254 BACTERIA IN THE SEA [PART III 



implanting this in a solution containing certain food materials 

 which can be utilised by the bacteria. Multiplication of the 

 organisms then takes place and their numbers become so 

 enormously augmented that their study is facilitated. Or if we 

 wish to estimate the numbers in which they occur in water or in 

 any other material a sample volume or weight of the latter is 

 taken and is distributed throughout the substance of a plate of 

 some solid food medium, such as nutrient gelatine or agar, or is 

 spread on the surface of the latter. Multiplication then takes 

 place as before ; each separate bacterium gives rise to a colony 

 which is composed of a vast multitude of individuals ; and since 

 the colonies are large enough to be easily visible to the naked eye 

 the number of individual bacteria in the original sample can be 

 deduced, for each bacterium has given rise to one colony. Usually 

 the colonies consist of individuals of the same species, so that the 

 different species present in the crude sample can be isolated from 

 each other and studied separately. The nature of the food media 

 on which bacteria grow and multiply ; the temperature at which 

 they grow best, or are killed ; the appearance of the colonies ; the 

 reactions of the germs to various chemical substances; and the 

 microscopic characters of the organisms ; all differ with the species, 

 and are made use of in determining the latter^. 



The bacteria may be classified according to their morphology 

 but this only affords a general scheme of grouping. Fischer 

 distinguishes the following families : 



(1) The Coccaceae, in which the germs are usually spherical 

 bodies of minute dimensions. The individuals may be separate 

 or arranged in little clusters, as in the case of the Staphylococci 

 of the pathologists. In addition to these micrococci we have the 

 Sarcinae and their allies, in which the cocci are arranged in 

 groups of four ; and the Streptococci^ in which they are arranged in 

 chains. These characters depend of course on the direction of the 



^ It is of course quite impossible to allude to the methods of the science here. 

 During the last forty years the methods of bacteriology have been dominated by 

 the immense importance which the study of micro-organisms has acquired from 

 the standpoint of medical science. The reader who is desirous of studying 

 bacteria from the point of view of the circulation of matter in nature cannot 

 do better than study A. Fischer's Structure and Functions of Bacteria (English 

 translation by Coppen Jones, Clarendon Press, 1900). 



