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.

but they do not say well nor do they assign a necessary cause who say simply that ‘it always happens so’, and imagine that this is a first principle in these cases. thus democritus of abdera says that ‘there is no beginning of the infinite; now the cause is a beginning, and the eternal is infinite; in consequence, to ask the cause of anything of this kind is to seek for a beginning of the infinite’. yet according to this argument, which forbids us to seek the cause, there will be no proof of any eternal truth whatever; but we see that there is a proof of many such, whether by ‘eternal’ we mean what always happens or what exists eternally; it is an eternal truth that the angles of a triangle are always equal to two right angles, or that the diagonal of a square is incommensurable with the side, and nevertheless a cause and a proof can be given for these truths. while, then, it is well said that we must not take on us to seek a beginning (or first principle) of all things, yet this is not well said of all things whatever that always are or always happen, but only of those which really are first principles of the eternal things; for it is by another method, not by proof, that we acquire knowledge of the first principle. now in that which is immovable and unchanging the first principle is simply the essence of the thing, but when we come to those things which come into being the principles are more than one, varying in kind and not all of the same kind; one of this number is the principle of movement, and therefore in all the sanguinea the heart is formed first, as was said at the beginning, and in the other animals that which is analogous to the heart.

from the heart the blood-vessels extend throughout the body as in the anatomical diagrams which are represented on the wall, for the parts lie round these because they are formed out of them. the homogeneous parts are formed by heat and cold, for some are put together and solidified by the one and some by the other. the difference between these has already been discussed elsewhere, and it has been stated what kinds of things are soluble by liquid and fire, and what are not soluble by liquid and cannot be melted by fire. the nutriment then oozes through the blood-vessels and the passages in each of the parts, like water in unbaked pottery, and thus is formed the flesh or its analogues, being solidified by cold, which is why it is also dissolved by fire. but all the particles given off which are too earthy, having but little moisture and heat, cool as the moisture evaporates along with the heat; so they become hard and earthy in character, as nails, horns, hoofs, and beaks, and therefore they are softened by fire but none of them is melted by it, while some of them, as egg-shells, are soluble in liquids. the sinews and bones are formed by the internal heat as the moisture dries, and hence the bones are insoluble by fire like pottery, for like it they have been as it were baked in an oven by the heat in the process of development. but it is not anything whatever that is made into flesh or bone by the heat, but only something naturally fitted for the purpose; nor is it made in any place or time whatever, but only in a place and time naturally so fitted. for neither will that which exists potentially be made except by that moving agent which possesses the actuality, nor will that which possesses the actuality make anything whatever; the carpenter would not make a box except out of wood, nor will a box be made out of the wood without the carpenter. the heat exists in the seminal secretion, and the movement and activity in it is sufficient in kind and in quantity to correspond to each of the parts. in so far as there is any deficiency or excess, the resulting product is in worse condition or physically defective, in like manner as in the case of external substances which are thickened by boiling that they may be more palatable or for any other purpose. but in the latter case it is we who apply the heat in due measure for the motion required; in the former it is the nature of the male parent that gives it, or with animals spontaneously generated it is the movement and heat imparted by the right season of the year that it is the cause.

cooling, again, is mere deprivation of heat. nature makes use of both; they have of necessity the power of bringing about different results, but in the development of the embryo we find that the one cools and the other heats for some definite purpose, and so each of the parts is formed; thus it is in one sense by necessity, in another for a final cause, that they make the flesh soft, the sinews solid and elastic, the bones solid and brittle. the skin, again, is formed by the drying of the flesh, like the scum upon boiled substances; it is so formed not only because it is on the outside, but also because what is glutinous, being unable to evaporate, remains on the surface. while in other animals the glutinous is dry, for which reason the covering of the invertebrates is testaceous or crustaceous, in the vertebrates it is rather of the nature of fat. in all of these which are not of too earthy a nature the fat is collected under the covering of the skin, a fact which points to the skin being formed out of such a glutinous substance, for fat is somewhat glutinous. as we said, all these things must be understood to be formed in one sense of necessity, but in another sense not of necessity but for a final cause.

the upper half of the body, then, is first marked out in the order of development; as time goes on the lower also reaches its full size in the sanguinea. all the parts are first marked out in their outlines and acquire later on their colour and softness or hardness, exactly as if nature were a painter producing a work of art, for painters, too, first sketch in the animal with lines and only after that put in the colours.

because the source of the sensations is in the heart, therefore this is the part first formed in the whole animal, and because of the heat of this organ the cold forms the brain, where the blood-vessels terminate above, corresponding to the heat of the heart. hence the parts about the head begin to form next in order after the heart, and surpass the other parts in size, for the brain is from the first large and fluid.

there is a difficulty about what happens with the eyes of animals. though from the beginning they appear very large in all creatures, whether they walk or swim or fly, yet they are the last of the parts to be formed completely, for in the intervening time they collapse. the reason is this. the sense-organ of the eyes is set upon certain passages, as are the other sense-organs. whereas those of touch and taste are simply the body itself or some part of the body of animals, those of smell and hearing are passages connecting with the external air and full themselves of innate spiritus; these passages end at the small blood-vessels about the brain which run thither from the heart. but the eye is the only sense-organ that has a bodily constitution peculiar to itself. it is fluid and cold, and does not exist from the first in the place which it occupies later in the same way as the other parts do, for they exist potentially to begin with and actually come into being later, but the eye is the purest part of the liquidity about the brain drained off through the passages which are visible running from them to the membrane round the brain. a proof of this is that, apart from the brain, there is no other part in the head that is cold and fluid except the eye. of necessity therefore this region is large at first but falls in later. for the same thing happens with the brain; at first it is liquid and large, but in course of evaporation and concoction it becomes more solid and falls in; this applies both to the brain and the eyes. the head is very large at first, on account of the brain, and the eyes appear large because of the liquid in them. they are the last organs to reach completion because the brain is formed with difficulty; for it is at a late period that it gets rid of its coldness and fluidity; this applies to all animals possessing a brain, but especially to man. for this reason the ‘bregma’ is the last of the bones to be formed; even after birth this bone is still soft in children. the cause of this being so with men more than with other animals is the fact that their brain is the most fluid and largest. this again is because the heat in man’s heart is purest. his intellect shows how well he is tempered, for man is the wisest of animals. and children for a long time have no control over their heads on account of the heaviness of the brain; and the same applies to the parts which it is necessary to move, for it is late that the principle of motion gets control over the upper parts, and last of all over those whose motion is not connected directly with it, as that of the legs is not. now the eyelid is such a part. but since nature makes nothing superfluous nor in vain, it is clear also that she makes nothing too late or too soon, for if she did the result would be either in vain or superfluous. hence it is necessary that the eyelids should be separated at the same time as the heart is able to move them. so then the eyes of animals are perfected late because of the amount of concoction required by the brain, and last of all the parts because the motion must be very strong before it can affect parts so far from the first principle of motion and so cold. and it is plain that such is the nature of the eyelids, for if the head is affected by never so little heaviness through sleepiness or drunkenness or anything else of the kind, we cannot raise the eyelids though their own weight is so small. so much for the question how the eyes come into being, and why and for what cause they are the last to be fully developed.

each of the other parts is formed out of the nutriment, those most honourable and participating in the sovereign principle from the nutriment which is first and purest and fully concocted, those which are only necessary for the sake of the former parts from the inferior nutriment and the residues left over from the other. for nature, like a good householder, is not in the habit of throwing away anything from which it is possible to make anything useful. now in a household the best part of the food that comes in is set apart for the free men, the inferior and the residue of the best for the slaves, and the worst is given to the animals that live with them. just as the intellect acts thus in the outside world with a view to the growth of the persons concerned, so in the case of the embryo itself does nature form from the purest material the flesh and the body of the other sense-organs, and from the residues thereof bones, sinews, hair, and also nails and hoofs and the like; hence these are last to assume their form, for they have to wait till the time when nature has some residue to spare.

the bones, then, are made in the first conformation of the parts from the seminal secretion or residue. as the animal grows the bones grow from the natural nourishment, being the same as that of the sovereign parts, but of this they only take up the superfluous residues. for everywhere the nutriment may be divided into two kinds, the first and the second; the former is ‘nutritious’, being that which gives its essence both to the whole and to the parts; the latter is concerned with growth, being that which causes quantitative increase. but these must be distinguished more fully later on. the sinews are formed in the same way as the bones and out of the same materials, the seminal and nutritious residue. nails, hair, hoofs, horns, beaks, the spurs of cocks, and any other similar parts, are on the contrary formed from the nutriment which is taken later and only concerned with growth, in other words that which is derived from the mother, or from the outer world after birth. for this reason the bones on the one hand only grow up to a certain point (for there is a limit of size in all animals, and therefore also of the growth of the bones; if these had been always able to grow, all animals that have bone or its analogue would grow as long as they lived, for these set the limit of size to animals. what is the reason of their not always increasing in size must be stated later.)

hair, on the contrary, and growths akin to hair go on growing as long as they exist at all, and increase yet more in diseases and when the body is getting old and wasting, because more residual matter is left over, as owing to old age and disease less is expended on the important parts, though when the residual matter also fails through age the hair fails with it. but the contrary is the case with the bones, for they waste away along with the body and the other parts. hair actually goes on growing after death; it does not, however, begin growing then.

about the teeth a difficulty may be raised. they have actually the same nature as the bones, and are formed out of the bones, but nails, hair, horns, and the like are formed out of the skin, and that is why they change in colour along with it, for they become white, black, and all sorts of colours according to that of the skin. but the teeth do nothing of the sort, for they are made out of the bones in all animals that have both bones and teeth. of all the bones they alone go on growing through life, as is plain with the teeth which grow out of the straight line so as no longer to touch each other. the reason for their growth, as a final cause, is their function, for they would soon be worn down if there were not some means of saving them; even as it is they are altogether worn down in old age in some animals which eat much and have not large teeth, their growth not being in proportion to their detrition. and so nature has contrived well to meet the case in this also, for she causes the failure of the teeth to synchronize with old age and death. if life lasted for a thousand or ten thousand years the original teeth must have been very large indeed, and many sets of them must have been produced, for even if they had grown continuously they would still have been worn smooth and become useless for their work. the final cause of their growth has been now stated, but besides this as a matter of fact the growth of the teeth is not the same as that of the other bones. the latter all come into being in the first formation of the embryo and none of them later, but the teeth do so later. therefore it is possible for them to grow again after the first set falls out, for though they touch the bones they are not connate with them. they are formed, however, out of the nutriment distributed to the bones, and so have the same nature, even when the bones have their own number complete.

other animals are born in possession of teeth or their analogue (unless in cases contrary to nature), because when they are set free from the parent they are more perfect than man; but man (also unless in cases contrary to nature) is born without them.

the reason will be stated later why some teeth are formed and fall out but others do not fall out.

it is because such parts are formed from a residue that man is the most naked in body of all animals and has the smallest nails in proportion to his size; he has the least amount of earthy residue, but that part of the blood which is not concocted is the residue, and the earthy part in the bodies of all animals is the least concocted. we have now stated how each of the parts is formed and what is the cause of their generation.