38d elementary biology. [chap. 



kept in a warm pLice, it will gradually become more and 

 more turbid, and, after a time, a scum of yeast will collect, 

 which may be many thousand, or million, times greater in 

 weight than that which was originally added. If the Toruhc 

 are examined as this process of multiplication is going on, it 

 will be found that they are giving rise to minute buds, which 

 rapidly grow, assume the size of the parent Tonila, and 

 eventually become detached; though, generally, not until 

 they have developed other buds, and these yet others. The 

 Tonihc thus produced by gemmation, one from the other, 

 are apt long to adhore together, and thus the heaps and 

 strings mentioned, as ordinarily occurring in yeast, are pro- 

 duced. No Toriila arises except as the progeny of another; 

 but, under certain circumstances, multiplication may take 

 place in another way. The Tonda does not throw out a bud, 

 but its protoplasm divides into (usually) four masses, each 

 of which surrounds itself with a cell-wall, and the whole are 

 set free by the dissolution of the cell-wall of the parent. 

 These c7idogaious reproductive bodies are termed spores^ 

 and retain their power of germination for a long time. 



As each of the many millions of Tontlcc which may thus 

 be produced from one Torula has the same composition as 

 the original progenitor, it follows that a quantity of Protein, 

 Cellulose and Fat proportional to the number of Toruhe 

 thus generated, must have been produced in tlie course of 

 the operation. Now these products have been manu- 

 factured by the Toruhc out of the substances contained in 

 the fluid in which they float and which constitutes their 

 food. 



To prove this it is necessary that this fluid should have 

 a definite composition. Several fluids will answer the pur- 

 pose, but one of the simplest (Pasteur's solution) is the 

 following : 



