226 ELEMENTS OP OBNITHOIOGT. 



vessels from tlie body ramify over the allantois and there receive 

 that purification and oxygenation vpherein 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 difierences 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 vertebrae are sketched out. 



In a later bi^t stUl young condition of the skeleton, the little 

 ossicles which are attached to the transverse processes of the 

 cervical vertebrsB 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 vertebrae later on anchylose together to form the 

 " Pygostyle " *, 



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. 



