VARIATION OF SPECIES 179 



shown that the cells may pass from the round shape to oval and 

 even elongated and curled cells. A great difference will also be 

 seen in the dimensions of the individual cells. In old cultures, yeasts 

 are generally smaller, on account of the scarcity of food which does 

 not permit the young cells to become fully developed. Also, such 

 cultures will give ascospores which germinate into smaller cells. We 

 have also pointed out how the cells of S. apiculatus may lose their 

 apiculate shape during a few generations. Hansen has shown that 

 the temperature may play a role in influencing the shape. For ex- 

 ample, in cultivating Saccharomyces carlsbergensis in beer wort at 27 

 and 7 C. this author obtained two very different shapes. Those 

 which formed at 27 C. presented a normal appearance; the others, 

 formed at 7 C., were very curious colonies made up of elongated cells 

 forming a sort of mycelium. The ascospores, themselves, may pre- 

 sent among the various individuals of the same species very different 

 shapes. With P. membranaefadens, for example, the ascospores are 

 quite spherical at times, and at others, may be egg-shaped. 



All of this simply points out that the cells in yeasts are not con- 

 stant in shape and may, depending upon the circumstances, take on 

 variable forms temporary or permanent. In a word, they are poly- 

 morphic. Although a species may present these various forms, there 

 usually is a predominant shape which, to a certain degree, is charac- 

 teristic and may be regarded as normal for the species under question. 

 In certain cases, one may note the predominance of abnormal forms 

 among the normal. 



Hansen has taken two series of cultures in which the cells are 

 distinctly different from a single cell of S. Carlsbergensis. One series 

 shows oval or round cells of the cerevisiae type while the other is of 

 elongated cells, more like the Pastorianus type. These last vegetations 

 are, then, abnormal, although they persist through a series of cultures. 

 Hansen has been able to preserve this variation for six months. It 

 appears, then, that in the life of the yeast diverse variations may 

 spring up which may endure for a time and give the yeast an abnor- 

 mal appearance. These are the sppntaneous variations which occur 

 without apparent cause and which may persist for a certain period 

 of time and recall the fluctuations or fluctuating variations which 

 are so often encountered with the higher plants and animals. 



Permanent Variations : Aside from the temporary variations 

 there are permanent ones which persist through a number of genera- 

 tions and very often become absolutely constant, creating new 

 varieties. The investigations of Lepeschkin l offer examples of more 



1 Lepeschkin, W. Zur Kenntniss der Ehrlichkeit bei der einzelligen Organ- 

 ismen. Cent. Bakt. 10, Abt. II, 1903. 



