THE FOREBRAIN OF THE ALLIGATOR 381 
of the latter nucleus is found on the outer surface of the ventral 
part of the lateral wall external to the striatum complex. 
What the factors were which produced these changes in form 
relation between these amphibian and reptilian brains it is quite 
impossible at present to say. The following account is offered 
as a possible suggestion of some of the ways in which these changes 
were brought about. Even in Amphibia one would expect the 
lateral part of the dorso-lateral area to be particularly closely 
tied up with the olfactory tract, for there the dorsal division of 
the lateral olfactory tract ends (Herrick, ’10, p. 523, fig. 40, tr. 
olf. d. lat.). In more highiy differentiated forms the same process 
probably occurred which is known to have happened in other 
parts of the brain, namely, that part of the cells will migrate 
outward, away from the general cell mass, in order to form a 
special receptive center for the incoming olfactory fibers (a nu- 
cleus of the lateral olfactory tract) while the remainder will come 
less directly under their influence. In Amphibia (Herrick, ’10) 
some thalamic somatic fibers reach the dorso-lateral area, al- 
though they are few in number compared with the olfactory fibers 
reaching that region. As one passes from amphibians to reptiles, 
there is a great increase in differentiation of the somatic thalamic 
regions, as has been said before, and this differentiation is ac- 
companied by an increase in the number of somatic fibers sent 
forward into the hemisphere by way of the lateral forebrain 
bundle. Some of the somatic fibers, passing dorsalward of the 
old limits of the striatum come into synaptic relation with the 
neurones corresponding with the old amphibian dorso-lateral 
area. Part of such fibers will form synapses with olfactory fibers 
and so a somatic-olfactory center, whose later representatives 
are the amygdaloid complex and the pyriform lobe cortex, will 
be formed. Others of these somatic fibers come into synaptic 
relations with the more medially placed neurones of this dorso- 
lateral area (i.e., those neurones less directly under the influence 
of the olfactory fibers). The entrance of this new mass of somat- 
ic fibers and the resultant somatic correlation will lead to an 
increase in both the cell number and cell dfferentiation and in 
this way a non-olfactory somatic correlation center can well 
