Generation from the egg proceeds in an identical manner with all
birds, but the full periods from conception to birth differ, as has
been said. With the common hen after three days and three nights there
is the first indication of the embryo; with larger birds the
interval being longer, with smaller birds shorter. Meanwhile the
yolk comes into being, rising towards the sharp end, where the
primal element of the egg is situated, and where the egg gets hatched;
and the heart appears, like a speck of blood, in the white of the egg.
This point beats and moves as though endowed with life, and from it
two vein-ducts with blood in them trend in a convoluted course (as the
egg substance goes on growing, towards each of the two circumjacent
integuments); and a membrane carrying bloody fibres now envelops the
yolk, leading off from the vein-ducts. A little afterwards the body is
differentiated, at first very small and white. The head is clearly
distinguished, and in it the eyes, swollen out to a great extent. This
condition of the eyes lat on for a good while, as it is only by
degrees that they diminish in size and collapse. At the outset the
under portion of the body appears insignificant in comparison with the
upper portion. Of the two ducts that lead from the heart, the one
proceeds towards the circumjacent integument, and the other, like a
navel-string, towards the yolk. The life-element of the chick is in
the white of the egg, and the nutriment comes through the navel-string
out of the yolk.