I02 JoURNA], OF CoMPARATnE NkUKOLOGV. 



gamated with the original cartilaginous cranium. The brain 

 may perhaps be depended on to reveal the true history of the 

 head. Balfour sugge&ted that the fore-brain is the unseg- 

 mented primordial encephalon of the invertebrate progenitor 

 of higher animals. Koelliker supposes that this unsegmented 

 portion is an outgrowth from the anterior part of the brain 

 proper, but the hypothesis which jiist now enjoys most 

 general acceptance is that of Kleinenberg and Dohrn, which 

 regards the fore-brain as the fused ganglia of a number of 

 primitive segments. The ingeneous theory of Gaskell will 

 be elsewhere alluded to. 



The solution of this problem of the metamerism of the 

 head which is most simple is that of Rabl,(') who believes 

 that morphologically and physiogenetically there are but two 

 divisions in the head of vertebrates; an anterior, larger, and 

 unsegmented portion, and a smaller, segmented posterior 

 portion. The boundary between these portions is the audi- 

 tory vesicle, which, nevertheless, belongs to the anterior 

 part. Although the mesoderm of the anterior portion may 

 be separated into several aggregates, they have no corre- 

 spondence, whether in the method of origin or relation to 

 the cranial nerves, with the true somatomeres. 



The anterior division has the following nerve pairs, the 

 olfactory, optic, trigeminus, and acustico-facialis, the third, 

 fourth and sixth l:)eing derivatives of the trigeminal system. 

 The primary nerves of the posterior division are the glosso- 

 pharyngeal and vagus dorsad and the hypoglossus ventrad. 

 The accessory is considered a portion of the vagus. Rabl 

 is much influenced by the fact that the trigeminus and 

 facialis-acusticus do not spring from a continuous nervous 

 ridge like the glosso-pharyngeal and sensory nerves gener- 

 ally, but distinct from it and distinct from each other. 



Anton Dohrn(") calls attention to the fact, long before 



1 C. Ram., Morph. J.ihrbuLli. xv, 2 Heft, 



2 Anatomischer Aiizeiger, i8go, p. 56. 



