There is a considerable difficulty in understanding how the plant is formed out of the seed or any animal out of the semen.Everything that comes into being or is made must (1) be made out of something, (2)be made by the agency of something, and (3) must become something.Now that out of which it is made is the material; this some animals have in its first form within themselves, taking it from the female parent, as all those which are not born alive but produced as a scolex or an egg; others receive it from the mother for a long time by sucking, as the young of all those which are not only externally but also internally viviparous.Such, then, is the material out of which things come into being, but we now are inquiring not out of what the parts of an animal are made, but by what agency.Either it is something external which makes them, or else something existing in the seminal fluid and the semen; and this must either be soul or a part of soul, or something containing soul.
Now it would appear irrational to suppose that any of either the internal organs or the other parts is made by something external, since one thing cannot set up a motion in another without touching it, nor can a thing be affected in any way by another if it does not set up a motion in it.Something then of the sort we require exists in the embryo itself, being either a part of it or separate from it.To suppose that it should be something else separate from it is irrational.For after the animal has been produced does this something perish or does it remain in it? But nothing of the kind appears to be in it, nothing which is not a part of the whole plant or animal.
Yet, on the other hand, it is absurd to say that it perishes after making either all the parts or only some of them.If it makes some of the parts and then perishes, what is to make the rest of them?
Suppose this something makes the heart and then perishes, and the heart makes another organ, by the same argument either all the parts must perish or all must remain.Therefore it is preserved and does not perish.Therefore it is a part of the embryo itself which exists in the semen from the beginning; and if indeed there is no part of the soul which does not exist in some part of the body, it would also be a part containing soul in it from the beginning.
How, then, does it make the other parts? Either all the parts, as heart, lung, liver, eye, and all the rest, come into being together or in succession, as is said in the verse ascribed to Orpheus, for there he says that an animal comes into being in the same way as the knitting of a net.That the former is not the fact is plain even to the senses, for some of the parts are clearly visible as already existing in the embryo while others are not; that it is not because of their being too small that they are not visible is clear, for the lung is of greater size than the heart, and yet appears later than the heart in the original development.Since, then, one is earlier and another later, does the one make the other, and does the later part exist on account of the part which is next to it, or rather does the one come into being only after the other? I mean, for instance, that it is not the fact that the heart, having come into being first, then makes the liver, and the liver again another organ, but that the liver only comes into being after the heart, and not by the agency of the heart, as a man becomes a man after being a boy, not by his agency.An explanation of this is that, in all the productions of Nature or of art, what already exists potentially is brought into being only by what exists actually; therefore if one organ formed another the form and the character of the later organ would have to exist in the earlier, e.g.the form of the liver in the heart.And otherwise also the theory is strange and fictitious.
Yet again, if the whole animal or plant is formed from semen or seed, it is impossible that any part of it should exist ready made in the semen or seed, whether that part be able to make the other parts or no.For it is plain that, if it exists in it from the first, it was made by that which made the semen.But semen must be made first, and that is the function of the generating parent.So, then, it is not possible that any part should exist in it, and therefore it has not within itself that which makes the parts.