VARIATION OF SPECIES 181 



of a mixture of sporogenic and asporogenic cells; thirdly, clear brown 

 colonies made up of only asporogenic cells. The asporogenic cells 

 could be maintained constantly in this state. Beijerinck effected a 

 separation of the types by heating at 56. The asporogenic type was 

 killed by this treatment, only the spores passing through. These 

 when grown on gelatin produced sporogenic cells with only about 1 

 per cent of asporogenic cells. These latter cells increase in propor- 

 tion as the cultures are kept in the laboratory. The yeast slowly 

 changes into an asporogenic type. 



Both of these types present different physiological and morpho- 

 logical characteristics. The sporogenic type is made up of cells 

 more elongated, and liquefies gelatin more quickly than the asporo- 

 genic variety, which presents round cells grouped like the Sardna. 

 Both varieties may be recognized macroscopically when treated with 

 iodine. The colonies made up of sporogenic cells are blued since the 

 membranes of the ascospores are impregnated with starch while the 

 asporogenic colonies are colored yellow. This is, then, a very definite 

 example of a transformation of a yeast to a permanent asporogenic 

 type. 



Similar results have been secured by Beijerinck (?) with Sch. 

 Pombe. By cultivating this yeast on nutrient gelatin, this investi- 

 gator noticed the formation of two kinds of colonies, one white and 

 composed of sporogenic cells, the other brown, and made up of 

 asporogenic cells. Here, then, we have both the sporogenic and 

 asporogenic cells. The loss of sporulation may be accompanied by 

 a loss of sexuality as has very often been noticed in the yeasts and 

 about which a little has been said in a preceding chapter. This 

 is true in a yeast secured from Beijerinck's laboratory under the name 

 of Sch. mellacei, in which the ascs form from ordinary cells without 

 undergoing any copulation. This yeast, which differs a little from 

 Sch. mellacei, seems to be a parthogenetic variety. Quite a similar 

 observation has been reported with S. Ludwigii. Guilliermond had 

 the opportunity to observe two types of this yeast from the same 

 source. Both came from Hansen's laboratory. With one the asco- 

 spores underwent a copulation at the moment of germination as is 

 the normal procedure for this species. In the other, the copulation 

 had entirely disappeared. All of the variations of sporogenic func- 

 tion and sexuality which we have mentioned up to this point super- 

 vene without apparent cause and may be regarded as true mutations. 



Other investigations by Hansen on the loss of sporulation give us 

 an example of variation produced by an accurately determined cause. 

 It is known that the maximum temperature of budding in a variety 

 of yeasts, is always a few degrees higher than the maximum tempera- 



