414 J. B. JOHNSTON 



The writer would substitute for this view the following simple 

 explanation of the septal region in the frog. Above and caudal 

 to the foramen the medial wall is all pallial. A part of this 

 pallial area has developed pyramidal cells and may be recognized 

 as a true hippocampus in a low stage of organization. The 

 remainder has retained the irregular arrangement of cells seen 

 in the selachian pallium and gives passage to the fimbria and 

 fornix fibers as in the reptile. Rostral to the foramen there has 

 been the same development of hippocampus but the residue of 

 hippocampal primordium is smaller and the line of demarcation 

 between this and the area parolfactoria has been obscured in 

 part. At the same time a new sulcus has developed in the 

 medial surface. This sulcus forms along the line of demarcation 

 between the pyramidal and irregular cells. The elongation of 

 the hemisphere has led to the formation of the fimbria just 

 below this sulcus, through the collection of many fibers of the 

 olfacto-cortical system, on their way to the caudal part of the 

 hippocampus. This sulcus is therefore the fimbrio-dentate sulcus. 



Each element in the above explanation is perfectly simple and 

 direct. There is no supposition that great masses of cells have 

 migrated half the length of the hemisphere without motive. The 

 only point of obscurity is the line of separation between the 

 primordium hippocampi and the area parolfactoria. Since this 

 is perfectly clear in reptiles and mammals it is quite legitimate 

 to accept the hypothesis that in the frog it has lost somewhat 

 in definiteness because of the small volume of the residual pri- 

 mordium hippocampi. 



GENERAL OBSERVATIONS AND SUMMARY 



When the results of the present series of studies are reviewed 

 it is seen that great confusion prevails in the morphological con- 

 ceptions and the nomenclature of this region of the brain. In 

 these studies for the first time there has been built up a connected 

 account of the evolution of the telencephalon beginning with 

 primitive brains and taking into consideration the factors and 

 processes by which the form of the mammalian telencephalon 

 have been determined. Briefly summarized, these processes are 



