122 JOURNAL OF CoMPARATIVE NEUROLOGY. 
tebrates, 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. 
7. Summary of the Cerebellum. 
The cerebellum of Mustelus is of relatively large size, and, 
although remaining in the primitively hollow condition, its wall 
has the same essential plan of structure as the organ in higher 
vertebrates. 
Neurones of PurKINJE form a zone, crowded at certain 
points, between the molecular and granular layers. Such a neu- 
rone has the structural features characteristic of its mammalian 
representative, but with a simpler branching of dendrites. Its 
office is to receive incoming equilibratory impressions. 
The neurones of the molecular layer are few in number. 
They are all of one variety. 
The neurones of the granular layer are of two kinds: Goer 
neurones and granular neurones. The former lie in the upper 
strata of the layer. The granular neurones are strikingly like 
those of higher vertebrates; their axis-cylinders branch ina 
T-shaped manner in the molecular layer, mediating between 
many incoming fibres and the dendrites of the neurones of 
PURKINJE. 
Ependyma is developed into a profusely branching fibre 
which extends through the granular layer, only. 
Neuroglia provides a supporting framework for the neu- 
rones of PuRKINJE and the molecular layer. Both astrocytes 
and BERGMANN’s fibres may be recognized, but there are tran- 
sitional forms indicating that the latter have been derived from 
the former. 
The large size of both the ear and the cerebellum of Mus- 
telus is to be explained by the swimming habits of the animal. 
The structures have an equilibratory value. 
The cerebellum has apparently arisen in the phylogeny of 
vertebrates as a fused outgrowth of the pair of tubercula acus- 
tica, a specialization of that part of the oblongata forming the 
original terminal station for the acustico-lateral system. 
