Nothing in all the range of nature is more remarkable than the adaptation of the two varieties of sexual organs in each species. This necessary provision is both a powerful means of securing the perpetuation of the species, and an almost impassable barrier against amalgamation.
The act of union, or sexual congress, is called coitus or copulation. It is accompanied by a peculiar nervous spasm due to excitement of special nerves principally located in the penis in the male, and in an extremely sensitive organ, the clitoris, in the female. The nervous action referred to is more exhausting to the system than any other to which it is subject.
[Union of the Ovum and Zoösperm.]—The zoösperms not only come in contact with the ovum, but penetrate the thin membrane which incloses its contents, and enter its interior, where they disappear, becoming united with its substance. In the ova of certain fishes, small openings have been observed through which the spermatozoa find entrance. Whether such openings exist in human ova is an undecided question; but it is probable that they do.
[Curious Modes of Reproduction.]—A peculiar kind of reproduction is observed in a variety of polyp, a curious animal which very much resembles a shrub in appearance. It attaches itself to some solid object, and then, as it grows, sends out little protuberances resembling buds. Some of these separate and fall off, swimming about as separate animals. These never become like the parent polyp; but they lay eggs, which hatch, and become stationary polyps like their grandparent, and in their turn throw off buds to form swimming polyps. In this case we have two kinds of generation combined, alternating with each other.
Plant-lice afford a curious illustration of a similar generation. Males and females unite and produce eggs. The creatures produced by the hatching of eggs are neither males nor perfect females. They are imperfect females. They are all alike, so that no sexual union occurs. Instead of laying eggs, they produce live young like themselves, which appear to be developed from internal buds similar to the external buds of the polyp. After this method of reproduction has continued for eight or ten generations, a few perfect individuals appear, and the first process is repeated.
The common honey-bee affords another illustration like the last. A virgin queen sometimes lays eggs, which always produce males, or drones. After union with a male, she lays eggs in the royal cells which become perfect females like herself. She also seems to have the power to lay, at will, unfecundated eggs, from which drones are produced.
[Human Beings Are Developed Buds.]—It has been very aptly suggested by an eminent physiologist that the ovum and zoösperm may be correctly considered as internal buds. Thus it would appear that generation is universally a process of budding. A child is but a compound bud, an offshoot from its parents. This idea is not a mere fancy, but has a scientific basis. As all the exquisite details of the most beautiful flower are in essence contained within the tiny bud which first makes its appearance, so is the developed human being, the full-grown man or woman, virtually contained within the tiny cell called the ovum after it has been impregnated or fecundated by the zoösperms. In short, men and women are blossoms in a strictly scientific sense.
[Fecundation in Hermaphrodites.]—The process of fecundation in hermaphrodite animals is very peculiar. In some cases, as in the snail, the union of two individuals is usually necessary, though each possesses both kinds of organs. In other cases, as in the tape-worm, the oyster, and numerous other mollusks, a single individual has the power to fertilize its own ova, thus being wholly independent. Human hermaphrodites are usually so deformed that fecundation is not effected, which is a fortunate safeguard against the multiplication of such monstrosities.
[Development.]—After the union of the two elements, known as fecundation or conception, if the conditions are favorable, development occurs, and the little germ is in due process of time developed into an individual which is an exact counterpart of its parents. During this developmental process, the embryonic being is variously treated by different classes of animals.
[Unprotected Development.]—Most fishes and reptiles discharge their ova before fecundation, or soon after, and pay no further attention to them. The fish deposits its eggs in a little hollow scooped out in the gravelly bed of a stream, or sows them broadcast upon the waters. The turtle buries its eggs in the sand, and leaves them to be hatched by the sun. The ostrich disposes of her eggs in the same way. Many other species of animals pay no regard to the protection of the germs which are destined, if placed under favorable conditions, to become individuals like themselves.