THE FISSURA HIPPOCAMPI 1S 
The olfactory bulb is in the process of actual detachment, 
so that its separation from the ventral sectors in the rostral pole 
has produced the fissura prima in the medial wall. Differentia- 
tion characteristic of the neopalltum appears in the dorsal lip 
of this fissure. There is in the other embryos, nos. H 173 (19 
mm.) and 460 (20 mm.) a small area dorso-frontal to the region 
where the fila olfactoris enter, which does not show the differ- 
entiation characteristic of the hippocampal anlage. 
The septum ependymale is divided into two layers, an outer 
clear zone and an inner matrix (figs. 38 and 39, Sept. epen.). 
This same tectonic arrangement is characteristic also of that 
tissue which adjoins the midvault of the foramen interventric- 
ulare. The change from a simple epithelium to the two cell 
layers described seems to be the first step in the process of differ- 
entiation in the lamina terminalis, a process which proceeds 
caudally. 
The progress of growth has added the following: 
1. Differentiation of the neopallium into three cell layers. 
2. Appearance of the intermediate cell layer in the dorsal 
part of the primordium hippocampi, a process whose develop- 
ment is more marked rostral to the terminal plate. 
3. The septum ependymale contains two cell layers through- 
out, thus differentiating it from the static area intercalata. 
The 32.1-mm. embryo, University of Chicago, H 41 
(figs. 40 and 41) 
At the level of the root of the olfactory bulb (fig. 40) the 
cortical differentiation extends ventrally as far as the fissura 
prima. ‘This groove with its dorsal differentiation continues for 
a short distance upon the rostral end of the medial wall. Caudal 
to the root of the olfactory bulb there is an area of unmistakably 
hippocampal formation. Here the intermediate cell layer has 
crept out of the matrix below the level of the fissura hippocampi 
and the pyramidal cell layer shows itself in the dorsal lip of the 
fissure. This migration is characteristic of the preterminal hip- 
pocampal formation. That part of the future hippocampus 
