DEVELOPMENT. 
203 
er). This is the allantois. It is full of blood-vessels, and 
it serves as the respiratory organ until the chick picks the 
shell and breathes by its lungs. 118 The chorion is the out¬ 
ermost part of the allantois, and the placenta of Mammals 
is the shaggy, vascular edge of the chorion. 
The alimentary canal is at first a straight tube closed at 
both ends, the middle being connected with the yolk-bag. 
As it grows faster than the body, it is thrown into a spi¬ 
ral coil; and at several points it dilates, to form the crop, 
stomach, gizzard, etc. The mouth is developed from an 
infolding of the skin. The liver is an outgrowth from 
the digestive tube, at first a cluster of cells, then of folli¬ 
cles, and finally a true gland. The lungs are developed 
on the third day as a minute bud from the upper part of 
the alimentary canal, or pharynx. As they grow in size, 
they pass from a smooth to a cellular condition. 
The skeleton at the beginning consists, like the noto¬ 
chord, of a cellular material, which gradually turns to car¬ 
tilage. Then minute canals containing blood-vessels arise, 
and earthy matter (chiefly phosphate of lime) is deposited 
between the cells. The primary bone thus formed is 
compact: true osseous tissue, with eanaliculi, laminae, and 
Haversian canals, is the result of subsequent absorption. 113 
Certain bones, as those of the face and cranium, are not 
preceded by cartilage, but by connective tissue: these are 
called membrane bones. Ossification, or bone-making, be¬ 
gins at numerous distinct points, called centres; and, the¬ 
oretically, every centre stands for a bone, so that there are 
as many bones in a skeleton as centres of ossification. 
But the actual number in the adult animal is much small¬ 
er, as many of the centres coalesce. 114 The development 
of the backbone is not from the head or from the tail, but 
from a central point midway between: there the first ver¬ 
tebrae appear, and from thence they multiply forward and 
backward. 
