PREPARATION OF PURE CULTURE. 43 



duced on to the surface or into the upper layer of the solidified 

 gelatine on a platinum spatula. Stab-cultures are those in 

 which a fraction of the growth is introduced by an inoculating 

 needle into a thick layer of solidified gelatine. Giant colonies 

 are formed by pouring a drop of the inoculated liquid on to 

 a stab in the solid gelatine. 



7. Preparation of the Pure Culture. 



To prepare an absolutely pure culture, it is necessary to 

 make sure by direct observation that the development begins 

 with a single cell, and that this is so completely isolated that 

 during the development no other cell can creep in and render 

 the culture impure. If such a pure culture is required for 

 experiments on a large scale or for actual fermentations, 

 special rules must be observed in order that the absolutely 

 pure growth at first developed shall be protected from every 

 infection during its further growth in a succession of larger 

 flasks. Care must, of course, be taken that the species is 

 developed under the most favourable conditions to secure a 

 vigorous and normal culture. The process in its later stages 

 is described in another section. We are here concerned with 

 the problem of securing the first absolutely pure culture as 

 the point of departure for the mass culture. 



The desideratum of direct observation presents difficulties 

 in the case of the smallest micro-organisms bacteria. Whilst 

 it has long proved possible to directly observe single cells of 

 yeasts and moulds on account of the size of their cells, this 

 has not been the case with the great majority of bacteria. 

 In such cases we must be content with methods which give a 

 certain probability for preparing a pure culture. It is only 

 quite recently that the technique has been sufficiently de- 

 veloped to allow of an approximately accurate solution of 

 this problem. 



Long before there was any attempt to work experimentally 

 with absolutely pure mass cultures, experiments in the culti- 

 vation of micro-organisms had been undertaken with a purely 

 botanical object, to discover what different forms a species 

 may assume, and with this object the development of single 

 cells was followed under the microscope. 



