16 C. M. CHILD. 



plasm, but it is possible for a cell or an organism to grow old 

 without any such increase in bulk of the cytoplasm, merely 

 in consequence of the increase in density and impermeability 

 of its membranes, or in consequence of loss of water. 



Moreover, I am unable to accept Minot's conclusion that 

 no cells except the egg undergo rejuvenescence. I have pointed 

 out above that rejuvenescence undoubtedly occurs in many 

 somatic cells in the lower organisms and I believe the same is 

 true, at least for certain cells, even in mammals. In gland 

 cells for example, the loading of the cell seems to me to repre- 

 sent in all respects a period of senescence, ending in almost 

 complete quiescence. With the beginning of the discharge of 

 the accumulated material rejuvenescence begins and at the end 

 of this period the cell is both morphologically and physio- 

 logically young, though not necessarily embryonic, that is to 

 say, the process of rejuvenescence which it has undergone does 

 not necessarily result in the disappearance of its characteristics 

 as a gland cell, but it has undoubtedly become a young gland 

 cell. 



According to Minot, the egg cell at the end of its growth 

 period is an old cell because the amount of cytoplasm in relation 

 to the nucleus is very great. According to my criteria of senes- 

 cence the egg cell is old and approaching death because the 

 accumulation of structural material in it is so great that metab- 

 olism is reduced almost to a minimum. We know that the 

 animal egg at the end of the growth period is almost quiescent. 

 Warburg ('10) has shown that its oxygen requirement is very 

 low as compared with that after fertilization. It is evident 

 that it is capable of but little further metabolic activity. After 

 its isolation from the body it undergoes at most only the two 

 maturation divisions and then unless fertilized or otherwise 

 stimulated to renewed activity it dies after a short time. 



Even the maturation divisions show certain features that 

 indicate a low rate of metabolism. In the first place they are 

 not separated by a period of nuclear growth: apparently nuclear 

 synthesis is impossible. And secondly, although we know 

 little of the physiological significance of the peculiar chromatic 

 phenomena, yet there are some indications that they too are 



