FOREBRAIN MORPHOLOGY 405 
cap on it, and dorsal to this space is seen a dense regular cortical 
layer (prim.c.). This is thicker in its lateral part than in its 
medial. The cortical layer represents the primordial cortex, the 
lateral part of which belongs to the pyriform lobe, the medial to 
the hippocampal rudiment. 
Below the ventricle is a clear space crossing over the ventral 
part of the brain and delimiting the nucleus olfactorius lateralis 
and the tuberculum olfactorium cortex which lie below it. This 
space, poor in cells, medially bends up dorsad to the medial 
border of the hippocampal rudiment. The dorsal part of this 
clear space in the future development is destined to become the 
zona limitans medialis (z.l.m.). The lateral part of the space in 
question also bends dorsad to the lateral border of the pyriform 
lobe and forms the zona limitans lateralis (z.1.1.). Thus at the 
front pole of the forebrain vesicle the zona limitans medialis and 
lateralis unite. The above-named cell-free space is present always 
from the 3.3-cm. stage, but now for the first time it is possible to 
see the future fate of it. 
B. Cross-section taken through the foremost part of the septum 
(fig. 12). The lateral ventricle is cut in its rostral part, the 
ventral as well as the dorsal part of the ventricle being opened 
by the section. The cross-section of the ventricle is more or less 
8-shaped. The cell-free space below the ventricle has disappeared 
but the medial and the lateral parts of it are seen as zonae limi- 
tantes, of which the medial (z.l.m.) is less conspicuous than the 
lateral (z.1.1.). The tuberculum olfactorium cortex covers the 
whole ventral surface of the section, and is continuous with the 
septal nucleus mentioned in preceding stages. 
In the pallial part of the forebrain the pyriform lobe (p.c.) 
occupies the lateral part and consists of a lateral rounded cell- 
mass, that sends a cortical lamina in the medial direction paral- 
lel to the dorsal surface of the brain vesicle. This cortical lamina 
is medially continued by a thin sheet of scattered cells (sc.c.) 
which medially merges with the hippocampal rudiment (h.c.). 
The lamella of scattered cells representing the outer layer of the 
general pallium is the backward extension of the cortical part 
connecting in the preceding sections the pyriform lobe with 
