10 THE HUMAN STERNUM 



In the chick, during this process of growth, there is a distinct 

 difference in the development of the sternum and of the ribs. When the 

 ribs are cartilaginous the sternum is still cellular ; when the latter first appears 

 the ribs are unconnected with it, and it is only secondarily and after the 

 sternum becomes cartilaginous that fusion occurs. 



The existence of two lateral halves in the chick's sternum is a feature 

 which is characteristic. It is to be associated, probably, with the early and 

 excessive development of the heart in the embryo bird, which has heavy 

 work to perform in the period of incubation ; and by its excessive size 

 delays the fusion of the parts, and causes a condition of embryonic 

 ectopia cordis. 



As in the frog, the sternum in its development succeeds the shoulder- 

 girdle and the ribs, and is in a cellular condition when both of the other 

 series have become cartilaginous. Goette^' mentions that in the chick (four 

 to five days) the sternal anlage was separate from the ribs. 



He regards the avian sternum as formed in two distinct parts, a 

 costal sternal plate and a clavicular anlage, the episternalia forming the crista 

 sterni. Both portions are paired and laid down as widely separated halves. 

 He considers that each half of the costal sternum arises probably from 

 rib-ends. 



Hoffmann^' also maintains that in the formation of the presternum 

 an interclavicular element is present uniting with the costal sternum, and 

 forming the keel. 



Miss B. Lindsay'* in a valuable paper, in which, however, the costal 

 view of the origin of the sternum is unquestionably accepted, describes in 

 the Gannet the occurrence (first stage) of eight ribs which end freely with 

 no trace of a sternum. Later (second stage), ' the sternum is indicated by 

 thick opaque aggregations in a layer of mesoblast.' In the next stage ' the 

 sternum is fully formed and fused with the ribs.' 'There are seven sternal 

 ribs.' In the fourth stage the lateral halves of the sternum are closed, and 

 the keel is formed. 



She describes more particularly in the ostrich the metasternum and 

 its processes as a backward extension from the mesosternum. 



