by Aristotle
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