ORIGINS OF THE CEREBRAL CORTEX 73 



simpler than that of most fishes. In the frog, the 

 highest of the amphibians, most of the gray matter 

 of the brain is arranged close to the ventricles, which 

 is the early embryonic position. A cross-section of 



olfactorq nerve 

 olfactory bulb 

 lateral ventricle 



corpus striatum 

 lamina "terminalis 

 inTervenrricufar foramen 

 third ventricle 

 optic lobe 

 ere be Hum 

 urth ventiicle 



c Clonal hemisphere 

 thalamus 

 midbrain 



medulla oblongata 



Spinal cond 



Fig. 8. — Diagrammatic representation of an amphibian brain from 

 which the roof of the thalamus and cerebral hemisphere has been dis- 

 sected off on the right side, exposing the third ventricle and one of the 

 lateral ventricles. The membranous roof of the fourth ventricle has also 

 been removed. There is a communication between third and fourth 

 ventricles which is not exposed in this dissection. 



the cerebral hemisphere of the frog (Fig. 9) shows this 

 primitive arrangement in the lateral wall. The medial 

 wall is more highly differentiated. The wall of this 

 hemisphere is structurally divided into four sectors. 

 Of these, the two dorsal comprise the region within 

 which cerebral cortex is differentiated in higher 



