i86 THE LOWER PRIMATES 



gradually drawn together into a compact bundle, coalesce to form a recog- 

 nizable structure. 



In the pontile levels, the reticular formation is separated from the deep 

 layer of the pontile nuclei by the trapezoid body. This structure at higher 

 levels forms a part of the lateral boundary of the reticular formation. 



The disposition of the reticular formation in marmoset is similar to that 

 in lemur. It supplies the main gray mass of the brain stem, affording a matri.x* 

 in which condensations of nuclear material appear as discrete nuclei. The 

 lateral nuclei of the reticular formation and the superior olivary nuclei are 

 but little better developed than in the lemur. In the region of the midbrain 

 the nucleus ruber appears as a special condensation of the reticular forma- 

 tion, surrounded by its fiber capsule derived from the superior cerebellar 

 peduncle and the striato-rubral tract. In its dorsomesial aspect there are 

 embedded successively the nuclei of the mesial somatic motor cell column, 

 the hypoglossal, the abducens, the trochlear and the oculomotor nuclei. 

 The vestibular nuclei appear, as in the lemur, in the dorsolateral angle of the 

 tegmentum. The nucleus of Deiters first appears as a wedge between the 

 reticular formation and the central gray matter. The cephalic extremity of 

 the reticular formation comes into rather intimate relation with the sub- 

 stantia ii'gra but does not fuse with it. Actual fusion takes place between 

 the reticular formation and the zona incerta of the diencephalon and with the 

 caudoventral pcM-tion of the central thalamic gray matter. 



The Pontile Nuclei 



In reconstruction these nuclear masses appear rather abruptly at the 

 level of the trapezoid body. There is no substantial indication of the arciform 

 nuclei in the oblongata which appear in higher primates as caudal prolonga- 

 tions of the pontile nuclei along the ventral surface of the pyramidal tracts. 

 The pontile nuclei appear as bilateral nuclear masses mainly disposed 



