CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
749 
In Acanthias, cells which have a decidedly fusiform shape are rare, the greater 
number being oval or rhomboidal. They occur in or on the radial fibres as in 
Scyllium. Most of the cells, however, are spherical, and are arranged in longitudinal 
groups, separated by intervals of granular neuroglia, in which are found fibres from 
the optic tract running from before backward ; but in the second layer they are 
divided by bundles of fibres belonging to the transverse commissure. Occasionally, 
but very rarely, a fusiform cell is found in the course of a longitudinal fibre, The 
fusiform cells are generally 20/r long by 6/x broad, from this they vary to 12p. by 
7/jl ; they are thus larger than the corresponding cells in Scyllium. The oval or 
round cells are usually about 8/x by 7p.. There is a gradation between the extremely 
elongated cells, and those which are nearly spherical. 
In Rhina (fig. 22) the fusiform cells are more numerous, they are also larger than 
in Acanthias. The rounded cells are also very numerous ■ as in the other species they 
give off processes to the surrounding network. 
The greater scarcity of cells in the optic lobe of the Rays is compensated for by its 
greater thickness. The fusiform cells are very scarce, most of those present being 
rounded, but some oval ones occur. They are on an average smaller than those which 
are found in Acanthias, and about equal in size to those present in the optic lobe of 
Scyllium. 
The second layer consists principally of bundles of transverse fibres, derived partly 
from the commissura ansulata, and to a small extent from the lateral columns of the 
medulla oblongata; these transverse fibres form many bundles, which are separated 
from each other by granular neuroglia, into which the rounded cells from the first 
layer penetrate; so that these two layers may be looked upon as overlapping each 
other, the cells of the first layer encroaching on to the territory of the second ; thus 
the limits of the two divisions are not strictly defined. In this stratum there occur 
also cells intermediate in character between the fusiform and the spherical cells. 
The third layer lies immediately internal to the endothelial lining of the ventricle; 
in it are found at the posterior end only some fibres which come forward from the 
cerebellum ; they do not extend far, and are not visible at the anterior end of the lpbe. 
The small cells from the first and second layers also penetrate into this one, and occur 
quite close to the endothelium at parts. In addition to these elements, large cells 
(fig. 22, Dachfyerne, R,ohon) occur, which are nearly equal in size to the motor cells 
of the ventral horns in the spinal cord, but they differ in shape and in the number of 
processes, and also probably in composition, since they much more easily become 
crumpled and lose shape. They are spherical or oval, and sometimes pear-shaped. 
They are provided with a conspicuous nucleus and nucleolus ; the cell contents are 
usually smooth, or only finely granular, the nucleus has a coarser appearance, and 
contains large granules, and the nucleolus is seen as a large granule with a dark 
border and lighter centre. They generally give off one process only, or very rarely 
