Part 2 (2/2)
In most instances the fecundation of flowers is chiefly effected through a purely mechanical process, though in these cases also we see a wonderful adaptation of parts to conditions.
When the male and female parts of flowers are situated on different plants, as is the case in the willow, the poplar, the melon vine, and many other species, the pollen of the male flower is wafted by the wind or gentle breeze to the stigma of the female flower, which will usually be found at no very great distance, although fertilization may take place in this way at very considerable distances. Bees, moths, and many other species of insects, serve a very important purpose in this work, transporting the fertilizing dust upon their wings, antennae, sucking-tubes, and feet. Small birds, and even the humble snail, which would scarcely be credited with any useful function, are also very serviceable in the same direction. The part performed by insects in the reproductive process of many plants is so great that they have been very poetically termed ”the marriage priests of flowers.”
Nature provides for thorough fecundation in these cases by placing the plants which bear the male and the female flowers near each other. This fact accounts for the unproductiveness of certain varieties of strawberries unless mixed with plants of some other variety, it being well known to nursery-men that some varieties produce only the female parts of flowers.
Modes of Fecundation in Animals.--The modes by which fecundation is effected in animals are still more various and wonderful than in plants.
In some of the lower animals, as in most fish and reptiles, both elements are discharged from the bodies of the parents before coming in contact, there being no contact of the two individuals. In this cla.s.s of animals the process is almost wholly a.n.a.logous to fecundation in those plants in which the male and female flowers are on different plants or different parts of the same plant. In the female fish, a large number of ova are developed at a certain season of the year known as the sp.a.w.ning season. Sometimes the number reaches many thousands. At the same time, the t.e.s.t.i.c.l.es of the male fish, which are contained within the abdominal cavity, become distended with developed zoosperms. When the female seeks a place to deposit her eggs, the male closely follows; and as she drops them upon the gravelly bottom, he discharges upon them the zoosperms by which they are fecundated. The process is a.n.a.logous to some species of frogs. When the female is about to deposit her eggs, the male mounts upon her back and rides about until the eggs are all deposited, discharging upon them the fertilizing spermatozoa as they are laid by the female.
In higher orders of animals, fecundation takes place within the generative organs of the female by contact between the male and the female organs. To effect this, there are necessitated certain accessory organs, the _p.e.n.i.s_ in the male and the _v.a.g.i.n.a_ in the female.
Nothing in all the range of nature is more remarkable than the adaptation of the two varieties of s.e.xual organs in each species. This necessary provision is both a powerful means of securing the perpetuation of the species, and an almost impa.s.sable barrier against amalgamation.
The act of union, or s.e.xual congress, is called _coitus_ or _copulation_.
It is accompanied by a peculiar nervous spasm due to excitement of special nerves princ.i.p.ally located in the _p.e.n.i.s_ in the male, and in an extremely sensitive organ, the _c.l.i.toris_, 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 Zoosperm.--The zoosperms 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 ill.u.s.tration 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 s.e.xual 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 ill.u.s.tration 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 zoosperm 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 zoosperms. 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 cla.s.ses 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.
Partial Protection of the Ovum.--There are some exceptions, however, to this general rule among fishes and reptiles. Even fishes manifest a degree of parental solicitude in certain cases. The male of a species of South American fish gathers up the eggs after fecundation has taken place, and carries them in his mouth until they are hatched. Another male fish carries the eggs of his mate in a little pouch upon the lower and posterior part of his body.
Certain species of frogs carry their eggs wound about their legs; others suspend them from the abdomen. Another variety carries its young upon its back. Prof. Wyman describes a ”swamp toad” which patiently takes the eggs of his mate, one by one, and fastens them upon her back, observing great regularity in arrangement. These several devices are evidently for the purpose of protecting, in some degree, the young individual during the helpless stage of its existence.
Development in the Higher Animals and Man.--Higher animals are less prolific, and their development is a more complicated process; hence, their young need greater protection, and, for this reason, the ova, instead of being discharged from the body of the female after fecundation, are retained.[3] As we have seen that a suitable receptacle is sometimes provided outside of the body, so now a receptacle is needed, and is provided in the interior of the body of the female. This receptacle is called
[Footnote 3: Curious examples of internal development sometimes occur in animals which usually deposit eggs. Snakes have been known to produce both eggs and living young at the same time. At the annual meeting of the American Society for the Advancement of Science, at Detroit, Mich., in August, 1875, we had the pleasure of examining a specimen, exhibited by Prof. Wilder, of a chick which had undergone a considerable degree of development within the ovary of the hen. It had a head, a rudimentary brain, and internal viscera, but no feathers nor limbs. It was, in fact, an egg hatched before it had been laid. The anomaly excited much interest at that time and since among biologists.]
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