164 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



a higher degree of pollution than the actual count appears 

 to indicate. For the colony in the depth is" only visible when 

 it concentrates the stain added to the medium, either physically, 

 or by a process of intra-vitam staining — this is a point of 

 theoretical interest that might be investigated — and numerous 

 colonies will deplete the jelly of the added nutritive substances. 

 The more rapidly growing colonies will, therefore, " starve 

 out " those which grow more slowly : we may safely assume 

 that there are such individual differences among bacteria 

 of even the same species and culture. 



As a matter of fact this is easily proved by making a series 

 of decimal dilutions of the same culture. In the higher 

 dilutions we shall find that the numbers of colonies in the plates 

 are multiples, or sub-multiples, by 10 of some number. But 

 the counts of the lower dilutions, if the original culture 

 were sufficiently concentrated, are not so much as 10, 100, times 

 those of the middle dilutions. It is important to note, there- 

 fore, that the estimates of the number of bacteria in the 

 substance being investigated should be based on the higher 

 rather than on the lower dilutions ; and that dilutions should 

 be made so as to have few colonies on the plate, apart 

 altogether from the question of ease in making the counts. 



IV. Methods of Sampling. 



Precautions in taking and forwarding samples of mussels 

 usually take the form of keeping the shellfish cool by packing 

 the vessels containing them in ice. It is assumed that the 

 micro-organisms contained in the shellfish may multiply 

 if the temperature is raised, and that this may, indeed, be 

 the case can often be shown by keeping the sample for a day 

 or more at laboratory temperature. But if it is the case 

 that intestinal bacteria do not continue to five and reproduce 

 in sea- water, as the experiments described on pp. 128-133. 



