62 FRANK BLAIR HANSON 



Furthermore, if the sternum is ossified from the ribs, seg- 

 mentation of that structure should be apparent at its earliest 

 appearance. On the contrary, however, the sternal bands ex- 

 hibit no trace of segmentation until a late period of develop- 

 ment. The bands may be followed carefully from section to 

 section, or the parts reconstructed in wax, but no one has ever 

 reported the slightest indication of an early division of the 

 bands into segments. 



The sternebrae may be interpreted as arising by a process of 

 segmentation in response to the demand for as great a measure 

 of elasticity on the ventral side of the animal as is allowed by 

 the more or less flexible vertebral column on the dorsal side. 



Sutures arising in this manner, as a response to strain, Avill 

 naturally appear at the weakest parts along the sternum. At the 

 points of attachment for the ribs the sternum is often deeply 

 notched, weakening this region, and here, as expected, occur 

 the lines of divisions of the sternum into segments or sternebrae. 

 That this is the cause and manner is indicated by the fact that 

 there are always the same number of sutures as there are pairs 

 of ribs attached to the sternum. By cutting a typical sternum 

 out of cardboard or a wax plate, and notching the sides for the 

 reception of ribs, it is possible by applying a lateral strain fo 

 produce sutures or cracks across the cardboard or wax sternum, 

 dividing it into sternebrae exactly as in the actual sternum. 



In many of the reptiles and such animals as the cats among 

 the mammals, where a long, lithe body in making its way through 

 thick undergrowth or over rough ground is often twisted into 

 almost an S-shape, the advantage of a segmented sternum is 

 obvious. How large a part this plays in breathing is not so 

 apparent, but doubtless has some bearing. 



In the Primates where the semi- and upright position obtains, 

 there is less need of flexibility, and the sternebrae tend to be- 

 come fused into the three typical parts of the primate sternum. 

 That the entire sternebral development is a secondary and late 

 acquisition and has no bearing on the origin of the sternum is 

 quite apparent. 



