100 BACTERIA. 



material, to produce other crops from this, and so on until a 

 sufficient quantity of pure yeast is obtained to bring about 

 the special fermentation in the large mass of wort. This 

 method has another very great advantage it enables an 

 investigator to take a single yeast-cell and to follow it in its 

 life-history ; in doing this Hansen completed the work that 

 Pasteur had commenced. 



As the process of obtaining pure cultures is of great interest not only to 

 scientific investigators but also to practical men, we may here give it in brief. 1 

 In a Pasteur flask containing wort, a cultivation of the yeast to be experi- 

 mented on is started and carried on as vigorously as possible. A quantity 

 of water, that has previously been sterilized by boiling, is added to the 

 growth ; the yeast-cells in a drop of given size are then counted under the 

 microscope. Let us suppose that ten cells are found ; a drop of similar size 

 is then transferred to a flask containing a known volume, say 20 c.c. of 

 sterilized water, so that we have one yeast- cell for each 2 c.c. of water. 

 The flask containing the 20 c.c. of water with the added 10 yeast-cells, 

 is now thoroughly shaken and then divided equally, I c.c. being placed 

 in each of twenty flasks containing sterilized wort. 2 If the separation has 

 been complete ten out of twenty flasks should contain one organism each ; 

 but this of course cannot be absolutely depended upon. After carrying 

 the process to this stage Hansen shakes the flasks very vigorously, by 

 which he separates the cells as far as possible from one another, puts them 

 in an incubator, and allows them to remain perfectly still, so that the cells 

 may sink to the bottom or become attached to the walls of the flask. If 

 there are more cells than one in any flask, they should have been completely 

 separated by the final shaking, and each will probably take up a separate 

 position on the wall or bottom of the flask, and at the end of several days 

 " the flasks are carefully lifted and examined, and it is noted whether one 

 or more white specks have been formed on the walls of the glass ; if only one 

 such speck is found we have then a pure culture." This method is especially 

 useful in the case where the yeast-plants are at all weakly, as all yeasts grow 

 much more luxuriantly and strongly in a fluid medium than they do on gelatine 

 plates, even when wort is added to the gelatine in order to render it more 

 suitable for the nutrition of the yeast. Where the species are mixed, but 

 where the individuals that we desire to separate are vigorous, the gelatine 

 method may be used with advantage, especially as the individual colonies 

 can then be examined from time to time, or continuously observed under 

 the microscope as they pass through their various phases of development. 



The characteristic appearances that were said at one time 

 to belong to the yeast-plants have been proved by Hansen 

 to be really non-existent, except in a very limited sense. 

 He maintains that the shape and relative size of the cells 

 and the appearance of the protoplasm of a yeast-plant are 



1 For the method of carrying on the process on a large scale see Jorgen- 

 sen's work. 



2 i c.c. or one cubic centimetre = about 16 minims. 



