The tongue has appeared on the floor of the mouth
as a bud of mesoblast covered by epiblast.
During the eighth, ninth, and tenth days the
embryo grows very rapidly, the head being still especially
large, and at the same time becoming more round, the
mid-brain not being so prominent.
From the eleventh day onwards the embryo suc-
cessively puts on characters which are not only
avian, but even distinctive of the genus, species and
variety.
So early as the ninth or tenth day the sacs con-
taming the feathers begin to protrude from the surface
of the skin as papille, especially prominent at first along
the middle line of the back from the neck to the rump,
and over the thighs, the sacs of the tail feathers being
very conspicuous. On the thirteenth day these sacs,
generally distributed over the body, and acquiring the
length of a quarter of an inch or more, appear to the
naked eye as feathers, the thin walls of the sacs allow-
ing their contents, now coloured according to the variety
of the bird, to shine through. ‘They are still however
closed sacs, and indeed remain such even on the nine-
teenth day, when many of them are an inch in length.
Feathers are epidermal structures. They arise from an in-
duration of the epidermis of papillw containing a vascular core.
On the eighth day a chalky-looking patch is ob-
servable on the tip of the nose. This by the twelfth
day has become developed into a horny but still soft
beak.
On the thirteenth day, nails are visible at the ex-
tremities, and scales on the remaining portions of the


Ix. ] OSSIFICATION, 283


toes. These on the sixteenth day become harder and
more horny, as does also the beak.
