354 ELIZABETH CAROLINE CROSBY 
Levi (04) has described cells of the double pyramid and small 
projection types in the hippocampus of reptiles. The account 
given here agrees substantially with the descriptions and figures 
given in his article. It is interesting to note that the medial side 
of the dorso-medial cortex, as Levi suggests, appears to be con- 
cerned mainly with the reception of incoming stimuli from the 
lower brain centers, while, on the other hand, the main projec- 
tion fibers which connect the hippocampus with the pyriform 
lobe (the alveus) and with the diencephalon (fornix and the 
tractus cortico-habenularis medialis) leave on the lateral side. 
In the turtle the hippocampal cortex lies close to the ventricle. 
Presumably, in that case, many of the efferent fibers leave on the 
medial side, but so far as is known, there is no literature on that 
subject. It is certain, however, that the dorso-medial region 
of the hippocampal cortex of the alligator represents a higher 
differentiation than the corresponding region in the brain of the 
turtle and that this differentiation is marked, not only by a more 
definite cell arrangement and possibly by a more specialized cell 
form, but also by a new position of the cell mass produced by a 
biotactic movement of the cell group away from the ventricular 
wall and in the direction of the incoming impulse. 
Meyer (92) and Levi (’04) claimed that the dorso-medial 
portion of the reptilian hippocampus was gyrus dentatus. This 
contention was denied by Ramon y Cajal, who, in an elaborate 
series of histological studies, showed that from its cell types and 
manner of connection, it could not be considered pure gyrus 
dentatus. In the Arris and Gale lectures (710), Elliot Smith 
ac'mits the correctness of the Cajal observations but says that the 
dorso-medial portion is undergoing a differentiation toward the 
production of gyrus dentatus and that it is the forerunner of 
that structure. This seems a fair statement of the conditions. 
The dorsal portion of the hippocampal cortex does not show 
the regular arrangement of cell layers found in the dorso-medial 
portion, and in general, its mass of cells shows a lighter staining 
reaction to toluidin blue. Figure 31 shows one type of corre- 
lation cell found in this dorsal region. This dorsal part appears 
to be concerned chiefly with the association of impulses rather 
