PREPARATION OF PURE CULTURE. 53 



yeast which can be distinguished from each other microscopi- 

 cally viz., Saccharomyces apiculatus, and a species of the 

 group 8. cerevisice. This mixture was introduced into 

 wort-gelatine, and after shaking was poured on to a glass 

 plate. Of the specks formed, about one-half contained one 

 species exclusively, the other half the other species, and in one 

 of the specks both species were found. 



A similar control was carried out for bacteria by Miquel 

 (1888), who introduced 100 colonies from a plate-culture 

 obtained in an air analysis into 100 flasks containing meat- 

 broth with peptone. The examination of the growths de- 

 veloped in the flasks showed that they contained 134 different 

 species of micro-organisms. This evidently depends upon 

 the fact that it is very difficult, and often quite impossible, 

 to separate all germs of bacteria and other organisms from 

 each other by shaking the gelatine mixture. 



Holm has subjected the method to a thorough analysis 

 (1891), in the case of a large number of yeast species, abso- 

 lutely pure cultures of which were prepared by the Hansen 

 method. The result of 23 series of experiments with different 

 mixtures was that only in a single case were 100 colonies 

 developed from 100 cells. In all the other series the method 

 proved faulty. In the most unfavourable case 100 colonies 

 were formed from 135 cells, and the average number obtained 

 was 100 colonies from 108 cells. This proves the plate method 

 to be defective also in the case of yeast. 



A modification of Brefeld's culture of a single cell in a 

 hanging drop is that known as the drop culture, introduced by 

 P. Lindner in 1893. It consists in conveying to a cover-glass 

 a number of small drops of a diluted culture in a nutritive 

 liquid by means of a mapping pen. The cover-glass is fastened 

 by a ring of vaseline on to a hollow-ground object glass, and 

 those drops are noted that contain only one cell. Care must, 

 therefore, be taken that the drops do not flow together before 

 the pure culture is conveyed to a flask. 



Burri attempted to solve the problem of preparing pure 

 cultures of bacteria under direct observation of single cells by 

 the help of his Indian ink point culture. He dilutes ordinary 

 liquid Indian ink with water in the proportion of 1 to 10, and 



