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  part is inside. And if we enclose many eggs together in a bladder or

  something of the kind and boil them over a fire so as not to make

  the movement of the heat quicker than the separation of the white

  and yolk in the eggs, then the same process takes place in the whole

  mass of the eggs as in a single egg, all the yellow part coming into

  the middle and the white surrounding it.

  We have thus stated why some eggs are of one colour and others of

  two.

  2

  The principle of the male is separated off in eggs at the point

  where the egg is attached to the uterus, and the reason why the

  shape of two-coloured eggs is unsymmetrical, and not perfectly round

  but sharper at one end, is that the part of the white in which is

  contained this principle must differ from the rest. Therefore the

  egg is harder at this point than below, for it is necessary to shelter

  and protect this principle. And this is why the sharp end of the egg

  comes out of the hen later than the blunt end; for the part attached

  to the uterus comes out later, and the egg is attached at the point

  where is the said principle, and the principle is in the sharp end.

  The same is the case also in the seeds of plants; the principle of the

  seed is attached sometimes to the twig, sometimes to the husk,

  sometimes to the pericarp. This is plain in the leguminous plants, for

  where the two cotyledons of beans and of similar seeds are united,

  there is the seed attached to the parent plant, and there is the

  principle of the seed.

  A difficulty may be raised about the growth of the egg; how is it

  derived from the uterus? For if animals derive their nutriment through

  the umbilical cord, through what do eggs derive it? They do not,

  like a scolex, acquire their growth by their own means. If there is

  anything by which they are attached to the uterus, what becomes of

  this when the egg is perfected? It does not come out with the egg as

  the cord does with animals; for when its egg is perfected the shell

  forms all round it. This problem is rightly raised, but it is not

  observed that the shell is at first only a soft membrane, and that

  it is only after the egg is perfected that it becomes hard and

  brittle; this is so nicely adjusted that it is still soft when it

  comes out (for otherwise it would cause pain in laying), but no

  sooner has it come out than it is fixed hard by cooling, the

  moisture quickly evaporating because there is but little of it, and

  the earthy part remaining. Now at first a certain part of this

  membrane at the sharp end of eggs resembles an umbilical cord, and

  projects like a pipe from them while they are still small. It is

  plainly visible in small aborted eggs, for if the bird be drenched

  with water or suddenly chilled in any other way and cast out the egg

  too soon, it appears still sanguineous and with a small tail like an

  umbilical cord running through it. As the egg becomes larger this is

  more twisted round and becomes smaller, and when the egg is

  perfected this end is the sharp end. Under this is the inner

  membrane which separates the white and the yolk from this. When the

  egg is perfected, the whole of it is set free, and naturally the

  umbilical cord does not appear, for it is now the extreme end of the

  egg itself.

  The egg is discharged in the opposite way from the young of

  vivipara; the latter are born head-first, the part where is the

  first principle leading, but the egg is discharged as it were feet

  first; the reason of this being what has been stated, that the egg

  is attached to the uterus at the point where is the first principle.

  The young bird is produced out of the egg by the mother's incubating

  and aiding the concoction, the creature developing out of part of

  the egg, and receiving growth and completion from the remaining

  part. For Nature not only places the material of the creature in the

  egg but also the nourishment sufficient for its growth; for since

  the mother bird cannot perfect her young within herself she produces

  the nourishment in the egg along with it. Whereas the nourishment,

  what is called milk, is produced for the young of vivipara in

  another part, in the breasts, Nature does this for birds in the egg.

  The opposite, however, is the case to what people think and what is

  asserted by Alcmaeon of Crotona. For it is not the white that is the

  milk, but the yolk, for it is this that is the nourishment of the

  chick, whereas they think it is the white because of the similarity of

  colour.

  The chick then, as has been said, comes into being by the incubation

  of the mother; yet if the temperature of the season is favourable,

  or if the place in which the eggs happen to lie is warm, the eggs

  are sufficiently concocted without incubation, both those of birds and

  those of oviparous quadrupeds. For these all lay their eggs upon the

  ground, where they are concocted by the heat in the earth. Such

  oviparous quadrupeds as do visit their eggs and incubate do so

  rather for the sake of protecting them than of incubation.

  The eggs of these quadrupeds are formed in the same way as those

  of birds, for they are hard-shelled and two-coloured, and they are

  formed near the hypozoma as are those of birds, and in all other

  respects resemble them both internally and externally, so that the

  inquiry into their causes is the same for all. But whereas the eggs of

  quadrupeds are hatched out by the mere heat of the weather owing to

  their strength, those of birds are more exposed to destruction and

  need the mother-bird. Nature seems to wish to implant in animals a

  special sense of care for their young: in the inferior animals this

  lasts only to the moment of giving birth to the incompletely developed

  animal; in others it continues till they are perfect; in all that

  are more intelligent, during the bringing up of the young also. In

  those which have the greatest portion in intelligence we find

  familiarity and love shown also towards the young when perfected, as

  with men and some quadrupeds; with birds we find it till they have

  produced and brought up their young, and therefore if the hens do

  not incubate after laying they get into worse condition, as if

  deprived of something natural to them.

  The young is perfected within the egg more quickly in sunshiny

  weather, the season aiding in the work, for concoction is a kind of

  heat. For the earth aids in the concoction by its heat, and the

  brooding hen does the same, for she applies the heat that is within

  her. And it is in the hot season, as we should expect, that the eggs

  are more apt to be spoilt and the so-called 'uria' or rotten eggs

  are produced; for just as wines turn sour in the heats from the

  sediment rising (for this is the cause of their being spoilt), so is

  it with the yolk in eggs, for the sediment and yolk are the earthy

  part in each case, wherefore the wine becomes turbid when the sediment

  mixes with it, and the like applies to the eggs that are spoiling

  because of the yolk. It is natural then that such should be
the case

  with the birds that lay many eggs, for it is not easy to give the

  fitting amount of heat to all, but (while some have too little)

  others have too much and this makes them turbid, as it were by

  putrefaction. But this happens none the less with the birds of prey

  though they lay few eggs, for often one of the two becomes rotten, and

  the third practically always, for being of a hot nature they make

  the moisture in the eggs to overboil so to say. For the nature of

  the white is opposed to that of the yolk; the yolk congeals in

  frosts but liquefies on heating, and therefore it liquefies on

  concoction in the earth or by reason of incubation, and becoming

  liquid serves as nutriment for the developing chick. If exposed to

  heat and roasted it does not become hard, because though earthy in

  nature it is only so in the same way as wax is; accordingly on heating

  too much the eggs become watery and rotten, [if they be not from a

  liquid residue]. The white on the contrary is not congealed by

  frost but rather liquefies (the reason of which has been stated

  before), but on exposure to heat becomes solid. Therefore being

  concocted in the development of the chick it is thickened. For it is

  from this that the young is formed (whereas the yolk turns to

  nutriment) and it is from this that the parts derive their growth

  as they are formed one after another. This is why the white and the

  yolk are separated by membranes, as being different in nature. The

  precise details of the relation of the parts to one another both at

  the beginning of generation and as the animals are forming, and also

  the details of the membranes and umbilical cords, must be learnt

  from what has been written in the Enquiries; for the present

  investigation it is sufficient to understand this much clearly,

  that, when the heart has been first formed and the great

  blood-vessel has been marked off from it, two umbilical cords run from

  the vessel, the one to the membrane which encloses the yolk, the other

  to the membrane resembling a chorion which surrounds the whole embryo;

  this latter runs round on the inside of the membrane of the shell.

  Through the one of these the embryo receives the nutriment from the

  yolk, and the yolk becomes larger, for it becomes more liquid by

  heating. This is because the nourishment, being of a material

  character in its first form, must become liquid before it can be

  absorbed, just as it is with plants, and at first this embryo, whether

  in an egg or in the mother's uterus, lives the life of a plant, for it

  receives its first growth and nourishment by being attached to

  something else.

  The second umbilical cord runs to the surrounding chorion. For we

  must understand that, in the case of animals developed in eggs, the

  chick has the same relation to the yolk as the embryo of the

  vivipara has to the mother so long as it is within the mother (for

  since the nourishment of the embryo of the ovipara is not completed

  within the mother, the embryo takes part of it away from her). So

  also the relation of the chick to the outermost membrane, the

  sanguineous one, is like that of the mammalian embryo to the uterus.

  At the same time the egg-shell surrounds both the yolk and the

  membrane analogous to the uterus, just as if it should be put round

  both the embryo itself and the whole of the mother, in the vivipara.

  This is so because the embryo must be in the uterus and attached to

  the mother. Now in the vivipara the uterus is within the mother, but

  in the ovipara it is the other way about, as if one should say that

  the mother was in the uterus, for that which comes from the mother,

  the nutriment, is the yolk. The reason is that the process of

  nourishment is not completed within the mother.

  As the creature grows the umbilicus running the chorion collapses

  first, because it is here that the young is to come out; what is

  left of the yolk, and the umbilical cord running to the yolk, collapse

  later. For the young must have nourishment as soon as it is hatched;

  it is not nursed by the mother and cannot immediately procure its

  nourishment for itself; therefore the yolk enters within it along with

  its umbilicus and the flesh grows round it.

  This then is the manner in which animals produced from perfect

  eggs are hatched in all those, whether birds or quadrupeds, which

  lay the egg with a hard shell. These details are plainer in the larger

  creatures; in the smaller they are obscure because of the smallness of

  the masses concerned.

  3

  The class of fishes is also oviparous. Those among them which have

  the uterus low down lay an imperfect egg for the reason previously

  given,' but the so-called 'selache' or cartilaginous fishes produce

  a perfect egg within themselves but are externally viviparous except

  one which they call the 'frog'; this alone lays a perfect egg

  externally. The reason is the nature of its body, for its head is many

  times as large as the rest of the body and is spiny and very rough.

  This is also why it does not receive its young again within itself nor

  produce them alive to begin with, for as the size and roughness of the

  head prevents their entering so it would prevent their exit. And while

  the egg of the cartilaginous fishes is soft-shelled (for they

  cannot harden and dry its circumference, being colder than birds),

  the egg of the frog-fish alone is solid and firm to protect it

  outside, but those of the rest are of a moist and soft nature, for

  they are sheltered within and by the body of the mother.

  The young are produced from the egg in the same way both with

  those externally perfected (the frog-fishes) and those internally,

  and the process in these eggs is partly similar to, partly different

  from that in birds' eggs. In the first place they have not the

  second umbilicus which runs to the chorion under the surrounding

  shell. The reason of this is that they have not the surrounding shell,

  for it is no use to them since the mother shelters them, and the shell

  is a protection to the eggs against external injury between laying and

  hatching out. Secondly, the process in these also begins on the

  surface of the egg but not where it is attached to the uterus, as in

  birds, for the chick is developed from the sharp end and that is where

  the egg was attached. The reason is that the egg of birds is separated

  from the uterus before it is perfected, but in most though not all

  cartilaginous fishes the egg is still attached to the uterus when

  perfect. While the young develops upon the surface the egg is consumed

  by it just as in birds and the other animals detached from the uterus,

  and at last the umbilicus of the now perfect fish is left attached

  to the uterus. The like is the case with all those whose eggs are

  detached from the uterus, for in some of them the egg is so detached

  when it is perfect.

  The question may be asked why the development of birds and

  cartilaginous fishes differs in this respect. The reason is that in

  birds the white and yolk are separate, but fish
eggs are one-coloured,

  the corresponding matter being completely mixed, so that there is

  nothing to stop the first principle being at the opposite end, for the

  egg is of the same nature both at the point of attachment and at the

  opposite end, and it is easy to draw the nourishment from the uterus

  by passages running from this principle. This is plain in the eggs

  which are not detached, for in some of the cartilaginous fish the

  egg is not detached from the uterus, but is still connected with it as

  it comes downwards with a view to the production of the young alive;

  in these the young fish when perfected is still connected by the

  umbilicus to the uterus when the egg has been consumed. From this it

  is clear that previously also, while the egg was still round the

  young, the passages ran to the uterus. This happens as we have said in

  the 'smooth hound'.

  In these respects and for the reasons given the development of

  cartilaginous fishes differs from that of birds, but otherwise it

  takes place in the same way. For they have the one umbilicus in like

  manner as that of birds connecting with the yolk,- only in these

  fishes it connects with the whole egg (for it is not divided into

  white and yolk but all one-coloured),- and get their nourishment from

  this, and as it is being consumed the flesh in like manner

  encroaches upon and grows round it.

  Such is the process of development in those fish that produce a

  perfect egg within themselves but are externally viviparous.

  4

  Most of the other fish are externally oviparous, all laying an

  imperfect egg except the frog-fish; the reason of this exception has

  been previously stated, and the reason also why the others lay

  imperfect eggs. In these also the development from the egg runs on the

  same lines as that of the cartilaginous and internally oviparous

  fishes, except that the growth is quick and from small beginnings

  and the outside of the egg is harder. The growth of the egg is like

  that of a scolex, for those animals which produce a scolex give

  birth to a small thing at first and this grows by itself and not