226 ELEMENTS OF ORNITHOLOGY, 
vessels from the body ramify over the allantois and there receive 
that purification and oxygenation wherein the process of respi- 
ration, as before said, consists. 
When ripe for leaving the shell, the young Bird pecks at and 
cracks it, being often aided, as in the chick, by a small hard 
prominence on the beak, which subsequently disappears. 
It may be useful here to note a few points as to the develop- 
ment of the arteries and the skeleton. 
Five pairs of arteries arch up on each side of the neck in the 
embryo, to form by their junction the aorta. The changes they 
undergo have been described as follows :— 
The first and second pairs soon disappear. 
The third pair gives rise to the carotid arteries. 
The right arch of the fourth pair persists as the arch and 
trunk of the aorta, and the left arch, as the left subclavian 
artery. 
The two arches of the fifth pair become the pulmonary 
arteries. 
Since, however, there are such great differences in the adult 
condition of these vessels, it is hardly to be expected that there 
should not also be divergences in the modes of their develop- 
ment. 
As to the skeleton, its primitive axis, the notochord, becomes 
invested with cartilage which subsequently segments, and then 
points of ossification begin to form the vertebral centra, and 
gradually the whole vertebrz are sketched out. 
In a later but still young condition of the skeleton, the little 
ossicles which are attached to the transverse processes of the 
cervical vertebre are all distinct, and show their essential nature 
as small ribs each with its tubercular and capitular process. 
The sacrum also reveals its essential composition by the dis- 
tinctness of its component parts, as may be well seen in the 
Ostrich (fig. 149). 
The caudal vertebre later on anchylose together to form the 
* nygostyle” *. 
The sternum is not at first a single osseous structure, but is 
made up typically of five parts. One of these forms the keel. 
Another on each side in front is the bone with which the 
sternal ribs articulate; while the hinder lateral parts of the 
sternum are formed by yet another on each side behind. 
* See ante, p. 175. 
