338 ELIZABETH CAROLINE CROSBY 
as follows. The lateral septal nucleus (Herrick ’10) equals 
the lateral parolfactory nucleus (Johnston ’13 and ’15) plus a 
part of the posterior portion of the primordium hippocampi 
(Johnston 713 and ’15) minus the nucleus accumbens (Herrick 
eb) 
Having had no opportunity to study reptilian embryological 
material, the writer is in no position to decide which nomen- 
clature is the better of the two. A most thorough study of the 
nuclei in the developing brain will be necessary before any- 
thing definite along that line can be determined. For con- 
venience the terminology of Johnston has been adopted except 
that the name nucleus accumbens has been retained. 
The essential point is that in the alligator, in the region of the 
fornix fibers and other descending hippocampal systems, there 
is a cell mass which serves as a place of synapse for many of the 
descending fibers. This cell group has been identified in a num- 
ber of reptiles besides Alligator mississippiensis. Further- 
more, so far as is now known, there are two possible sources of 
origin for this cell mass. The first theory is that it is a special- 
ization of a portion of the primordium hippocampi as a place of 
synapse between the hippocampus and the basal centers. The 
second theory implies that cells, situated in the basal region 
and serving as places of synapse for descending fibers, moved 
upward toward their source of stimulation according to the 
principle of neurobiotaxis (Kappers, 714) and invaded the re- 
gion of the primordiun hippocampi. 
Tuberculum olfactorium (figs. 5, 6, 7, 15). This nucleus be- 
gins in the hemisphere a short distance behind the olfactory 
crus. It is ventro-medial in position and is continuous anteri- 
orly with the nucleus olfactorius anterior, from which it can be 
distinguished by the cortex-like arrangement of its outer layer 
of cells. Its inner portion is made up of small groups of cells 
which show, though not so clearly as in some forms, the arrange- 
ment into islands so characteristic of the highly developed 
tuberculum olfactorium. The cortical and non-cortical layers 
of the tuberculum olfactorium are shown in the drawings of the 
toluidin blue sections from this region (figs. 5 to 7). 
