The spinal marrow, and the vertebræ which contains it, appear to be the real axis, to which we must refer all the double parts of the animal, for they seem to derive their origin, and to be only symmetrical branches issuing from this trunk or common base, for we see the ribs shoot out on each side of the vertebræ in the young chicken as the young branches shoot out from the principal branch of a tree. In all embryos the middle of the head and vertebræ appear to be the first formed; afterwards we see on the two sides of a vesicle which forms the middle of the head two other vesicles which appear to proceed from the first. These two vesicles contain the eyes and the other double parts of the head; so likewise we perceive little tubercles shoot out in equal numbers from each side of the vertebræ, which extend by degrees and form the ribs, and other double parts of the trunk. On the side of this trunk already formed, as the conclusion, the legs and arms appear. This first expansion is very different from that which is made afterwards; it is the production of parts which appear for the first time; that which succeeds is only a growth of all the parts already created.
This symmetrical order of all the double parts found in every animal, the regularity of their position, the equality of their extension and growth, and the perfect resemblance between them, seem to indicate that they derive their origin from the simple parts; that there must reside in these simple parts a power which acts equally on each side, or, which answers the same meaning, they are the fixed points against which the power that produces the expansion of the double parts is exercised. That the power which acts on the right is equalled by that of the left side, and consequently they are counterbalanced by this re-action.
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.