126 MARION HINES 
will enable the growth of the medial hemisphere wall to be in- 
terpreted more wisely. It may also give new data upon the de- 
velopment of the new parts of the cortex lying on the medial 
wall of the hemisphere. 
Area epithelialis 
The area epithelialis lies between the telencephalon medium 
and the sulcus limitans hippocampi. In the 11.8-mm. embryo 
this tissue is almost neglible in the rostral division of the hemi- 
sphere, but it becomes markedly greater in the caudal portions 
of that evagination. This difference is one of fundamental 
importance (figs. 1 to 6, sketches 2 and 3). Along the dorsal 
margin of the area epithelialis on the opposite side of the sulcus 
limitans hippocampi, the fornix is developed. This sulcus is 
regarded as the ventral limit of the cerebral cortex and the epithe- 
lial tissue is the derivative of the most dorso-medial portion of 
the roof of the primitive evaginated telencephalon. In such a 
primitive condition, the fornix formed a fringe along its lateral 
border and the anlage of the hippocampus lay laterally of it 
(i.e., morphologically ventral to it). This condition is almost 
exactly realized in the brain of Ichthyomyzon concolor (Herrick 
and Obenchain, 713). In the process of evagination the relations 
of the area epithelialis to the fornix and the hippocampus are 
reversed; that is, the two latter areas are turned first outward, 
then upward, so that in the medial wall of the hemisphere they 
come to lie dorsally of the area epithelialis. This seems to be 
a true statement of the case, because in the 11.8-mm. brain the 
sulcus limitans hippocampi is coextensive with the epithelial 
Fig.9 This is the anterior view of the model shown in figures 7 and 8, no. 1121 
of the Mall Collection. X 163. There is a slight depression in the midline 
separating the initial evagination into two halves. 
Fig. 10 This is the anterior view of the same model as that shown in figures 
13 and 14. This 14-mm. embryo belongs to the Mall Collection in Baltimore, 
no.940. X 16%. 
Fig.11 Thisisapen-and-ink sketch of the same view as that shown in figure 9, 
no. 1121. X 162. The areas indicated in figure 8 are shown in their dorsal 
extent. The planes of section of figures 21 to 25 are indicated. 
Fig.12 Anterior view of no. 940, same embryo as figure 10. X 163. 
