_Conditions of the Medium for Immortal Cells._—As for the cells which are less differentiated, the protophytes and the protozoa situated one degree lower in the scale than the infusoria, we must admit the possibility of that perfect and continuous equilibrium which would save them from senile decrepitude. And it is quite understood that this privilege remains subordinated to the perfect constancy of the appropriate medium. If the latter changes, the equilibrium is broken, the small insensible perturbations of nutrition accumulate, vital activity decays, and in sole consequence of the imperfection of the extrinsic conditions or of the medium, the living being finds itself once more dragged down to decay and to death. _Immortal Elements of the Metazoa._—All the preceding facts and considerations refer to isolated cells, to monocellular beings. But, and this is what makes these truths so interesting, they may be extended to all cells grouped in collectivity—i.e., to all the animals and living beings that we know. In the complicated edifice of the organism, the anatomical elements, at any rate the least differentiated, would have a continual brevet of immortality. Generally speaking, this would be the case for the egg, for the sexual elements, and perhaps, too, for the white globules of the blood, the leucocytes. And, further, around each of these elements must be realized the invariably perfect medium which is the necessary condition. This does not take place. _Elements in Accidental and Remediable Death._—As for the other elements, they are like the infusoria, but without the resource of conjugation. The ambient medium becomes exhausted and intoxicated around each cell, in consequence of the accidents which happen to the other cells. Each therefore undergoes progressive decay, and finally they perish—the decay and destruction being perhaps in principle accidental, but, in fact, they are the rule. The different anatomical elements of the organism are more or less sensitive to those perturbations which cause senescence, necrobiosis, and death. There are some more fragile and more exposed. Some are more resisting, and finally, there are some which are really immortal. We have just said that the sexual cell, the ovum, is one. It follows that the metazoan, man for instance, cannot entirely die. Let us consider one of these beings. Its ancestors, so to speak, have not entirely disappeared; each has left the fertile egg, the surviving element from which has issued the being of which we speak; and when it in its turn has developed, part of that ovum has been placed in reserve for a new generation. The death of the elements is not therefore universal. The metazoan is divided from the beginning into two parts. On the one hand are the cells destined to form the body, _somatic_ cells. They will die. On the other hand are the _reproductive_, or _germinal_, or _sexual_ cells, capable of living indefinitely.