From hence we may infer, that if there is any defect or excess in the matter which is to serve for the formation of the double parts, as the powers which impel them on each side are equal, the defect or excess must be formed the same both on the right and left; for example, if, from a defect of matter, a man has but two fingers instead of five on the right hand, he will have but two on the left hand; or if, by an excess of matter, he has six fingers on one hand, he will have six on the other; or if the matter be vitiated, and causes an alteration in the right part, it will be the same on the left. This fact is very often seen. Most monsters are made with symmetry; the disarrangement of the parts of monsters appears to be made with order: Nature, therefore, even in her errors, mistakes as little as possible.
This harmony of position in the double parts of animals is found also in vegetables; branches shoot out from buds on every side; the veins in the leaves are equally disposed as to the principal vein; and although symmetrical order appears to be less exact in vegetables than in animals, it is only because it is more varied, and its limits are more extended, and less precise; but we may nevertheless easily discover this order, and distinguish the simple and essential parts from those which are double, and the latter we must regard as having taken their origin from the former. We shall more fully discuss this point, as far as relates to vegetables, when we come to treat of them.
It is not possible to determine under What form the double parts exist before expansion, nor in what manner they are folded, nor what figure results from their position by connection with the simple parts. The body of the animal, in the instant of formation, certainly contains every part which is to compose it; but the relative position of these parts must be very different then from what it becomes afterwards. It is the same with vegetables, for if we observe the expansion of a young leaf, we shall perceive that it is folded on both sides the principal vein, and that its figure does not resemble at that time what it afterwards assumes.
When we amuse ourselves by folding paper to form crowns, boats, &c. the different folds of the paper seem to have no resemblance to the form which must result by the unfolding; we only see that these folds are always made in an uniform order, and exactly the same on one side as that we have made on t he other; but it would be a problem beyond known geometry, to determine the figures which may result from all the unfoldings of a certain given number of folds. All what immediately relates to the position, is beyond our mathematical sciences. This art, which Leibnitz calls Analysis Situs, is not yet found out; though the art, which would shew us the connections that result from the position of things, would perhaps be more useful than that which has only bulk for its object, for we have often more need to know the form than the matter.
In the unfolding of Nature's productions, not only the folded parts take new positions, but they acquire, at the same time, extent and solidity. Since we cannot therefore determine the result of the simple unfolding of a folded form, in which, as in a piece of folded paper, there is but one change of position between the parts, without any augmentation or diminution of the bulk or mass of the matter, how is it possible for us to judge of the complex unfolding of the body of an animal, in which not only the relative position of the parts, but also their mass of matter, undergoes considerable changes? We cannot, therefore, reason upon this subject, but by drawing some inductions from the examination of the things at the different periods of their unfolding, and by assisting ourselves with the observations that we have had the opportunity to make.
It is true we see the chick in the egg before incubation; it floats in a transparent liquor, contained in a small purse, formed by a very fine membrane in the centre of the cicatrice; but this chick is then only a particle of inanimate matter, in which we cannot discern any organization, nor any determined figure. We judge by the external form that one of the extremities is the head, and the rest to be the spine of the back. It appears that this is the first product of fecundation resulting from the mixture of the seed of the male and female; nevertheless, before asserting this as a fact, there are many things should be considered. When the hen has cohabited with the cock for a few days, and afterwards separated from him, the eggs she produces for a month after separation are as fertile as those she produced during the time of cohabitation with the male, and unfold at the same time; they only require twenty-one days sitting, and the embryo of the one will be as forward and as completely formed as that of the other. From hence we might think, that this form, under which the chick at first appears to us in the egg, does not immediately proceed from a mixture of the two liquors, but that it existed in other forms during the time the egg remained in the body of the mother; for the embryo in the form we see it before incubation, requires only heat to unfold and bring it forth. Now, if it had this form twenty days, or a month before, when the egg was first fecundated, why was it not hatched by the internal heat of the hen? and why is not the chicken perfectly formed in those eggs which are fecundated twenty-one days before the hen lays them?
This difficulty is not so great as it appears; for we must conceive, that in the time of the cock's cohabitation with the hen, each egg receives in its cicatrice, wherein the female liquor is contained, a small portion of the semen of the male. The egg attached to the ovary is in oviparous females, what the glandular substance is in the testicles of viviparous females. The cicatrice of the egg corresponds with the glandular bodies in which the seminal liquor of the female resides; that of the male penetrates and mixes there with it; from this mixture, the formation of the embryo instantly results. The first egg which the hen lays after coition is fecundated, and capable of producing a chicken; those which she lays afterwards were fecundated at the same instant; but as there is still wanting essential parts to this egg, the production of which is independent of the seed of the male, as the white, membranes, and shell, the young embryo contained in the cicatrice cannot unfold in this imperfect egg, although assisted by the internal heat of the mother. It remains, therefore, in the cicatrice in the state in which it was formed, until the egg has acquired all the parts necessary to the growth and nourishment of the chicken: and it is not till the egg has attained its perfection that the embryo begins to unfold: this unfolding is performed by the external heat of incubation; but it is certain, if the egg could be confined within the body of the hen for 21 days after it was completely formed, the chicken would be produced, unless the internal heat of the hen should prove too powerful, for the degrees of heat necessary to hatch chickens are not very extended, and the least defect or excess is equally prejudicial to their unfolding. The last eggs the hen lays, containing the same as the first, proves nothing more than that the egg must acquire entire perfection before the embryo can unfold itself; and for want of the heat necessary to this unfolding, eggs may be kept a considerable time before incubation, without preventing the produce of the chickens they contain.
It appears, therefore, that the state of the embryo, when the egg is laid by the hen, is the first state which succeeds fecundation; that the form under which we see it is the first form resulting from the intimate mixture, and form the penetration of the two seminal liquors; and consequently by following, as Malpighius has done, this unfolding from hour to hour, we discover all that is possible to be known, unless we could see the two liquors mix before our eyes, and how the first arrangement of the particles are made, which produces the first form of the embryo.
If we reflect on this fecundation (which is made at the same time) of these eggs, which are laid successively, and along time after each other, we shall find new arguments against the existence of eggs in viviparous animals; for if the females of viviparous animals, or if women contained eggs, like hens, why are there not many fecund at the same time? why are not some of them produced in nine months, and others at distant periods? and when women have two or three children, why do they all come into the world at one time? If these fœtuses were produced by the means of eggs, would not they come successively, according as the eggs come to perfection, after the time of impregnation? And would not super-fœtation be as frequent as they now are scarce, or as natural as they appear to be accidental?
We cannot follow the unfolding of the fœtus in the matrix as we pursue that of the chick in the egg; the opportunities of observing it are few, and we can only know what anatomists, surgeons, and midwives have written thereon. It is by collecting all their particular observations, and by comparing their remarks and their descriptions, that we have made the following abridged history of the human fœtus.