10 



COMPARATIVE PHYSIOLOGY. 



The elements of which, yeast is composed are C, H, O, N, S, 

 P, K, Mg, and Ca; but chiefly the first four. 



Physiological. — If a little of the powder obtained by drying 

 yeast at a temperature below blood-heat be added to a solution 



of sugar, and the lat- 

 ter be kept warm, 

 bubbles of carbon di- 

 oxide will be evolved, 

 causing the mixture 

 to become frothy ; and 

 the fluid will acquire 

 an alcoholic charac- 

 ter (fermentation). 



If the mixture be 

 raised to the boiling- 

 point, the process de- 

 scribed at once ceases. 



It may be further 

 noticed that in the 

 fermenting, saccha- 

 rine solution there is 

 a gradual increase of 

 turbidity. All of these 

 changes go on per- 

 fectly well in the to- 

 tal absence of sun- 

 light. 



Yeast - cells are 

 found to grow and 

 reproduce abundant- 

 ly in an artificial food 

 solution consisting of 

 a dilute solution of 



Fie. 4.— Farther development of the forms represented certain salts tOffether 

 in Fig. 3. . ' & . 



With sugar. 



Conclusions. — What are the conclusions which may be legiti- 

 mately drawn from the above facts ? 



That the essential part of yeast consists of cells of about the 

 size of mammalian blood-corpuscles, but with a limiting wall 

 of a substance different from the inclosed contents, which latter 

 is composed chiefly of that substance common to all living 

 things — protoplasm ; that like other cells they reproduce their 



FiQ. 2.— Varions stages in the development of brewer's 

 yeast, seen, with the exception of the first in the 

 series, with an ordinary high power (Zeiss, B. 4) of 

 the microscope. The first is greatly magnified 

 {Gundlach's A Immersion lens). The secondseries 

 of four represents stages in the division of a single 

 cell ; and the third series a branching colony. 

 Everywhere the light areas indicate vacuoles. 



Fig. 3.— The endogonidia (ascospore) phase of repro- 

 duction — i. e,, endogenous division. 



