Houser, Zhe Neurones of a Selachian. 115 
neurones which characterize this layer in higher vertebrates. 
The morphology is somewhat more simple in the selachian, as 
should be expected, but it is a suggestive fact that the struc- 
tural plan of higher forms is here outlined in its essential 
features. 
4. Supporting Elements. 
The supporting elements of the cerebellum are referable 
to both the ependymal and the neurogliar series, the former 
being limited to the granular layer, and the latter to the molec- 
ular layer. The structure in each instance appears to be partic- 
ularly adapted to the supporting of the nervous mechanisms 
peculiar to these regions. 
a. The Ependyma.—The ependyma of the cerebeJlum pre- 
sents the usual palisade of closely crowded cells forming the 
membrana limitans interna (Fig. 18). The cell-body is irregu- 
larly pyramidal in form, the sides rarely tapering evenly to the . 
apex but exhibiting more or less bold curvatures of outline. 
From the apex of the cell-body, a process, the ependymal 
fibre, arises, and this pursues a course through the structures 
of the granular layer to near the outer limit of that zone. I 
have not found a single instance where one of these fibres 
passed beyond the granular into the molecular layer, a fact of 
some possible phylogenetic value, indicating the more ancient 
character of the internal region. 
The ependyma-fibre is relatively stout, but its diameter is 
far from uniform. There are fibres which have portions of the 
length four or five times the thickness of the intervening parts ; 
see Fig. 18, 6. Occasionally, knobs and other rounded thick- 
enings are found, particularly at angles where the course of the 
fibre changes abruptly. The trend of the fibre is never con- 
spicuously irregular, only such slight turns and windings occur- 
ring as might be expected where obstructions are present dur- 
ing the period of growth. The general trend is directly toward 
the surface of the cerebellar fold, the several fibres lying more 
or less nearly parallel with each other. 
The degree of branching exhibits wide diversity. Certain 
