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  have an external discharge; this is put into form by the power of

  the male residing in the semen secreted by him, or, as is clearly seen

  to happen in some insects, by the part in the female analogous to

  the uterus being inserted into the male.

  It has been previously stated that the discharge accompanying sexual

  pleasure in the female contributes nothing to the embryo. The chief

  argument for the opposite view is that what are called bad dreams

  occur by night with women as with men; but this is no proof, for the

  same thing happens to young men also who do not yet emit semen, and to

  those who do emit semen but whose semen is infertile.

  It is impossible to conceive without the emission of the male in

  union and without the secretion of the corresponding female

  material, whether it be discharged externally or whether there is only

  enough within the body. Women conceive, however, without

  experiencing the pleasure usual in such intercourse, if the part

  chance to be in heat and the uterus to have descended. But generally

  speaking the opposite is the case, because the os uteri is not

  closed when the discharge takes place which is usually accompanied

  by pleasure in women as well as men, and when this is so there is a

  readier way for the semen of the male to be drawn into the uterus.

  The actual discharge does not take place within the uterus as some

  think, the os uteri being too narrow, but it is in the region in front

  of this, where the female discharges the moisture found in some cases,

  that the male emits the semen. Sometimes it remains in this place;

  at other times, if the uterus chance to be conveniently placed and hot

  on account of the purgation of the catamenia, it draws it within

  itself. A proof of this is that pessaries, though wet when applied,

  are removed dry. Moreover, in all those animals which have the

  uterus near the hypozoma, as birds and viviparous fishes, it is

  impossible that the semen should be so discharged as to enter it; it

  must be drawn into it. This region, on account of the heat which is in

  it, attracts the semen. The discharge and collection of the

  catamenia also excite heat in this part. Hence it acts like

  cone-shaped vessels which, when they have been washed out with hot

  water, their mouth being turned downwards, draw water into themselves.

  And this is the way things are drawn up, but some say that nothing

  of the kind happens with the organic parts concerned in copulation.

  Precisely the opposite is the case of those who say the woman emits

  semen as well as the man, for if she emits it outside the uterus

  this must then draw it back again into itself if it is to be mixed

  with the semen of the male. But this is a superfluous proceeding,

  and Nature does nothing superfluous.

  When the material secreted by the female in the uterus has been

  fixed by the semen of the male (this acts in the same way as rennet

  acts upon milk, for rennet is a kind of milk containing vital heat,

  which brings into one mass and fixes the similar material, and the

  relation of the semen to the catamenia is the same, milk and the

  catamenia being of the same nature)- when, I say, the more solid

  part comes together, the liquid is separated off from it, and as the

  earthy parts solidify membranes form all round it; this is both a

  necessary result and for a final cause, the former because the surface

  of a mass must solidify on heating as well as on cooling, the latter

  because the foetus must not be in a liquid but be separated from it.

  Some of these are called membranes and others choria, the difference

  being one of more or less, and they exist in ovipara and vivipara

  alike.

  When the embryo is once formed, it acts like the seeds of plants.

  For seeds also contain the first principle of growth in themselves,

  and when this (which previously exists in them only potentially) has

  been differentiated, the shoot and the root are sent off from it,

  and it is by the root that the plant gets nourishment; for it needs

  growth. So also in the embryo all the parts exist potentially in a way

  at the same time, but the first principle is furthest on the road to

  realization. Therefore the heart is first differentiated in actuality.

  This is clear not only to the senses (for it is so) but also on

  theoretical grounds. For whenever the young animal has been

  separated from both parents it must be able to manage itself, like a

  son who has set up house away from his father. Hence it must have a

  first principle from which comes the ordering of the body at a later

  stage also, for if it is to come in from outside at later period to

  dwell in it, not only may the question be asked at what time it is

  to do so, but also we may object that, when each of the parts is

  separating from the rest, it is necessary that this principle should

  exist first from which comes growth and movement to the other parts.

  (Wherefore all who say, as did Democritus, that the external parts of

  animals are first differentiated and the internal later, are much

  mistaken; it is as if they were talking of animals of stone or wood.

  For such as these have no principle of growth at all, but all

  animals have, and have it within themselves.) Therefore it is that

  the heart appears first distinctly marked off in all the sanguinea,

  for this is the first principle or origin of both homogeneous and

  heterogeneous parts, since from the moment that the animal or organism

  needs nourishment, from that moment does this deserve to be called its

  principle or origin. For the animal grows, and the nutriment, in its

  final stage, of an animal is the blood or its analogue, and of this

  the blood-vessels are the receptacle, wherefore the heart is the

  principle or origin of these also. (This is clear from the

  Enquiries and the anatomical drawings.)

  Since the embryo is already potentially an animal but an imperfect

  one, it must obtain its nourishment from elsewhere; accordingly it

  makes use of the uterus and the mother, as a plant does of the

  earth, to get nourishment, until it is perfected to the point of being

  now an animal potentially locomotive. So Nature has first designed the

  two blood-vessels from the heart, and from these smaller vessels

  branch off to the uterus. These are what is called the umbilicus,

  for this is a blood-vessel, consisting of one or more vessels in

  different animals. Round these is a skin-like integument, because

  the weakness of the vessels needs protection and shelter. The

  vessels join on to the uterus like the roots of plants, and through

  them the embryo receives its nourishment. This is why the animal

  remains in the uterus, not, as Democritus says, that the parts of

  the embryo may be moulded in conformity with those of the mother. This

  is plain in the ovipara, for they have their parts differentiated in

  the egg after separation from the matrix.

  Here a difficulty may be raised. If the blood is the nourishment,

  and if the heart, which first comes into being, already contains

  blood, and the nourishment comes from outside
, whence did the first

  nourishment enter? Perhaps it is not true that all of it comes from

  outside just as in the seeds of plants there is something of this

  nature, the substance which at first appears milky, so also in the

  material of the animal embryo the superfluous matter of which it is

  formed is its nourishment from the first.

  The embryo, then, grows by means of the umbilicus in the same way as

  a plant by its roots, or as animals themselves when separated from the

  nutriment within the mother, of which we must speak later at the

  time appropriate for discussing them. But the parts are not

  differentiated, as some suppose, because like is naturally carried

  to like. Besides many other difficulties involved in this theory, it

  results from it that the homogeneous parts ought to come into being

  each one separate from the rest, as bones and sinews by themselves,

  and flesh by itself, if one should accept this cause. The real cause

  why each of them comes into being is that the secretion of the

  female is potentially such as the animal is naturally, and all the

  parts are potentially present in it, but none actually. It is also

  because when the active and the passive come in contact with each

  other in that way in which the one is active and the other passive (I

  mean in the right manner, in the right place, and at the right time),

  straightway the one acts and the other is acted upon. The female,

  then, provides matter, the male the principle of motion. And as the

  products of art are made by means of the tools of the artist, or to

  put it more truly by means of their movement, and this is the activity

  of the art, and the art is the form of what is made in something else,

  so is it with the power of the nutritive soul. As later on in the case

  of mature animals and plants this soul causes growth from the

  nutriment, using heat and cold as its tools (for in these is the

  movement of the soul), and each thing comes into being in

  accordance with a certain formula, so also from the beginning does

  it form the product of nature. For the material by which this latter

  grows is the same as that from which it is constituted at first;

  consequently also the power which acts upon it is identical with

  that which originally generated it; if then this acting power is the

  nutritive soul, this is also the generative soul, and this is the

  nature of every organism, existing in all animals and plants. [But

  the other parts of the soul exist in some animals, not in others.] In

  plants, then, the female is not separated from the male, but in

  those animals in which it is separated the male needs the female

  besides.

  5

  And yet the question may be raised why it is that, if indeed the

  female possesses the same soul and if it is the secretion of the

  female which is the material of the embryo, she needs the male besides

  instead of generating entirely from herself. The reason is that the

  animal differs from the plant by having sense-perception; if the

  sensitive soul is not present, either actually or potentially, and

  either with or without qualification, it is impossible for face, hand,

  flesh, or any other part to exist; it will be no better than a

  corpse or part of a corpse. If then, when the sexes are separated,

  it is the male that has the power of making the sensitive soul, it

  is impossible for the female to generate an animal from itself

  alone, for the process in question was seen to involve the male

  quality. Certainly that there is a good deal in the difficulty

  stated is plain in the case of the birds that lay wind-eggs, showing

  that the female can generate up to a certain point unaided. But this

  still involves a difficulty; in what way are we to say that their eggs

  live? It neither possible that they should live in the same way as

  fertile eggs (for then they would produce a chick actually alive),

  nor yet can they be called eggs only in the sense in which an egg of

  wood or stone is so called, for the fact that these eggs go bad

  shows that they previously participate in some way in life. It is

  plain, then, that they have some soul potentially. What sort of soul

  will this be? It must be the lowest surely, and this is the nutritive,

  for this exists in all animals and plants alike. Why then does it

  not perfect the parts and the animal? Because they must have a

  sensitive soul, for the parts of animals are not like those of a

  plant. And so the female animal needs the help of the male, for in

  these animals we are speaking of the male is separate. This is exactly

  what we find, for the wind-eggs become fertile if the male tread the

  female in a certain space of time. About the cause of these things,

  however, we shall enter into detail later.

  If there is any kind of animal which is female and has no male

  separate from it, it is possible that this may generate a young one

  from itself without copulation. No instance of this worthy of credit

  has been observed up to the present at any rate, but one case in the

  class of fishes makes us hesitate. No male of the so-called erythrinus

  has ever yet been seen, but females, and specimens full of roe, have

  been seen. Of this, however, we have as yet no proof worthy of credit.

  Again, some members of the class of fishes are neither male nor

  female, as eels and a kind of mullets found in stagnant waters. But

  whenever the sexes are separate the female cannot generate perfectly

  by herself alone, for then the male would exist in vain, and Nature

  makes nothing in vain. Hence in such animals the male always

  perfects the work of generation, for he imparts the sensitive soul,

  either by means of the semen or without it. Now the parts of the

  embryo already exist potentially in the material, and so when once the

  principle of movement has been imparted to them they develop in a

  chain one after another, as the wheels are moved one by another in the

  automatic machines. When some of the natural philosophers say that

  like is brought to like, this must be understood, not in the sense

  that the parts are moved as changing place, but that they stay where

  they are and the movement is a change of quality (such as softness,

  hardness, colour, and the other differences of the homogeneous parts);

  thus they become in actuality what they previously were in

  potentiality. And what comes into being first is the first

  principle; this is the heart in the sanguinea and its analogue in

  the rest, as has been often said already. This is plain not only to

  the senses (that it is first to come into being), but also in view

  of its end; for life fails in the heart last of all, and it happens in

  all cases that what comes into being last fails first, and the first

  last, Nature running a double course, so to say, and turning back to

  the point from whence she started. For the process of becoming is from

  the non-existent to the existent, and that of perishing is back

  again from the existent to the non-existent.

  6

  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