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 perfec- 

 tion, 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 i° " 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 ex- 

 periences evolution, decay, and death. Thus the 



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