LETHALITY OF THE META^OA. 34I 



fundamental condition of invariable youth and 

 of immortality fails in all metazoa. The vital 

 wastes accumulate in all through the insufficiency 

 or the imperfection of nutritive absorption or of 

 excretion. Life decays ; the organism progressively 

 alters, and thus is constituted that state of de- 

 crepitude by atrophy or chemical modification 

 which we call senescence, and which ends in death. 

 To sum up, old age and death may be attributed to 

 cellular differentiation. 



Possible Alimentary Rejuvenescence of the Differ- 

 entiated Cells — Conditions of Medium. — We must add, 

 however — as the teaching of experiments in general 

 and in particular as the teaching of the experiments 

 of Loeb and of Calkins — that a slight change of the 

 environment, made at the right time, is capable of 

 re-establishing equilibrium and of completely re- 

 juvenating the infusorian. Senescence has not in 

 this case a definitive any more than an intrinsic 

 character; a modification in the composition of the 

 alimentary medium will successfully resist it. If we 

 are allowed to generalize this result, it may be said 

 that senescence, the declining trajectory, the evolu- 

 tion step by step down to death, are not for the 

 cells considered in isolation an inevitable and essen- 

 tially inherent in the organism, and a rigorous con- 

 sequence of life itself They preserve an accidental 

 character. In senescence and death there is no 

 really natural, internal cause, inexorable, and irre- 

 mediable, as was claimed in the past by J. Muller, 

 and more recently by Cohnheim in Germany and 

 Sedgwick Minot in America. 



Conditions of the Medium for Immortal Cells. — As 

 for the cells which are less differentiated, the proto- 



