120 J. B. JOHNSTON, 
make up the great bulk of the whole saccus wall (Phot. 18). The 
blood spaces are relatively much larger in the brain of older fish. 
The blood corpuscles are closely packed into these spaces, as they 
are in most of the venous spaces of the brain. The corpuscles take 
a green stain in methylene blue and acid fuchsin preparations, so 
that the saccus is sharply distinguished from the hypophysis in which 
the spaces are small and the corpuscles inconspicuous. The basement 
membrane is strong but of varying thickness. It takes a red stain 
in common with the con- 
nective tissue. Within 
the basement membrane 
is an incomplete layer of 
nerve fibres running be- 
tween the bases of the 
epithelial cells. The 
Hap ere course of these fibres 
7 will be described below. 
In the walls of the saccus they form bundles which are sometimes 
seen cut across, sometimes running parallel with the plane of section. 
These bundles vary in size and give off fibres to all parts of the 
saccus epithelium. 
The epithelium lining the saccus extends forward also on the 
floor of the corpus mammillare for the greater part of its length. 
Compare Phots. 19, 20 and 21 with Phot. 3. The epithelium con- 
sists of closely set columnar cells of two kinds, supporting cells and 
ciliated cells (Fig. L). The supporting cells are very slender and 
are so few in number in some parts of the saccus as to be made 
out with difficulty. They have small nuclei which are crowded either 
near to the surface or to the basement membrane. The cells are 
so slender as to appear to be little more than fine fibres, they are 
stained blue and always reach the basement membrane. In parts 
of the wall where thick nerve bundles are present these slender 
supporting cells are very clearly seen as blue threads traversing the 
layer of red-stained nerve fibres. In such places the ciliated cells 
do not penetrate the nerve fibre layer, athough elsewhere they usually 
reach or approach close to the basement membrane. The ciliated 
cells are many times thicker than the supporting cells, being 8—10 u 
in thickness and 12—24 u in height. The cells are shorter in the 
smaller sacs or pouches than in the main cavity of the saccus and 
are short wherever they rest upon a nerve bundle. They have large 
