ORIGINS OF THE CEREBRAL CORTEX 75 



question has never been adequately tested. In the 

 frog the case is different, for decerebration is followed 

 by profound depression and loss of initiative or so- 

 called spontaneity. In the absence of external stimu- 

 lation the animal tends to squat motionless to the 

 end of life. Most of the ordinary reflex responses to 

 external stimulation (except olfactory) are, however, 

 unimpaired. The cerebral hemispheres here do seem 

 to exert some sort of a dynamogenic influence upon 

 behavior as a whole (see p. 210), though its precise 

 nature has not been determined. 



THE SAUROPSID TYPE 



The reptilian cerebral hemisphere is considerably 

 enlarged as compared with any fishes or amphibians. 

 Figure 10 illustrates the general form of the forebrain 

 of a turtle as seen in longitudinal section, and Figure 

 23 (p. 106) shows a cross-section taken through the 

 middle of the hemisphere. It is evident that the 

 enlargement over the condition found in the frog 

 (Fig. 9) is chiefly in the lateral and dorsal walls; 

 laterally, we have the corpus striatum complex and 

 dorsally true cortex. The relations of these two parts 

 will be discussed more in detail in the next chapter. 



In the most generalized existing reptilian brains 

 (turtles), clearly differentiated cortex has emerged 

 from the reflex centers of the cerebral hemisphere as 

 three separate sheets of superficial nerve cells on the 

 dorsal convexity of the hemisphere. Figures 22-24 



