Replicated Ref# 14 and 29] (Wikisource contributor note)
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[1] shown that the interchange between the nucleus and the protoplasm is a condition of assimilation. I have likewise shown that only the general protoplasm can enter the nucleus and that the products of differentiation are excluded from it. The progressive 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 differentiation 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 dedifferentiation 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 dedifferentiation is also capable of renewing its youth. It has long been known that encystment and accompanying loss of differentiation lead to rejuvenescence. Jacobs,[2] 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[3] 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.
