ScHAPER, Structure of Selachian Cerebellum. 3 



shark already approaches that of the higher vertebrates in its 

 morphological structure. This, however, is not the case. On 

 a closer examination we soon learn tJiat we have to do only 

 with a plate throtvn into transverse folds and enclosing a relatively 

 roomy cavity, not with a solid organ like that of bii'ds and 

 maimnals. The essential feature in the cerebellum of the latter 

 is the massive development of the white matter, which goes 

 hand in hand with an extraordinarily complicated surface fold- 

 ing of the cortex, whence the formation of the ^ ^ arbor vitae " 

 so characteristic of birds and mammals. There is none of this 

 present in the selachian cerebellwn. There is expressed, indeed, 

 in the folding an evident tendency to an extension of the sur- 

 face, yet these folds cannot be directly (also not genetically) 

 homologized with the^ "convolutions" and furrows of the cere- 

 bellum of the higher vertebrates. In the first case we have, 

 in general, at least, to do with actual folds, i. e. with structures 

 where an infolding on one side of the lamella corresponds to an 

 outfolding on the other, while in the second case we are con- 

 cerned with solid ridges and protuberances separated by fur- 

 rows. 



Concerning the finer structure, the presence of the three 

 cerebellar layers typical for all vertebrates — the molecular, Pur- 

 kinje and granular — has been already demonstrated by the ear- 

 lier authors, as Viault (8), Sanders (3) and Rohon (2). I have 

 to add, however, that the granular layer does not everywhere 

 participate to the same extent in the structure of the cerebellar 

 lamella. Indeed in definite and extensive portions of the latter 

 it is entirely absent, in consequence of which the cortical matter 

 in these places consists of the molecular and Purkinje layers only 

 and the latter is separated from the ependymal layer by only a 

 relatively thin zone of fibers. The Purkinje cells likewise are 

 absent in certain regions of the lamella. This relation of the 

 layers is in various respects of especial interest but I cannot 

 enter into this here. The accompanying schema of a transverse 

 section through the anterior portion of the cerebellum of Mus- 

 telus may serve to illustrate this condition. 



