ONTOGENY AND PHYLOGENY OF THE STERNUM 73 



rotes; for if the sternum of the human embryo were to be demonstrated 

 apart from the costal girdles, one diagram would serve to explain both 

 that and what we find in this little snake-like lizard .... and 

 if ribs had not been arrested we might have seen the counterpart of 

 the ribs of the mammalian embryo. 



The figure is of an adult, and, as Parker says, it might also be de- 

 scribed as that of the mammalian embryo, except that no ribs are 

 present and the sides of the presternum are closely applied to the 

 coracoids. Now, if Paterson and the author be correct in their con- 

 tention that the presternum and coracoids in the rat and mouse 

 are continuous at an early stage, and this stage precedes the 

 fusion of ribs and sternum, then the adult Chirotes sternum is a 

 structure that far more closely approximates the early embry- 

 onic sternum of mammals than Parker suspected when he made 

 the comparison above quoted between the two. This is a more 

 striking parallel than is usually met between the adult structure 

 in a lower group and an embryonic stage of the same structure 

 in a higher group. 



While probably all will admit the above argument, since both 

 are Amniotes, the question may be raised as to whether there is 

 any evidence for relating such a sternum as occurs in Chirotes 

 with the amphibian sternum, for the crux of the whole matter of 

 sternal homology lies between these two groups. It would seem 

 that a direct comparison might be made with the Calamites 

 sternum, and this structure, as has already been indicated, by a 

 fusion of the sternal bars, and a retention of the sutural relation 

 to the epicoracoids might be metamorphosed directly into the 

 adult sternum of Chirotes, and this is especially strong evidence 

 when we consider that in neither the reptile nor amphibian com- 

 pared does the question of ribs enter at all, since there are none 

 in either form in the region of the sternum. 



Using the crocodile (fig. 31) as a contrast to Chirotes in show- 

 ing the extreme of fusion of ribs and sternum in reptiles, it may 

 be remarked at once that aside from the presence of ribs this 

 sternum is directly comparable with the last one considered. The 

 difference is due to the ventral growth and attachment of ribs to 

 the sternum, otherwise it is essentially the same as more primitive 



