144 THE LOWER PRIMATES 



special tegmental nuclear condensation of the mesencephalon, the red 

 nucleus. The reticular formation in the mid-mesencephaiic region extends 

 around the central gray matter in the form of a thin lamina which separates 

 the central gray matter from the inferior and superior colliculi, and forms a 

 support for the ventrolateral angle of both colliculi. 



Above the level of the substantia nigra the reticular formation again 

 appears between the interpeduncular gray matter and the lateral geniculate 

 as a massive development, extremely irregular in its arrangement and extend- 

 ing upward to become continuous with the nuclei and reticular formation of 

 the diencephalon. 



The Pontile Nuclei 



The pontile nuclei appear at about the midvcntricular level as small, 

 isolated collections of gray matter on the ventral surface of the brain stem. 

 These collections of gray matter develop chielly near the midline, but scat- 

 tered nuclear masses can be seen extending outward to form small discrete 

 accumulations. Above the midvcntricular level, the pontile nuclei become 

 somewhat larger and form a definite lamina of gray matter which assumes a 

 fair degree of thickness and a relatively extensive lateral disposition, covering 

 a small segment of the ventral aspect of the gray matter of the brain stem. 

 This mass of pontile nuclei gradually develops out of the isolated groups of 

 gray matter which form the pontile nuclei in the lower portions of the meten- 

 cephalon. They rapidly extend laterally, continue upward and then rapidly 

 diminish by drawing toward the midline, where they end in contact, if not by 

 fusion with the undifferentiated interpeduncular gray matter. The pontile 

 nuclei, where they assume any proportions at all, appear only as a simple 

 lamina of gray matter forming the ventral contour of the gray matter of the 

 brain stem. They show no tendency whatsoever to establish any degree of 

 complexity and do not interlace with the bundles of the pyramidal tract 

 which lie in a compact group dorsal to the middle of the pontile lamina. 



