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Giacominr!), while regarding it as the specially modified “ourlet” 
of the mantle, does not attempt to further define its homologies. Hırr's ?) 
hypotheses are too ill-defined and too purely speculative to be seri- 
ously discussed. 
The opinion generally held at the present time is a modification, 
or rather an extension, of that which ScHwALBeE has stated. Thus 
Ramon Y CAyAaL?) regards the hippocampus as an inrolling of two 
convolutions, the marginal of which — the Fascia dentata — is com- 
posed of (a) a superficial molecular layer, (b) the stratum granulosum 
and (c) a layer of polymorphous cells. 
In reviewing the literature of the subject STRASSER?) gives ex- 
pression to a somewhat similar idea. He describes the Fascia den- 
tata as being separated from the cortex Ammonis and pushed into its 
concavity, so that a picture of two U’s is developed, to use SALA’S?) 
comparison. 
The conclusion at which I have arrived is a modification of that 
expressed by Ramon y CAJAL, from which however it differs in several 
essential points. 
Our attention will be confined to the anterior region of the hippo- 
campus of the reptile and monotreme, where the facts which are at 
work in the evolutionary process can be so clearly demonstrated. (But 
the conclusions thus arrived at are of general application to any region 
of the hippocampus of the higher mammal.) 
In the region under consideration the ventral margin of the hippo- - 
campal formation is not free, but is directly continuous with an area 
of the mesial wall of the hemisphere, which I have distinguished as 
“the precommissural area” ®). 
In the reptile brain the mesial region of the dorsal cortex and 
the adjacent region of the mesial cortex (throughout the whole length 
of the hemisphere) constitute the hippocampal formation. In a trans- 
1) “Fascia dentata du grande hippocampe”. Arch. ital. de biologie, 
T. V, p. 411. 
2) “The Hippocampus’. Phil. Trans., 1893 B. 
3) “Nuovo concepto de la histologia de los centros nerviosos”. Bar- 
celona, 1893. 
4) “Alte und neue Probleme der entwickelungsgeschichtlichen For- 
schung etc.” MERKEL und Bonner’s Ergebnisse, Bd. II, 1892, p. 579. 
5) loc. eit. 
6) In a paper entitled “The Morphology of the True Limbic Lobe, 
Corpus Callosum, Septum Pellucidum and Fornix”, Journ. of Anat. and 
Phys., Vol. XXX, Part 1, October 1895, and Part 2, January 1896, I have 
discussed the general morphology of this region. 
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