CALLITHRIX JACCHUS, THE MARMOSET 175 



the case of lemur, this primordial auditory organization appears to be in the 

 interest of rapid automatic reactions in response to auditory stimuH. It 

 provides for immediate readjustments of body posture against impending 

 attack or other peril, the approach of which is indicated by sounds. Also, as 

 in the case ot lemur, this provision for immediate reaction in response to 

 sound imposes a delinite degree of hmitation in the range and scope of motor 

 activity. It deprives the animal of certain dehberative and reflective ele- 

 ments which furnish a period ol latency in the selection of alternative courses 

 of action designed to meet emergencies which may develop. The superior 

 cerebellar peduncle (Sep) appears as a crescent extending inward and 

 forward preparatory to its decussation. The pyramid (Py) still presents 

 a disseminated appearance and interspersed among its fasciculi are clusters 

 of the pontile nuclei. Lateral to the mesial fillet (Mf ) is a long, sweeping 

 mass of fibers on their way to the inferior colliculus, the axons of which 

 constitute the lateral fillet (Lf ). Dorsolateral to the mesial fillet is a bundle 

 of fibers constituting the rubrospinal tract and spinothalamic tract. The 

 central gray matter (Cen) has greatly increased in size and now almost 

 surrounds the small cavity constituting the caudal orifice of the Sylvian 

 aqueduct whose roof is formed by the superior medullary velum and by the 

 decussating fibers of the trochlear nerve. Between the two inferior colliculi, 

 resting upon the decussation of the trochlear nerve, are the lingular folia of 

 the cerebellum. 



LEVEL OF THE SUPERIOR CEREBELLAR PEDLINCULAR DECUSSATION (FIG. S"^) 



The next cross section is introduced to show the decussation of the 

 superior cerebellar peduncle (^XScp) preparatory to its entrance into the 

 red nucleus. The section selected discloses this decussation at its maximum 

 and afl'ords another illustration of the relatively poor development of the 

 cerebello-rubro-spinal connections, which implies a low degree of organiza- 



