302 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



of the fifth month, 1*1. XXXV, fig. 6. Its after development may be traced in 

 PL XXXV, figs. 9, 11, 13, 15, 17 and PL XXXVI, figs. 2 and 4. The tendency 

 to break up into a number of similar parts which are mere vegetative repetitions 

 of one another in appearance, is also plainly indicated as development proceeds, 

 until finally it attains its most complex development in the adult human brain. 



It is well developed in all the Primates with the exception of the marmosets 

 and Cheiromys. In Lemur nigrifrons, Plate XXXIV, fig. IT, and Indris, fig. 8, it 

 is a straight or slightly curvilinear continuous furrow. In Propithecus diadema it 

 appears in one specimen as a somewhat tortuous fissure, Plate XXXIV, fig. 23, 

 which is somewhat broken up in another, fig. 22. In Avahis, Plate XXXIV, fig. 

 20, it becomes continuous with a fissure extending in a curve around and on the 

 mesial surface of the occipitotemporal lobe closely following the curve of the mesial 

 arched fissure; the calcarine being separated from this and only represented by a 

 posterior portion continuous with what would be, in this case, a secondary mesial 

 arched fissure. It would appear from this that the production of both the mesial- 

 occipito-frontal and the mesial occipitotemporal fissures are produced by the action 

 of the same resultants of the pressure forces which gave rise to the mesial arched 

 fissure itself and, indeed, to be merely a vegetative repetition of this fissure, 

 separated into two portions by the action of the resisting, forces of the developing 

 occipital lobe. 



In most of the Simiadas it consists of one or more somewhat straight or more 

 or less tortuous fissures, as may be seen in PL XXXVII, fig. 5; PL XXXVIII, fig. 

 12. Posteriorly as may be seen in PL XXXVII, figs. 5, 7 and 14; PL XXXVIII, 

 figs. 12 and 13, is seen a fissure marked m.o.f." It separates the superior internal 

 pit de passage from the inferior internal pit de passage, two small bridging convolu- 

 tions running from the mesial surface of the occipital lobe to join the mesial parietal 

 portion (praecuneus) of the occipito-frontal lobe. This fissure corresponds with the 

 posterior portion of the mesial occipitotemporal fissure (collateral) and in PL 

 XXXVII, fig. 5 and PL XXXVIII, fig. 12, the exact similarity of these two fissures 

 and their identical relations with the two extremities of the primary occipital arch 

 can be plainly seen. I regard them, therefore, as morphologically homologous. 

 Evidently the conditions of growth must have been similar. This small fissure, 

 m.o.f." represents, therefore, the most posterior extremity of the mesialo ccipito- 

 frontal fissure, and if we examine the mesial surface of the brain of Chrysothrix, 

 PL XXXVIII, fig. 8, we see a single continuous fissure extending from a position 

 just anterior to the mesial extremit} 7 of the superior branch of the primary occipital 

 arch, forward to the end of the occipito-frontal lobe. 



From these observations it will be seen that the morphological type of the 

 mesial surface of the brain is a perfectly definite and symmetrical one and corres- 

 ponds to the diagram, figure 10. Additional reasons will be given in support of 

 this view, under the head of the convolutions of the occipital lobe and the signifi- 

 cation of the pits de passage. 



The mesial occipito-frontal fissure includes the following named furrows : 



