8 THE HUMAN STERNUM 



bone, or the separate centres of ossification, the cartilaginous model, or 

 the pre-chondral tissue which is the most important feature in its history ? 

 Obviously, the only fair picture can be obtained by a composite view. All 

 these phases in the growth of the bone must be taken into account, and it 

 is only from a complete knowledge of its development, structure and 

 variations that we can form a proper notion of its morphological position. 



B. Early Development of the Sternum 



{a) Fishes. 



Elasmobranchs. In Acanthias vulgaris (PL III, Figs. 25, 26), I find that 

 in young embryos the appearance of the shoulder-girdle and fore limb is 

 foreshadowed by the occurrence of a conglomeration of cells such as has 

 been described above, which is ultimately converted into hyaline cartilage. 

 This stretches across the median ventral line and forms a continuous bar, 

 without any differentiation into sternum and coracoids. 



(V) Amphibia (^Anurd). 



In the frog, the sternum (Marshall*^) appears to be developed 

 subsequently to the formation of the limb girdles. After the approximation 

 of the precoracoid and coracoid bars, and the formation of the longitudinal 

 epicoracoid elements on each side, further growths occur anteriorly and 

 posteriorly. The omosternum is formed in front by the growth of bilateral 

 processes, as continuations forward of the epicoracoids, which fuse to form 

 a median cartilage. The sternum and xiphisternum are formed in the same 

 way by an extension backwards and fusion together of similar bilateral 

 projections from the hinder end of the epicoracoids. 



(c) Reptiles. 



Rathke" describes two distinct bilateral strips in Lacerta agilis, which 

 unite to form the sternum. They are not at first continuous with the ribs ; 

 and in Anguis fragilis he finds the embryonic sternum further removed from 

 the ribs in earlier than in later stages, thus indicating a separate origin for 

 the sternum. The two halves of the sternum fuse together from before 

 backwards. 



