1896 THE MICROSCOPE. 23 



the leavened doiigli is saved usually for the uext bak- 

 ing to facilitate the action. 



Another form of this spontaneous fermentation is car- 

 ried on in what is known as a salt-rising bread. In this 

 the fermentation is brought about by micro-organisms, 

 it not being known which one or ones produce the alco- 

 holic fermentation. In some experiments which I made 

 on salt-rising bread, in no case was yeast or any of the 

 budding fungi found, but in every case large numbers of 

 bacteria, four different kinds being constant. That is, in 

 plate cultures made from rising dough, there were four 

 forms that differed from one another in various respects 

 that were always found. Other forms beside these were 

 found sometimes, but were not constant. 



In the manufacture of bread as ordinarily carried on 

 some form of "pressed yeast" is used, either the moist 

 or the dry. The dry cakes usually have a small amount 

 of yeast and a large amount of corn meal or some flour. 

 The moist has a large amount of yeast, and a small 

 amount of starch, usually that of the potato. The yeast 

 is S. cerevisise, or brewers' yeast. 



Yeast is a unicellular plant, without chlorophyll, thus 

 belonging to the fungi. The cells are round, oval, or in 

 some species very much elongated. The cells consist of 

 a definite cell-wall, which encloses a mass of granular 

 protoplasm. The wall consists of cellulose, which is some- 

 what delicate in young growing cells, but much thicker 

 in old or resting cells. In the protoplasm are a number 

 of vacuoles, a nucleus is present, but is difficult to see, so 

 that special treatment has to be given for its demonstrat- 

 ion. Yeast rejiroduces ordinarily by budding. 



When the yeast is placed under favorable conditions 

 for growth, the wall of the cell, usually near one the 

 poles, bulges out ; into this, protoplasm presses. The 

 cell now consists of two unequal parts, the smaller one, 



