THE FISSURA HIPPOCAMPI 109 
tion to the tip of the temporal pole (see the stippled area in 
fig. 16). The other components of the vesicle have shifted their 
places in the sector formation, but continue to maintain their 
fundamental relationships. ‘The future hippocampus now oc- 
cupies the centro-medial area, in levels anterior to the lamina 
terminalis (figs. 29 and 30, Prim. hip.). This tissue swings 
gradually upward to those levels further caudalward (figs. 31 
and 32, Prim. hip). The ventral margin is limited throughout 
by the sulcus limitans hippocampi (figs. 29 to 32, Sul. vent.). 
The groove which outlines the future hippocampus on its medial 
surface is the fissura hippocampi (figs. 29 to 32, Fis. hip.). Not 
only is the future hippocampus distinguished by the cell-free 
outer zone, the marginal velum, but also, by the narrower matrix, 
and a greater thickness of the brain wall itself. Immediately 
opposite the sulcus limitans hippocampi lies a small group of 
cells which appear to be migrating into the marginal velum from 
the matrix. This group of cells is coincident with the extent 
of the limiting sulcus, and it is the primordial fascia dentata 
(figs. 29 to 31, Fas. den.). It lies wholly within the hippocampal 
formation above the limiting sulcus. 
The remainder of the dorsal medial wall is histologically the 
same tissue as that composing the dorso-lateral one. It is 
the neopallium (figs. 29 to 32, Neopal.) and is characterized at 
this stage of development by a dense matrix and a marginal 
velum, filled with migrating neuroblasts. In amount it is pro- 
portionally and actually greater in the rostral than in the caudal 
division of the telencephalon. 
The division of the ventro-medial sector into two areas, similar 
to those described in no. 940 (the 14-mm.), is clearly seen in 
the section immediately posterior to the level of the tuberculum 
olfactorium (fig. 31). The more dorsal slender wall is the 
septum ependymale (fig. 31, Sept. epen.), and the more ventral, 
the septum (fig. 31, Sepi.). In the latter region a minute differ- 
entiation in the form of a row of cells lying in the marginal velum 
may be identified. This is the nucleus medialis septi, which also 
appears in levels through the tuberculum olfactorium (figs. 
29 and 30, Nuc. med. sept.). This cell grouping is continuous 
