MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 315 



ins. the future fissura centralis, appears somewhat earlier than the other two repre- 

 sented by the foetal brain at the middle of the sixth month, PI. XXXV, fig. 7. The 

 anterior, f.\ tig. 8, becomes the inferior frontal fissure, together with its vertical 

 branch, the inferior vertical frontal or fissura pnvcentralis. The posterior, i p., 

 develops into the fissura parietalis. Differentiation takes place as seen in figs. 10 

 and 12, until finally, at a little past the beginning of the eighth month, fig. 10, this 

 region assumes the appearance as found in the higher Simiadse and in Man. In the 

 meantime, as can he seen in fig. 12. the superior frontal fissure has developed. In 

 figs. 14 and 16 the gradual increase in tortuosity and complexity can he noted, 

 together with the evolution of fissures, f. 3 and f 4 , the superior and inferior vertical 

 branches respectively of the first and second frontal fissures. Increase in complexity, 

 due to the formation of numerous sulculi, takes place even after birth, but the gen- 

 eral type is still adhered to, until finally we have the varied and complicated config- 

 uration as found in the adult human brain. 



In the Lemurs the fissures of the lateral frontoparietal region retain their primi- 

 tive structure, a definite development of the fissura centralis having as yet not taken 

 place, although indications of its future appearance can be readily seen in Cheiromys, 

 fig. 24. Usually, however, this surface of the occipitofrontal lobe is divided up by 

 two longitudinally-running fissures, which are more or less interrupted in their course 

 and which seperate three occipitofrontal convolutions, OF 1 , OF 2 , OF 3 . Indications 

 can, however, be distinguished, as in Propitheais diadema, fig. 21, of the tendency 

 to produce a vertical arrangement, f. 3 and f. 4 being slightly represented. The fissura- 

 tion as a whole of the entire lateral surface of the hemisphere exhibits a primitive 

 homogeneous and scarcely characteristic form of development. 



In the Marmosets, PI. XXXIV, figs. 11, 12 and 13, no signs of fissuration exist 

 on the lateral surfaces of either the occipitofrontal or occipitotemporal lobes, repre- 

 senting a condition similar to that found in the human foetus at the beginning of the 

 sixth month. 



In the lowest forms of Platyrrhina or New World monkeys, such as Chrysothrix, 

 we find but a slight advance in development over the conditions as exhibited in the 

 brain of the Arctopithecini. The absence, or at most slight fissuration of the cere- 

 bral surface of these animals, is undoubtedly due to the enormous antero-posterior 

 expansion of the skull, which in these forms reaches its highest relative develop- 

 ment ; so much so that in Chrysothrix sciurejis the posterior extremity of the 

 hemisphere extends back of the cerebellum to an extent relatively much greater than 

 in that of any other Primate, not even excepting man. In PI. XXXVIII, figs. 6 

 and 9, we have views of the upper and inferior lateral surface of the brain of this 

 monkey, and it will be seen that the only fissures represented are the Sylvian and 

 the external perpendicular portion of the fissura occipitalis superior, which are con- 

 tinuous, and the superior occipitotemporal, o.t. 1 . Faint indications can, however, be 

 discerned of the fissura centralis ; otherwise the brain is entirely smooth upon the 

 lateral surface. The convolution al development of these brains is, therefore, simpler 



