148 Journal of Comparative Neurology. 



in regions in which there is rapid destruction of nervous 

 matter or activity is partially dormant. Such nuclei are 

 variously interpreted, but recent studies have convinced 

 the author that many of them are simply dormant nerve 

 cells which may become functional at a later period. In 

 the tectum opticum, for example, the transition between 

 the granules and functional ganglion cells is complete. It 

 has also been shown that, of the vast number of granules in 

 the inner layer of the cerebellum, many at least are of a 

 nervous character and possess a certain amount of proto- 

 plasm with the usual processes. Mr. Turner and the 

 writer have suggested that the rosette clusters of cells in 

 the axial lobe of reptiles and birds may also be of the 

 nature of proliferating centres. 



From the anatomical standpoint, therefore, it is reason- 

 able to admit the hypothesis that, although the number of 

 brain cells is to a certain extent predetermined and they 

 are separately outlined in an early stage, yet they do not 

 become functional until exigencies require it and the period 

 and rate of becoming functional may depend largely upon 

 such matters as nutrition, mental exercise, etc. It seems 

 probable that these newer cells are at first to a certain ex- 

 extent indifferent in function and that, by the interpenetra- 

 tion of their terininal brushes with the reticulum formed by 

 "dentrites" of other cells, they come to participate in the 

 general functions of the reticulum and the elements of 

 which it is formed. Such cells might easily substitute for 

 other cells that had been injured or removed by accident 

 or worn out by use. Nevertheless it seems improbable 

 that a complete renewal of cells takes place, for, in that 

 case, the physical basis of memory would remain quite as 

 obscure as it has hitherto seemed. It is true that almost 

 the only hints we have respecting the physical basis of 

 memory are such as grow out of the known connection 

 between cell-reproduction and the chromatin and other 



