i86 AGE, GROWTH, AND DEATH 



nuclei, we must be very circumspect, and not fancy 

 too easily that we have reached a safe conclusion un- 

 less we have taken into consideration all the possible 

 factors by which the size may have been varied. 



In what I have said to you hitherto in regard to 

 the power of growth, I have directed your attention 

 chiefly to the power of growth as it exists in a cell in 

 consequence of that cell's condition. When the cell 

 is in the young state, it can grow rapidly ; it can mul- 

 tiply freely ; when it is in the old state it loses those 

 capacities, and its growth and multiplication are cor- 

 respondingly impeded, and if the organisation is car- 

 ried to an extreme, the growth and the multiplication 

 of the cell cease altogether. 



We find, however, that it is not merely a question of 

 the capacity of the cells, but also of the exercise of that 

 capacity, which we must deal with. Here enters a factor 

 of which we learn from the study of regeneration. The 

 phenomena of regeneration are important and very in- 

 structive. We shall come to them presently. It will 

 make our study of regeneration clearer, more signifi- 

 cant, I think, if we pause for a moment to consider 

 certain fluctuations in the natural development of the 

 organism. We see, for instance, in the brain that early 

 the cells begin to assume the character of nerve cells and 

 that thereafter their multiplication ceases. But, cur- 

 iously, there will be a spot in the spinal cord, for ex- 

 ample, where the change of the cells into nerve cells 

 has not taken place, and from that growth will go on. 

 Cells will migrate from that spot and reach their ulti- 



