THE FISSURA HIPPOCAMPI BES 
Already in the marginal velum in embryos H 173 and 460 a thin 
layer of cells, the nucleus medialis septi, was evident. Now, 
lying between this nucleus and the matrix is a large diffuse group 
of cells, the nucleus lateralis septi (fig. 48, Nuc. lat. sept.). Here, 
also, rostral to the anterior commissure, the separation of the 
ventro-lateral from the ventro-medial sector is emphasized by a 
prolongation of the angulus ventralis (fig. 43, Ang. vent.). Its 
accentuation in this embryo is due to the growth of the medial 
nucleus of the caudate complex. The ventricular angle mark- 
ing the boundary between the dorso-lateral and the ventro- 
lateral sectors has become more acute because of changes in 
the corpus striatum, namely, a ventricular extension of the 
lateral root of the caudate complex and the appearance of the 
anlage of the lentiform nucleus (figs. 43 and 44, Nuc. lent.). 
Examining the relationships of tissue at a level passing through 
the more caudal part of the lamina terminalis (fig. 44, Lam. 
term.) we find the septum is narrow. However, three groups of 
cells may be seen, the inner matrix, a middle diffuse group, the 
nucleus lateralis septi (fig. 44, Nuc. lat. sept.), and an outer thin 
layer, the nucleus medialis septi (fig. 44, Nuc. med. sept.). Their 
development will be considered in the second study. ‘The hip- 
pocampus joins this tissue dorsally. Within the hippocampus 
the pyramidal cell layer extends past the middle of the fissura 
hippocampi, while the intermediate cell group lies in its ventral 
lip, just dorsal to the level of the sickle-shaped sulcus limitans 
hippocampi (fig. 43, Sul. vent.). Along the extreme ventral 
margin of the hippocampus extending dorsally from the region of 
the sulcus mentioned lies the fascia dentata (fig. 438, Fas. den.). 
In the space between the fascia dentata and the matrix lie the 
fornix fibers. These fibers also occupy the space between the 
medial and lateral septal nuclei. From this level, caudally, 
throughout the compass of the hippocampus, the fascia dentata 
grows dorsally along its most ventral margin (figs. 43 to 46, 
Fas. den.). 
In figure 45, a section through the paraphysis, the lateral 
choroid plexuses join the lateral limbs of the paraphyseal arch 
(Par. ar.). The small tubules lying in cross-section on either 
