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. 112 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 canaliculi, 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. 



