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On The Generation Of Animals   


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
through any attachment to the parent. The reason is similar to that of
the growth of yeast, for yeast also grows great from a small beginning
as the more solid part liquefies and the liquid is aerated. This is

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