On account of this it results that in man alone
among animals the period of gestation is irregular; whereas the period
is fixed in the rest, there are several periods in man, for children
are born at seven months and at ten months and at the times between,
for even those of eight months do live though less often than the
rest. The reason may be gathered from what has just been said, and the
question has been discussed in the Problems. Let this explanation
suffice for these points.
The cause why the parts may be multiplied contrary to Nature is
the same as the cause of the birth of twins. For the reason exists
already in the embryo, whenever it aggregates more material at any
point of itself than is required by the nature of the part. The result
is then that either one of its parts is larger than the others, as a
finger or hand or foot or any of the other extremities or limbs; or
again if the embryo is cleft there may come into being more than one
such part, as eddies do in rivers; as the water in these is carried
along with a certain motion, if it dash against anything two systems
or eddies come into being out of one, each retaining the same
motion; the same thing happens also with the embryos. The abnormal
parts generally are attached near those they resemble, but sometimes
at a distance because of the movement- taking place in the embryo, and
especially because of the excess of material returning to that place
whence it was taken away while retaining the form of that part
whence it arose as a superfluity.
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