ORIGINS OF THE CEREBRAL CORTEX 87 



the hemisphere both hippocampal and pyriform 

 cortex of the opossum become very compHcated, but 

 certain features of the reptihan pattern are always 

 preserved. Figure 13 shows the arrangement of cells 

 in the cortex in the middle of the pyriform lobe of the 

 opossum. The most characteristic neurons of all parts 

 of the reptilian cortex and of all parts of the archipal- 

 lium of mammals are the double pyramids to which 

 reference was made above. Typical forms of these 

 cells are shown in Figure 15. 



The double pyramids of the pyriform cortex are 

 arranged in a single compact layer (II, of Fig. 13). 

 One big dendrite extends toward the outer surface 

 of the brain. Here in the plexiform layer (I, of Fig. 

 13) synaptic junctions are made with various incom- 

 ing fibers, chiefly from subcortical olfactory centers. 

 A second tuft of dendrites is directed inward and 

 effects synaptic junctions with various associational 

 fibers, chiefly from other cortical fields. This type of 

 polarization of these cells is possible only because the 

 dense cortical sheet is so thin that each neuron can 

 span its entire thickness, thus receiving incoming 

 nervous excitations of different physiological sorts at 

 outer and inner poles. Each of these cells is a correla- 

 tion neuron of the physiological type already illus- 

 trated in a simpler form in the amphibian midbrain 

 (Fig. 3, p. 48). Adequate switchboard space for the 

 larger number of elements is provided by removal of 

 the cells from their primary position next the ventricle 



