THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 311 



the mesial olfactory nucleus are homologous with the tractus 

 olfacto-corticalis, and the region to which the upper commissure 

 is distributed must be considered as a part of the epistriatum. 

 The part of the epistriatum which receives tertiary olfactory 

 fibers is the olfactory cortex or hippocampus. A part of the 

 epistriatum which does not receive such fibers covers the inner 

 surface of the striatum and in all higher vertebrates is regarded 

 as an integral part of that body. 



The amphibian hemispheres differ in two chief ways from those 

 of selachians. First, the secondary olfactory centers are less 

 massive, the wall is thinner and the ventricle larger, and the 

 epistriatum is relatively larger. Second, there is a more complete 

 folding of the lateral wall than in selachians. In selachians the 

 lateral walls rose up, arched over and fused. In amphibia the 

 lateral walls rose up and folded over without fusing with one 

 another. The result is to form on each side a lateral hem- 

 isphere whose ventricle is a continuation of the primitive 

 lateral or olfactory ventricle and whose dorso-mesial wall was 

 primitively the dorsal part of the lateral wall of the median 

 ventricle. The lateral ventricle of the hemisphere is not 

 completely surrounded by nervous walls as is the primitive 

 olfactory ventricle. In amphibia the mesial wall of each 

 hemisphere is connected with its fellow over the median 

 ventricle by a membranous roof, which is homologous with 

 that in fishes. The diagrams in Figure 151 will show how the 

 amphibian hemispheres have been formed by folding over of 

 the lateral walls, and also the history of the epistriatum in fishes 

 and amphibia. The epistriatum is in reality no more or less than 

 the central gray matter of the forebrain in fishes. The dorsal 

 part of this central gray in selachians receives olfactory fibers of 

 the third order and in consequence becomes differentiated as the 

 olfactory cortex. This region first becomes distinct in the amphib- 

 ian brain as the result of the folding described. The caudal 

 portion of the forebrain wall has not been folded into the lateral 

 hemisphere and the epistriatum retains the position which it has 

 in the caudal part of the forebrain in fishes. In higher forms 

 this body becomes reduced but is still recognizable in mammals 



