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that too when he saw their teeth were white. The reason of the

  whiteness of semen is that it is a foam, and foam is white, especially

  that which is composed of the smallest parts, small in the sense

  that each bubble is invisible, which is what happens when water and

  oil are mixed and shaken together, as said before. (Even the ancients

  seem to have noticed that semen is of the nature of foam; at least

  it was from this they named the goddess who presides over union.)

  This then is the explanation of the problem proposed, and it is

  plain too that this is why semen does not freeze; for air will not

  freeze.

  3

  The next question to raise and to answer is this. If, in the case of

  those animals which emit semen into the female, that which enters

  makes no part of the resulting embryo, where is the material part of

  it diverted if (as we have seen) it acts by means of the power

  residing in it? It is not only necessary to decide whether what is

  forming in the female receives anything material, or not, from that

  which has entered her, but also concerning the soul in virtue of which

  an animal is so called (and this is in virtue of the sensitive part

  of the soul)- does this exist originally in the semen and in the

  unfertilized embryo or not, and if it does whence does it come? For

  nobody would put down the unfertilized embryo as soulless or in

  every sense bereft of life (since both the semen and the embryo of an

  animal have every bit as much life as a plant), and it is

  productive up to a certain point. That then they possess the nutritive

  soul is plain (and plain is it from the discussions elsewhere about

  soul why this soul must be acquired first). As they develop they also

  acquire the sensitive soul in virtue of which an animal is an

  animal. For e.g. an animal does not become at the same time an

  animal and a man or a horse or any other particular animal. For the

  end is developed last, and the peculiar character of the species is

  the end of the generation in each individual. Hence arises a

  question of the greatest difficulty, which we must strive to solve

  to the best of our ability and as far as possible. When and how and

  whence is a share in reason acquired by those animals that participate

  in this principle? It is plain that the semen and the unfertilized

  embryo, while still separate from each other, must be assumed to

  have the nutritive soul potentially, but not actually, except that

  (like those unfertilized embryos that are separated from the mother)

  it absorbs nourishment and performs the function of the nutritive

  soul. For at first all such embryos seem to live the life of a

  plant. And it is clear that we must be guided by this in speaking of

  the sensitive and the rational soul. For all three kinds of soul,

  not only the nutritive, must be possessed potentially before they

  are possessed in actuality. And it is necessary either (1) that they

  should all come into being in the embryo without existing previously

  outside it, or (2) that they should all exist previously, or (3), that

  some should so exist and others not. Again, it is necessary that

  they should either (1) come into being in the material supplied by the

  female without entering with the semen of the male, or (2) come from

  the male and be imparted to the material in the female. If the latter,

  then either all of them, or none, or some must come into being in

  the male from outside.

  Now that it is impossible for them all to preexist is clear from

  this consideration. Plainly those principles whose activity is

  bodily cannot exist without a body, e.g. walking cannot exist

  without feet. For the same reason also they cannot enter from outside.

  For neither is it possible for them to enter by themselves, being

  inseparable from a body, nor yet in a body, for the semen is only a

  secretion of the nutriment in process of change. It remains, then, for

  the reason alone so to enter and alone to be divine, for no bodily

  activity has any connexion with the activity of reason.

  Now it is true that the faculty of all kinds of soul seems to have a

  connexion with a matter different from and more divine than the

  so-called elements; but as one soul differs from another in honour and

  dishonour, so differs also the nature of the corresponding matter. All

  have in their semen that which causes it to be productive; I mean what

  is called vital heat. This is not fire nor any such force, but it is

  the spiritus included in the semen and the foam-like, and the

  natural principle in the spiritus, being analogous to the element of

  the stars. Hence, whereas fire generates no animal and we do not

  find any living thing forming in either solids or liquids under the

  influence of fire, the heat of the sun and that of animals does

  generate them. Not only is this true of the heat that works through

  the semen, but whatever other residuum of the animal nature there

  may be, this also has still a vital principle in it. From such

  considerations it is clear that the heat in animals neither is fire

  nor derives its origin from fire.

  Let us return to the material of the semen, in and with which

  comes away from the male the spiritus conveying the principle of soul.

  Of this principle there are two kinds; the one is not connected with

  matter, and belongs to those animals in which is included something

  divine (to wit, what is called the reason), while the other is

  inseparable from matter. This material of the semen dissolves and

  evaporates because it has a liquid and watery nature. Therefore we

  ought not to expect it always to come out again from the female or

  to form any part of the embryo that has taken shape from it; the

  case resembles that of the fig-juice which curdles milk, for this

  too changes without becoming any part of the curdling masses.

  It has been settled, then, in what sense the embryo and the semen

  have soul, and in what sense they have not; they have it potentially

  but not actually.

  Now semen is a secretion and is moved with the same movement as that

  in virtue of which the body increases (this increase being due to

  subdivision of the nutriment in its last stage). When it has

  entered the uterus it puts into form the corresponding secretion of

  the female and moves it with the same movement wherewith it is moved

  itself. For the female's contribution also is a secretion, and has all

  the arts in it potentially though none of them actually; it has in

  it potentially even those parts which differentiate the female from

  the male, for just as the young of mutilated parents are sometimes

  born mutilated and sometimes not, so also the young born of a female

  are sometimes female and sometimes male instead. For the female is, as

  it were, a mutilated male, and the catamenia are semen, only not pure;

  for there is only one thing they have not in them, the principle of

  soul. For this reason, whenever a wind-egg is produced by any

  animal, the egg so forming has in it the parts of both sexes

  potentially, but has not the principle in question, so that it
does

  not develop into a living creature, for this is introduced by the

  semen of the male. When such a principle has ben imparted to the

  secretion of the female it becomes an embryo.

  Liquid but corporeal substances become surrounded by some kind of

  covering on heating, like the solid scum which forms on boiled foods

  when cooling. All bodies are held together by the glutinous; this

  quality, as the embryo develops and increases in size, is acquired

  by the sinewy substance, which holds together the parts of animals,

  being actual sinew in some and its analogue in others. To the same

  class belong also skin, blood-vessels, membranes, and the like, for

  these differ in being more or less glutinous and generally in excess

  and deficiency.

  4

  In those animals whose nature is comparatively imperfect, when a

  perfect embryo (which, however, is not yet a perfect animal) has

  been formed, it is cast out from the mother, for reasons previously

  stated. An embryo is then complete when it is either male or female,

  in the case of those animals who possess this distinction, for some

  (i.e. all those which are not themselves produced from a male or

  female parent nor from a union of the two) produce an offspring which

  is neither male nor female. Of the generation of these we shall

  speak later.

  The perfect animals, those internally viviparous, keep the

  developing embryo within themselves and in close connexion until

  they give birth to a complete animal and bring it to light.

  A third class is externally viviparous but first internally

  oviparous; they develop the egg into a perfect condition, and then

  in some cases the egg is set free as with creatures externally

  oviparous, and the animal is produced from the egg within the mother's

  body; in other cases, when the nutriment from the egg is consumed,

  development is completed by connection with the uterus, and

  therefore the egg is not set free from the uterus. This character

  marks the cartilaginous fish, of which we must speak later by

  themselves.

  Here we must make our first start from the first class; these are

  the perfect or viviparous animals, and of these the first is man.

  Now the secretion of the semen takes place in all of them just as does

  that of any other residual matter. For each is conveyed to its

  proper place without any force from the breath or compulsion of any

  other cause, as some assert, saying that the generative parts

  attract the semen like cupping-glasses, aided by the force of the

  breath, as if it were possible for either this secretion or the

  residue of the solid and liquid nutriment to go anywhere else than

  they do without the exertion of such a force. Their reason is that the

  discharge of both is attended by holding the breath, but this is a

  common feature of all cases when it is necessary to move anything,

  because strength arises through holding the breath. Why, even

  without this force the secretions or excretions are discharged in

  sleep if the parts concerned are full of them and are relaxed. One

  might as well say that it is by the breath that the seeds of plants

  are always segregated to the places where they are wont to bear fruit.

  No, the real cause, as has been stated already, is that there are

  special parts for receiving all the secretions, alike the useless (as

  the residues of the liquid and solid nutriment), and the blood, which

  has the so-called blood-vessels.

  To consider now the region of the uterus in the female- the two

  blood-vessels, the great vessel and the aorta, divide higher up, and

  many fine vessels from them terminate in the uterus. These become

  over-filled from the nourishment they convey, nor is the female nature

  able to concoct it, because it is colder than man's; so the blood is

  excreted through very fine vessels into the uterus, these being unable

  on account of their narrowness to receive the excessive quantity,

  and the result is a sort of haemorrhage. The period is not

  accurately defined in women, but tends to return during the waning

  of the moon. This we should expect, for the bodies of animals are

  colder when the environment happens to become so, and the time of

  change from one month to another is cold because of the absence of the

  moon, whence also it results that this time is stormier than the

  middle of the month. When then the residue of the nourishment has

  changed into blood, the catamenia tend to occur at the above-mentioned

  period, but when it is not concocted a little matter at a time is

  always coming away, and this is why 'whites' appear in females while

  still small, in fact mere children. If both these discharges of the

  secretions are moderate, the body remains in good health, for they act

  as a purification of the secretions which are the causes of a morbid

  state of body; if they do not occur at all or if they are excessive,

  they are injurious, either causing illness or pulling down the

  patient; hence whites, if continuous and excessive, prevent girls from

  growing. This secretion then is necessarily discharged by females

  for the reasons given; for, the female nature being unable to

  concoct the nourishment thoroughly, there must not only be left a

  residue of the useless nutriment, but also there must be a residue

  in the blood-vessels, and this filling the channels of the finest

  vessels must overflow. Then Nature, aiming at the best end, uses it up

  in this place for the sake of generation, that another creature may

  come into being of the same kind as the former was going to be, for

  the menstrual blood is already potentially such as the body from which

  it is discharged.

  In all females, then, there must necessarily be such a secretion,

  more indeed in those that have blood and of these most of all in

  man, but in the others also some matter must be collected in the

  uterine region. The reason why there is more in those that have

  blood and most in man has been already given, but why, if all

  females have such a secretion, have not all males one to correspond?

  For some of them do not emit semen but, just as those which do emit it

  fashion by the movement in the semen the mass forming from the

  material supplied by the female, so do the animals in question bring

  the same to pass and exert the same formative power by the movement

  within themselves in that part from whence the semen is secreted. This

  is the region about the diaphragm in all those animals which have one,

  for the heart or its analogue is the first principle of a natural

  body, while the lower part is a mere addition for the sake of it.

  Now the reason why it is not all males that have a generative

  secretion, while all females do, is that the animal is a body with

  Soul or life; the female always provides the material, the male that

  which fashions it, for this is the power that we say they each

  possess, and this is what is meant by calling them male and female.

  Thus while it is necessary for the female to provide a body and a

  material mass, it is not necessary for the male, bec
ause it is not

  within the work of art or the embryo that the tools or the maker

  must exist. While the body is from the female, it is the soul that

  is from the male, for the soul is the reality of a particular body.

  For this reason if animals of a different kind are crossed (and

  this is possible when the periods of gestation are equal and

  conception takes place nearly at the same season and there is no great

  difference in the of the animals), the first cross has a common

  resemblance to both parents, as the hybrid between fox and dog,

  partridge and domestic fowl, but as time goes on and one generation

  springs from another, the final result resembles the female in form,

  just as foreign seeds produce plants varying in accordance with the

  country in which they are sown. For it is the soil that gives to the

  seeds the material and the body of the plant. And hence the part of

  the female which receives the semen is not a mere passage, but the

  uterus has a considerable width, whereas the males that emit semen

  have only passages for this purpose, and these are bloodless.

  Each of the secretions becomes such at the moment when it is in

  its proper place; before that there is nothing of the sort unless with

  much violence and contrary to nature.

  We have thus stated the reason for which the generative secretions

  are formed in animals. But when the semen from the male (in those

  animals which emit semen) has entered, it puts into form the purest

  part of the female secretion (for the greater part of the catamenia

  also is useless and fluid, as is the most fluid part of the male

  secretion, i.e. in a single emission, the earlier discharge being in

  most cases apt to be infertile rather than the later, having less

  vital heat through want of concoction, whereas that which is concocted

  is thick and of a more material nature).

  If there is no external discharge, either in women or other animals,

  on account of there not being much useless and superfluous matter in

  the secretion, then the quantity forming within the female

  altogether is as much as what is retained within those animals which