The formation of the foetus is, then, made by the union of the organic particles contained in the mixture of the seminal liquor of both sexes; this union produces the local establishment of the particles, which determines them to arrange themselves as they were in the individuals which furnished them; insomuch, that the molecules, which proceed from the head, cannot, by virtue of these laws, place themselves in the legs, or any other part of the foetus. All these molecules must be in motion when they unite, and in a motion which must cause them to tend to a kind of centre, about which the union is made. This centre, or fixed point, which is necessary to the union of the molecules, and which, by its re-action and inertia, fixes the activity, and destroys the motion, is, probably, the first assemblage of the molecules which proceed from the sexual parts of the other individual; they must arrange under the form of an organized body which will not be another foetus, for the reasons we have before given.[AC] [AC] In this, as in some other places, our author has gone into a diffuse repetition which we have considered unnecessary and therefore avoid. On the whole, I conceive there are organic particles of the sexual parts, which serve as a fixed point, or a centre of union, around which all the other parts that form the embryo collect. I speak of it only as probable; but as they are the only particles which differ, I have thought it more natural to imagine, that it is around these different particles the union is formed than those which are common to both sexes. We have before observed, that those who have imagined the heart was the first formed, are deceived: those who say it is the blood, are no less so. All is formed at the same time. If we only consult observation, the chicken is seen in the egg before it has been sat upon; we perceive the spine of the back and the head, and, at the same time, the appendages which form the placenta. I have opened a great number of eggs, before and after incubation; and I am convinced, by my sight, that the chicken exists entirely in the middle of the cicatrice, the moment it comes from the body of the hen. The heat, communicated to it by incubation, only expands the parts by setting the liquors in motion; but it is not possible to determine which parts of the foetus are fixed in the instant of formation.