Page:Life and death (1911).djvu/354

 CHAPTER VI.

LETHALITY OF THE METAZOA AND OF DIFFERENTIATED CELLS.

Evolution and death of metazoa.—Possible rejuvenescence of the differentiated cells by the conditions of the medium.—Conditions of the medium for immortal cells.—The immortal elements of metazoa.—The element in accidental and remediable death.—Somatic cells and sexual cells.

Evolution and Death of Metazoa.—We have seen that the infusoria are no longer animals in which material exchanges take place with sufficient perfection, and in which cellular division, the consequence of growth, is produced with sufficient precision and equality for life to be carried on indefinitely in a perfect equilibrium in the appropriate medium without alteration or check. A fortiori we no longer find the perfect regularity of nutritive exchange in the classes above them. In a word, starting from this inferior group, there are no animated beings in the state of existence which Le Dantec calls "condition 1^o" of manifested life?" Living matter, instead of being continually kept identical in conditions of identical media, is modified in the course of existence. It becomes dependent on time. It describes a declining trajectory; it experiences evolution, decay, and death. Thus the