300 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



Development of Occtpito-temporal Fissures. 



The first fissure to make its appearance upon the occipitotemporal lobe is the 

 mesial occipitotemporal. On Plate XXXV, fig. 6, it may be seen marked m. o. t., 

 whilst by comparing fig. 5, it will be seen that the rest of the lobe, together with 

 the whole lateral surface of the hemisphere, is entirely smooth at this period of 

 development, which is about the end of the fifth month. Above may be seen the 

 mesial occipitofrontal which makes its appearance at about the same time, though 

 somewhat earlier. This is undoubtedly due to the fact that the occipito-frontal 

 lobe grows more rapidly than the occipito-temporal and as a result there is an 

 increased amount of pressure in this region. 



By the beginning of the seventh month, Plate XXXV, fig. 8, there has 

 appeared a well marked superior or first occipito-temporal, and this is followed 

 by the appearance about the middle of the seventh month of the posterior portion 

 of the inferior or second occipito-temporal fissure. These grow deeper and better 

 defined and gradually become slightly tortuous as development proceeds, until 

 toward the end of the ninth month they assume the appearance as found in Plate 

 XXXVI, figs. 3 and 4, which may be compared with the condition as found in the 

 chimpanzee, figures 7 and 8, same plate. 



FISSURES AND CONVOLUIONS OF THE OCCIPITO-TEMPORAL LOBE AS FOUND 



IN THE PRIMATES. 



1. In the Lemuridaj we find all of these fissures well developed except that in 

 some cases the inferior occipito-temporal is but slightly developed. Indeed the 

 type of configuration of the cerebral surfaca of these animals corresponds perfectly 

 to the brain of the human foetus, at the middle of the seventh month, as may be 

 well seen by comparing Plate XXXV, figs. 10 and 11, with those of Lemicr nigri- 



frons., Plate XXXIV, figs. 16 and 17; Propithecus diadema, Plate XXXIV, figs. 15, 

 22 and 23; Propithecus Edwardsii,¥\aXe XXXIV. fig. 15; Avahis, Plate XXXIV, 

 figs. 18, 19 and 20; Indris, Plate XXXIV, figs. 6, 8 and 10. 



2. In Arctopithecini or marmosets, Plate XXXIV, figs. 11, 12 and 13, we find 

 the brain surface entirely smooth, with the exception that in some species, as 

 Hapale jacchus, fig. 13, the mesial occipito-temporal and frontal are partly outlined, 

 corresponding with the condition as formed in the human foetus at the end of the 

 fifth month, Plate XXXV, fig. 6. 



3. In the Platyrrhini and Catarrhini or simians of the Old and New World, 

 we can trace two corresponding lines of fissural and convolutional evolution. In 

 Chrysothrix, Plate XXXVIII, figs. 8 and 9, and Callithrix, Plate XXXIV, fig. 26, the 

 mesial occipito-frontal and temporal are well developed, extending, as indicated in 

 the morphological tj^pe laid down in fig. 9, from the position of the extremities of the 

 primary occipital arch, O 1 and O 2, forward to the end of the lobes, correspond- 

 ing with the human foetal brain as found in the middle of the sixth month, Plate 

 XXXV, fig. 7. In Cebus, Plate XXXVIII, fig. 12, we have a stage corresponding 



