YEAST 29 



but the physiological processes are the same in essence, and we 

 have in the unicellular protozoon a complete organism, vitally 

 no less an organism than a bird, fish or mammal. The several 

 grades in complexity present different aspects of biological 

 principles and justify our division into organisms of one cell, 

 organisms of tissues, and organisms of organs. 



The physiological activities of single-celled organisms, while 

 undoubtedly simpler than those of many-celled animals, are 

 nevertheless marvellously complicated and only a little advance 

 in knowledge has been made, and to this advance the minute 

 organisms known as the yeasts, have contributed no small part. 



A. THE ORGANIZATION AND VITALITY OF YEAST CELLS 



Yeasts are widely spread in nature, occurring either as " wild" 

 forms or as cultivated commercial types. Wild yeasts live 

 normally upon the surfaces of fruits of various kinds, or on 

 fruit juices; in addition to this habitat, however, there are a 

 great many wild yeasts that live normally in the digestive tract 

 or body cavities of different kinds of animals. A drop of sweet 

 cider gives a good idea of certain species. Baker's yeast, mixed 

 with water, shows myriads of minute yeast cells which cause 

 the milky appearance of the fluid. Brewer's yeast contains 

 several species, two well-marked kinds form the "top" and 

 " bottom" yeast of commerce, the former being used for the 

 manufacture of ales, stout, porter, etc., the latter for "lager beer." 



Microscopical examination of baker's or Brewer's yeast shows 

 that the individual yeast cells have but little structure. A tiny 

 bit of gray protoplasm is enclosed in a definite double-contoured 

 membrane which by appropriate treatment may be shown to 

 consist of cellulose. The cells are spherical or spheroidal in 

 form, and the protoplasm contains, in addition to the ordinary 

 granules of protoplasm, small or larger refractile dots probably 

 of the nature of fat. Several vacuoles are present and a nucleus. 

 All of these constituents of the cell vary according to conditions. 

 In old cells the cellulose membrane is thicker than in young 

 cells, as can be easily demonstrated by the use of aqueous solu- 



