RECONSTRUCTION OF PITHECUS RHESUS 399 



and almost immediately after its first identification the typical appearance 

 of the pontile nuclei develops. In its most caudal portions the pontile nucleus 

 appears as an incomplete ring of nuclear material deficient on the lateral 

 aspect and surrounding the relatively undivided pyramidal tract fibers. As 

 this structure is traced upward, the ring soon becomes complete, surrounding 

 the pyramidal tract and the lowest fibers of the pallio-pontile system of fibers 

 and, at the same time, there are distinctly developed the two layers of the 

 pontile nucleus (the superficial and deep layers), which meet mesially and 

 laterally the two buttresses of the pontile nuclei. 



There is some indication of migration of masses of nuclear material in 

 between the fibers of the two longitudinally coursing systems, producing a 

 poorly developed, lace-like appearance. The pontile nuclei are materially 

 heavier in formation and more extensive in distribution than in the preceding 

 forms. As the mesencephalon is approached the mass of the pontile fibers 

 gradually diminishes and the superficial layer contracts and extends further 

 upward for only a short distance ventral to the developing cerebral peduncle. 

 The deep layer becomes increasingly closer in proximity to the mesencephalic 

 reticular formation and gradually is transformed into the substantia nigra. 



The Vestibular Complex 



The vestibular complex makes its appearance first as a wedge-shaped 

 mass of gray matter inserted between the subependymal gray matter forming 

 the floor of the fourth ventricle above the cephalic extremity of the nucleus 

 of the column of Coll, and the mesial surface of the nucleus of the column of 

 Burdach. Rapidly increasing in size, it replaces the nucleus of the column 

 of Goll and displaces laterally the nucleus of the column of Burdach, thus 

 bringing about through the agency of the opening of the fourth ventricle 

 that lateral displacement which is characteristic of all of the dorsal medullar}- 

 nuclei of the brain stem. 



