ONTOGENY AND PHYLOGENY OF THE STERNUM 51 



Since the observations of Whitehead and Waddell accord with 

 neither those of Ruge nor of Paterson, the formulation of a new 

 theory of sternal origin was necessary. 



Their discovery of the single median rudiment would not allow 

 of Ruge's conclusion, for he thought the sternum a product of 

 the costal extremities; neither would they square with Pater- 

 son's view, for he derived the sternal bands from the median 

 rudiment; and further, it was apparently not a product of the 

 sternal ends of the clavicles. So recourse was had to either of 

 two other theories, each possible so far as their observations go: 

 " first, it may be formed 'in situ,' or, second, it may be derived 

 from the anterior ends of the sternal bands by each of them send- 

 ing a prolongation medialward to join its fellow in the median 

 plane. The fact that we never found this rudiment in a paired 

 condition, but always as a single band of cells uniting the an- 

 terior ends of the sternal bands leads us to believe that the first 

 interpretation is the more probable." 



Thus Whitehead and Waddell say there are two possi- 

 bilities remaining for the formation of the anterior median rudi- 

 ment, either that it arises 'in situ' or as a derivative of the sternal 

 bands. They overlooked another possibility, its derivation from 

 and relation to the shoulder-girdle. This may not have been an 

 unnatural error in view of the fact that they studied chiefly the 

 pig where the clavicle and that associated coracoidal mesen- 

 chymatous material of the early embryo is lacking. Their 

 view-point was derived from the developmental stages in one or 

 two mammals only, and they paid little attention to the com- 

 parative anatomical and the phylogenetic side. 



This median rudiment is considered by Whitehead and Wad- 

 dell to be the homotype of the presternum of monotremes, but 

 no reasons or arguments for such a belief are set forth. This con- 

 clusion, however, is clearly invalid, for it is impossible to reconcile 

 the theory of 'in situ' for this anterior rudiment with their state- 

 ment that it finds its homologue in the presternum of lower forms 

 or the so-called prosternum of monotremes. If these be homol- 

 ogous, then the presternum and prosternum also arise 'in situ,' 

 and no morphologist believes that they do. 



