THE FOREBRAIN OF THE ALLIGATOR So! 
glomeruli but have never been observed entering into the for- 
mation of a glomerulus. They intermingle with the dendrites 
of other mitral cells and granule cells and so make one of the 
important elements of the plexiform layer. This plexiform 
layer, then, provides an additional mechanism for the increasing 
and the summating of stimuli. 
The axones of the mitral cells arise from their ventricular 
border. A short distance from the cell body, the axones of the 
larger cells divide into two main branches of approximately 
equal size. One branch enters the granule cell layer and comes 
into synaptic relations with its neurones. The other branch 
runs caudad in the tractus olfactorius, giving off, at various 
levels, numerous fine collaterals into both the granule cell and 
the plexiform layers. The first branch and fine collaterals of 
the second branch are chiefly (although not entirely) to pro- 
vide a mechanism for the summation and strengthening of stim- 
uli. The main part of the second branch provides for the con- 
duction of the impulse to the secondary centers. In some of 
the mitral cells apparently only one of the branches may be 
present. When this is the case, it is usually the one into the 
tractus olfactorius which is represented. 
Granule cell layer (figs. 138 and 25 to 28)* The inner granule 
cells occupy the inner portion of the bulb about the ventricle, 
next to its ependymal lining. Extending caudad, at about 
the beginning of the olfactory crus, this layer is replaced by the 
cells of the nucleus olfactorius anterior, although the material 
at hand has not permitted the drawing of so precise a line of 
demarcation as Johnston (’15) is able to do in Cistudo carolina. 
In teleosts, Sheldon (’12) has called the whole mass a part of 
the nucleus olfactorius anterior and then explained that certain 
neurones function as granule cells. The conditions found in 
the alligator represent an advance in differentiation over those 
described for teleosts; yet even here it does not appear that the 
granule cell layer is so physiologically distinct from the nucleus 
olfactorius anterior since, in part, its cells still serve as second- 
ary olfactory neurones. 
