THE FOREBRAIN OF THE ALLIGATOR ooo 
From the.standpoint of their types of synaptic connection ap- 
parently three functions are served by the neurones of the granule 
cell layer. ‘The first of these is that of diffusing and summating 
the incoming olfactory impulses and so strengthening the dis- 
charge into the hemispheres. This purpose is served by the 
type II cells, the stellate cells, and, in part, by the goblet cells 
(particularly those found in the anterior part of the bulb). All 
these cells receive their impulses by way of the mitral cells and 
do not send their axones into the tractus olfactorius. 
A second group of these stellate and goblet cells send their 
dendrites into the glomeruli and their axones into the tractus 
olfactorius and so, from a functional standpoint, are practically 
mitral cells. 
The third function served by neurones of the granule cell 
layer is that of acting as the cells of secondary olfactory nuclei. 
Such cells receive impulses from the mitral cells and send their 
axones into the tractus olfactorius. The goblet and stellate 
cells offer examples of this type of neurone. 
Judging from what is known of the development and spe- 
cialization of the centers of the central nervous system, it seems 
but fair to suppose that, in phylogeny, the centers of the olfac- 
tory bulb arose from undifferentiated central gray. Johnston 
(98) has shown that in Petromyzon and in Acipenser, neurones 
of this mitral cell type are found all through the central gray. 
The same author (15) has described, in Cistudo carolina, a 
granule cell layer in which are cells functioning as mitral cells. 
Certain cells of the central gray (on the whole those nearer 
the periphery) will receive a larger number of the incoming 
olfactory impulses. Under the operation of neurobiotaxis 
(Kappers, ’14) such cells will be drawn toward the periphery 
and, in this way, a mitral cell layer will be formed. Accom- 
panying such a migration toward the surface and the conse- 
quent higher specialization, there will be a differentiation in 
form and in size to meet the greater demands. 
Not all the cells left in the central gray will lose their connec- 
tion with the fila olfactoria and so certain goblet cells and prob- 
ably some of the stellate cells (although the proof for this is not 
