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On The Generation Of Animals   

soft like a true 'skin', as with the scaly integument of fish and
reptiles, then the testes must needs be internal. Therefore they are
so in dolphins and all the cetacea which have them, and in the
oviparous quadrupeds among the scaly animals. The skin of birds also
is hard so that it will not conform to the size of anything and
enclose it neatly. (This is another reason with all these animals for
their testes being internal besides those previously mentioned as
arising necessarily from the details of copulation.) For the same
reason they are internal in the elephant and hedgehog, for the skin of
these, too, is not well suited to keep the protective part separate.

[The position of the uterus differs in animals viviparous within
themselves and those externally oviparous, and in the latter class
again it differs in those which have the uterus low and those which
have it near the hypozoma, as in fishes compared with birds and
oviparous quadrupeds. And it is different again in those which produce
young in both ways, being oviparous internally and viviparous
externally. For those which are viviparous both internally and
externally have the uterus placed on the abdomen, as men, cattle,
dogs, and the like, since it is expedient for the safety and growth of
the foetus that no weight should be upon the uterus.]


The passages also are different through which the solid and liquid
excreta pass out in all the vivipara. Wherefore both males and females
in this class all have a part whereby the urine is voided, and this
serves also for the issue of the semen in males, of the offspring in
females. This passage is situated above and in front of the passage of
the solid excreta. The passage is the same as that of the solid
nutriment in all those animals that have no penis, in all the ovipara,
even those of them that have a bladder, as the tortoises. For it is
for the sake of generation, not for the evacuation of the urine,
that the passages are double; but because the semen is naturally
liquid, the liquid excretion also shares the same passage. This is
clear from the fact that all animals produce semen, but all do not
void liquid excrement. Now the spermatic passages of the male must
be fixed and must not wander, and the same applies to the uterus of
the female, and this fixing must take place at either the front or the
back of the body. To take the uterus first, it is in the front of
the body in vivipara because of the foetus, but at the loin and the
back in ovipara. All animals which are internally oviparous and
externally viviparous are in an intermediate condition because they
participate in both classes, being at once oviparous and viviparous.
For the upper part of the uterus, where the eggs are produced, is
under the hypozoma by the loin and the back, but as it advances is low
at the abdomen; for it is in that part that the animal is
viviparous. In these also the passage for solid excrement and for
copulation is the same, for none of these, as has been said already,
has a separate pudendum.

The same applies to the passages in the male, whether they have
testes or no, as to the uterus of the ovipara. For in all of them, not
only in the ovipara, the ducts adhere to the back and the region of
the spine. For they must not wander but be settled, and that is the
character of the region of the back, which gives continuity and
stability. Now in those which have internal testes, the ducts are
fixed from the first, and they are fixed in like manner if the
testes are external; then they meet together towards the region of the

The like applies to the ducts in the dolphins, but they have their
testes hidden under the abdominal cavity.

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