For, in sanguineous animals, both
heart and liver are visible enough when the body is only just
formed, and while it is still extremely small. For these parts are
to be seen in the egg sometimes as early as the third day, being
then no bigger than a point; and are visible also in aborted
embryos, while still excessively minute. Moreover, as the external
organs are not precisely alike in all animals, but each creature is
provided with such as are suited to its special mode of life and
motion, so is it with the internal parts, these also differing in
different animals. Viscera, then, are peculiar to sanguineous animals;
and therefore are each and all formed from sanguineous material, as is
plainly to be seen in the new-born young of these animals. For in such
the viscera are more sanguineous, and of greater bulk in proportion to
the body, than at any later period of life, it being in the earliest
stage of formation that the nature of the material and its abundance
are most conspicuous. There is a heart, then, in all sanguineous
animals, and the reason for this has already been given. For that
sanguineous animals must necessarily have blood is self-evident.
And, as the blood is fluid, it is also a matter of necessity that
there shall be a receptacle for it; and it is apparently to meet
this requirement that nature has devised the blood-vessels.
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