THE FISSURA HIPPOCAMPI - 89 
sulcus, separate the thalamic wall into three parts. The dis- 
tance between the pronounced dorsal sulcus and thesulcuslimitans 
has increased both absolutely and relatively, so that this middle 
portion of the thalamus is becoming the most extensive of the 
three divisions. The anterior extension of the sulcus limitans 
cannot be traced to the neighborhood of the optic evagination. 
A new ventricular suleus has made its appearance. It lies 
between the medial limb of the corpus striatum and hypo- 
thalamus, arising in the recessus preopticus (fig. 16, Rec. preop.) 
and loosing itself in the floor of the foramen interventriculare 
(fig. 16, For. int.). Immediately dorso-caudal to this sulcus 
lies the midregion of the thalamus, which now forms the posterior 
boundary of the foramen (fig. 15). The dorsal boundary of this 
midthalamic division can be followed rostrally to the region of 
the velum transversum. 
However, in the younger stage, the 14-mm. embryo (fig. 13), 
the posterior boundary of the foramen is here formed by the 
velum transversum, whose diencephalic limb is continuous with 
the epithalamus, and its floor is formed by the corpus striatum. 
In the epithalamus the epiphyseal evagination is more con- 
stricted. The roof of the whole is a little thicker. In the 
hypothalamus the infundibular recess is wide and the posterior 
lobe is not constricted. The floor and the walls have increased 
in thickness. In the midline the bed for the optic chiasma has 
increased in volume by growth, not only antero-posteriorly, 
but also dorso-ventrally. This region is separated from the 
massive portion (fig. 16, P. c.) of the lamina terminalis by a 
deep groove, the recess preopticus (fig. 16, Rec. preop.). 
In the picture of this model (fig. 16) the most striking aspect 
of the telencephalon medium lying between this recess and the 
velum transversum is the sharp angle in the midline. This 
angle, the angulus terminalis (fig. 16, Ang. term.), was commented 
upon in the description of the 14-mm. embryo. Here the anterior 
boundary of the telencephalic midline suddenly changes its direc- 
tion and becomes the vaulted roof of the foramen interventric- 
ulare. Is this angle a landmark? Can it be used as a fixed point 
from which to measure change? Further, can it be used as a 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 
