98 MICRO-ORGANISMS IN RELATION TO MAN, 
which developed from the caterpillars, but also in the eggs 
they laid, and he found that when any one of these three 
forms was affected, the organisms were passed on to the next 
stage. He was able to show that they increased in the body, 
that they were the cause of the disease, and that by careful 
examination and destruction of the affected eggs, and the 
preservation of the healthy ones, the disease could gradually be 
eliminated. Pasteur thus succeeded in showing for the first 
time, the causal relationship between a certain specific 
micro-organism, endowed with definite morphological and 
physiological characters, and a specific and characteristic 
disease. It is, of course, almost superfluous to add that 
Pasteur’s conclusions were not received without considerable 
opposition by his contemporaries, and one objection which 
was vigorously urged was, in effect, that he had not employed 
pure cultures of any single micro-organism. 
I must here make a short digression to explain what 
bacteriologists mean by the term ‘ pure-culture.” 
Although, as previously stated, bacteria vary much in size 
and shape, yet the simple examination under the microscope 
of the form of the cells composing any growth has been shewn 
by experience to be insufficient for the determination of the 
purity, or otherwise, of the growth. It thus became necessary 
to devise a method by means of which absolute certainty 
could be obtained that any particular growth consisted of 
one species or variety only. This was first of all done by 
diluting the liquid containing the organism under examination 
to an enormous extent with sterilized water, and then adding 
a small definite volume of the diluted liquid to each of a series 
of flasks containing a suitable nutritive liquid. The volume of 
liquid to be added to each flask was so arranged that it should 
only contain on an average one single cell; and therefore any 
growth in the flasks was supposed to proceed from the one 
cell. A comparison of the physiological and chemical proper- 
ties of the different growths then enabled an opinion to be 
