238 LIFE OF ELIE METCHNIKOFF 



digestive organs, were too small in size for physio- 

 logical experiments. Thus, those two examples of 

 natural death among multicellular beings were un- 

 suitable to the projected study. 



He found a more favourable subject in the moth 

 of the silk-worm (Bombyx mori) ; the rudimentary 

 buccal organs of that insect make all feeding im- 

 possible and predestine it to a natural death. The 

 dimensions of the silk-worm moth are large enough 

 and it has a life duration of twenty-five or thirty 

 days, therefore sufficient to allow the study of the 

 mechanism by which its death is brought about. 

 Metchnikoff procured a quantity of silk-worms, and 

 soon the moths hatched and covered all the mantel- 

 pieces and tables in Norka with white flakes. He 

 ascertained that it was not hunger which brought 

 about the death of the moths, for their organism was 

 not in the least exhausted. 



The nutrition of the latter takes place at the 

 expense of the fatty substance which remains after 

 the metamorphosis of the chrysalis into a moth. The 

 dissolution of this fatty substance produces toxins 

 which pass into the urine. Thus the obvious cause 

 of the death of the moth is an acid intoxication by 

 toxic urine secreted in the bladder. As the latter 

 does not empty itself, uraemia becomes inevitable. 



The majority of moths contain no micro-organisms 

 which could suggest death by infection. 



The only theoretic objection against a natural 

 death might consist in the existence of " invisible 

 microbes." Indeed, the question of invisible microbes 

 revealed in certain infections perturbed Metchnikoff's 

 mind to such an extent that, during his last illness, he 

 used to say that it would have been a curse to his 



