THE FISSURA HIPPOCAMPI Si 
Fascia dentata 
In the medial margin of the hemisphere wall in the 19.1-mm. 
embryo opposite the sulcus imitans hippocampi lies a group of 
cells, the fascia dentata. This group of cells does not appear 
in the younger embryos, but persists in the rest of the series as 
a mass of cells, which seem to have migrated out of the matrix 
lying opposite the dorsal limit of the suleus. This differentiation 
begins anteriorly and passes posteriorly, following in develop- 
ment the initial differentiation of the future hippocampal region 
into the outer marginal velum and inner matrix layers. These 
cells slip along the marginal velum of the developing hippocampus. 
In the 39.1-mm. embryo and the 43-mm. they form a slender 
band almost coextensive on the medial wall ventro-caudally 
with the development of the hippocampal cortex. The rostral 
limit of the fascia dentata lies in close proximity to the rostral 
limit of the sulcus limitans hippocampi. The four sketches, 
in figure 51, present comparable levels through the region under 
discussion from four different embryos. A, a 16-mm. embryo, 
shows no differentiated fascia dentata, but in B, a 20-mm., 
a group of differentiated cells opposite the sulcus limitans hippo- 
campi may be seen. In C, a 39.1-mm., these cells have slipped 
along the marginal velum of the hippocampus; while in D, an 
85-mm.. they show the characteristic crescentic line-up. In the 
levels delimited only that of the 85-mm. demonstrates the re- 
lation of the sulcus fimbrio-dentatus to the growing fascia dentata. 
Here the fornix fibers lie ventral to the dentate band, among 
undifferentiated cells of the primordium hippocampi, separated 
from that gyrus by the sulcus fimbrio-dentatus. 
Figures 47 to 50 These drawings were made from a coronal series of the 
forebrain of a 43-mm. human embryo, no. 886, of the Mall Collection. Refer to 
figures 19 and 20 for the medial sagittal section of the model and the planes of 
these sections. X 62. 
_ Fig. 47 Through the hippocampus, showing that tissue in both its caudal 
and rostral aspects. 
Fig. 48 A section through the upper border of the thalamus, the septum 
ependymale, and the anterior and the posterior divisions of the hippocampus. 
Fig.49 Through the septal region and the posterior part of the hippocampus. 
Note the cellular groups in the septum. 
Fig. 50 Through the caudal hippocampus, the middle of the thalamus and 
the olfactory bulb. 
