158 MARION HINES 
separates cortical from subcortical regions; but throughout the 
regions bordering the post-terminal area epithelialis it separates 
the thin area intercalata and (in the adult) the choroid fissure 
from the hippocampal formation. In the series presented no 
cortical areas are found ventral to it, nor are there any cells which 
can be proved to be neuroblasts ventral to it. But in figure 51, 
sketch D, it seems impossible to determine just where the bound- 
ary between the primordium hippocampi and the lamina epi- 
thelialis should be drawn. In the sketches it has been assumed 
to lie as indicated, below the area containing undifferentiated 
cells of the primordium hippocampi. On this interpretation 
that portion of the fimbria marked Fornix (fig. 51, D) is a segment 
of the lamina epithelialis which has been secondarily thickened 
by the invasion of fornix fibers; but possibly, it should be regarded 
as belonging within the primordium hippocampi, a possibility 
which cannot be disregarded until the methods of neurological 
technique have demonstrated otherwise. Doctor Herrick sug- 
gests to me that a close series of developmental stages of this 
region in reptiles or lower mammals would probably be favorable 
material for the solution of this question. 
Fissura hi ppocampr 
Having established the homology of the fissura hippocampi 
with the reptilian fissura arcuata of Herrick, we are desirous 
of clearing the situation as it exists in the history of the embry- 
ology of this region. As development proceeds, the primitive 
hippocampus presents a smooth contour from its earliest defini- 
tion (9 mm.) to 16 mm. From that length to approximately 
24 mm. the fissure extends into the medial wall from the base 
of the olfactory evagination to the end of the hippocampal 
primordium as a shallow groove involving the whole of this 
peculiarly differentiated area. For approximately the next 10 
mm. of growth in greatest length, the fissure grows progressively 
shallow in the region above the area chorioidea, so that from the 
surface it appears to be divided into two segments. However, 
there is no interruption of the hippocampal formation itself. 
