DEVELOPMENT OF NERVE ELEMENTS. 169 



when developed are typically dineuric, and hence the 

 number of fibres emerging from either end of the 

 ganglion should be equal to one another, and also equal 

 to the number of cells in the ganglion. Nelson x first, 

 and then Hodge, undertook to determine the number 

 of fibres in the dorsal roots, and also the number of cells 

 in the associated ganglia. Nelson found in small frogs 

 the number of cells more than twice that of the fibres. 

 Hodge has obtained figures which are similar. 2 These 

 figures suggest that all the cells which are there present 

 have not yet sent out their neurons, and that in the 

 larger and older animals the proportion of the developed 

 cells in the ganglion would increase, and hence the pro- 

 portion of cells to fibres would diminish ; but for this 

 explanation good evidence is still wanting. 



By comparison it appears that during early life the 

 development of nerve cells goes on more actively in man 

 than in the frog, but the difference is one of degree only, 

 and, as we shall later see, in the cerebral cortex at least 

 it continues in man for a very long time. The data to 

 be presented will be useful in interpreting the size of the 

 brain, but as a preface to such an interpretation the 

 relative development of the grey and white substance is 

 first to be determined. 



During early fcetal life there is no distinction between 

 white and grey matter, the entire system being grey, 

 but with increasing age the medullary substance appears, 

 the change first occurring in the cord. According to the 

 estimates of De Regibus, the grey matter forms 58*5 per 

 cent, of the entire weight of the cerebral hemispheres. 

 What is designated " grey " matter in these cases is 

 really a mixture of cells and fibres, and the cells, 



1 His records were destroyed when the Science Hall of the 

 University of Wisconsin was burned in 1884. 

 3 Hodge, Am. Journ. of Psychology, 1889. 



