THE FISSURA HIPPOCAMPI 105 
The pars tenuis of the lamina terminalis and the septum ependy- 
male (fig. 23, Sept. epen.). 
Region of the paraphyseal arch (figs. 24 and 25h ebsi ne gla en 
Velum transversum (figs. 7 and 8, Vel. trans.). 
In the region of the bed of the optic chiasma (fig. 21, F’. b. op. 
ch.) the evaginating vesicles form two arcs of a circle, their in- 
tersection being marked by a small depression in the midline. 
Following this are of the telencephalic brain wall around to 
its point of union with the diencephalon, it appears fairly uniform 
in histological structure. In figure 22 the vault of the brain 
wall is more extensive. Within its outer margin a barely discer- 
nible clear zone appears, the Randschicht of His, or the marginal 
velum (Man. 1). This zone is more clearly defined a few milli- 
meters (measured on the figure itself) lateral of the midline. The 
wall at this point shows a very slight local thickening. In figure 
23 the lateral expanse of the vesicle is greater and the marginal 
velum more sharply defined. The union of the two vesicles 1s 
made by a thin epithelial plate of cells. In this figure, approxi- 
mately 6 mm. on either side of this thin line, no mantle zone can 
be identified in the brain wall (figs. 9 and 11, Sept. epen.). But 
beyond this region, for 10 mm. on either side, the marginal velum 
reaches its maximum definition. 
Figure 24 is a section through the caudal portion of the tel- 
encephalic roof plate, at the level of what appears to be the 
paraphyseal arch (figs. 8 and 24, Tel. r. pl.). This small arch in 
the midline is one feature of a remarkable histological differen- 
tiation in the telencephalon. Joining the lateral limbs of this 
arch is a slender, homogeneous epithelial tissue. Immediately 
lateral to this tissue is an area in which the marginal velum is 
clearer and wider than any other portion of the vesicle. Again 
lateral to this area is a region of the brain wall which forms the 
telencephalic evagination (Tel. evag.) or incipient cerebral hemis- 
phere, extending backward to the point of junction with the 
thalamus at the di-telencephalic groove. Upon the ventricular 
surface the first and second areas are separated by a shallow 
sulcus, which will be referred to as the sulcus limitans hippocampi 
(fig. 24, Sul. vent.). 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, No. 1 
