382 ELIZABETH CAROLINE CROSBY 
grow up within areas primarily olfactory in type. With the en- 
trance of a larger and larger number of somatic fibers within the 
area and the corresponding increase in the number and size of 
the cells, two changes in form relations occur in the lateral hemi- 
sphere wall. One change is the pushing outward and down- 
ward of the cell masses associated with the lateral olfactory tract 
so that they come to occupy secondarily a position superficial 
to the striatal region (the area occupied in the figures by the 
pyriform lobe and the nucleus of the lateral olfactory tract). 
The other change is the bulge medialward into the ventricle of 
the dorso-lateral somatic area which is so characteristic of the 
forebrains of the turtle and alligator. 
The cortex of the pyriform lobe has arisen as a differentiation 
from the general cell mass of the forebrain which serves as a 
nucleus for the lateral olfactory tract. In the lower reptiles this 
cortex has appeared, although it is less differentiated than the 
olfactory cortex of the medial wall. Johnston (’16) has given very 
briefly some of the main features of the embryonic development 
of the lateral olfactory area and the cortex of the pyriform lobe 
in turtles. He finds the olfactory areas differentiating in the 
ventro-lateral part of the hemisphere and believes that the pyri- 
form lobe cortex arises from cells of this region which have pro- 
liferated and perhaps migrated dorsalward, so that they came to 
lie external to the dorsal ventricular ridge. He isnot certain, how- 
ever, that they have not developed in situ. So far as the writer is 
aware, the history of the embryonic development of the pyriform 
lobe cortex in the alligator is unknown, but in all probability it 
is very similar to that of the development in the turtle. Of 
course, the question at once arises as to the factors operating to 
produce the specialized pyriform cortex from the general nucleus 
of the lateral olfactory tract and the reason for its new migration ~ 
dorsalward (if that occurs as Johnston believes). In attempt- 
ing to find a solution for the question one must look for the en- 
trance into this region of fibers carrying a different type of im- 
pulse, for differentiation within an area is not dependent upon 
an increase in the number of fibers bearing the same sort of im- 
pulse but upon the introduction into the region of fibers bearing 
