328 



The fenestra praecerebralis and cavum praecerebrale, one or both, 

 are thus of such wide distribution in fishes and amphibians that 

 they must represent important ancestral structures, and such structures 

 would be found in the primordial mouth and its related parts if the 

 vertebrate is developed from an annelid ancestor in the manner 

 recently proposed by Delsman (1913). The fenestra praecerebraHs 

 would then represent the primordial mouth, or an atrial formation 

 related to it, and the cavum praecerebrale would be a preoral space, 

 above the "Scheitelplatte" of Dblsman's descriptions, where the 

 archencephalon was wholly or in part developed but later withdrawn 

 within the cranial cavity. The concentration, in this region in 

 selachians, of the ampullae of Lorenzini would then be related to 

 sensory tissues that probably encircled this primordial oral opening. 



Literature. 

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Delsman, H. C. 1913. Der Ursprung der Vertebraten. Mitt. zool. Stat. Nap. Bd. 20. 



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Parker, W. K, 1873. On the Structure and Development of the Skull in the 

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