THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 



303 



the basal and lateral wall and on its dorsal and internal surface 

 is the epistriatum (Fig. 149). The olfactory lobe forms the 

 whole thickness of the wall at the front part of the forebrain and 

 covers the lateral surface of the striatum, but does not extend up 

 into the roof. In the sturgeon a few nerve cells and fibers are 

 found in the membranous roof which may be vestiges of the 

 massive roof in selachians. In bony fishes the lateral olfactory 

 nucleus is better developed than in ganoids and forms the nucleus 

 thaeniae. The olfactory bulb lies forward as in selachians and is 

 connected with the forebrain by a distinct olfactory tract. The 



Epistriatum 



Tr. olf.-haben. 

 Olf. tract fibers 



Tr. olf.-hypothal. lat. 

 Tr. strio-thalam. 



Tr. olf.-hypothal. med. 



Xuc. praeopticus 



Fig. 149. — A transverse section of the brain of the sturgeon behind the anterior 

 commissure. Compare Fig. gg 



olfactory tract in ganoids (Fig. loi) is distributed to all parts of 

 the olfactory lobe and a part of it decussates in the cephaHc part 

 of the lamina terminalis to end in the epistriatum of the other 

 side. In bony fishes the olfactory tract is in two bundles, a medial 

 and a lateral. The lateral bundle ends in the lateral olfactory 

 nucleus (nucleus thaeniae) and in some cases a part of it decussates 

 in the anterior commissure. The medial bundle always decus- 

 sates and is distributed to the lateral olfactory nucleus. In both 

 ganoids and bony fishes there arises from the anterior part of the 

 olfactory lobe a tractus olfacto-hypothalamicus medialis, and from 

 the lateral part of the lobe (nucleus thaeniae) a tractus olfacto- 

 hypothalamicus lateralis. From the nucleus thaeniae of bony 



