ORIGI N AND STRUCTURE OF THE HEAD 155 



without the somites themselves moving in one direction 

 or in the other, or fusing or being split up. We have to 

 do simply with a displacement of differentiation and not 

 of the cell material itself. 



The hypoglossus-musculature and the hypoglossus itself, 

 originating always from the anterior post-branchial somites 

 and having a constant relation to the branchial basket, 

 which also has not always the same extension, must 

 therefore be considered as homologous throughout the 

 whole series of Craniates, from the Cyclostomes on to the 

 Amniotes. The length of the skull, however, being inde- 

 pendent from the number of gill-slits, i.e. from the length 

 of the branchial segmental level, the cranio-vertebral limit 

 has no constant relation with the hypoglossus-roots. In 

 the same way e.g. the paired and the unpaired fins in 

 different groups of fishes may vary in their situation inde- 

 pendently from each other. 



