EXAMINATION OF YEASTS 241 



The apparatus can be placed in an incubator if any special 

 temperature is required, or may be kept at the ordinary 

 temperature of the room. 



Hansen found that the formation of spores takes place 

 slowly at low temperatures, more rapidly as the tempera- 

 ture is raised to a certain point ; when this point is passed 

 their development is again retarded, until finally a tempera- 

 ture is reached at which it ceases altogether. The mass on 

 the plaster plate is carefully examined from time to time. 

 After a certain lapse of time, which varies with the dif- 

 ferent species, roundish plasma particles appear in the 

 cells, and these are the first indications of spores. In 

 their further development they become surrounded by a 

 wall, which is seen more or less distinctly in the different 

 species. The spores may expand to such an extent that 

 the pressure which they exert on each other whilst they 

 are still enclosed in the mother-cell brings about the forma- 

 tion of the so-called partition-walls. During the further 

 development a complete union of the walls may take place, 

 so that a true partition-wall results ; the cell then becomes 

 a compound spore divided into several chambers. During 

 germination the spores swell, and the wall of the mother- 

 cell, which was originally thick and elastic, becomes 

 stretched thinner, and finally becomes ruptured, and then 

 remains as a loose shrivelled skin partially covering the 

 spores, or it becomes gradually dissolved during germina- 

 tion. The importance of this method of examination is 

 seen from the following fact : The Sacch. cerevisice does not 

 form spores until a period of ten days has elapsed ; whilst, 

 on the other hand, the Sacch. pastorianus II. (the most 

 common ' wild ' yeast), when kept under exactly the same 

 conditions, gives evidence of the commencement of spore- 

 formation after seventy-seven hours. 



(c) The Formation of Films. — Hansen subjected the films 

 which appear on the surface of fermenting liquids to a 



16 



