ScHAPER, Stnidiire of Selachian Cerebellum. 15 



with which an individual fiber can be followed from one layer 

 to the other in sections is now clear from this characteristic of 

 the neuraxons, in the transition zone the fibers usually leave the 

 plane of the section and are thereby withdrawn from further 

 observation. It thus comes about that we frequently see in 

 silver preparations that the majority of the fibers ascending and 

 descending through both layers end in the intermediate zone as 

 though cut off, of which figures 11 and 18 (Plates II and III) 

 furnish a clear illustration. 



These observations on the arrangement and course 

 of nerve-fibers are still of a meager nature and require much 

 amplification by means of further investigations. The study of 

 these relations in the cerebellum of selachians is rendered 

 considerably more difficult and complicated through the varia- 

 tions in the combination of the different layers and especially 

 through the entire absence of a granular layer in extensive re- 

 gions of the cerebellar plate. The selachian cerebellum pre- 

 sents conditions, owing to these peculiarities, very unlike, as 

 far as I know, the cerebella of all other vertebrates. 



TJic Neuroglia. 



The neuroglia of the selachian cerebellum is in various re- 

 spects, both from the morphological and phylogenetic point of 

 view, of especial interest. The supporting substance as a whole 

 undoubtedly stands plylogenetically on a very low scale. Sau- 

 erbeck has already succeeded in demonstrating ti'iie ependyma 

 cells in the cerebellum of Mustelus, " zvelchc der membrana lijn- 

 itans interna ansitzen deren Fortsatzc sicJi bis zur membrana lim- 

 itans externa verfolgen lassen." He conjectured that they form 

 the principal constituent of the whole supporting substance, not 

 having succeeded in demonstrating other elements belonging 

 to the neuroglia type. I can confirm the observation of Sauer- 

 beck's in so far as that the ependyma cells play a leading role 

 in the constitution of the supporting tissue of the selachian cere- 

 bellum ; I have to add, though, that the majority of them are 

 much modified in their morphological appearance, that they 

 retain their connection with the membrana limitans externa 



