82 FRANK BLAIR* HANSON 



human embryo accept Ruge's view, and the reason is apparent 

 when we consider that in working exclusively with the highest 

 mammals, one is at the disadvantage of not knowing what stages 

 may have been suppressed in evolution. It would seem that, 

 beginning with the lowest forms and working up through each 

 successive group, as is attempted in this present paper, a foun- 

 dation is laid upon which to interpret the greatly reduced history 

 in the higher mammals. 



Does the complete girdle, sternum, coracoid, and scapula, cross 

 the midline in forms higher than the marsupial? Since the per- 

 manent adult coracoid articulating with the sternum in the mono- 

 treme is reduced to only a fetal stage in the marsupial, we might 

 expect this stage to be very transitory or entirely suppressed 

 higher in the scale. Since the marsupials, edentates, rodents, 

 and insectivores are all ancient orders and probably lie not far 

 from the monotreme stem, among them such a stage might be 

 found, if present. Paterson was the first to discover this in an 

 early embryo of the rat, and I found the same thing (section on 

 ontogeny) in the mouse of 7.75-mm. and 17.2-mm. human em- 

 bryo. This is shown in figures 5 and 6, where the girdles are 

 very similar indeed to that of a shark (fig. 2). The next stages 

 indicate that scapulae, coracoids, and sternum are all derivatives 

 of this single mesenchymatous element. This stage is exceedingly 

 brief in the mouse, as compared with the marsupial, for it cannot 

 be detected with certainty in a 6-mm. mouse, and is hardly rec- 

 ognizable in the 8.75-mm. mouse. This rapid suppression would 

 lead us to suppose that possibly no such stage is present in the 

 human embryo, yet figure 6 shows man to have retained it iden- 

 tically as in the rodents. 



Huntington ('18) identifies the costocoracoid ligament of man 

 as indicating "'the original path of the sternal extension of the 

 coracoid." It is interesting in this connection that the costo- 

 coracoid ligament often contains fibrocartilaginous nodules. 

 Figure 42 shows this ligament in relation to the other parts in 

 man on the left side of the figure, while on the right side, the fun- 

 damental plan of the vertebrate girdle as illustrated in the Anura 

 is shown. Huntington does not say whether he considers the 



