v.] METHODS OF INOCULATION. 51 



medium. The various species, owing to difference of growth, 

 form clusters differing in aspect, size, and arrangement. 



A good plan of ascertaining the relative number of certain 

 bacteria present in given samples of water is that by means 

 of plate-cultivations. A definite small quantity of the 

 water is added to a definite quantity of liquefied sterile 

 nutrient gelatine contained in a sterile test-tube ; shake well 

 and pour out on a glass plate or glass dish to be kept under 

 bell-glass in a moist chamber, i.e. after the manner of an 

 ordinary plate cultivation. After being kept at a temperature 

 of about 21 to 22 C. notice the number of colonies that 

 have sprung up, the number of different colonies (size, shape, 

 colour, and whether liquefying the gelatine or not), and from 

 these an index is obtained of the approximate number of 

 such bacteria that have been originally present in a given 

 quantity of the water. But it must be borne in mind that 

 the number of colonies is no absolute index of the number 

 of bacteria originally present in the water, for the following 

 reasons : (a) not every colony that makes its appearance on 

 the plate cultivation even granted it is due to the growth of 

 a single species, but which is not always the case owes its 

 origin to one single individual, since for instance micrococci 

 bacteria and bacilli may occur in the original water as zoogloea 

 and chains, and these cannot by any amount of shaking be 

 broken up into single elements ; (b) not all bacteria introduced 

 into the gelatine come up as colonies, since not all of them 

 are capable of growing in the gelatine, and not all can thrive 

 at the temperature at which the gelatine remains solid (c) 

 the liquefaction of the gelatine by some of the colonies and 

 not by others does not indicate different species, since this 

 depends sometimes on the nature of the nutrient gelatine, 

 and to the fact whether the growth takes place in the depth 

 or on the surface; (d) accidental contamination with organisms 



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