210 EMBRVOLOGY 
and knee are formed, but all the limbs are still exceedingly alike in 
shape. Most of the skeletal parts make their appearance as car- 
tilages ; especially the cartilaginous cranium, the visceral arches 
like the jaws and hyoids, the vertebree with the ribs, and the frame- 
work of the limbs. The changes affecting the vascular system have 
already been mentioned. 
Changes from the 6th day onwards.— 
The sixth day marks a new epoch in the development of the 
chick, for distinctly avine characters then first make their appear- 
ance, the embryo of a Bird being until this period strikingly like 
that of any other amniotic vertebrate, for instance, that of a Mammal 
or still more like that of a four-footed Reptile. The avine specializa- 
tion begins of course very gradually. But on the sixth day for the 
An. ae 
EmpBryo or Fowt ON THE SIxTH Day. UMBILICAL REGION OF THE SAME. 
Natural size and position, Magnified. 
Am. Amnion; Al. Allantois; G, Gut; 7, Tail; IV, Wing; y.s. Yolk-sac. 
first time become visible the main features of the characteristically 
avine wing and foot; the crop and the intestinal ceca make their 
appearance, the stomach is differentiated into a proventriculus and 
a gizzard; the nose begins to develop into a beak; and the com- 
mencing bones of the skull arrange themselves after an avine 
type. 
During the seventh day the amniotic cavity has assumed con- 
siderable dimensions, the fluid increasing within it ; obvious move- 
ments begin to appear in the amnion itself as slow vermicular 
contractions which creep rhythmically over it. The amnion in fact 
begins to pulsate slowly and rhythmically, and by its pulsations the 
embryo is rocked to and fro in the egg. The allantois, which at a 
later period shews similar movements, has spread out rapidly in 
all directions and is filled with fluid. The vascular area loses 
its terminal sinus; both vitelline arteries and veins now pass to 
and from the body of the chick as single trunks, assuming more 
