136 THE LOWER PRIMATES 



opposite the middle of the inferior eolliculus, continuing upward and increas- 

 ing gradually in widtli is joined by the lateral geniculate which rapidly 

 develops into a large mass applied to the lateral surface of the midbrain. The 

 superior eolliculus is unusually large and appears as a direct continuation 

 upward of the collicular plate, presenting a flat plateau-like structure which 

 extends to the upper limit of the reconstruction. The ventral surface presents 

 a marked development of what corresponds to the interpenduncular graj- 

 matter, in this case, however, protruding between and ventral to the pedun- 

 cles. The interpeduncular gray matter continues forward as a protrusion 

 which extends vcntrally from the surface of the brain stem for a considerable 

 distance. The major portion of the brain stem is formed by the reticular 

 formation which has a particularly undifferentiated appearance and serves as 

 a support for the more specialized structures of the stem which develop upon 

 its several surfaces. 



The presence of a small quantity of reticular formation in the angle 

 between the ventral and dorsal gray horns, indicates that at the lowest levels 

 of the reconstruction the decussation of the pyramidal tract has already 

 begun. The first sections of the reconstruction correspond to the lowermost 

 limit of the medulla oblongata and have an appearance almost typically that 

 of the spinal cord, with the exception of the small mass of the reticular for- 

 mation already mentioned. The central gray matter is distinctly V-shaped in 

 appearance, presenting definite ventral gray horns and dorsolateral exten- 

 sions with a distinct cap of gray matter, the substantia gelatinosa of Rolando. 

 For a short distance the substantia gelatinosa of Rolando continues upward 

 on both sides of the midline separated by a rather deep groove in which lie 

 the dorsal white columns of the spinal cord and medulla oblongata. The 

 dorsal horns, however, soon show a tendency to deviate outward, the tips 

 of the horns first turning laterally and then somewhat ventrally, as the dorsal 

 gray horns separate themselves one from the other. The ventral gray column 



