7C, 



PEOCEEDINO8 OF THK ANATOMICAL AND ANTHROPOLOGICAL 



As regards the left twin (see Fig. 8) the fourth right aortic arch 

 has been entirely suppressed. The ventral roots of the fourth arches 

 have fused together producing the carotid trunk. The right sub- 

 clavian has grown out independently from the fourth left arch in con- 

 sequence of the suppression of its own the fourth arch. The left 

 subclavian artery has arisen from its correct position, whilst the 

 ductus arteriosus is also in its normal position. Here, as in the right 



I 



Fig. 8. Diagram to illustrate the evolution of the great vessels in the case of the left twin. 



twin, the vascular abnormalities are entirely due to the suppression 

 of that particular arch which has been concerned with the fused upper 

 limb. As the left upper limb of the one twin is fused to the right 

 upper limb of the other twin it necessarily follows that the corre- 

 sponding aortic arches have become suppressed by fusion, and to this 

 may be referred the various vascular abnormalities described. 



Whilst embryologists may not agree with all the remarks made 



