MENTAL EVOLUTION OF MAN 215 



lished as a primitive nerve axis, which at first is nearly 

 uniform throughout its entire length and displays no 

 differentiation into brain and spinal cord. Before long 

 an enlargement of the anterior end expands and de- 

 velops into a primitive three-parted brain. It is not 

 yet a real brain, however, and it is entirely incapable 

 of functioning in such a way as to justify the use of the 

 word mental for the results of its operations. We know 

 that it is only in the cerebral hemisphere of the adult 

 brain that the processes of true human consciousness 

 go on. But it is not until long after the three-parted 

 stage that the cerebral hemispheres make their appear- 

 ance; therefore we cannot speak of mind as present 

 when the cell and tissue basis of mind is not present. 

 When, now, the cerebral hemispheres do appear, they 

 are small bean-shaped structures no larger relatively 

 than those of a fish. Later they enlarge so as to attain 

 the relative size of the cerebral hemispheres of an 

 amphibian, and still later they are like those of a 

 reptilian brain. Continuing to enlarge, they begin to 

 fold so that the total surface is increased without very 

 much addition to their bulk. At this time the cerebral 

 hemispheres of the brain of the human embryo are like 

 those of an adult cat or dog. The process of general 

 enlargement and of progressive convolution are con- 

 tinued, and stages are reached and passed which corre- 

 spond with the monkey and ape conditions. 



Nothing in human development is more impressive 

 than the origin of the cerebrum and its development 

 by passing through successive stages which are counter- 

 parts in the main of the adult brains of other and lower 

 animals. The alteration of a tissue-mechanism con- 



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