172 



THE CEREBRUM. 



status, hence chemicals, electricity and diseases appear to exer- 

 cise a selective power and affect certain neurones without in- 

 fluencing others. 



Development (Figs. 52 and 128). Every neurone is the de- 

 rivative of an epiblastic cell, called the germinal cell of His, which 

 is located in the neural tube or crest. This germinal cell, in 

 the neural tube, sends out a process from what was originally 

 its deep end, and becomes a neuroblast. This process forms the 

 axone and, later, dendrites are produced. During development 



A 3 



Fig. 52. Diagram showing development of neurones in the spinal cord. 

 (McMurrich after Schd/er.~) 



The circles, indifferent cells ; circles with dots, neuroglia cells; shaded cells, germinal cells; 

 circles with cross, germinal cells in mitosis; black cells, nerve-cells. 



the cell-body wanders more or less from its original position, and 

 thus reaches its adult location. The neurones are in this manner 

 aggregated into nuclei and cortex. The germinal cells of the 

 neural crest form bipolar neuroblasts, which send out a process 

 from each extremity. The axone appears first, the dendrite later, 

 as in the neural tube. The resulting neurones make up the 

 cerebral, spinal and sympathetic ganglia. In the spinal ganglia 

 and in the common sensory cerebral ganglia, the neurones become 

 converted into the unipolar form by the lateral growth of the cell- 

 body and the fusion, for a short distance, of the two processes 



