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On The Generation Of Animals   

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.


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.

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