140 THE LOWER PRIMATES 



separated, one from the other, by the pyramidal tracts which lie on the 

 immediate ventral aspect of the brain stem. These nuclei continue upward 

 in this same position, increasing to a moderate extent chiefly by an increment 

 in their most ventral portion which assumes a somewhat chib-shaped appear- 

 ance. There is but little indication of the development of the accessory and 

 main olivary nuclei, the entire nucleus apparently consisting of a single 

 lamina of gray matter. There is not the slightest indication of the formation 

 of a fundus; neither is there any intimation of plication in the arrangement of 

 the lamina. At its upper extremity the inferior olivary nucleus is the most 

 ventral of all of the structures of the brain stem. The nucleus continues 

 upward to a point somewhat below the level of the greatest width of the 

 ventricle. It then rapidly diminishes and comes to an abrupt termination. 



The Reticular Formation 



This mass of nuclear material makes its first appearance m the lower- 

 most level of the reconstructicMi as a small accumulation of gray matter 

 situated in the angle between the dorsal and ventral gray columns. It con- 

 tinues upward in this location, very gradually increasing in size, and replaces 

 the ventral gray column through the merging of the latter into the undiffer- 

 entiated reticular formation. From this point upward the reticular formation 

 assumes the position of the major constituent of the brain stem, being rela- 

 tively massive and supporting the various specializations which appear on its 

 several surfaces. From the point of the disappearance of the ventral gray 

 column, the reticular formation rapidly increases in size both laterally and 

 ventrally being separated in the midline by the longitudinally coursing fiber 

 bundles which form the raphe and by the inferior olivary nucleus which 

 appears as a special condensation in the ventromesial angle of the formation. 

 Above the level of the inferior olivary nucleus, the reticular formation of 

 either side closely approaches the midline, the space occupied by the raphe 



