HANSEN'S METHOD OF SINGLE-CELL CULTURE. 219 



nishing true pure cultures when the experimenter has succeeded, 

 by shaking, in distributing the sample sown in the nutrient 

 gelatin so uniformly that the cells are all embedded separately in 

 the solidified gelatin stratum. Despite the opinion of G. TOPF (I.) 

 to the contrary, this condition is not always attainable. By 

 means of an artificial mixture of beer yeast with Sacch. apiculutus 

 which is recognisable from the peculiar shape of its cells 

 HANSEN (XII.) showed that about 2 per cent, of the colonies on 

 the resultant plate cultures were impure, i.e., contained cells of 

 both species. In a special experiment with a series of pure yeast, 

 partly alone and partly in artificially prepared mixtures, J. 0. 

 HOLM (IV.) showed that from 108 to 135 cells formed the basis 

 of 100 colonies obtained on nutrient gelatin plates by the Koch 

 method. Still more unfavourable are the conditions in cases 

 occurring in laboratory practice, where natural mixtures have to 

 be separated in which the mutual connection of different species 

 is, for various reasons, of more frequent occurrence and more 

 intimate. Thus P. MIQUEL (IX.) found, in a case of air analysis, 

 that out of 442 colonies of which 385 consisted of bacteria 

 only 136 contained a single species, whilst 87 contained two, 75 

 three, and 87 four or more species, and LAFAR (I.) has shown 

 that mixed colonies, formed of yeast and bacteria together, also 

 occur. Hence, in view of the small dimensions of the cells, the 

 only way to remedy tne unreliability of the plate culture method 

 in the case of bacteria is by makii)g repeated cultures, as recom- 

 mended in vol. i., chapter xi. With yeast, on the other hand, it 

 is much easier to overcome the defects of the method and obtain 

 cultures undoubtedly arising from a single cell, the cells being 

 large enough (usually 5-10 /i) to enable a low-power objective to 

 be used, the longer focus obviating the risk of contact with the 

 gelatin layer under examination. Consequently the freshly 

 moulded plate, inoculated with the sample to be subdivided, can 

 be examined with a low power (40-60) for the purpose of dis- 

 covering cells that, in addition to being alone, are far enough from 

 their neighbours to ensure the isolation of the resulting colonies 

 and enable re-inoculations to be made from these without incur- 

 ring the risk of including members from other colonies in the 

 transfer. The position of these suitable cells in the gelatin 

 stratum is noted down at once, so that they may be readily identi- 

 fied later and used for cultures that shall be indisputably descended 

 from a single cell and therefore pure cultures in the strictest 

 sense of the term. 



At present we can only deal briefly with the technique of the 

 preparation of single-cell cultures, the reader being referred for 

 more complete details and modifications of the method to the 

 special works on the subject, notably the instructions given by 

 E. C. HANSEN (XV. and XXXV.) himself, and A. KLOCKER'S 

 book (VII.) detailing the experience gained in this branch at the 



