100 THE LOWER PRIMATES 



ventricle has its usual boundaries but shows in a most indefinite way these 

 surface markings which characterize it in Anthropoidea. This dorsal surface of 

 the brain stem, however, assumes great prominence because of the develop- 



FIG. 48. VENTRAL SURFACE OF BRAIN STEM OF TARSIUS SPECTRUM. 

 [Actual Length, 16 mm.] 



Key to Diagram, cerebr.-peduncle, Cerebral Peduncle. 



mcnts in the mesencephahc roofplate. Here the mfenor colliculus and mesial 

 geniculate body are larger and better defined than in all other primates. The 

 superior colliculus attains dimensions almost warranting the designation 

 of optic lobe, but in any event much more conspicuous than in lemurs, 

 monkeys, apes or man. 



Thus, in the feeble development of certain features whose progressive 

 expansion characterizes the line of evolutional development in the primates, 

 tarsius appears to be an expression of this process m its simplest terms. It 

 indicates this primitiveness quite as much in the superior and inferior colliculi 

 of the midbrain. In spite of the fact that the animal has made such notable 



