RESEARCHES ON YEASTS. 79 



scopic appearance, the difference noticeable is that the spores of 

 culture yeasts contain a less dense and less refractive plasma : 

 as a rule they contain vacuoles, and often have the appear- 

 ance of being empty, in which case their walls are sharply 

 defined. The spores of the wild yeasts, on the other hand, 

 are completely filled with a uniform, strongly refractive 

 plasma. A short time after I had drawn attention to this 

 in the lecture mentioned below, communications reached 

 me from other investigators, who stated that they had made 

 similar observations. It was found, however, as is usually 

 the case when a question is submitted to a thorough experi- 

 mental examination, that the subject was more complicated 

 than at first appeared, and that a long series of studies is 

 necessary for its solution. Apart from the difficulties which 

 the microscopic examination itself offers when this is intended 

 to give definite information with regard to the said structure, 

 the matter is further complicated in other respects. Namely, 

 low-fermentation brewery yeasts can also, under certain 

 conditions, give bright spores filled with a dense plasma, and 

 inversely, the wild yeasts can give spores having a structure 

 like that mentioned as belonging to the culture yeasts. 

 Whilst discussing the differences between species, it will be 

 of interest here to point out that whilst some species e. g. 

 SaccJi. Pastorianus I. can very easily be made to yield spores 

 having the appearance of being empty, this is, on the other 

 hand, more difficult in the case of others e. g. Sacch. ellip- 

 soideus II. That there is a peculiar condition of structure 

 which has its practical significance in the analysis of beer 

 yeast is evident from all investigations which I have hitherto 

 carried out. 



Whilst the spores of most species are round, oval or 

 kidney-shaped, those of others are, on the other hand, hat- 

 shaped. There are, likewise, morphological differences notice- 

 able with reference to their germination, but these only 

 seldom occur. Differences between the species may also 



