ONTOGENY AND PHYLOGENY OF THE STERNUM 77 



dition as here found. The sternum of Vanellus is not a highly 

 complex structure, as such structures go in the higher birds, but 

 shows many affinities to the reptilian stock." 



It would be absurd to think that these large, heavy sternal 

 bands were originally derived from the feeble and loosely attached 

 pairs of ribs, while it is entirely plausible to suppose that in an 

 earlier stage a continuous sheet of cartilage on either side was 

 subsequently differentiated into sternal bands, coracoid, and 

 scapula. That this is actually the state of affairs in early embryos 

 of several mammals will soon be shown. 



That the development of the sternum is largely independent of 

 ribs in the birds, as in reptiles, is further shown by Parker's ob- 

 servations on Apteryx: 



In the earliest stage in which the sternum is present it extends 

 backwards to the level of the third thoracic rib; the first two ribs are 

 united to it by joints, the third loosely attached by connective tissue. 

 In the next stage, the first three ribs are attached by joints, and the 

 fourth by connective tissue, that is, as it appears to me, the portion of 

 the sternum corresponding to the third and fourth ribs is formed by 

 a backward growth of the anterior region and quite independently of the 

 last two ribs (italics mine), the union of which with it is a secondary 

 process. 



This is just about what Paterson says concerning the devel- 

 opment of the sternum in the rat, except that it leads him to 

 conclude that the sternum is not of costal origin, while Parker, 

 giving all the evidence necessary to substantiate Paterson's 

 view, is nevertheless himself oblivious of the logic of his own work, 

 and makes the surprising statement that "the relation of the 

 shoulder girdle to the sternum is altogether secondary, and forms 

 no part of the axial skeleton, as the Transcendentalists vainly 

 teach." 



The above account of conditions in the birds seems to be suf- 

 ficient to relate genetically the sternum in birds to that of rep- 

 tiles and mammals, and while of little or no philogenetic value 

 in this connection, yet the evidence shows the avian sternum to 

 be homologous with the sterna throughout the entire vertebrate 

 series, one of the theses for which this paper contends. 



