346 RALPH S. LILLIE. 



spon>i\eness, as already pointed out. implies a state of the plasma 

 membrane in which changes of permeability and of electrical 

 polarization are produced with difficulty. The increasingly 

 resistant character of the membranes implies altered composition. 

 Possibly the condition in over-ripe eggs, as well as in senescence, 

 results from a progressive accumulation or adsorption at the 

 phase-boundaries, i. e., in the membranes, of materials which for 

 some reason are not readily eliminated by the organism. 1 A 

 ^imilar view has recently been expressed by Child. 2 According 

 to his theory "senescence in nature consists physiologically in a 

 decrease in the- rate of metabolism, and this is determined mor- 

 phologically by the accumulation in the cell of structural ob- 

 stacles to metabolism, e. g., decrease in permeability, increase in 

 density, accumulation of relatively inactive substances, etc." 

 Minot's 3 theory that senescence is the expression of a progres- 

 --ively increasing differentiation of cells, i. e., of an increase in the 

 proportion of separated solid structural material, bears a certain 

 resemblance to this view. The view which I have suggested 

 above is distinctive only in so far as it attributes the essential 

 change to a modification of the membranes. It is, theoretically at 

 leaM, within the possibilities of physiological science to prevent 

 or retard this accumulation of inert materials in the membranes 

 and so to delay senescence. Or the already modified membranes 

 might, it not too profoundly altered, be restored to a normal 

 condition by certain forms of treatment. The simpler the metab- 

 olism and the less widely differentiated the tissues, the greater 

 would appear to be the possibility of such "rejuvenescence" <>l 

 the organism as a w r hole. In one large group, tin- Protozoa 

 senescence seems not to be an inevitable occurrence; the earlier 

 interpretation of conjugation as a process whose essential role 

 is to counteract an innate tendency to senescence has been 

 discredited by the work of Calkins and Woodruff. The condi- 

 tions in Mcta/oa differ from those in Protozoa chielly in their 

 greater complexity, but probably in no other essential respect. 



'Probably certain colloidal (i. e., iixliitu-iblr i ,m<l . -li. mirully n-tractory by- 



l>i''xlucts of nn-uilinli-iii. 



2 Cf. Child, Ariliir ftir Enlwicklungsmechanik, HJM. Y<>1. .u. !> 5,i7- 



3 C. S. Minot, "The Problem ol Age, Growth .in<! D.-.uli." NYw York ami 



London, 1908. 



