THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 101 



human cortex, and probably is its precursor, though this apparently 

 is not its only relationship. 



In all amphibian brains these sectors of the pallial field can be 

 identified, though no superficial cortical gray is present in any of 

 them, as illustrated in figures 96-99. On the lateral aspect thepiriform 

 area (figs. 85, 86, 111, p.pir.) shows no evidence of cortical differenti- 

 ation; it is, in fact, the chief secondary olfactory nucleus (nuc.ol.d.L); 

 nevertheless, its location and connections identify it unmistakably 

 as the primordium of the piriform cortex of reptiles and lower mam- 

 mals. Its neurons are small, simple, and similar to those most com- 

 monly seen in the brain stem (figs. 98, 99, 105). On the dorsal con- 

 vexity there is an undifferentiated and poorly defined field (p.p.d.), 

 which is the precursor of the reptilian dorsal cortex. 



The medial sector — primordium hippocampi (p.hip.) — shows a 

 first step toward cortical differentiation, for here the compact central 

 gray layer is dispersed by outward migration of the cells throughout 

 the thickened wall, and these are imbedded in dense neuropil. These 

 neurons vary from small to quite large and, in general form, resemble 

 those of other parts of the pallium, though they are evidently more 

 specialized (figs. 97, 98, 99, 105). One to several thick and thorny 

 dendrites arise from the cell body and spread widely, some reaching 

 the external limiting membrane. The axon may arise from the cell 

 body, but usually from the base of one of the dendrites, It may 

 divide, sending one branch into the dorsal pallium and one to the 

 septal nuclei or medial forebrain bundle (fornix). Some axons are 

 short, branching freely within the area of spread of the dendrites 

 (fig. 105; '396, fig. 44), but most of them send one or more long 

 fibers from this arborization into tracts which leave this field. Close 

 to the surface are a few tangential neurons which at one time I re- 

 garded as precursors of the reptilian cortical cells. Similar cells are 

 found also throughout the brain stem, and in the. pallium (fig. 98) 

 their axons take short courses as correlation fibers. They are more 

 numerous in the frog (P. Ramon y Cajal, '22, figs. 6, 7). The dif- 

 ferentiation of the hippocampal cells is further advanced in Amblys- 

 toma than in Necturus ('33a, p. 183) and less so than in anurans. 

 The hippocampal neuropil increases in density and complexity as we 

 pass from Necturus to Amblystoma and the frog. 



The four layers of neuropil characteristic of the brain stem (p. 30) 

 are very unequally developed in the amphibian hemisphere (Nec- 

 turus, '336, p. 176). The periventricular neuropil is everywhere 



