SPONTANEOUS VARIATIONS 37 



But if the fungus changes on long-continued cultivation, it usually 

 changes in the direction indicated by the arrows. The so-called 

 black yeasts may go through the first three stages, finally appearing 

 as a mold of the genus Cladosporium. See page 111. Ringworm 

 fungi may pass through the last three stages. Although transforma- 

 tions in the direction of the arrows are seen far more frequently than 

 in the reverse direction, these variations are not entirely irreversible. 

 Punkari and Henrici,-^ studying variations in the yeast Crypto- 

 coccus pulchertimus, called attention to the similarities between the 

 variations and those exhibited by bacteria as they change from 

 smooth to rough forms. As the yeast develops into rudimentary 

 mycelium, the colonies become rough and wrinkled. Negroni -* made 

 a similar comparison in the case of Candida albicans. Since the 

 basic mechanisms are unknown in both cases, no definite statement 

 can be made, but one is justified in assuming that the common varia- 

 tions in fungi, as indicated in the diagram above, are of the same 

 general character as the variations commonly observed in colonies 

 of bacteria and usually designated microbic dissociation. 



Sectors and Secondary Colonies. These variations have been 

 described as appearmg rather gradually in the cultures, but it seems 

 probable that what actually occurs is a rather sudden change in a 

 cell here and there, and then a gradually developing dominance of 

 these new types over the normal types as the strain is continued in 

 further subcultures. Exactly similar transformations may be ob- 

 served to occur rather suddenly in portions of a single culture. As 

 with bacteria, such changes are best seen in colonies, especially giant 

 colonies, i.e., a single large colony allowed to grow for a long time 

 on a Petri plate or in a flask of agar. The occurrence of variation 

 in these giant colonies is usually manifested by the occurrence of 

 sectors, wedge-shaped areas differing in color or texture from the 

 body of the colony. Such sectors represent the growth of the fungus 

 from a cell which has undergone a transformation. As the colony 

 spreads, the growth from this cell differs in appearance from that of 

 other cells at the periphery of the colony. Sometimes little tufts of 

 mycelium or little papillae of yeast cells will occur in an older part 

 of the colony, which are different in character from the rest of the 

 colony. These tufts or papillae correspond to the secondary colonies 

 of bacteria. 



Spontaneous Variations. Little is known regarding the mechan- 

 isms involved in these variations, but it seems likely that both in- 

 ternal and external factors are involved, and that external agencies 

 merely increase the rate of a change that tends to occur sponta- 



