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?