CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
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of the protoplasm are more loosely arranged than in an ordinary cell ; or the body in 
the centre might be regarded as a cell, surrounded by a mass of granules for rein- 
forcing or extending its nervous energy. 
In Mugil cephalus these bodies occupy the whole of the “ nucleus rotundus,” but in 
Crenilabrus there is a space left in the centre occupied only by granules and a network 
of fibres. 
In Crenilabrus the circumference of this body is bounded by a layer of small cells 
(fig. 22, c.), varying in length from O' 010 millim. to 0'016 millim., and in width from 
0'004 millim. to 0'005 millim. ; they are elongated in shape, and have a circular 
nucleus, which contains several granules instead of a single nucleolus. This nucleus 
often projects from one end of the cell, giving to the latter a high-shouldered appear- 
ance ; the side of the cell opposite its nucleus ends in one or two processes, and is 
always turned towards the centre of the “ nucleus rotundus.” These cells absorb 
colouring matter to a greater extent than most others. In Mugil they exist, but do 
not form a connected layer round the body in question, as is the case in Crenilabrus. 
Cerebellum. 
The cerebellum presents a structure comparable to that found in Mammalia. In a 
section (fig. 23) through this tuberosity, four layers are observable from the external 
surface to the centre. 
The first layer (fig. 23, a.) consists of straight fibres, arranged perpendicularly to the 
long axis of this division of the brain ; these fibres are embedded in a coarse granular 
neuroglia. 
The second layer (fig. 23, b.) is composed of large bipolar cells of peculiar structure, 
corresponding to the cells of Purkinje in the cerebellum of Mammalia. These form a 
stratum three or four cells in thickness throughout the cerebellum, except at the 
superior and inferior angles, where they are accumulated to a greater extent. 
The third layer (fig. 23, c.) is constituted by a mass of minute cells, and ramifications 
of fine fibrillse. 
The fourth layer is formed by a bundle of fibres which go into the anterior crus 
cerebelli. 
The bipolar cells have the usual structure of those of Purkinje (fig. 23, d.) in the 
mammalian cerebellum. They are elongated bodies, containing a large nucleus, in 
which a well-developed nucleolus is observable ; one end tapers into a fine process, 
while the other passes off more gradually to a thick one. This latter, after passing 
some distance in the same layer, either towards the anterior or posterior end, parallel 
to the long axis of the cerebellum, enters the external layer, and, dividing dichoto- 
mously, becomes finer and still more fine, as it continues at right angles to the surface, 
and passes towards the outer edge, where it eventually disappears from view. At the 
point of bifurcation of these fibres a swelling occurs which fills up the angle formed by 
