THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 309 



part of the brain, namely, in the lateral wall of the median ventricle 

 anterior to the central gray of the thalamus. The composition 

 and distribution of the commissure is shown in Figure 150. As 

 it runs up behind the foramen of Monro it divides into two parts, 

 the larger going into the medial wall of the hemisphere and the 

 smaller turning into the lateral wall of the median ventricle. The 

 latter bundle runs for a short distance ventral to and distinct from 

 the tractus olfacto-habenularis and spreads out in the central 

 gray cephalo-ventral to the nucleus habenulae. This region of 

 central gray is clearly seen in Cryptobranchus, Necturus and in 

 tadpoles of Amblystoma. It is a very compact body whose cells 

 are arranged in rows next the ventricle and whose outer part is 

 a fiber layer formed by the commissural bundle. In its structure, 

 position and its relation to the tractus olfacto-habenularis, this 

 body corresponds to the epistriatum of Petromyzon and to the 

 caudal part of the epistriatum in Acipenser. As the cell layer 

 is traced forward it curves through the foramen of Monro, forming 

 the dorsal border of the foramen, and becomes continuous with the 

 ventricular cell layer of the dorsal half of the mesial wall. Inas- 

 much as the commissure is distributed to both the lateral wall of 

 the median ventricle and to the mesial wall of the lateral ventricle 

 and as the ventricular cell layer is continuous throughout these 

 regions, it is just to conclude that they form a structural and 

 functional unit. The composition of the commissure gives further 

 ground for regarding this whole region as homologous with the 

 epistriatum in fishes, with which a part of it agrees in position. 

 The epistriatum in fishes receives an ascending tract from the 

 hypothalamus which decussates in the anterior commissure. 

 Such a tract is not certainly found in the lower commissure in 

 amphibia but is present in the upper commissure. These fibers 

 presumably constitute the tractus lobo-epistriaticus and the area 

 in which they end is the epistriatum. In selachians it has been 

 noticed that the region called the epistriatum receives a tertiary 

 olfactory tract from the mesial olfactory nucleus, the tractus 

 olfacto-corticalis. In selachians and ganoids a part of this tract 

 crosses to the opposite side. In amphibia the tractus olfactorius 

 septi and the bundle which enters the upper commissure from 



