ORIGINS OF THE CEREBRAL CORTEX 79 



It evidently can influence behavior mediated by the 

 lower centers while this is in process, but apparently 

 little machinery has been elaborated for taking the 

 initiative in the control of this behavior through 

 processes intrinsic within the cortex itself. Accord- 

 ingly, when a turtle is decorticated there is no very 

 obvious change in the behavior under ordinary 

 laboratory conditions. Doubtless there is an effect 

 upon behavior, but experiments to reveal it have not 

 yet been published. 



A type of cerebral hemisphere similar to that seen 

 in turtles seems to have been the point of departure 

 for further evolution of the vertebrate cerebrum. 

 This evolution evidently took two divergent direc- 

 tions: one leading to birds, the other to mammals. 

 The avian line leads up through the higher reptiles, 

 notably the crocodiles, and can readily be followed. 



Figure 25 (p. no) illustrates the brain of the 

 alligator. The subcortical striatal mass is much larger 

 than in turtles and the cortex is more extensive. The 

 three cortical fields are not so well separated ana- 

 tomically as in the turtle, and a small field at the 

 anterior end is electrically excitable (Bagley and 

 Richter, 1924). In birds the enlargement and compli- 

 cation of the striatum complex has advanced to its 

 culmination in the vertebrate series, but the cerebral 

 cortex has made no corresponding advance. In fact, 

 the avian cortex is more simply organized than the 

 reptilian. 



