264 CHORDATE ANATOMY 



body and the neopallium. These two regions are not clearly differentiated 

 in cyclostomes, but are distinguishable in fishes. Of the two, the pallium 

 changes more. The hemispheres of teleost fishes have a thin epithelial 

 pallium or mantle. Compared with the mantle of teleosts, that of elasmo- 

 branchs and dipnoans, which are more dkectly in the fine of mammaUan 

 ancestry, is relatively thick. Homologies with the palHum of higher 

 vertebrates are difficult on account of the lack of differentiation. In the 

 palHum of fishes the cellular gray matter is adjacent to the ventricle, while 

 the external layer is fibrillar. Even in the palUum of fishes, however, some 

 cells migrate from the gray into the fibrillar zone. (Fig. 323) 



The palHum of Amphibia, taking Rana as a type, is thick, and is differ- 

 entiated into a median archicortex and a lateral paleocortex, both asso- 



A ' ^ B C ' ■ D 



Fig. 323. — Horizontal diagrams of ichthyopsid brains. A, sturgeon; B, elasmo- 

 branch; C, teleost; D, amphibian. Primitive fore-brain wall stippled; telencephalic 

 evaginations horizontally lined; thalamus vertical lines, c, common ventricle, /, inter- 

 ventricular foramen; /, lateral ventricle; m, mid-brain; o, olfactory bulb; t, terminal 

 lamina; II, optic nerve; 3, third ventricle. (From Kingsley's " Comparative Anatomy 

 of Vertebrates," after Herrick.) 



ciated with olfactory fibers. In reptiles the number of cell layers in the 

 pallium increases to three. The medio-dorsal region of each hemisphere 

 forms an archicortex or hippocampus. In the lateral pallium, dorsal to 

 the striate body, are possibly the beginnings of a neocortex. (Fig. 324) 



A true many-layered neocortex appears in all mammals and enlarges 

 so much that the paleocortex is crowded into a ventral position and the 

 archicortex pushed dorsally toward the median plane. The number of 

 cell layers has increased until five are distinguished in most, if not all, 

 mammals. 



The evolution of the cortex is accompanied by cellular changes. In 

 the pallium of lower vertebrates, cell bodies lie close together. The 

 thickening of the cortex in mammals is correlated with separation of the 

 cells, which, however, retain connexion with one another by means of 

 elongated dendritic processes, the number of interconnexions with adjacent 



