After this, as said already, the internal parts come into being before the external.The greater become visible before the less, even if some of them do not come into being before them.First the parts above the hypozoma are differentiated and are superior in size; the part below is both smaller and less differentiated.This happens in all animals in which exists the distinction of upper and lower, except in the insects; the growth of those that produce a scolex is towards the upper part, for this is smaller in the beginning.The cephalopoda are the only locomotive animals in which the distinction of upper and lower does not exist.
What has been said applies to plants also, that the upper portion is earlier in development than the lower, for the roots push out from the seed before the shoots.
The agency by which the parts of animals are differentiated is air, not however that of the mother nor yet of the embryo itself, as some of the physicists say.This is manifest in birds, fishes, and insects.For some of these are separated from the mother and produced from an egg, within which the differentiation takes place;other animals do not breathe at all, but are produced as a scolex or an egg; those which do breathe and whose parts are differentiated within the mother's uterus yet do not breathe until the lung is perfected, and the lung and the preceding parts are differentiated before they breathe.Moreover, all polydactylous quadrupeds, as dog, lion, wolf, fox, jackal, produce their young blind, and the eyelids do not separate till after birth.Manifestly the same holds also in all the other parts; as the qualitative, so also the quantitative differentia comes into being, pre-existing potentially but being actualized later by the same causes by which the qualitative distinction is produced, and so the eyelids become two instead of one.
Of course air must be present, because heat and moisture are present, the former acting and the latter being acted upon.
Some of the ancient nature-philosolphers made an attempt to state which part comes into being after which, but were not sufficiently acquainted with the facts.It is with the parts as with other things; one naturally exists prior to another.But the word 'prior' is used in more senses than one.For there is a difference between the end or final cause and that which exists for the sake of it; the latter is prior in order of development, the former is prior in reality.Again, that which exists for the sake of the end admits of division into two classes, (1) the origin of the movement, (2) that which is used by the end; I mean, for instance, (1) that which can generate, (2) that which serves as an instrument to what is generated, for the one of these, that which makes, must exist first, as the teacher before the learner, and the other later, as the pipes are later than he who learns to play upon them, for it is superfluous that men who do not know how to play should have pipes.Thus there are three things: first, the end, by which we mean that for the sake of which something else exists; secondly, the principle of movement and of generation, existing for the sake of the end (for that which can make and generate, considered simply as such, exists only in relation to what is made and generated); thirdly, the useful, that is to say what the end uses.Accordingly, there must first exist some part in which is the principle of movement (I say a part because this is from the first one part of the end and the most important part too); next after this the whole and the end; thirdly and lastly, the organic parts serving these for certain uses.Hence if there is anything of this sort which must exist in animals, containing the principle and end of all their nature, this must be the first to come into being- first, that is, considered as the moving power, but simultaneous with the whole embryo if considered as a part of the end.
Therefore all the organic parts whose nature is to bring others into being must always themselves exist before them, for they are for the sake of something else, as the beginning for the sake of the end;all those parts which are for the sake of something else but are not of the nature of beginnings must come into being later.So it is not easy to distinguish which of the parts are prior, those which are for the sake of another or that for the sake of which are the former.For the parts which cause the movement, being prior to the end in order of development, come in to cause confusion, and it is not easy to distinguish these as compared with the organic parts.And yet it is in accordance with this method that we must inquire what comes into being after what; for the end is later than some parts and earlier than others.And for this reason that part which contains the first principle comes into being first, next to this the upper half of the body.This is why the parts about the head, and particularly the eyes, appear largest in the embryo at an early stage, while the parts below the umbilicus, as the legs, are small;for the lower parts are for the sake of the upper, and are neither parts of the end nor able to form it.