110 MARION HINES 
over the angulus terminalis with those cells already identified 
as the primordial fascia dentata, though these two cell masses 
originate in different sectors and have very different morphological 
significance. The lamina epithelialis, above the foramen in- 
terventriculare, has invaginated now to form the lateral choroid 
plexus. 
The last sector, the ventro-lateral, shows plainly the be- 
einnings of the medial and lateral limbs of the caudate complex. 
The separation of the caudate nucleus from the neopallium is 
made evident by a shallow ventricular groove. The angulus 
ventralis, due in the main to the enlargement of the ventricular 
eminence of the caudate’s medial complex (figs. 30 and 31, Cor. 
str. med.), bounds the septum laterally. 
The following histological changes have taken place between 
the stage last described, the 14-mm., and the 19.1-mm.: 
1. The expansion of a mantle zone in the septal region. 
2. The acceleration of growth in the neopallium, accompanied 
by the complete incorporation of the hippocampal anlage in the 
medial wall. 
3. The appearance of the primordial fascia dentata opposite 
the sulcus limitans hippocampi. It is continuous with a similar 
group of cells in the septum, the nucleus medialis septi. 
4. The appearance of the plexus chorioideus ventriculi lateralis. 
5. The obvious indentation of the hippocampal wall, the 
fissura hippocampi. 
The 20-mm. embryo, Mall Collection, 460 (figs. 17, 33 to 37) 
The morphological and histological differentiation of the tel- 
encephalon of this embryo resembles in large part that of the 
19.1-mm.; the boundary lines of the various areas remain un- 
changed and they occupy the same relative positions in the four 
sectors. The fissura hippocampi is a broad bow-shaped bend 
of the medial wall of the hemisphere involving practically the 
entire extent of the hippocampal formation and extending from 
the base of the bulbus olfactorius to the tip of the temporal pole 
(fig. 17, stippled area). Rostral to the terminal plate, the greatest 
depth of this fissure lies in the center:of the hippocampal forma- 
