306 



MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



of the manner of its formation. Thus, in Lemur nigrifrons, fig. 16, Propithecus 

 diadema and Edward sii, figs. 14, 15 and 21, Ava/izs, figs. 18 and 19, and Indris, 

 figs. 6, 7, 9 and 10, it is not present, but we find two parallel running fissures, more 

 or less broken up, which correspond in position, relation and direction to the two 

 occipitofrontal fissures, o f 1 and o f 2 , and I designate these as the superior and 

 inferior occipito-frontal fissures. 



In Chiromys or Aye-Aye, Plate XXXIV, fig. 24, the superior frontal will 

 be seen curving perpendicularly downward at its anterior extremity, producing a 

 corresponding separation of the inferior occipito-frontal into two portions, an ante- 

 rior which corresponds with the inferior frontal fissure of the higher Primates, and 

 a portion extending backward into the temporal lobe, the anterior portion of which 

 represents the fissura interparietalis. 



In the Gibbon, fig. 25, the step will be seen by means of which the curving 

 fissure of the Aye-Aye becomes the fissura centralis of the higher Primates. In 

 this brain the posterior portion of the inferior occipito-frontal fissure arches around 

 the posterior extremity of the fissura Sylvii, and becomes continuous with the 

 superior occipito-temporal, o t 1 , whilst the anterior separated portion becomes the 

 inferior frontal fissure. The interparietal in such brains is represented by the 

 curved fissure surrounding the posterior extremity of the fissure of Sylvius. 



The primitive condition, therefore, of the lateral surface of the occipito-frontal 

 lobe as found in the Lemurs would be as in the diagram, fig. 11, and this gives, I 

 believe, the original morphological type of the lateral surface of the occipito-frontal 

 lobe as illustrated in fig. 11, which is variously modified by separation of single 



Fig. ii. 



Fig. 12. 



fissural lines into one or more parts and bendings due to the generic and specific 

 forms of cerebral and skull evolution. Fig. 12 shows the conditions produced by 

 the separation and bending downward of the posterior part of the fissure, o f 1 , to 

 form the fissura centralis, and it can be readily seen that as this vertical type of 

 development, which is undoubtedly due to the enormous antero-posterior develop- 

 ment of the occipito-frontal lobe, progresses, the condition as found in Man and the 

 higher Primates is brought about. (Compare, with figs. 1 and 4, Plate XXXIV.) 

 Thus it will be found that the pressure forces produced by the enormous elongation 



