34 



finiiaf^ of Purkinje neurones here. Tnese neurones nan lonii. 

 supposed to charicterize the ceraballani alone, anJ their stri- 

 kingly peculiar appearance makes them readily identified. The 

 neurones of Purkinje from the acusticum of Mustelus agree with 

 those from the cerebellum as to size, general form, shape and 

 character of dendritic top, and even in the presence of the 

 spiny gemmules so characteristic of this nervous element. Ther.; 

 can be no question as to their morphological identity in 'viuste- 

 lus. Johnston ('98b) has found Purkinje neurones in the acus- 

 ticum of Acipenser, somewhat smaller and simpler than those of 

 the cerebellum, but undoubtedly equivalent. Similar results 

 will probably be obtained in all of the simpler vertebrates. 



It may be concluded, in the light of these embryological 

 and structural facts, that the cerebellum has arisen in the 

 phylogeny of vertebrates as a fused outgrowth of the pair of 

 tubercula acustica. The acusticum is the primary end-station, 

 as ie have seen, for the nerves of the ear and the lateral 

 line organs. The cerebellum has been differentiated from the 

 primary ending as a special centre for presiding over equili- 

 bration. Parallel with the increasing development of the equi- 

 librial sense in vertebrates, the cerebellum has Gradually 

 acquired associations with other than the original source, so 

 that the fibres entering the organ have ever been growing more 

 numerous, and the bulk of the fibrous centre consequently more 

 massive. 



