78 



The peculiar pattern formed by the lunate and paramesial sulci 

 in the human brain (Fig. 1) is frequently exhibited in the brains of 

 the Gorilla and Hylobates. In most other Apes the V-shaped pattern 

 formed by these two furrows becomes obliterated (or, at any rate, 

 hidden) by the expansion of the occipital operculum. To make this 

 clear I shall give a brief sketch of the evolution of the lateral occipital 

 sulci. (More complete data will be found in the works of Cunningham, 

 ZucKERKANDL aud the writer, to which I have already referred.) 



The sulcus occipitalis transversus is the common inheritance of 

 the Mammalia: it is the sulcus postlateralis of the Carnivora: I have 

 seen the typical Primate form of this furrow in an old $ Lemur 

 fulvus. The parieto-occipital sulcus is the common property of the 

 Primates, for, though absent in the Hapahdae and sometimes even in 

 the small Cebidae, it is invariably present in the Prosimiae. 



The sulcus lunatus first makes its appearance in the larger Cebidae 

 as a simple,, semilunar furrow placed very obliquely near the lateral 

 margin of the occipital region. (This is well shown in Zuckerkandl's 

 memoir, op. cit.) In the larger Cebidae its posterior lip becomes 

 operculated and the furrow becomes prolonged mesially as the oper- 

 culum expands, so that in some examples of the genus Cebus the 

 sulcus becomes transverse and the operculum covers over the sulcus 

 occ. transversus and the dorsal part of the parieto-occipital sulcus. 

 In Cebus the inferior occipital sulcus is present: in Lagothrix the 

 superior occipital sulcus is also developed. 



In all the Cercopithecidae there is invariably a large occipital 

 operculum, which springs not only from the posterior lip of the sulcus 

 lunatus but also from the dorso-mesial edge of the hemisphere: it 

 always overlaps the sulcus transversus, which may even in some cases 

 become confluent with the sulcus lunatus or in other cases become 

 aborted. In the smallest species of Ccrcopithecus the superior occipital 

 sulcus may be absent. 



In Semnopithecus (which points the way to the development of 

 the Simiidae through the Hylobates-stage) the mesial part of the oper- 

 culum begins to dwindle, leaving the parieto-occipital sulcus and the 

 arcus parieto-occipitalis exposed. In Hylobates this is carried a stage 

 further and the small sagittal mesial operculum formed by the inner 

 lip of the sulcus paramesialis becomes completely separated from the 

 great operculum which overlaps the sulcus lunatus. The Chimpanzee 

 and, as a rule, the Orang also retain the Cercopithecoid arrangement, 

 whereas the condition exhibited in many Gorilla's brains is a real 

 transition-stage from the Gibbon-condition to that exliibited by such 



