332 MORPHOLOGY OF THE CEREBRAL CONVOLUTIONS. 



the posterior mesial portion of the gyrus occipitalis superior represents it. It only- 

 becomes evident as we ascend toward Man, in whom it attains its highest devel- 

 opment. This evolution of the cuneus appears, according to my observation, to- 

 take place as follows. We have seen that in the monkeys a small convolution, 2. 

 develops from the floor of the first occipital fissure, O 1 , and if figs. 5, 7, 14, 17, 

 Plate XXXVII, and fig. 12, Plate XXXVIII, be compared, it will be seen that be- 

 sides this convolution there is growing from the posterior walls of the occipital cleft, 

 in front of the scroll-like portion of the gyrus occipitalis primus (superior), a small 

 swelling marked en. This is the cuneus, and through its development we have the 

 fissura occipitalis interna pushed forward, in the same manner, that on the lateral 

 surface its outer continuation the fissura occipitalis externa (transversa of Ecker), 

 has been pushed backward by the development of the gyrus, 2, the pli de passage sup- 

 erieur externe. Thus we see that even in many of the lower Simiadee the external and 

 internal perpendicular fissures are not entirely continuous but are more or less 

 separated in the depths of the occipital cleft, O 1 , by the concealed convolution, 2, fig. 12, 

 PI. XXXVIII. The external portion of O 1 has been displaced backward to some 

 extent, which displacement, as in the human brain, can be seen when the sides of the 

 fissure are widely separated. Thus in the brain of Macacus nemestrinus, fig. 5, PI. 

 XXXVII, the internal extremity of the upper branch of the occipital arch may be 

 seen ending at O 1 , back of the internal portion, po, or parietal occipital, and separ- 

 ated from it by the posterior root of the convolution, 2, the further development of 

 which becomes, as we have seen above, the cuneus, and I have marked upon it Cn. 

 as showing its relation to the same part in the human brain. In another brain of 

 Macactis nemestrinus, PL XXXVII, fig. 7, we find the same condition of affairs only 

 farther advanced. In this brain the relations that I have indicated are distinctly 

 seen in the natural and undisturbed condition of the hemisphere. The backward pro- 

 longation of the fissura occipitalis superior is indicated by O 1 , whilst its anterior wall 

 is seen marked po. The walls of the primitive fissure have evidently been pushed 

 aside and a gap produced which is filled up by the developing cuneus and plis de passage, 

 a condition similar to that taking place on the lateral surface by means of which 

 the so-called gyrus and fissura par-occipitalis of Wilder have been produced. By 

 comparing figs. 5 and 7, PI. XXXVII, and fig. 12, PL XXXVIII with fig. 5, 

 PL XXXVI, it will be seen the parieto-occipital fissure is really a new fissure 

 of the same type as the par-occipital fissure of Wilder, being produced in the 

 same way, namely, by the development upward from the floor of a deep fissure, 

 of an arched gyrus, and in PL XXXVII, fig. 16 can be seen the way in which 

 the parieto-occipital is actually formed. It is marked po., and is really a new 

 velopment, formed only in those brains where the convolution, 2, exists (the superior 

 external pli de passage), which, when it reaches the lateral surface, arches around its 

 lateral extremity. In this condition it is not confluent with the calcarine fissure, as 

 it is in Man, but an indication of forces at work which will eventually produce such 

 a connection can be seen in small furrows found in this region in many brains (fig. 5, 

 PL XXXVII. A somewhat different condition is present in fig. 14, PL XXXVII, 



