441 



to be the sulcus lunatus in more than 200 human brains and a 

 large series of Apes and have been able to demonstrate its value as 

 the one decisive criterion in the sure identification of the "Affenspalte". 

 It is clear that it is the rapid expansion of the area striata which 

 calls the sulcus lunatus into being. 



The other effects of its growth vary considerably in different 

 genera. 



In most Apes the whole of the mesial part of the area striata 

 becomes deeply involuted so that a vertical section through its posterior 

 (i. e. broader) part (e. g. in a Cercopithecus aethiops) presents the 

 peculiar appearance represented in Figure 5. None of the area striata 

 is exposed on the surface but the whole of the walls of the deep furrow 

 which most writers would call "calcarine" (s. i. m.) is formed of cortex 

 containing the stripe of Gennari. The result of this infolding of the 

 whole breadth of the area striata is the formation of a very deep fossa 

 which extends forward to the region where the stria-bearing region is 

 much narrower: consequently the walls of the anterior part of the 

 "fossa striata" (as we may call it) are not wholly formed of area striata, 

 because the latter is much too narrow and forms only the deeper part 

 of the furrow (Fig. 6). Still further forward the fossa is prolonged 



s. 1. m. 



foss. str. —'- , _, 



str. Gennan 



s. 1. m. 



str. Gennari 



Fig. 5. Fig. 6. 



Fig. 5. Transverse section (vertical) througli the posterior half of the so-called 

 "calcarine" sulcus in an Ape (Cercopithecus aethiops), to show that the furrow is re- 

 ally a complete involution of the conical area containing Gennari's stria — a fossa 

 striata (occipitalis). 



Fig. 6. A corres2>onding section througli the same hemisphere further forward. 

 The area striata is narrower but the furrow is the forward prolongation of the complete 

 striate involution, hence its walls are only partially formed of the stria-bearing cortex. 



beyond (i. e. in front of) the area striata into the gyrus fornicatus. 

 The study of such a section as that represented in Figure 5 can leave 

 us in no doubt as to the causal relationship between the so-called 

 "calcarine" furrow and the folding of the area striata. In other words 



