THE FOREBRAIN OF THE ALLIGATOR 379 
pointed out (see discussion of hippocampus), there is a clearly 
defined arrangement of a considerable part of the hippocampal 
formation into definite cortex-like layers, these layers have not 
moved out from the ventricle as in higher forms, but still form 
a ventricular mass. 
The hippocampal cortex of the alligator represents another 
step in advance in differentiation, for here the cortex has moved 
away from the ventricle and accompanying this differentiation 
has been the specialization, at least to a considerable extent, 
of its medial aspect to serve as the afferent side of the cortex and 
its lateral aspect to serve as the efferent side. One of the causes 
at least, for the outward migration of cells of the dorso-medial 
area to form the hippocampal cortex is probably to be found in 
the operation of the law of neurobiotaxis (Kappers, 714). Ac- 
cording to this law, cell bodies tend to migrate along their den- 
drites toward their. source of stimulation. The medial olfac- 
tory tracts and other tracts bearing afferent impulses to the 
hippocampus are on the medial surface of the hemisphere and 
the cells of the developing cortical layers move out toward the 
surface of the hemisphere in order that they may come into 
eloser relationship with the incoming impulses. 
To recapitulate, the following steps appear to have lead from 
the primordial hippocampal type to the relatively simple type 
of cortex found in part of the hippocampal area in the alligator. 
In Amphibia (Herrick, ’10) the afferent and efferent fibers 
spread out all through the dorso-medial area. Following a 
higher differentiation of the diencephalic and telencephalic sub- 
cortical correlation centers, there is a higher differentiation in 
the dorso-medial area so that the arrangement of the cells into 
cortex-like layers, such as we find in the turtle, occurs. This 
second step is followed in other reptiles by a further specializa- 
tion of a part of the hippocampal cortex, so that it has an afferent 
medial side and an efferent lateral one. 
The non-olfactory diencephalic fibers, which enter the telen- 
cephalon for the purpose of forming correlations with the incom- 
ing olfactory impulses, are partly visceral and partly somatic 
in type. Those ascending from the hypothalamus by way of 
