SEPTUM, HIPPOCAMPUS, PALLIAL COMMISSURES 417 



Elliot Smith and Levi have shown, the most medial border of 

 this gives rise to the fascia dentata. Dorsal to the fascia den- 

 tata the hippocampus folds inward, on account of pressure from 

 the expanding general cortex and perhaps other causes. This 

 infolding produces the hippocampal fissure, which is known in 

 embryos by the name fissura arcuata. This fissure is not present 

 in the reptiles studied by the writer. 



6. The septum pellucidum in lower mammals consists of a 

 thick mass of gray matter which imbeds the fornix system and 

 its commissure, becomes continuous with the hippocampal forma- 

 tion beneath the splenium of the corpus callosum and is con- 

 nected with the supracallosal hippocampus by columns of cells 

 between the bundles of the commissures. Rostrad the septum 

 is continuous with the indusium around the genu corporis callosi 

 and extends forward ventral to the precallosal hippocampus to 

 the olfactory peduncle. This septum pellucidum represents a 

 part of the roof of the selachian forebrain (primordium hippo- 

 campi) which has remained in a low stage of development. In 

 this rostral continuation of the septum pellucidum there runs a 

 longitudinal bundle of fibers which are collected from the basal 

 olfactory centers and reach the hippocampus either through the 

 fimbria or by way of the stria Lancisii. Between this fimbria- 

 bundle and the fascia dentata a sulcus appears in reptiles and 

 mammals which is the fimbrio-dentate sulcus of human anatomy. 



7. The use of the term primordium hippocampi in the sense 

 which is given to it in this paper requires some comment. The 

 term is used by me to denote that lowly organized pallial mass 

 in the selachian brain from part of which the hippocampal forma- 

 tion has developed. It is also used for the homologous mass in 

 cyclostomes where it is situated in the telencephalon medium. It 

 is also used for the residue which is left from this mass in higher 

 vertebrates after the hippocampal formation has been developed. 

 This residue is the body long known as the septum pellucidum. 

 The hippocampal formation and septum together constitute a con- 

 tinuous mass of gray matter which thickens the lamina supra- 

 neuroporica and forms the bed for the hippocampal commissure 

 and corpus callosum. 



