ONTOGENY AND PHYLOGENY OF THE STERNUM 79 



but by some the precoracoid. That the two structures, i.e., the 

 epicoracoid of the monotremes and this " sheet of mesenchy- 

 matous cells" are homologous is the belief of Broom. In the 

 larger specimens (37 mm. and over) the coracoid becomes de- 

 tached from the sternum by a process of degeneration, and this 

 continues until the well-known adult condition is reached (fig. 37) 

 where coracoids and sternum are far apart. 



However, in a mammary fetus 23 mm. long an intermediate 

 condition was found. As Broom describes it: 



The coracoid process is similar to that in the large foetus, but from 

 it there is produced backwards and inwards a small cartilaginous proc- 

 ess, which nearly meets the outer process of the presternum. It may 

 thus be concluded that during the later intra-uterine development of 

 Trichosurus, and probably of other marsupials (later verified in other 

 marsupials), there is a well developed coracoid, which, as in the adult 

 Monotremes, most reptiles, birds, and amphibians, articulates with the 

 sternum, and that shortly after birth, the coracoid loses its attachment 

 with the sternum, and becomes rapidly absorbed, only the anterior 

 part remaining as the coracoid process. 



From this description it appears that the degeneration of the 

 coracoid begins in its middle part and absorption progresses 

 toward each end. In the marsupials that part of the coracoid 

 attached to the sternum is completely absorbed and no trace of 

 it is found in the adult, while the half connecting with the scapula 

 is represented in the adult by the coracoid process. In this con- 

 nection mention may be made of a peculiar structure I find in 

 the mouse and rat, and an almost constant structure in rodents, 

 as figured by Parker ('68) . In most of his figures of the Rodentia, 

 there is a small bony process on either side of the presternum 

 between the juncture of the clavicle and the first rib with the 

 sternum (fig. 38). Parker calls this process the epicoracoid and 

 says it was left by the retreating coracoids of the lower forms. It 

 would seem from my observations on the mouse, described above, 

 that Parker is correct to the extent that this is the median end 

 of the coracoid, but wrong in making it simply an hereditary 

 rudiment. It is, rather, the end of a complete embryonic cora- 

 coid, which in the rat, mouse and man, as in the marsupial, 

 extends across to the sternum in an early stage. Whereas in the 



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THE AMERICAN JOURNAL OF ANATOMY, VOL. 26, NO. 1 



