108 MARION HINES 
corpus striatum and the lateral olfactory areas. The particular 
area which is of immediate interest is the dorso-medial sector, 
the future hippocampus (fig. 26, Prim. hip.). The peculiar 
histological character of this tissue, already noted in the 11.8-mm. 
embryo but accentuated here, (that is, the clear practically cell- 
free marginal velum and the slightly thinner matrix) extends from 
the region just dorsal of the future olfactory evagination, sickle- 
like, over the foramen interventriculare to the caudal tip of the 
hemisphere (see the stippled area in figs. 12 and 14). 
Passing caudally, the future hippocampus occupies a progress- 
ively more dorsal position in the telencephalic wall (figs. 27 
and 28). The amount of neopallial tissue present in the caudal 
portion of the hemispheric evagination is less than that found at 
its rostral pole. In figure 27, a section through the paraphyseal 
arch (Par. ar.), there lies between the arch and the future hippo- 
campal cortex (Prim. hip.) a curved ependymal wall with. the 
concavity directed outward. This tissue, which joins the hippo- 
campus at the suleus limitans hippocampi (Sul. vent.) and the 
lateral limb of the paraphyseal arch, is the first stage in the 
development of the sulcus of the lateral choroid plexus (figs. 
27 and 28, Sul. fu. pl. ch. ven. lat.). 
The regions differentiated as histologically distinct areas are 
as follows: The neopallium, the hippocampus, the corpus stria- 
tum, the septum ependymale, the septum, the two divisions of 
the area chorioidea, the tela chorioidea telencephali medii, and 
the paraphysis, together with their respective contiguous struc- 
tures, the area intercalata and the lateral choroid plexus. Be- 
sides these, there is the marked ventro-caudal growth of the 
hippocampus. 
The 19.1-mm. embryo, University of Chicago, H 173 
(figs. 15, 16, 29 to 32) 
The cell arrangement within the telencephalon of this embryo 
bears little semblance: to that found in the 14-mm. embryo, with 
one exception, the future hippocampus. This region of rela- 
tively cell-free marginal velum can be identified on the medial 
wall from the region of the base of the olfactory bulb evagina- 
