314 MORPHOLOGY OF THE CEREBRAL CONA r OLUTIONS. 



volution, AC, then, becomes directly continuous with the superior frontal convolu- 

 tion, and represents morphologically, if we should divide the brain in this state 

 into a frontal and parietal lobe, a superior parietal convolution. The middle fron- 

 tal, F 2 , figs. 14 or 15, becomes directly continuous with the superior parietal 

 convolution, P 1 , and the inferior frontal, F 3 , with the inferior parietal, P 2 P 2 

 whilst the interparietal fissure Avould be the posterior portion of the inferior frontal 

 frontal fissure, f 2 , separated from its anterior portion by the downward development 

 of the posterior part of the superior occipito- frontal fissure, o. f. 1 , forming the so- 

 called fissura centralis or fissure of Rolando. This relation shows that the convo- 

 lution P 1 , which, following Pansch, we have called the gyrus parietalis superior, is 

 really morphologically, in the primitive type of Primate brain, a middle parietal con- 

 volution, and such I believe is its real significance. However, as I have remarked 

 in speaking of the division of the cerebral hemispheres into lobes, I consider the 

 division of that portion of the brain lying above the fissure of Sylvius into a frontal 

 and a parietal lobe as merely convenient for purposes of description, and since in all 

 Primates above the Lemurs, Marmosets, and lowest of the Simiadae the fissura cen- 

 tralis has become a well differentiated portion of the primitive superior occi pi to-frontal 

 fissure, I think it best to retain the name of superior parietal convolution, because it 

 really represents the condition as found in the brain of Man and the higher 

 Simiadae. Moreover, the instant we give the fissure of Rolando its true primitive 

 morphological position, the distinction between the parietal and frontal divisions 

 or lobes entirely disappears, and we merely have three lateral occipitofrontal 

 convolutions extending from the occipital lobe, 0, to the anterior extremity of the 

 frOntal lobe, F, and as such morphologically, it appears to me, they should be 

 considered. The evolution of this region of the cerebral surface takes place as 

 follows: Of the three lateral occipito-frontal convolutions, fig. 17, the superior 

 O F, 1 differentiates into the superior frontal, F 1 , and the anterior central, A.C.; 

 the middle, F 2 , into the middle frontal, F 2 , the posterior central, P C, and the 

 superior parietal, P 1 , fig. 16; the third, F 3 , becomes the inferior frontal, F 3 , and the 

 inferior parietal, P 2 P 2 . 



The two lateral occipito-frontal fissures separate into a superior frontal, o.f. 1 , and 

 the fissura centralis, c, and the second into an inferior frontal, o.f. 2 , and the interpa- 

 rietal, i.p. This is exactly the arrangement found in many of the Lemurs, and we 

 can trace through the curious synthetic and aberrant form Cheiromys resem- 

 blances to the convolutional structure of the brains of the Carnivora. 



Development of the Fissures and Convolutions of the Occipito-Frontal Lobe in the Human 

 fcetus and their appearance and relations in the brains of the slmiad^e. 



The lateral surface of the occipito-frontal lobe at the beginning of the sixth 

 month is entirely smooth in the human foetus, PI. XXXV, fig. 5. Shortly afterward 

 there appears upon its surface three radiating furrows, which constitute three of the 

 primary furrows of Pansch, PI. XXXV, fig. 8. Of these the central one, represents 



