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.