for the urine, pros tatic liquor, and semen. In some few species, the passage which conducts the two former fluids is distinct from that of the seminal liquor. The bifid fork-like glans, of the opossum has three openings, one at the point of bifurcation for trans mitting the urine ; and two for the semi nal fluid at the two extremities of the glans. The short urethra of the ornitho rynchus paradoxus opens directly into the cloaca, and the large penis of the ani mal serves merely to conduct the semi nal fluid. It divides into two parts at its extremity, and each of these is furnished with sharp papilla, which are perforated for the passage of the semen. A similar structure obtains in the ornithorhynchus hystrix, where the penis divides into four glands.
In some species of the cat-kind the glans is covered with retroverted which, as these animals have no vesiculm seminales, may enable the male to hold the female longer in his embraces.
Lastly, it deserves to be mentioned, that in some mammalia, the male penis, while unerected, is turned backwards ; so that the urine is voided in the male in the same direction as in the female. The hare, lion, and camel, afford instances of this structure. But the statement which has been so often repeated since the time of Aristotle, that these retromingentia co pulate backwards, is erroneous.