THE SIZE OF ORGANISMS 197 



metabolism and is brought about in nature by the removal in one way 

 or another of structural obstacles to metabolism." 



It is well known that constructive metabolism can not take place in 

 the absence of a nucleus, and I have elsewhere" shown that the inter- 

 change between the nucleus and the protoplasm is a condition of assimi- 

 lation. I have likewise shown that only the general protoplasm can 

 enter the nucleus and that the products of differentiation are excluded 

 from it. The progressiva increase of such products and corresponding 

 decrease in the general protoplasm lessen this interchange between 

 nucleus and cell body and thus decrease the power of constructive 

 metabolism. 



In conclusion it may be said that there are several factors which 

 produce senescence, but that the chief of these is the progressive differ- 

 entiation of the protoplasm. As Minot has well said " Old age and 

 death are the price which we pay for our differentiation." If we could 

 find means by which this progressive differentiation could be stopped or 

 reversed when it has gone too far, we might hope to attain potential 

 immortality. That the possibility of this is not a mere delusion is 

 shown by the fact that there are many animals which either in whole 

 or in part are capable of rejuvenescence. In Protozoa the dedifferentia- 

 tion which usually precedes or accompanies division is a process of 

 rejuvenescence, and where such dedifferentiation and division are long 

 delayed even protozoans show signs of old age. The same is true of 

 germ cells; the mature egg and sperm are senile cells not because the 

 one has a very large nucleus and the other a very small one, but because 

 both are loaded with products of differentiation which interfere with 

 constructive metabolism. When the sperm enters the egg and either 

 leaves behind its old cell body or dissolves it, and its nucleus gets a new 

 protoplasmic body, it is rejuvenated; likewise when the egg begins to 

 dissolve the yolk and other products of differentiation with which it 

 has been loaded it begins to live anew. 



Similarly any adult animal or plant which is capable of dedifferen- 

 tiation is also capable of renewing its youth. It has long been known 

 that encystment and accompanying loss of differentiation lead to 

 rejuvenescence. Jacobs,^" working under my direction, found that 

 when the rotifer, Philodina, becomes senescent, it may be rejuvenated 

 if it is completely dried up and afterwards put back into water ; in this 

 treatment it evidently undergoes dedifferentiation. 



Child^^ found that after planarians in a condition of apparent 

 extreme senility had been starved for some time, they afterward became 

 young within a few hours or days. Evidently the starving served to use 

 up a part of the structural substance which prevented rapid metabolism. 



•"Jacobs, "The Effects of Desiccation on the Rotifer Philodina roseola, 

 Jour. Exp. Zool., 6, 1909. 



