430 Profs. Percy Frankland and Marshall Ward. 



matter, which I had to test in various ways before deciding on a 

 satisfactory plan for it must be borne in mind that one has to so 

 arrange matters that the necessary phase of growth shall occur at a 

 time convenient for observation and measurement, otherwise it would 

 be imperative on one to be ready to take the measurable phase at any 

 hour of the day or night it may therefore suffice to say that further 

 reflection on all the circumstances suggests that the following con- 

 ditions would have to be fulfilled before satisfactory results could be 

 obtained. It will be obvious that some of these conditions can not 

 be completely satisfied, so that observers will have to be content to 

 approach them as nearly as possible. I give the ideas as they arise, 

 but perhaps other investigators can suggest improvements. 



1. The temperatures employed should be constant, and this condi- 

 tion can be so nearly approached that we may regard practice as 

 according with theory. The difficulties have been sufficiently dis- 

 cussed as they arose. 



2. The food-material and medium employed should be constant. 

 This condition can also be approximately carried out, because although 

 (1) the water-contents of the hanging-drop vary at different tem- 

 peratures, and (2) no food-medium remains exactly the same from 

 day to day, however carefully prepared and kept, considerable 

 accuracy can be attained. This difficulty cannot be got over by 

 using a fresh brew each time, because no two brews can be exactly 

 alike. With great care, the nearest approximation to uniformity 

 must be aimed at. 



3. The spores used must be from the same culture. This can be 

 realised, and, apart from theoretical differences as the age of spores 

 increases, we may regard this condition as satisfied. 



4. The culture-cells used must be prepared in the same way, and 

 be loaded with the same water supply, and havethe same air-contents. 

 Practically this condition is also met. 



5. The culture-drops should always be of the same size, and supply 

 equal amounts of food-materials. Here we cannot do more than 

 approximate as closely as possible to the requirements, though much 

 can be done by practice, using the same loop, and so on. 



6. Strictly speaking, the drop ought always to contain the same 

 number of spores preferably, no doubt, one spore but it would be 

 impossible to meet this condition exactly, in working with large 

 numbers of cultures, as I have to do, and so approximation only can 

 be hoped for. I regard the matter of importance, however, for it is 

 impossible to overlook the probability amounting to all but cer- 

 tainty that each filament is to some extent an antagonist to all its 

 neighbours : competing with them for oxygen, water, and food- 

 materials, and, no doubt, affecting the medium injuriously as its 

 metabolism proceeds. With great care and long practice it is 



