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 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.

Somatic and Sexual Cells.—In this sense we may say with Weismann that there are two things in the animal and in man—the one mortal, the soma the body, the other immortal, the germen. These germinal cells, as in the case of the protozoa we mentioned above, possess a conditional immortality. They are imperishable, but on the contrary, are fragile and vulnerable. Millions of ova are destroyed and are disappearing every moment. They may die by accident, but never of old age.