I -Q THE GROWTH OF THE BRAIN. 



An attempt to combine these two views, one based 

 on embryology, and the other on the histology of the 

 adult cell, would lead us here too far afield ; suffice it to 

 say that both views assume that the incoming and out- 

 going impulses travel by separate pathways. Thus the 

 suggestion that a given branch may contain more than 

 one pathway is supplementary and not contradictory to 

 the accepted view. 



Though complete observations are wanting, we know 

 in a general way that the nerve elements begin as small 

 neuroblasts and grow large, the change in size involving 

 all parts of the cell. The increase is at first rapid, then 

 slow, but the time at which it ceases in any given cell 

 cannot be exactly determined. It is, however, not pos- 

 sible to identify the growing period of all the cells with 

 the growing periods of the individual, because the 

 histological study of the nerve tissues shows that all the 

 cells do not enlarge simultaneously. 



There is reason to think that the nerve cells which 

 begin to develop early increase in size as long as general 

 growth continues, itself a period of uncertain length, 

 while those cells which begin to develop later, increase 

 up to the end of the same period. But the dates at 

 which development may commence can be far apart, 

 and hence the periods during which the cells are grow- 

 ing can have very different lengths. The discussion of 

 the increase which occurs during development should be 

 carried on in the light of all the changes which are then 

 taking place. On examining a section of the developing 

 spinal cord in which the germinal cells are still dividing, 

 it will at once be seen that the germinal cells appear 

 larger than the neuroblasts by which they are surrounded, 

 and they also have a different character. The nucleus of 

 the germinal cell or its immediate descendant is sur- 

 rounded by cytoplasm clearly evident on all sides. In 



