THE FISSURA HIPPOCAMPI £55 
eal site of development supports the generalization of Elliot 
Smith (’96) for monotremes, that this tissue is the fringe of the 
cortex. 
Hippocampus 
Coincident with the appearance of the small area characterized 
by thicker marginal velum in the dorsal wall of the evaginating 
hemisphere in the 11.8-mm. embryo, there is a slight thickening 
of the wall itself. These two features furnish the first differentia- 
tion of the primordial hippocampus. The histological and mor- 
phological differentiation is more apparent in the 14-mm., and in 
the 19.1-mm. and the 20-mm. the whole extent of this tissue is 
involved in a groove, the fissura hippocampi. Here its wall is 
slightly thicker than the wall of any other part of the cortex. 
There is little cell migration from the matrix into the marginal 
velum. In fact, at this stage of development of the cerebral 
vesicle, this is the only region where a true cell-free margin is 
found. The tissue which lies immediately dorsal to the hippo- 
campus is neopallium and that which is ventral is either septum 
or a derivative of the area epithelialis. This fissure, in all proba- 
bility, is not formed by an invagination due to more rapid growth 
of the central part of this tissue, but rather by a buckling of the 
wall on itself as the result of the appositional growth of the neo- 
pallium between the endorhinal fissure and the dorsal limit of 
the hippocampus, plus perhaps the lack of support, except at 
one point, by the epithelial tissue ventral to it. The fissure is 
deeper and less broad where the ventral support is narrow. Since 
this fissure or groove lies above the sulcus limitans hippocampi 
and the fascia dentata and involves the whole wall containing 
the primitive hippocampus, there can be little doubt as to its 
identity. It is the fissura hippocampi or the fissura arcuata of 
His, the Bogenfurche of other authors. This fissure must not 
be confused with the sulcus limitans hippocampi or the sulcus 
fimbrio-dentatus. It corresponds to the fissura arcuata of Herrick 
(10) in reptiles, Johnston (’13) to the contrary notwithstanding, 
if the evidence above be accepted, namely, that the definitive 
hippocampus is derived from the reptilian dorso-medial cortex. 
