FACTS 151 



tion of invisible microbes. They are beings so small that, 

 under the greatest magnifying power, they are impercepti- 

 ble. And yet it has been possible to cultivate them in 

 bouillons where their multiplication can be observed owing 

 to their action on certain colloids or on certain living beings. 

 They are known only by what they do. We are sure they 

 are living because, by mixing a drop of the culture in which 

 they are supposed to be alive with as much bouillon as we 

 please, we can obtain just so much new culture having the 

 same pathogenic or diastasic properties as the first. These 

 very minute beings are, consequently, known only by the 

 fundamental phenomenon of assimilation. 



Certain authors think that these mysterious beings, en- 

 dowed with assimilative powers, are not visible because they 

 have no form or, in other words, because their living sub- 

 stance is soluble in the liquids in which they live. Had it 

 been possible to verify such an assertion, it would have 

 unsettled the whole general notion of cellular structure. 



This, it seems, must be given up. 



By using filters with pores sufficiently fine it is possible 

 to stop on the way all invisible microbes known and to steri- 

 lize their culture. Until we know more, therefore, it must 

 be taken as established that all living beings have a form, 

 even when they are too small to be known directly by 

 microscopic observation. 



Nevertheless, the existence of species so extremely small 

 takes all its value from the idea of cellular dimension. 

 There are unicellular rhizopods as large with relation to the 

 invisible microbes as the camel is with relation to a globule 

 of its own blood. 



One thing remains of importance in the idea of the cell, 

 after putting aside Haeckel's monera and species too small 

 for their structure to be known. It is the notion of small 



