20 



minute bacteria. But granting that two micrococcus varie- 

 ties are actually, as well as apparently, identical in form, 

 that alone proves identity of function as conclusively as 

 the morphological identity of two sugar-coated pills, con- 

 taining arsenic and bread respectively, establishes the 

 unity of their effects. 



I have considered this subject at such length because 

 it will be necessary, in the course of these lectures, to 

 criticise certain experiments, and reject as not proven 

 certain conclusions, because attained by this method of 

 cultivation in liquids. The solution of many problems 

 would be, in my judgment, materially hastened, and 

 much conflict and confusion avoided by the substitution 

 for this still popular method of one demonstrably far 

 more accurate, to be presently described. Flask- and 

 tube-cultures are certainly convenient for cultivation en 

 masse, but evidently imperfect for investigations as to the 

 role of bacteria in disease. 



The fact that bacteria growing under different condi- 

 tions, and apparently functionally different, are morpho- 

 logically identical and all so widely distributed and 

 ubiquitous, renders it evident that one condition must be 

 fulfilled in order to demonstrate conclusively and finally 

 that the organisms present in a culture at a given moment 

 are the progeny of those previously planted by the ex- 

 perimenter ; the former must be observed to proceed by 

 continuity of structure, from the original bacteria. In 

 this way, and thus only, can all doubt and objection be 

 silenced ; all possibility of misinterpretation be elimin- 

 ated. The first requisite for the execution of this method 

 is evidently continuous observation of the growing bacte- 

 ria through the microscope ; a method long used. A 

 drop of nutrient liquid is placed on a cover-glass, the 

 bacteria sown therein, and the cover inverted over a 

 hollow slide or a cell of glass cemented to the slide ; the 

 edge of the cover is then or previously smeared with oil, 

 for the double purpose of limiting evaporation from the 

 droplet and of preventing the intrusion of foreign sub- 

 stances or organisms. 



