198 COMPARATIVE ZOOLOGY. 
to protect the embryo. Meanwhile, another important or- 
gan is forming on the other side. A portion of the lower, 
or internal, blastodermic layer elongates downward, and 
then upward, spreading out over the whole inner surface 
of the shell, so that it surrounds both embryo and amnion. 
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.“ The chorion is the outer- 
most part of the allantois—in other words, the envelope of 
the ovum; 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- 
‘al coil; and at several points it dilates, to form the crop, 
stomach, gizzard, ete. 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 trne gland. The lungs are developed 
on the third day as minute buds 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 gelatinous material, which gradually becomes 
condensed and cellular, turning to cartilage. Then mi- 
nute canals containing blood-vessels arise, and earthy mat- 
ter (chiefly phosphate of lime) is deposited between the 
cells. The primary bone thus formed is compact: true 
osseous tissue, with canaliculi, laminee, and Haversian ca- 
nals, is the result of subsequent absorption.”* 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, begins at 
numerous distinct points, called centres ; and, theoretical- 
ly, every centre stands for a bone, so that there are as 
