88 FRANK BLAIR HANSON 



6. That the number of ribs reaching the sternum varies from zero 

 to a large number; sometimes the ribs are attached to the anterior 

 part of the sternum, again exclusively to the posterior part, but 

 apparently whatever the number or relation of ribs, the sternum 

 remains unaffected, indicating strongly its independence of the 

 costal cartilage. 



7. That the evidence presented seems to bear out the homology 

 of the sterna throughout the vertebrates; therefore, to classify 

 them as coracoidal for the lower groups, and costal for the higher 

 is unnecessary and artificial, for in the Amniota the sternum is as 

 truly coracoidal in origin as it is in the Ichthyopsida. 



8. That the mesosternum and xiphisternum are two back- 

 ward prolongations of the coracoidal presternum, sometimes 

 uniting in the midline (some reptiles, birds, and mammals), 

 again remaining distinct as horns or bars (some Amphibia, some 

 reptiles) . 



V. SUMMARY 



At the close of the section on the ontogeny of the sternum a 

 number of conclusions were listed, and likewise at the close of the 

 section on phylo'geny. These results may now be gathered up 

 in the three main theses of this paper, which are stated thus : 



1 . That the sternum is an homologous structure throughout all 

 groups of vertebrates, and occurs in forms ranging from Hexanchus 

 up to the highest mammals. 



2. That the anterior element of the sternum has its origin in 

 common with the shoulder-girdle, and in the embryo or through- 

 out life is in intimate relation to the coracoids. 



3. That the sternal bands are derivatives of the anterior median 

 rudiment, and may be secondarily, but never genetically, as- 

 sociated with ribs. 



