MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 25 



phases of development, morphological investigation, or the object 

 may be to study the vital functions of a growth developed 

 from a single cell, biological and physiological investigations. 

 As these two methods of investigation are of different character, 

 the means employed must likewise differ. 



(a) Pure cultures for morphological investigations. 

 After the discovery had been made, by means of the micro- 

 scope, 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 morpho- 

 logical examination of a pure culture was made. For this 

 purpose it became necessary to guard against such disturbances 

 as would arise if other cells hindered the selected one from 

 multiplying or concealed 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 careful observations of this kind. 

 The propagation of yeast-cells was observed by MITSCHERLICH, 

 KUETZING (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 develop- 

 ment 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 inves- 

 tigation 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 tube of the 

 microscope in order to keep out foreign germs as much as 

 possible. When the development took place in ordinary fluid 

 drops, the preparation was placed in the interval between two 



