ONTOGENY AND PHYLOGENY OF THE STERNUM 87 



'intercepted cartilage' interpreted by Howes ('91), Parker ('91), 

 and myself as the rudiment of the presternum. If two dotted 

 lines be added to the Hexanchus girdle (fig. 1), a sternum com- 

 parable to that of Rana is produced, with epicoracoids meeting 

 in the midline and the fundaments of the omosternum and ster- 

 num present. 



9. Conclusions 



This somewhat lengthy review and discussion of the phylogeny 

 of the sternum in the different classes of vertebrates, together 

 with the accompanying figures, which have been adapted and 

 modified from various sources, has revealed very clearly several 

 facts of outstanding importance in relation to the problem of 

 sternal origin: 



1. That there is present a median ventral rudiment, derived 

 from the coracoids, which may be identified as a presternum as 

 far back in the vertebrate series as the shark, and can be followed 

 up through a ganoid, a teleost, a dipnoan, and from there on 

 through the Tetrapoda. 



2. That in all cases of vertebrates, and as high as man in the 

 Mammalia, there is in the embryo or throughout life a contin- 

 uous girdle across the ventral side and connecting the two scap- 

 ulae above. 



3. That this girdle in its ventral aspect is in the most intimate 

 relation to the anterior part of the sternum; sometimes the mes- 

 enchymatous material passes over insensibly from one structure 

 into the other without any line of demarkation; or at most, 

 adults of Amphibia and Reptilia, there being but a suture between 

 the two parts. 



4. That in all these forms from the lowest to the highest, the 

 relation of sternum and ribs is purely secondary and the result of 

 a comparatively late fusion of the two structures in the embryo. 

 The presence or absence of ribs does not seem to affect the devel- 

 opment or size of the sternum in any degree. 



5. That plural ribs extending to the ventral side of the body 

 are a recent acquisition of the vertebrates, while the shoulder- 

 girdle and sternum are coexistent and intimately related from 

 the earliest appearance of the Gnathosomes. 



