EMBRYOLOGY 211 
and more the appearance of mere branches of the by this time 
advanced mesenteric vessels. The yellow yolk has become quite 
fluid, and its bulk has increased owing to its having absorbed much 
of the rapidly-diminishing white of the egg. During the next days 
the yolk diminishes rapidly in bulk, it being taken up by the 
abundantly-developed blood-vessels ; the yolk-sac becomes flaccid, 
and by the eleventh day is thrown into a series of internal folds. 
The intestine has by elongation formed a number of convolutions 
and loops, some of which are hanging down into the somatic stalk, 
but by the fifteenth day these loops are gradually withdrawn into 
the cavity of the abdomen, the walls of which have by this time 
been definitely established like the walls of the chest. The allantois 
now fills most of the amniotic cavity and lies close under the shell, 
being separated from the shell membrane only by the thin false 
amnion and the remains of the vitelline membrane, with which it 
fuses. When the egg is opened the pulsations of the allantoic 
arteries at once attract attention. By the nineteenth day the white 
of the egg has entirely disappeared, and the yolk-sac is withdrawn 
into the abdomen. 
Concerning the changes of the embryo itself it is to be observed, 
that by the seventh day the head, which is still disproportion- 
ately large, ceases to grow more rapidly than the body. The 
tongue appears on the floor of the mouth; the visceral clefts on the 
sides of the now more distinctly-marked neck are closed. On the 
eighth day a white patch of calcareous matter appears on the tip 
of the nose; the latter is by the twelfth day transformed into a 
horny but still soft beak. On the following day nails are visible 
at the ends of the toes and some of the fingers, and scales on the 
anterior surface of the feet and toes. Beak, claws, and scales become 
harder and more horny by the sixteenth day. Feathers begin to 
protrude as early as the ninth day from the surface of the skin as 
papille, especially prominent on the middle line of the back and on 
the thighs. By the thirteenth day the feathers are distributed over 
most parts of the body, and acquiring the length of a quarter of an 
inch appear to the naked eye as feathers, their thin horny sheaths 
allowing their pigmented contents to shew through. The sheaths 
are not pierced until a day or two before hatching, when some of 
them are nearly an inch in length. The cartilaginous skeleton is 
completed by the thirteenth day, and the muscles can be made out 
with tolerable clearness. Ossification begins already much earlier 
in various parts of the bones of the limbs, but much of the skeleton 
remains cartilaginous or imperfectly ossified long after the bird has 
been hatched. The whole embryo changes its position on the 
fourteenth day so as to lie lengthways in the egg, with its beak 
touching the shell-membrane where this forms the inner wall of 
the rapidly-increasing air-chamber at the broad end. On about the 
