[AD] Vide Comment. Acad. Petropol, vol IV. page 261 and 262.
But even should we refuse this idea of penetration, we cannot deny that the liquor of the female, which flows from the glandular bodies of the testicles, may enter by the opening at the extremity of the trunk, as that of the male does by the orifice of the viscera; and that consequently these two liquors may mix of themselves in this cavity, and form there the fœtus in the manner we have explained.
OF THE EXPANSION, GROWTH, AND DELIVERY OF THE FŒTUS, &C.
In the expansion of the fœtus, two different degrees of growth make different kinds of expansion. The first, which succeeds immediately after the formation of the fœtus, is not proportionable in all the parts of which it is composed. The more distant it is from the formation, the more in proportion are its parts, and it is only after it has quitted the womb of its mother that the growth of the parts is made in nearly an equal manner. It must not be imagined that the figure of the fœtus, at the moment of formation, is absolutely like that of an adult. It is certain that the embryo contains every part which, must compose a man, but they differ in their successive expansion.
In an organized body, as that of an animal, we may suppose some parts are more essential than others, and though some may be useless or superfluous, there are some on which the rest seem to depend for their expansion and disposition. We must consider some as fundamental parts, without which the animal cannot exist, and which are more accessory and external, and appear to derive their origin from the first, and which seem to be formed as much for the ornament, symmetry, and external perfection of the animal, as for the necessity of its existence, and the exercise of the essential functions of life. These two kinds of different parts expand successively, and are almost equally apparent when the fœtus quits the womb; but there are others which Nature seems to keep in reserve, as the teeth, which do not appear for some time, and also the glandular bodies in the testicles of females, the beards of males, &c. which do not shew themselves till the age of puberty.
In order to discover the fundamental and essential parts of an animal body, we must pay attention to the number, situation, and nature of the whole; those which are simple, those whose position is invariable, and those without which the animal cannot exist, will be the essential parts; those, on the contrary, which are double, or in a greater number, those whose size and position vary, and those which may be retrenched from the animal without destroying or even doing it an injury, may be looked upon as less necessary, and more accessory, to the animal machine. Aristotle has said that the only parts essential to animals were those with which they take their nutriment, and throw out the superfluous parts of it from the body. From the mouth to the arms are simple parts, which no other can supply. The head and spine of the back are also simple parts, whose position is invariable. The spine of the back serves for a foundation to the fabric of the body; and it is from the marrow which it contains that the motion and action of most of the members and organs proceed; it is also this part which appears one of the first in the embryo. Now these simple parts which appear the first are all essential to the existence and form of the animal. There are many more double than simple parts in the body of an animal, and seem to be produced on each side of the simple parts by a kind of vegetation; for these double parts are similar in form, and different in position. The left hand exactly resembles the right, because it is composed of the same number of parts; nevertheless, if it was placed in the situation of the right, we could not make use of it for the same purposes, and should have reason to regard it as a very different member. It is the same with respect to the other double parts; they are similar as to form, and different as to the position which is connected to the body of the animal; and by supposing a line to divide the body into two equal parts, the position of all the similar parts would refer to this line as a centre.
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.