MORPHOLOGY AND ANATOMY OF YEAST CELLS. 299 



When the cell is fully grown the strength of the membrane 

 depends on the concentration of the nutritive fluid. It has 

 a tendency to thicken in liquids with a high precentage of 

 extract ; especially marked thickening is met with in the 

 resting cells occurring in films. By treatment with concen- 

 trated hydrochloric acid a division of the cell wall can often 

 be observed. 



The gelatinous network first observed by Hansen may be 

 regarded as a special development of the membrane, remini- 

 scent of the zoogloea formation of bacteria. Under certain 

 conditions, which have not yet been defined, the colonies 

 brought about by the budding of yeast cells may com- 

 bine to form irregular clots which sink more rapidly than 

 individual cells (" break " and clarification in the brewery). 

 This doubtless stands in relationship to a feature of the de- 

 velopment of the yeast cell discovered by Hansen in 1884, 

 He found that both Saccharomycetes and other budding 

 fungi may secrete a gelatinous network which may take 

 the form of strands or plates in which the cells are em- 

 bedded (Fig. 50, A, B). If, for example, some thick brewery 

 yeast is placed in a glass and allowed to remain under cover 

 in such a way that it slowly dries, and then a trace of this 

 yeast is mixed in a drop of water, the network can be clearly 

 seen (Fig. 50, A). The formation also occurs in the gypsum 

 block and gelatine cultures. The author has frequently 

 observed this formation in the yeast samples despatched to 

 his laboratory in filter paper enclosed in envelopes.* Hansen 

 also found it in the film-formations of nearly all species. An 

 ordinary microscopic examination of the pitching yeast in a 

 brewery does not show this fermentation ; with the help of 

 staining, however, its presence can be readily detected (Fig. 

 50, B). When the yeast is repeatedly washed, it is no longer 

 possible to detect the network by staining ; but if the water 

 is removed, and the yeast set aside for a time and then suitably 

 treated, the gelatinous masses can be readily seen. By varying 



* This method of preserving a sample of yeast is very convenient. A small 

 piece of filter paper is rapidly passed through a flame several times, and a few 

 drops of yeast are poured on to it ; it is then folded up, and afterwards wrapped 

 in several layers of paper which have been similarly treated. 



