Fishes also, as was above stated, are provided with wombs, and
wombs of diverse kinds. The oviparous genera have wombs bifurcate in
shape and low down in position; the cartilaginous genus have wombs
shaped like those of O birds. The womb, however, in the
cartilaginous fishes differs in this respect from the womb of birds,
that with some cartilaginous fishes the eggs do not settle close to
the diaphragm but middle-ways along the backbone, and as they grow
they shift their position.
The egg with all fishes is not of two colours within but is of
even hue; and the colour is nearer to white than to yellow, and that
both when the young is inside it and previously as well.
Development from the egg in fishes differs from that in birds in
this respect, that it does not exhibit that one of the two
navel-strings that leads off to the membrane that lies close under the
shell, while it does exhibit that one of the two that in the case of
birds leads off to the yolk. In a general way the rest of the
development from the egg onwards is identical in birds and fishes.
That is to say, development takes place at the upper part of the
egg, and the veins extend in like manner, at first from the heart; and
at first the head, the eyes, and the upper parts are largest; and as
the creature grows the egg-substance decreases and eventually
disappears, and becomes absorbed within the embryo, just as takes
place with the yolk in birds.
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