MIDBRAIN AND THALAMUS OF NECTURUS 217 



As we pass from the lower mammals to the reptiles, the true 

 cerebral cortex shrinks abruptly to relatively small dimensions; 

 in reptiles, however, the subcortical centers of the cerebral 

 hemisphere (striatum complex and olfactory apparatus) are 

 relatively enormously developed. Again, as we pass from the 

 reptiles to the amphibians, the true cortex (i.e., the superficial 

 gray layer of the cerebral hemispheres) disappears entirely, a 

 small trace of it being present in the primordial hippocampal 

 area of the frog, and none at all in urodeles. The striatum 

 complex also is greatly diminished, and the olfactory apparatus 

 remains as the dominant functional system of the cerebrarl 

 hemisphere. In all types of fishes the dominance of the ol- 

 factory apparatus and the reductidji of the striatum complex 

 ('somatic area' of Johnston) are still more pronounced. 



The complex mesencephalic and diencephalic structures of 

 fishes are chiefly intrinsic apparatuses, i.e., correlation centers 

 developed in situ for the performance of the most complex 

 correlations of which these animals are capable ; in the reptiles, 

 birds and mammals, however, the increasing complexity of 

 the thalamic centers is directly related with the elaboration of 

 the mechanisms of the cerebral hemisphere. These parts of 

 the thalamus, therefore, constitute here a cortical dependency 

 in the sense of von Monakow, and are termed by Edinger the 

 neothalamus by reason of their functional relationship with the 

 neopallium. 



In the urodele Amphibia the midbrain and thalamus are in a 

 relatively generalized form, lacking the extreme specializa- 

 tion of the higher fishes, on the one hand, and of the amniote 

 vertebrates, on the other hand. The recent Amphibia them- 

 selves are, it is true, somewhat aberrant with reference to the 

 main line of vertebrate descent from fishes to mammals. 

 Nevertheless in the consideration of the morphogenesis of the 

 cerebral hemispheres these forms are very instructive, for here 

 the picture is uncomplicated by the great neencephalic systems 

 and the primitive apparatus of the brain stem is not elaborately 

 specialized in any direction. 



