404 NILS HOLMGREN 
is filled up by an elongate, vertical cell-mass belonging to the 
nucleus olfactorius lateralis, which basally merges with the tu- 
berculum olfactorium cortex. The cells of this septal nucleus, 
however, assume a much paler color than the tuberculum cells. 
In the stage of 4.9 cm. the septal nucleus consists as before of a 
dense cell-mass on each side of the medial line and begins basally 
to detach from the tuberculum cortex. Between the two septal 
nuclei, the mid-part of the septum is filled up by rather scattered 
cells. Ventrally this cell-lamina spreads out between the medial 
parts of the tuberculum olfactorium cortex. Against the fora- 
men monroi where the septal nuclei are confluent below the neuro- 
poric recess the vertical cell-lamina disappears, and the basal 
part of the same is found ventral to the united basal parts of the 
septal nuclei. 
The inversion of the forebrain vesicle is not essentially more 
advanced than before. The zona limitans lateralis is tolerably 
well marked, but the zona limitans medialis is not visible at 
all. 
6. Stage of 5-cm. body length (measured after fixation in Car- 
noy’s fluid). This stage is much more advanced than indicated 
by the body length. This was undoubtedly much greater in 
the living specimens than in the preserved. I think that the 
difference may be estimated at 3 to 4 mm. in favor of the living 
specimen. 
In this stage the essentials of the pallial structure are differen- 
tiated. Therefore I will deal with it in a little more detail than 
with the preceding stages. I here will adopt the method of de- 
scribing a number of transverse sections, cut at different levels 
of the forebrain. 
A. Cross-section taken just in front of the foremost part of the 
lateral ventricle (fig. 11). The wall of the ventricle is just 
touched by the section. At this level the hemispheres are quite 
free from each other. The ventricular neuroblastic layer is 
touched a little below the middle of each hemisphere. Dorsal 
to this point is a great dense cell-mass, representing the ventricular 
cell-layer which covers the dorsal portion of the ventricle fron- 
tally. Dorsal to this cell-mass is a great cell-free space lying as a 
