498 



COMPARATIVE ANATOMY 



tionately. Vision in primates is more important than smell, and brain 

 changes express relative functional values. 



Cortex. Two parts of the primitive vertebrate brain participate 

 specially in the great enlargement of the cerebral hemispheres, the striate 

 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 

 elasmobranchs and dipnoi, which are more directly in the line of mam- 



B C ' ' D 



Fig. 413. — Horizontal diagrams of ichthyopsid brains. A, sturgeon; B, elasmo- 

 branch; C, teleost; D, amphibian. Primitive forebrain wall stippled; telencephalic 

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

 interventricular foramen; I, lateral ventricle; m, midbrain; o, olfactory bulb; t, terminal 

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

 of Vertebrates," after Herrick.) 



malian ancestry, is relatively thick. Homologies with the pallium of 

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

 In the pallium of fishes the cellular gray matter is adjacent to the ventricle, 

 while the external layer is fibrillar. Even in the pallium of fishes, how- 

 ever, some cells migrate from the gray into the fibrillar zone. (Fig. 413) 



The pallium of amphibia, taking Rana as a type, is thick, and is 

 differentiated into a median archicortex and a lateral paleocortex, both 

 associated 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. 



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. (Fig. 414) 



The evolution of the cortex is accompanied by cellular changes. 

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



