MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 23 



logical investigation; or the object may be to study the 

 vital functions of a growth developed from a single cell, 

 biological and physiological investigation. As these two 

 methods of investigation are of different natures, the means 

 to be employed must likewise differ. 



(a) Pure Cultures for Morphological investigations. 

 After the discovery had been made, by means of the microscope, 

 that yeast consists of cells, it was not long before the attempt 

 was made to determine, by closely observing one of these cells, 

 the way in which they multiply, and in what forms the new 

 generations occur. In other words, a morphological examina- 

 tion of a pure culture was made. For this purpose it became 

 necessary to guard against such disturbances as would arise 

 from other cells hindering the selected one from multiplying 

 or withdrawing it from the observer's view. On the other 

 hand, it would not matter if foreign cells occurred in other 

 portions of the preparation. 



Ehrenberg^ as early as 1821, observed the germination of 

 the spores of some fungi by means of investigations of this 

 kind. Later, the propagation of yeast-cells was observed by 

 Mitscherlich, Kutzing (1851), and F. Schulze (1860), in the 

 same way. A small quantity of high-fermentation yeast was 

 diluted with beer-wort until it contained only one or two 

 yeast-cells ; from a drop of this an ordinary preparation was 

 made, the cover-glass was cemented fast on the glass slide, 

 and the development of the cell was watched under the 

 microscope. The same method was employed, in its main 

 features, by Tulasne (1861) and De Bary (1866) in their 

 famous researches on the germination of the spores of the 

 fungi. The investigation was carried further by Brefeld, who 

 followed the development of the mycelium until it in its turn 

 again formed spores. He sowed the spores on the object-glass. 

 When his investigation was to extend over a longer space of 

 time, during which an ordinary drop of liquid would evaporate, 

 he added gelatine to the liquid, and placed a small shade of 

 paper over the apparatus; this shade was attached to the 



