GENERATION

GENERATION, in physiology, the act of procreating and producing a being similar to the parent.

According to Aristotle, the male animals contain the principle, and the female the matter, of generation; for though both are furnished indeed with a seminal liquor, yet the semen of the males alone is prolific. The moderns, on the other hand, as well those who contend for the system of generation from eggs, as they who adopt that of the animalcules in the male-seed, pretend that females have no such seminal liquor at all, and that what has been commonly taken for it is some other animal-fluid.

Harvey is of opinion, that all females are furnished with eggs; and that the embryos, or young animals, are formed in the same manner as a chick in the egg of any bird. Generation, according to this celebrated physician, is effected wholly by means of the uterus, or womb, which conceives the fetus by a kind of contagion communicated to it by the male-seed, much in the same way as the load-stone communicates magnetism.

Generation. magnetism to iron. This contagion, he thinks, acts not only on the uterus, but is communicated to the whole body of the female, which is altogether prolific; though the uterus, he acknowledges, is the only part that is capable of conceiving the fetus, just as the brain is alone capable of forming ideas and notions. Agreeable to this doctrine of Harvey, Steno and other anatomists have pretended to discover certain eggs in the ovaries or testicles of women; which Mr Buffon denies to be the case, affirming, that there are no such eggs to be found there.

We cannot enter into a detail of the reasonings for and against the system of generation from eggs; and shall therefore only observe, that its advocates pretend to have discovered eggs in all the females on which they made observations; that the largest of those found in women did not exceed the bigness of a pea; that they are extremely small in young girls under fourteen, but that age and commerce with men makes them grow larger; that there are more than twenty such eggs in each ovary or testicle; that they are fecundated in the ovary by the spirituous and volatile part of the male-seed; that they afterwards are detached and fall into the uterus through the Fallopian tubes; that here the fetus is formed of the internal substance of the egg, and the placenta of the exterior part.

Leeuwenhoek is the author of another system of generation, from animalcules in the male-seed. He tells us, he discovered many thousands of these in a drop less than a grain of sand. They are found in the semen of all males whatever, but not in that of females; and are so small, that 3,000,000,000 of them are not equal to a grain of sand, whose diameter is but the hundredth part of an inch. When any of these animalcules gets into an egg fit to receive it, and this falls into the womb through the Fallopian tubes, the humours which distil through the vessels of the womb, penetrating the coats of the egg, swell and dilate it, as the sap of the earth does feed thrown into it. The placenta begins to appear like a little cloud, upon one side of the external coat of the egg; and, at the same time, the spine of the embryo-animalcule is grown so big, as to become visible; and a little afterwards, the cerebrum and cerebellum appear like two bladders; and the eyes stand next, goggling out of the head; then the beating of the heart, or punctum saliens, is plainly to be seen; and the extremities discover themselves last of all.

These animalcules are of different figures, some like tadpoles, and others like eels*. In the semen of a man, and in that of a dog, there have been discovered two different kinds of them, the one supposed to be males, and the other females. Some even pretend to have seen animalcules disengage themselves from the membranes that surround them; and that they then appeared perfectly like men, with legs, arms, &c. like those of the human body!

All the advocates for the system of generation from animalcules strongly oppose that from eggs. They contend, that these animalcules cannot be looked upon as the inhabitants of the semen, since they were of greater extent than the liquor itself; not to mention, that no such animals are found in any other liquors of the body; and since females have nothing similar to these animals, they think it manifest that the prolific

principle resides in males. When they are asked, To Generation what purpose serves such an immense profusion of human animalcules? they answer, that it is agreeable to the ordinary course of nature, both in the animal and vegetable part of the creation. They likewise strengthen their system, by alledging the many examples we have of similar transformations in the insect-class of animals, which, from caterpillars and small worms, become winged animals of the butterfly or fly kinds.

By this system, says Mr Buffon, the first woman cannot be said to have contained the whole race of mankind, as being all, according to it, the true posterity of the first man, and in their animalcule state contained only in him. On this principle, he proceeds to invalidate the system of generation from animalcules: for supposing the size of a man to be 1, then will that of one of the spermatic animalcules be $\backslash frac\{1\}\{1000000000\}$; and as a man is to an animalcule of the first generation in the same ratio that this animalcule is to an animalcule of the second generation, it follows, that this last will be expressed by the fraction $\backslash frac\{1\}\{1000000000000\}$. In this manner he computes the size of the animalcules of several generations, all supposed to be living animals, notwithstanding that their minuteness exceeds the power of imagination to conceive; and then tells us, that the system of generation from eggs is liable to the same objections, whereof the detail may be seen in his Hist. Natur. tom. ii. p. 157 et seq.

As to Buffon's own system, he thinks that every part, both of animals and vegetables, contains an infinite number of organic molecules; that these molecules assume successively different forms, and are put into different motions, according to the circumstances they are in; but that they are much more numerous in the seminal liquors of both sexes, and the seeds of plants, than in other parts; that these organic molecules make the matter of nutrition; that this matter is always active, and tends to organization, forming itself into different shapes, according to the moulds it meets with. When the quantity of this organic matter is but small, as in man and most large animals, generation only takes place at the age of maturity, and even then the number of animals produced is but small. The case is just the reverse in animals which abound with this matter, as in fishes, and most birds.

With respect to the generation of mankind, the same author thinks it a certain fact, that the male-seed is received into the womb of the woman; and that, for this purpose, it is highly probable the internal orifice opens during the act of coition. The female-seed also makes its way into the womb, where, being mixed with that of the male, they both together contribute to the formation of the fetus; which is either male or female, according as the seed of the man or woman abounds most with organic molecules; and the infant resembles either father or mother, according to the different combinations of these molecules. Both these seminal liquors he thinks equally active in the formation of the fetus, and that they fix and counterbalance each other; the molecules of each parent being thereby determined to form similar parts to those of the individual that furnished them, as the head, trunk, arms, legs, &c. He thinks the molecules proceeding from the genital parts fix themselves first; and that the other molecules arrange themselves successively

Generation. successively round these, in the same order which they before occupied in the parent. When a great quantity of the feminal liquors of both sexes is received into the womb, there are formed different spheres of attraction, in different parts of these liquors; the consequence of which is, that several foetuses are formed at the same time.

Nearly akin to Mr Buffon's system, is that of Mr Maupertuis, which he has explained in his Venus Physique. He observes, that all the variety observable among mankind, may have been accidental at first; but being once established in the constitution of the parents, they become natural to their posterity. To illustrate this, he gives an instance of a sexdinary family at Berlin, who had six fingers, or six toes, and frequently both; and that this peculiarity was transmitted equally by the father and mother, but was lost by alliances with those who had only the usual number of fingers or toes.

He farther observes, that most animals, excepting mankind, have stated seasons for procreation; and that the females go with young some a longer, others a shorter time. Mares go from eleven to twelve months; cows and hinds go nine months, as do also women; foxes and wolves, five months; and hitches go only seven weeks; cats nine weeks; and rabbits but thirty-one days. Most birds are hatched in twenty-one days; the canary-birds, and some others, are hatched in thirteen or fourteen days. It appears, therefore, that there is an endless variety in the time and manner of the generation of animals.

Whoever reads this short sketch of the different theories of generation that have hitherto been invented, will probably require no other arguments to convince him, that physicians and philosophers are still as ignorant of the nature of this mysterious operation as they were in the days of Noah.

Concerning the formation and nutrition of the foetus after the female has conceived, there have also been great disputes; but as this subject is more easily investigated, and in some measure falls under the notice of our senses, there is much less uncertainty concerning these matters than generation, or the manner in which the embryo is originally formed. The following particulars are confirmed by the greatest number of observations.

About the seventh day, the eye may discover the first lineaments of the foetus; but these lineaments are as yet very imperfect. Two little vesicles appear in an almost transparent jelly, the largest of which is destined to become the head of the foetus, and the other smaller one is reserved for the trunk. But at this period no extremities are to be seen; the umbilical chord appears only as a very minute thread, and the placenta does not as yet absorb the red particles of the blood. At the end of fifteen days, not only the head but the features of the face begin to be developed.—The nose

appears like a small prominent line; and we are able to distinguish another line under it, which is destined for the separation of the lips. Two black points appear in the place of eyes, and two minute holes mark the ears. At the sides of the trunk both above and below, we see four minute protuberances, which are the rudiments of the arms and legs. At the end of three weeks the body of the foetus is somewhat augmented, and both the hands and feet are to be distinguished. The upper extremities are found to increase faster than the lower ones, and the separation of the fingers is accomplished sooner than that of the toes.

Towards the end of the first month, the foetus is about an inch long, and the human form may be desicively ascertained: all the parts of the face may be distinguished; the shape of the body is clearly marked out, the haunches and the abdomen are elevated, the fingers and toes are separated from each other, and the intestines appear like minute threads. After six weeks the foetus is grown much longer, and the human figure appears to be more perfect, but the head is still larger in proportion than the other parts of the body.

At the end of the second month, the foetus measures two inches and a quarter; at the end of the third month, three inches and a half; and about the fourth or fifth month, usually about five inches; and from that time to the end of the ninth month, it gradually increases to about the length of twelve inches, sometimes more, and sometimes not quite so much.

The foetus during all this time assumes an oval figure, which corresponds with the shape of the uterus. Its chin is found resting on its breast, with its knees drawn up towards its chin, and its arms folded over them. But it seems likely that the posture of some of these parts is varied in the latter months of pregnancy, so as to cause those painful twitches which its mother usually feels from time to time.—In natural cases, its head is probably placed towards the os tincæ from the time of conception to that of its birth; though formerly it was considered as being placed towards the fundus uteri, till about the eighth or ninth month, when the head, by becoming specifically heavier than the other parts of the body, was supposed to be turned downwards.

The capacity of the uterus increases in proportion to the growth of the foetus, but without becoming thinner in its substance, as might naturally be expected.—The nourishment of the foetus, during all this time, seems to be derived from the placenta, which appears to be originally formed by that part of the ovum which is next the fundus uteri. The remaining unconnected part of the ovum, and likewise the surface of the placenta, are covered by a membrane called chorion (A), within which is another pellucid membrane called amnios (B); and these two include a watery fluid, which is the liquor amnii, in which the foetus

Generation fetus floats till the time of its birth (c).—In the first months of pregnancy, the involucrea bear a large proportion to their contents, but this proportion is afterwards reversed as the fetus increases in bulk.

The placenta, which is the medium through which the blood is conveyed from the mother to the fetus, and the manner in which this conveyance takes place, deserve to be clearly described, as being a subject not generally understood.—Without such an explanation it might perhaps be readily supposed, that the arteries of the uterus pass into the substance of the placenta; and that the blood, after being conveyed through the umbilical arteries to the fetus, is returned back by the umbilical vein to the placenta, and from thence to the uterus.—Such an idea, however, would be a very erroneous one, and we shall point out the true manner in which this process is conducted.

The placenta is a broad, flat, and spongy substance, like a cake, closely adhering to the inner surface of the womb, usually near the fundus, and appearing to be made up by the ramifications of the umbilical arteries and vein. The arteries of the uterus discharge their contents into the spongy cells of this cake; and the veins of the placenta, absorbing the blood from these cells in the same manner as they absorb it in the corpora cavernosa penis, at length form the umbilical vein, which passes on to the liver, and from thence to the heart of the fetus, by the vena cava. Its circulation, however, through the heart is not conducted in the fetus as it is in the adult: in the latter, the blood is carried from the right auricle of the heart through the pulmonary artery, and is returned to the left auricle by the pulmonary vein; but a dilatation of the lungs is essential to the passage of the blood through the pulmonary artery, and this dilatation cannot take place till after the child is born and has respired. This deficiency, however, is supplied in the fetus, by an immediate communication between the right and left auricle of the heart, through an oval opening in the septum, which divides the two auricles, called foramen ovale. The blood is returned again from the fetus, through two arteries called the umbilical arteteries, which sometimes arise from the iliacs, and sometimes from the aorta descendens. These two vessels taking a winding course with the vein, form with that, and the membranes by which they are surrounded, what is called the umbilical cord. These arteries, after ramifying through the substance of the placenta, open and discharge their blood into its cells, from whence it is absorbed by the veins of the uterus; so that a constant deposition and absorption are carried on, and the fetus is found to have a circulation independent of its mother.

GENERATION of Fishes. See FISH.

GENERATION of Plants. See BOTANY, sect. v.

GENERATION of Insects. See INSECTS.

PARTS of GENERATION. See ANATOMY, n^o 371, 372.