EARLY DEVELOPMENT OF HEMISPHERES 289 
ventral portion of the anterior commissure. This is the first 
evidence of the medial forebrain bundle. 
From this point forward the pars lateralis of the iy arenes 
shows little change except in the increase in size, from the fun- 
damental pattern already laid down (figs. 23 and 24). The pars 
dorso-medialis, however, shows at stage 42 (8 mm.) the first signs 
of differentiation into the primordium hippocampi. The single 
layer of ependymal cells lining the contiguous portion of the 
ventricle has, through proliferation, been converted into a rather 
thin, but nevertheless distinctly stratified layer (fig. 26). The 
ependymal cells constitute a clearly defined inner layer, peripheral 
to which lies a layer several cells thick more loosely packed than 
the inner layer. Among the outer loosely packed cells, in the 
caudal region may be seen the fibers of the fimbria complex form- 
ing dorsally a clearly defined lamina separating the telencephalon 
from the diencephalon. The stria medullaris complex cannot be 
distinguished at this stage. . 
The pars ventro-medialis shows the thickening anteriorly of 
the septum ependymale. An increase in the number of mitotic 
figures in this region indicates an area of rapid proliferation. 
Concomitant with this growth, there is an invasion of the sep- 
tum by fibers of the tractus olfactorius ventro-medialis. The 
enlargement thus formed constitutes the nucleus medialis septi. 
It seems doubtful, from the material at hand, that this nucleus 
develops as a result of the invasion of this region by cells from 
below and in front, as suggested by Herrick (’10). Rather, it 
would appear that the growth into the septum ependymale of 
the tractus olfactorius ventromedialis initiates a period of rapid 
proliferation of the neuroblasts there formed, resulting in the 
development in situ of the nucleus. The subsequent entrance of 
association fibers via the columna fornicis from the primordium 
hippocampi is concomitant with its later increase in size. 
The first signs of the future lateral choroid plexuses appear 
at this time as lateral evaginations of the medial wall of the 
hemisphere where the septum ependymale becomes continuous 
with the ventral leaf of the velum transversum (fig. 24). 
