MORPHOLOGY OF THE FOREBRAIN 485 



grey matter by a clearly marked zona limitans relatively free from 

 nuclei. Each of these parts of the cerebral wall is differentiated 

 at the expense of its own ventricular grey and neither is in the 

 ontogeny to any extent derived from, the other so far as its cellular 

 material is concerned. It is my opinion that the same is true when 

 we consider the origin of the primordium hippocampi phylogenet- 

 ically. 



It is, on the other hand, equally evident that the physiological 

 motive which has led to the further differentiation of tissue within 

 thercstral part of the primordium hippocampi in Anura is the func- 

 tional connection with the underlying precommissural body. The 

 size and structural complexity of the primordium hippocampi and 

 of the precommissural body vary in relation to each other and 

 the richness of the connecting fiber systems. The precommissu- 

 ral body serves as a way-station between the primary olfactory 

 centers and the hypothalamus and hippocampus, as the avenue 

 of discharge into the hippocampus of ascending fibers from the 

 hypothalamus and as a pathway for efferent impulses from the 

 hippocampus having a collateral connection from the fornix 

 with the precommissural cells and discharging chiefly into the 

 hypothalamus. In the Amphibia these functions are imperfectly 

 localized, but in mammals the researches of Cajal show that the 

 nucleus medianus septi is devoted chiefly to the olfacto-hypothala- 

 mic connections and the nucleus lateralis to the collateral fornix 

 fibers. 



Ram6ny Cajal ('04, p. 1050) is of the opinion that the mammal- 

 ian corpus striatum is a center for the reinforcement (by means of 

 collateral discharge) of the impulses sent out from the cortex by 

 the efferent projection fibers of the internal capsule and that the 

 nuclei of the septum (particularly the lateral nucleus) bear a 

 similar relation to the fibers of the commissura hippocampi and 

 fornix. He figures in the rat with great clearness collaterals both 

 from the commissure and from the fornix to the cells of the septum, 

 especially its lateral nucleus. Elliot Smith ('10) has arrived at a 

 similar conclusicn. Though clearly this collateral relation with 

 the efferent fibers from the hippo can pal formation was not the 

 primary functional motive in the differentiation of the septal 



