THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 103 



pallial fields seen in Amphibia; and the dorsal and lateral cortex 

 (general cortex and piriform cortex) are related to a massive sub- 

 cortical thickening of the lateral wall of the hemisphere, which was 

 called the "dorsal ventricular ridge" by Johnston and "hypopal- 

 lium" by Elliot Smith. The thalamic radiation, comparable with the 

 amphibian tractus thalamo-frontalis, is large in turtles. It ends 

 chiefly in the rostral part of the hypopallium, but some of these fibers 

 pass through without synapse into the dorsal or general cortex. The 

 latter fibers are true thalamic sensory projection fibers with connec- 

 tions of neopallial type. In front of this region there is a "pallial 

 thickening," from which motor cortical projection fibers go out to the 

 cerebral peduncle — again a neopallial type of connection — and this 

 part of the cortex is electrically excitable (Johnston, '16). 



In the alligator the topographic relations are very different. The 

 dorsal, or general, cortex has no contact with the hypopallium except 

 at its rostral end in the region of the pallial thickening of turtles, 

 which Crosby ('17, pp. 358, 381, figs. 5, 6) calls "primordial general 

 cortex." This primordium she regards as the germinal tissue or focal 

 point in the differentiation of the general cortex, and she gives a clear 

 statement of the factors which probably were operative in the dif- 

 ferentiation of this general cortex. 



The dorsolateral sector of the anterior olfactory nucleus of Nec- 

 turus, together with some adjoining tissue, is just such an area of 

 basal, i.e., subpallial, type as Crosby postulated; it is in the exact 

 position with reference to other parts of the hemisphere as her pri- 

 mordial general cortex; and it receives especially strong fascicles of 

 the strio-pallial association, which turn far forward to reach it. It is 

 significant that in the alligator this primordium is the only region 

 where both projection fibers of the lateral forebrain bundle and 

 shorter fibers from the hypopallium and corpus striatum can connect 

 with the general cortex (Crosby's figs. 5-9, 12-19, 37). Bagley and 

 Langworthy ('26) have shown that in the alligator this area and 

 parts of the cortex adjoining are electrically excitable, thus furnish- 

 ing experimental proof that true motor projection fibers of neopallial 

 type arise from it. The underlying hypopallium was tested and found 

 to be unexcitable. Ariens Kappers ('29, p. 140) accepts Crosby's inter- 

 pretation of the reptilian primordial neopallium and states that in 

 the lizard, Varanus, a small number of thalamic projection fibers 

 ascend directly to this area. This, he says, is the source of the 



