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real development, and give occasion to the diversity which is occasionally perceptible amongst them.
If we admit, however, that the germs of the eniozoa are always received from without, their occurrence, in different stages of development, in the foetus in utero is a circumstance some-, what difficult of explanation under that theory. Small, indeed, must be the germ, which, when received into the digestive organs of the mother, can pass into her circulation, be transmitted into the vessels of the foetus, be deposited in some viscus and there undergo its full development; yet such cases have occurred, if the theory be correct. Certain it is, that, however the fact may be accounted for, worms have been found in the foetus by individuals whose testimony cannot be doubted. Frommann saw the distoma hepaticum in the liver of the foetal lamb; Kerckring, ascarides lumbricoides in the stomach of a fœtus six and a half months old; Brendel, tæniæ in the human foetus in utero; Heim, tæniæ in the new-born infant; Blumenbach, tæniæ, in the intestine of the newborn puppy; and Goeze, Bloch, and Rudolphi, the same parasite in sucking lambs.
Perhaps the conclusion of Cuvier is the soundest and most consistent with analogy, that these parasites "propagate by germs so minute as to be capable of transmission through the narrowest passages; so that the germs may exist in the infant at birth." We have seen, that not simply the germs, but the animals themselves have been found at this early period of existence.
Still there are many distinguished naturalists, who conceive it probable, that spontaneous generation may occur in the lowest divisions of the living scale. Amongst these Lamarck and Geoffroy are perhaps the greatest names; and Adelon seems to accord with them; but the facts, which he adduces in support of the position are singularly inapposite, and feeble.
The views of Lamarck, regarding the formation of living bodies, are strange in the extreme; and exhibit to us, what we so frequently witness, that in order to get rid of a subject, which is difficult to the comprehension, the philosopher will frequently explain facts, or adopt suppositions, that require a much greater stretch of the imagination to invent, and present stronger obstacles to belief than those for which they are substituted. M. De Lamarck maintains, that the first organized beings were formed throughout by a true spontaneous generation; owing their existence to an excitative cause of life, probably furnished by the circumambient medium, and consisting of light and the electric fluid. When this cause met with a substance of a gelatinous consistence and dense enough to retain fluids, it organized the substance into cellular tissue, and a living being resulted. This process, according to Lamarck, is occurring daily at the extremity of the vegetable and animal kingdoms.
The being, thus formed, manifested three faculties of life;nutrition, growth, and reproduction,—but only in the most simple
manner. The organization soon, however, became more complicated, for it is, he remarks, a property of the vital movement to tend always to a greater degree of development of organization, to create particular organs, and to divide and multiply the different centres of activity; and, as reproduction constantly preserved all that had been acquired, numerous and diversified species were, in this manner, formed, possessing more and more extensive faculties. So that, according to this system, nature was directly concerned only in the first draughts of life; participating indirectly in the existence of living bodies of a more complex character; and these last proceeded from the former after the lapse of an enormous time, and an infinity of changes in the incessantly increasing complication of organization;—reproduction continuing to preserve all the acquired modifications and improvements.
The simplest kind of generation does not require sexual organs. The animal, at a certain period of existence, separates into several fragments, which form so many new individuals. This is called fissiparous generation, or generation by spontaneous division. We have examples of it in the infusory animalcules—as the vinegar eel or vibrio aceti.
A somewhat more elevated kind of reproduction is the gemmiparous, which consists in the formation of buds or germs on some part of the body. These, at a particular period, drop off, and form as many new individuals. According as the germs are developed at the surface of the body, or internally, the gemmiparous generation is said to be external or internal.
In these two varieties the whole function is executed by a single individual.
Higher up in the scale we find special organs for the accomplishment of generation—male and female. In those animals, however, that possess special reproductive organs, some have both sexes in the same individual, or are hermaphrodite or androgynous, as is the case in almost all plants, and in some of the lower tribes of animals.
In these, again, we notice a difference. Some are capable of reproduction without the concourse of a second individual; others, again, although possessing both attributes, require the concourse of another; the male parts of the one uniting with the female parts of the other, and vice versa. Both, in this way, become impregnated. The helix hortensis or garden snail affords us an instance of this kind of reproduction. They meet in pairs, according to Shaw, and stationing themselves an inch or two apart, launch several small darts, not quite half an inch long, at each other. These are of a horny substance, and sharply pointed at one end. The animals, during the breeding season, are provided with a little reservoir for them, situated within the neck, and opening on the right side. On the discharge of the first dart, the wounded snail immediately retaliates on its aggressor, by throwing a similar dart; the other re
news the battle, and in turn, is again wounded. When the darts are expended, the war of love is completed, and its consummation succeeds.
In the superior animals each sexual characteristic is possessed by a separate individual,—the species being composed of two individuals, male and female, and the concourse of these individuals, or of matters proceeding from both sexes, being absolutely necessary for reproduction.
But here, again, two great differences are met with in the process. Sometimes the fecundating fluid of the male sex is not applied to the ovum of the female, until after its ejection by the latter, as in fishes. In other cases, the ovum cannot be fecundated after its ejection, and the fluid of the male sex is applied to it whilst still within the female, as in birds and the mammalia. In such case, the male individual is furnished with an organ for penetrating the parts of the female, and in this kind of generation there must be copulation.
Again, where there is copulation, the following varieties may exist. First. The ovum, when once fecundated, may be immediately laid by the female, and may be hatched out of the body, constituting oviparous generation. Secondly. Although the process of laying may commence immediately, the fecundated ovum may pass so slowly through the excretory passages, that it may be hatched there, and the new individual may issue from the womb of the parent possessing the proper formation. This constitutes ovoviviparous generation, of which we have an example in the viper. Lastly. The fecundated ovum may be detached from the ovary soon after copulation, but, in place of being ejected, it may be deposited in a reservoir, termed a womb or uterus; be fixed there; attract fluids from the organ adapted for its development, and thus increasing at the expense of the mother, be hatched, as it were, in this reservoir, so that the new individual may be horn under its appropriate form. In such case, moreover, the new being, after birth, may be for a time supported on a secretion of the motherthe milk. These circumstances constitute viviparous generation; in which there are copulation, fecundation, gestation or pregnancy, and lactation or suckling.
There is a very considerable difference in animals as regards the nurturing care afforded by the parents to their young. Amongst the oviparous animals, many are satisfied with instinctively depositing their ova in situations, and under circumstances favourable to their hatching, and then abandoning them, so that they can never know their progeny. This is the case with insects. Others again, as birds, subject their ova to incubation, and, after they have been hatched, administer nourishment to their young during the early period of existence. In the viviparous animal, these cares are still more extensive; the mother drawing from her own bosom the nutriment needed by the infant, or suckling her young.
There are some other varieties in the generation of animals. In VOL. II.
some, it can be performed but once during the life of the individual; in others, we know it can be effected repeatedly. Sometimes one copulation fecundates but a single individual; at others, several generations are fecundated. A familiar example of this fecundity occurs in the common fowl, in which a single access will be sufficient to fecundate the eggs for the season. In the insect tribe this is still more strikingly exemplified. In the aphis puceron or green-plant louse, through all its divisions, and in the monoculus pulex, according to naturalists, a single impregnation suffices for at least six or seven generations. There is in this case another strange deviation from the ordinary laws of propagation, viz. that in the warmer summer months the young are produced viviparously, and in the cooler autumnal months oviparously.
A single impregnation of the queen bee will serve to fecundate all the eggs she may lay for two years at least. Huber believes for the whole of her life, but he has had numerous proofs of the former. She begins to lay her eggs forty-six hours after impregnation, and will commonly lay about three thousand in two months, or at the rate of fifty eggs daily. Lastly, the young are sometimes born with the shape which they have always to maintain; at others, under forms, which are, subsequently, materially modified, as in the papilio or butterfly genus.
The reproduction of the human species requires the concourse of both sexes; these sexes being separate, and each possessed by a distinct individual—male and female. All the acts comprising it may be referred to five great heads. 1. Copulation, the object of which is to apply the fecundating principle, furnished by the male, to the germ of the female. 2. Conception or fecundation, the prolific result of copulation. 3. Gestation or pregnancy, comprising the sojourn of the fecundated ovum in the uterus, and the development it undergoes there. 4. Delivery or accouchement, which consists in the detachment of the ovum; its excretion and the birth of the new individual; and lastly, lactation, or the nourishing of the infant on the maternal milk.
Of the Generative Apparatus.
The part, taken by the two sexes in the process of generation, is not equally extensive. Man has merely to furnish the fluid, necessary for effecting fecundation, and to convey it within the female. He consequently participates only in copulation and fecundation; whilst, in addition, the acts of gestation and lactation are accomplished by the female. Her generative apparatus is therefore more complicated, and consists of a greater number of organs.
1. Of the Genital Organs of the Male.
The generative apparatus of the male comprises two orders of parts: those which secrete and preserve the fecundating fluid, and
those which accomplish copulation. The first consist of two similar glands—the testes—which secrete the sperm or fecundating fluid from the blood. 2. The excretory ducts of those glands—the vasa deferentia. 3. The vesiculæ seminales, which communicate with the vasa deferentia and urethra; and 4. Two canals, called ejaculatory, which convey the sperm from the vesiculæ seminales into the canal of the urethra, whence it is afterwards projected externally. The second consists of the penis, an organ essentially composed of erectile tissue, and capable of acquiring considerable rigidity. These parts will require a more detailed notice.
Testes. The testicles are two glands situated in a bag, suspended beneath the pubes, called the scrotum; the right being a little higher than the left. They are of an ovoid shape, compressed laterally, their size being usually that of a pigeon's egg, and their weight about seven and a half, or eight drachms.
Like other glands, they receive arterial blood by an appropriate vessel, which communicates with the excretory duct. The spermatic artery conveys the blood, from which the secretion has to be operated, to the testicle. It arises from the abdominal aorta at a very acute angle, is small, extremely tortuous, and passes down to the abdominal ring, through which it proceeds to the testicle. When it reaches this organ, it divides into two sets of branches, some of which are distributed to the epididymis, others enter the testicle at its upper margin, and assist in constituting its tissue.
The excretory ducts form, in the testicle, what are called the seminiferous vessels or tubuli seminiferi. These terminate in a white cord or nucleus, situated at the upper and inner part of the organ, where the excretory duct commences, and which is called the corpus highmorianum or sinus of the seminiferous vessels. Besides these anatomical elements of the testes, there are also—1. Veins, termed spermatic, which return the superfluous blood back to the heart. These arise in the very tissue of the organ, and form the spermatic plexus, the divisions of which collect in several branches, that pass through the abdominal ring, and unite into a single trunk, which subsequently divides again into another plexus, termed corpus pampiniforme. This has been described as peculiar to the human species, and as a diverticulum for the blood of the testicle, whose functions are intermittent. These veins ultimately terminate on the right side in the vena cava, and on the left in the renal vein. 2. Lymphatic vessels, in considerable number, the trunks of which, after having passed through the abdominal ring, open into the lumbar glands. 3. Nerves, partly furnished by the renal and mesenteric plexuses and by the great sympathetic, partly by the lumbar nerves, and which are so minute as not to be traceable as far as the tissue of the testicle. 4. An outer membrane or envelope to the whole organ, called tunica albuginea or peritestis. This is of an opaque white colour, of an evidently fibrous and close texture, and envelopes and gives shape to the organ. It also sends into the interior of the testicle numerous filiform, flat