THE EIBS, THE STERNUM, AND THE THOEAX 



151 



Ossification. Each sternebra appears to be developed from a 

 single centre of ossification ; but there is ground for believing that 

 these centres are derived from paired primary centres. 



VARIATIONS IN THE STERNEBR.E. 

 VARIATIONS IN SIZE AND DEVELOPMENT. 



The size of the entire sternum is largely dependent upon the size 

 of the animal ; it is influenced, however, to a great extent by the size 

 of its variable terminal xiphisternum. 



The following table exhibits the maximum lengths and widths of 

 the sternebrre in ten skeletons. The measurements are given in milli- 

 metres, and do not include the intersternebral cartilages. 



As a rule, the sternebrse are regular and symmetrical ; some varia- 

 bility in form is frequently shown by the seventh and by the xiphi- 

 sternum. 



VARIATIONS IN NUMBER. 



The number of sternebrse may be increased to nine by the addition 

 of a short cubical element between the seventh and the xiphisternum. 

 In some sterna this abnormal eighth piece is so far reduced in size 



