DEVELOPMENT OF THE STERNUM 105 



the clavicle. In the case of the pig there is, of course, no ques- 

 tion of the relation of the two structures, as the pig has no clavicle; 

 in the cat the sternum is only secondarily and temporarily con- 

 nected with the clavicle; is there also a stage in the development 

 of the human sternum when the median portion of the sternal 

 anlage is not connected with the clavicle? We think it quite 

 probable that such a stage does exist, though we were unable to 

 find it in the material at our command. However, as already 

 pointed out, this failure may have been due to the loss of a num- 

 ber of sections from the very series in which, a priori, we should 

 have expected to find such a stage. 



DISCUSSION AND CONCLUSIONS 



Our findings make it clear that the theory of Paterson, accord- 

 ing to which the mammalian sternum begins as a single median 

 blastema which bifurcates and sends a prolongation caudalward 

 on each side, is untenable. For in all three forms studied by us 

 the appearance of the paired portion of the anlage, the sternal 

 bands, antedates that of the median portion. 



The e\idence is almost as strong against the view of Ruge and 

 Mueller that the sternal bands are derived from the ventral ex- 

 tremities of the ribs. If the formation of these bands is due to 

 the proliferation of cells composing the ventral ends of the ribs, 

 i.e., if they are segmental in the beginning, one would reasonably 

 expect to find in the early period of their development breaks in 

 their continuity; yet we were not able to detect interruptions in 

 any of the many series of the sternal bands in their precartilag- 

 inous stages. A much stronger argument than this, however, 

 is the fact that the more anterior ribs did not reach the sternal 

 hands in the earliest stages that we examined. It is undoubtedly 

 true, in our opinion, that continuity of tissue between the ventral 

 ends of the ribs and the sternal bands does exist at an early stage 

 in the development of the sternum; but in the earliest stages in 

 both the pig and the cat such continuity is lacking between the 

 bands and the first and second ribs, while in the human embryo 

 at 10.5 mm. none of the ribs directly reached the sternal band. 



