8 MICIIO-ORGANISMS IN WATER 



centres many seemed to be different varieties, some 

 being yellowish, brown, red, white, grey in colour, 

 whilst in shape they also presented wide divergences. 



On microscopically examining the nature of these 

 centres he ascertained that each consisted of one kind 

 of micro-organism ; that some, for example, were made 

 up of large micrococci, some of small micrococci, some 

 of bacilli, and so on ; that, in point of fact, each ceiitre 

 was in reality a colony or pure cultivation of one jiar- 

 ticular organism. 



If, instead of a potato, a surface of liquid culture 

 material, in area equal to that of the potato, was ex- 

 posed to the air, Koch found that, although undoubtedly 

 similar organisms gained access to the liquid as to the 

 potato, yet their development proceeded in a very 

 different manner. Thus, on submitting a portion to 

 microscopic examination, the hquid was found to 

 be teeming with all sorts and shapes of organisms 

 mixed up one with the other in inextricable confusion, 

 there being not the faintest approach to anything which 

 could be designated a pure cultivation. 



The difference in the development of the micro- 

 organisms in the two instances was not far to seek, in 

 the one case the culture material being solid, those 

 individual bacteria which gained access to it were im- 

 prisoned by rigid surroundings, and, being unable to 

 move from the spot, commenced to multiply, there 

 yielding in course of time a colony visible to the naked 

 eye. ^ On the other hand, those which were collected in 

 the liquid had no such restrictions imposed upon their 

 movements, and, being free to traverse the whole extent 

 of the hquid, multiphed indiscriminately in all directions, 

 and hence the medley of forms which was visible under 

 the microscope. Here, then, we have the first observa- 

 tions which led Koch to the elaboration of his beautiful 



