THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 307 



minal body), and a portion above the foramen of Monro which 

 together with the adjacent dorsal and caudal parts of the hemi- 

 sphere forms the hippocampus. 



The fibers of the olfactory tract are distributed to the whole of 

 the lateral wall and to the front part of the roof, base and septum 

 and also to the nucleus praeopticus. A part of the olfactory tract 

 decussates in the lower of the two commissures and goes to the 

 lateral wall. As in selachians, then, the lateral and front walls 

 of the hemispheres constitute the olfactory lobe. The base of the 

 hemisphere is the corpus striatum. Fibers from the lateral wall 

 pass back to the thalamus mingled with the tractus strio-thalami- 

 cus. They constitute a tractus olfacto-hypothalamicus lateralis, 

 homologous with that in fishes. From the septum and part of 

 the roof and base at the front end arise fibers which curve for- 

 ward and downward and form a tract which runs backward near 

 the mid-ventral line and enters the hypothalamus (tractus cortico- 

 medialis of P. Ramon in the frog). This corresponds to the tractus 

 olfacto-hypothalamicus medialis in fishes. From the ventral and 

 lateral part of the lateral wall, an area corresponding to the nucleus 

 thaeniae in fishes, arises the tractus olfacto-habenularis which runs 

 as in fishes. From the septum, which belongs to the olfactory 

 lobe, fibers arise which run up and back and end in the caudal 

 part of the mesial wall and roof, the tractus olfactorius septi of 

 authors. This is a tertiary olfactory tract and corresponds to the 

 tractus olfacto-corticaHs in selachians, and should be given the 

 same name. 



The commissures are difficult to analyze and compare with 

 those of other vertebrates. The lower commissure contains fibers 

 of the tractus olfacto-hypothalamicus from the cephalic and lower 

 half of the mesial wall (precommissural or paraterminal body, 

 mesial olfactory nucleus), fibers of the tractus olfactorius which 

 end in the recessus praeopticus, and fibers of the tractus strio- 

 thalamicus. The lower commissure is placed in the base and 

 lateral walls in very much the same position as the anterior com- 

 missure in fishes and it is usually called the anterior commissure 

 proper. The upper commissure is very peculiar in its position 

 and composition. It crosses from side to side in the lamina 



