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, th'ere 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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