IX 



THE VERTEBKATE HEAD 



501 



that its extension backwards has taken place by successive steps. 

 In the most ancient recognizable stage the cranium (Palaeocranium 

 Fiirbringer) extended no farther back than the vagus nerve. 

 T\\\^ phase is represented either persistent or revertive in the 



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adult stage of modern Cyclostomes : it is also seen in the young 

 Lepidosiren of stage 34 (see Fig. 154, B, p. 309). 



The next phase is seen in the adults of such relatively primitive 

 groups as the Elasmobranchs, the dipneumonic Lungfishes and the 

 Amphibians, in which an occipital region has been added on to the 

 palaeocranium. 



