216 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



Island of Eeil and its opercula are only well developed in the 

 higher primates. Figs. 178, 179 and 180 represent the stages 

 in its evolution. In Fig. 178 the condition in dog-like apes is 



sup. front 



intra-parietal „„„„„„ j? 

 par-oca fis. 



affenspalte 



island of Reil 

 ant lim. sulcus (fronto-orbital) 



1st temp. fis. 



occip. fis. 



Fig. 178. — The Island of Reil and Fissures on the lateral Aspect of the Brain of a 

 dog-like Ape. 



represented. Only the upper and lower limiting sulci of the 

 Island are present. The Island, which is small, is continuous 

 anteriorly with the frontal lobe. In anthropoids (gorilla, etc.) 

 the Island is larger ; the upper and lower limiting sulci are 

 present ; an imperfect anterior limiting sulcus (fronto-orbital 

 fissure) is present and partially separates the Island from 

 the orbital surface of the frontal lobe. In man all three 

 limiting sulci are present and completely isolate the Island, 

 and occasionally this is the condition (Fig. 179 E) in the 

 higher anthropoids, but it is in man only that the orbital 

 operculum grows up and meets with the other opercula. 

 This can be the more easily understood when it is remembered 

 that the orbital part of the 3rd frontal convolution is connected 

 with speech. 



Temporary Fissures. — In the 3rd month of foetal life the 

 cerebral vesicles have thin walls, and when extracted from the 

 head show several fissures. These fissures, unlike those which 

 appear at the sixth and seventh months on the cortex of the 

 brain, are really inflections of the whole thickness of the cerebral 

 wall and are variable in number and position. It is possible 



