146 J. B. JOHNSTON, 
fibres which is not to be reckoned with the olfactory lobe proper, 
but belongs to the olfactory nerve. This zone is thickest at the 
cephalic end and is much thicker on the lateral than on the other 
surfaces. Many more fibres enter the lateral parts of the lobe and, 
as will be seen below, the glomeruli are larger and more numerous 
in that part of the lobe. The zone of olfactory glomeruli occupies 
the external one-third of the olfactory lobe proper. The inner two- 
thirds of the lobe comprises a zone of cells and fibres in which no 
layers or special collections of either cells or fibres are to be seen. 
To this zone I have given the name of granular zone, since it 
contains the cells known as granule cells. These zones have no 
special significance in Acipenser, since both contain cells of similar 
functions, but the recognition of the zones is convenient for topo- 
graphical purposes. 
a) Cells of Olfactory Lobe (Fig. O). 
1) Large mitral cells. — These are the largest and most 
characteristic cells in the olfactory lobe. They are found in the 
glomerular zone and are much more numerous in the lateral part 
of the lobe. The cell bodies are the largest (actual volume) in the 
brain of Acipenser. They measure 16—48 by 52—240 u, and are 
usually much more compact than the PURKINJE cells, which alone 
approach them in size. The cell body is very variable in form, 
although it does not present such great irregularites as do the 
PURKINJE cells. The cells bear two or more very large dendrites 
which spread widely in the glomerular zone. They are frequently 
much curved and usually divide into several branches. Sometimes 
there is a main stem which runs parallel with the surface and gives 
rise to several short, thick branches which end in glomeruli. The 
number of large dendrites is sometimes as great as five and I have 
seen as many as four large glomeruli borne by the branches of a 
single dendrite. It is probable that a single cell sometimes supplies 
as many as ten or twelve large glomeruli. The usual number sup- _ 
plied by the medium-sized and smaller cells is from three to five. 
The dendrites break up into an extremely complicated set of 
branches which help to form a glomerulus (see description of glome- 
ruli below). The large mitral cells bear other, small dendrites which 
end freely in the glomerular zone without forming glomeruli. I have 
found these non-glomerular dendrites in only a few cases and believe 
that they occur on only a part of the cells. The neurite arises from 
