THE FISSURA HIPPOCAMPI 1a} 
This last is essentially the reptilian condition, and, since some 
of these fornix fibers in both cases enter the alveus in the adult, 
it follows that the reptilian dorso-medial cortex is comparable 
with the human hippocampal cortex rather than with the fascia 
dentata, as supposed by Meyer (’92) and Levi (94). More- 
over, the embryological evidence presented by these embryos 
is decisive in favor of the same conclusion. The fascia dentata 
arises from cells of the matrix immediately dorsal to the sulcus 
limitans hippocampi, from which position its cells migrate dor- 
salward along the outer margin of the hippocampal formation. 
This mode of origin corresponds in all major particulars with 
Sketch B. x 352. No. 460, 20 mm. (section 15-1-2), Carnegie Institution, 
Baltimore. The histological characters of the hippocampus are the same as 
those described for the 16-mm. embryo. Thearea itself is greater dorso-ventrally 
and the fissura hippocampi, although shallow, inciudes a greater sweep of tissue. 
The lamina epithelialis is thinner, composed of only a few rowsof epithelial cells. 
Opposite the sulcus limitans hippocampi within the marginal velum lie a group 
of cells. the undifferentiated fascia dentata. 
SketchC. X78. No.H163,39.1 mm. (section 147-14), University of Chicago 
Collection. The medial wall in the region of the hippocampal primordium 
bulges prominently into the ventricle, its ventricular convexity being greater 
than its medial concavity. Within the dorsal lip of the fissura hippocampi 
three cortical lamina are present: 1) the inner, or matrix; 2) the middle, or the 
intermediate cell layer, and, 3) the outer or row of young pyramids. The inter- 
mediate cell layer, like the matrix, is never as wide in the center or ventral lip 
of the fissure as it is in the dorsal lip or in the neopallium. The lamina epi- 
thelialis ismore convoluted and presents at this level a concave surface outward, 
an indication, the writer believes, of the sulcus fimbrio-dentatus. The fascia 
dentata has migrated dorsally in the marginal velum of the hippocampus. The 
locus from which these cells come shows no cortical lamination, although between 
the fascia dentata and the matrix a few undifferentiated cells remain. 
Sketch D. xX 10%. No. 1400-23, 85 mm. (section 34-14). A brain belonging 
to the private collection of Dr. George L. Streeter, studied at the Carnegie 
Laboratory of Embryology, Baltimore. The hippocampal formation lies in the 
depth of the fissure. The fascia dentata seems caught in a characteristic twirl. 
Ventral to the fascia dentata lies a small sulcus, the sulcus fimbrio-dentatus. 
The fornix fibers are intermingled with undifferentiated cells, the persistent 
primordium hippocampi. It takes no leap of fancy to bridge the growth process 
from this stage to the adult. 
Note. In the untouched photographs from which figures 29-31, 33-36, 38, 39 
and 41 were reproduced the histological differentiation of the facsia dentata was 
clearly visible, as indicated in the pen drawings, figures 42-45, 47-51. This 
detail in some of the photographs is lost in the reproduction. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 34, NO. 1 
