146 MICRO-ORGANISMS AND FERMENTATION. 



kept moist by half filling the dish with water. 1 Tf it is 

 desired merely to bring about the formation of spores, the 

 apparatus may be allowed to remain at the ordinary room- 

 temperature. 



HANSEN was the first to give an accurate description of the 

 structure of spores and a detailed account of their evolution 

 founded upon observations of individual spores ; and he dis- 

 tinguished three typically different groups of Saccharomycetes 

 which are characterised either by the mode of germination or 

 by the form of the spores. 



After a certain lapse of time, which varies with the different, 

 species, roundish plasma-particles appear in the cells and these 



FIG. 37. The first stages of development of the spores of Saach. cerevisiae I. (after 

 HANSEN): a, b, c, d, e, rudiments of spores, where the walls are not yet distinct : ./', y, 

 h, i, j, completely-developed spores with distinct walls. 



are the first indications of spores (Fig. 37). In their further 

 development, they are surrounded by a wall, which is seen 

 more or less distinctly in the different species. 



In the first type, to which Saccharomyces cerevisiae I. belongs, 

 the spores may expand during the first stages of germination 

 to such an extent that the pressure which they exert on each 

 other while they are still enclosed in the mother cell, brings 

 about the formation of the so-called partition walls (Fig. 38). 

 This causes more or less plasma to be squeezed or wedged 

 between the spores, or the walls of the spores may be brought 

 into contact. During further development, a complete union 



1 Ascospores can also be obtained when yeast is spread upon sterilised 

 solidified gelatine, prepared with or without a nutrient solution, kept in a 

 damp place ; likewise in yeast-water and in sterilised water ; finally, spore- 

 forming cells occur in the films of the Saccharomycetes. The method is 

 evidently not dependent upon these different substrata but upon the know- 

 ledge of the factors which render it possible for the cells to exercise this 

 function of forming spores. 



