282 F. H. EDGEWORTH. 



The first fourteen of the phenomena recorded appear to be 

 referable to a common cause ; if a muscle spreads into one or 

 more neighbouring- segments, that portion tends to be inner- 

 vated by the corresponding nerve or nerves. The backward 

 extension of the origin of the Xltli appears to be referable 

 to the same canse. 



It is not 3^et known what happens Avithin the central 

 nervous system — whether there is a corresponding migration 

 of motor neuroblasts or whether new ones are locally formed. 



The cause of the phenomenon cited under (15) above is 

 much more obscure. The muscle is the interarcualis ven- 

 tralis of the first branchial segment, and is homologous with 

 the similarly developed muscle of Amphibia, some Teleos- 

 tomi, and some Mammalia, and yet, unlike them, it is inner- 

 vated by spino-occipital nerves and not by the IXth, just as 

 if it were a coraco-branchialis I. 



(b) The possibility of the independent development of 

 similar secondary changes in the various groups arises in the 

 case of the hypobranchial spinal muscles, the hypobranchial 

 cranial muscles, the levatores arcuum branch ialium, and 

 trapezius, the hyoid bar and related muscles, the adductor 

 mandibulae. 



In Ceratodus and in Scyllium the hind end of the genio- 

 hyoid secondarily extends backwards to the shoulder-girdle. 

 The question arises whether this feature is inherited from a 

 common ancestor or whether it has been independently 

 acquired. In favour of the second view are the facts that 

 Avithiu the group of the Teleostomi all conditions exist 

 between that of a genio-hyoid whicli has slightly extended 

 backwards and a coraco-mandibularis. 



A similar question arises in regard to the formation of 

 coraco-branchiales in Ceratodus and Scyllium. Again, within 

 the group of the Teleostomi all va,riations exist between inter- 

 arcuales ventrales and their homologues, coraco-branchiales. 



These secondary modifications in the hypobranchial-spinal 

 and hypobranchial-cranial muscles appear to be moipho- 

 logical expressions of an increased need of tying the 



