STRUCTURAL CULMINATION 957 



In most ol tin- lemurs the post-S\I\ian sulcus is a simjilc Inicar lurrow 

 representing the parallel or su]:)fn()r teinjjoral sulcus m apes and man. 



\\ ith reference to the central (Rolandic) fissure in lemurs, there is some 

 dilierence of opinion. The slight indentation between the frontal and parietal 

 areas has been called by Jacobsohn and Flatau the precentral sulcus, a view 

 which disregards the fact that such a sulcus ne\er occurs in the absence 

 oi the more stable central hssure. The tendency in all lemuroids is toward 

 a fusion ol this Irontal indentation with the caudal extremity of the coronal 

 sulcus which [produces that great transverse fissure, the central (Rolandic) 

 sulcus of apes and man. 



The iissural pattern ol tarsius is simple in the extreme, ])rcsenting as it 

 does but a single indentation, the fissure of Sylvius. This is undoubtedly 

 the suj^rarhinal fold corresponding to the mammalian pseudo-Sylvian sulcus. 

 In this sense the neopallium of tarsius is the most generalized of all the 

 lower primates. It thus stands closest to some subprimate lissencephalic 

 brain which was the forerunner of the simian cerebrum. Tarsius at the same 

 time IS more pithecoid than the lemurs in the development of its visual 

 cortex and in the appearance ol a posterior cornu in its lateral ventricle. 

 \\ ith this combination of generalized characters and pithecoid tendencies, 

 the brain ol tarsius ser\'es particularl\ wi'll as the transition stage which 

 carru'd upward from sonn' insecti\ ore-like type the basic characters of 

 mammalian cerebral organization but also felt the decided impetus toward 

 simian di'\elopment. It has remained dynamically more plastic than the 

 brain ot lemurs, which latter (however primate in their procli\ ities) have 

 experienced a specialization quite their own, due, it may be, to secondary 

 retrogressive modifications. 



If, as first suggested by Huxley and subsequently insisted upon with 

 added emphasis by Osborn, the ancestral type of placental mammal was an 

 animal resembling the tree shrew (Tupaia), then it is probable that the 



