THE FISSURA HIPPOCAMPI 85 
The ventricular communication between the diencephalon and 
the telencephalon has suffered a tremendous change in its con- 
tour. Now, for the first time, the future relationships are deter- 
mined. The dorsal and terminal boundaries of the foramen 
interventriculare are no longer an arc of a circle, although they 
are not as yet roof and terminus meeting each other in an angle 
as described for H173. Instead of the smooth ventricular di- 
telencephalic union, that junction is marked by a slight eminence, 
which is continued forward into the floor of the cerebral hemis- 
phere, the corpus striatum (fig. 14, Corp. str.). 
The point of entrance of the fila olfactoria (fig. 14, Fil. olf.) 
into the cerebral hemisphere enables us to locate the region of 
the future olfactory bulb evagination. 
The telencephalic vesicle itself extends far beyond the midline, 
anteriorly and dorsally. Consequently, the great longitudinal 
fissure now divides the telencephalon into two parts, the cere- 
bral hemispheres, with a small residual telencephalon medium 
between. 
In the 14-mm. embryo the differentiation of the telencephalon 
medium and adjacent parts of the cerebral hemisphere has 
advanced to the point where most of the morphologically sig- 
nificant regions can be delineated. In the following paragraphs 
these will be described and defined. 
The telencephalon medium lying between the velum trans- 
versum and the preoptic recess (fig. 14) may be divided into 
dorsal and ventral moieties, the area chorioidea (A. ch.) and the 
lamina terminalis (Lam. term.), each of which is again sub- 
divided into two parts. The lamina terminalis ventrally is 
thick and massive (pars crassa, p. c.) and dorsally is a thin 
epithelium (pars tenuis, p. t.). In the course of development 
the massive part enlarges dorsalward at the expense of the thin 
part. The area chorioidea consists of two morphologically dis- 
tinct portions, the tela chorioidea telencephali medii (Tel. ch. 
tel. med.) anteriorly and the paraphyseal arch (Par. ar.) pos- 
teriorly. In this brain, it is impossible to draw the dividing line 
between the tela chorioidea telencephali medii and the lamina 
