234 THE LOW ER PRIMATES 



central gray column tends further to separate this latter structure from the 

 midhne. As the mass of the dorso-medullarv nuclei reaches its maximum, 

 the sufjstantia gelatinosa Rohindi which at this point is passing over into 

 the sulxstantia gelatinosa trigemini is shifted into its fixed lateral position. The 

 heavy dorsal mass of the nucleus of Burdach overhangs laterally the sub- 

 stantia gelatinosa trigemini. The central gray matter at this point has 

 merged to a considerable extent with the reticular formation. The transition 

 between the substantia gelatinosa Rolandi and the substantia gelatinosa 

 trigemini is essentially without a definite line of demarcation. As the latter 

 structure is traced upward, however, it constantly increases in size except for 

 the constriction found at the midolivary level, termed the waist of the 

 trigeminal nucleus. 



The Inferior Olivary Nucleus 



In the reconstruction, the inferior olivary nucleus shows a moderate 

 advance over that found in the Lemur mongoz and the Callithrix jacchus. A 

 slight indication of secondary plication now appears in the dorsal and ventral 

 branches of the u-shaped nucleus. The ventral accessory olivary nucleus 

 appears at about the point at which the dorsal sensory nuclei begin to assume 

 proportions of any size. It first manifests itself as a round collection of 

 gray material which flattens out obliquely from before backward and inward. 

 At a somewhat higher level, isolated gray islands appear which coalesce 

 and form a similar parallel flattened band, the dorsal accessory olivary 

 nucleus. The chief nucleus appears between the lateral extremities of this 

 accessory nucleus as a rounded mass which rapidly assumes the form of a 

 loop, the ventral branch of which fuses with the lateral extremity of the 

 ventral accessory nucleus, while the dorsal branch is continued mesially 

 parallel to the dorsal accessory nucleus. The mesial extremities of the 

 accessory nuclei then fuse together. The accessory nuclei attenuate and dis- 



