28 MORPHOLOGY AND DEVELOPMENT OF YEASTS 



Thus, these examples indicate that, in many of the yeasts, the 

 ascogenous cells which represent gametes, develop by parthenogenesis, 

 n ^ preserving, nevertheless, a little of their sexual 

 attraction; this is insufficiently developed to insure 

 copulation. These species include the yeasts which 

 have completely lost all traces of sexuality, and in 

 which sexuality is definitely established, such as 

 the Saccharomyces and the majority of yeasts. 



We have seen that certain yeasts, as S. Lud- 

 wigii, Johannisberg II and Willia Saturnus, after 

 having lost their primitive sexuality, have experi- 

 enced the need of compensating this by a parthenog- 

 amy which consists in the nuclear and protoplasmic 

 fusion of ascospores, two by two. But even this 

 secondary sexuality seems 



a* 



Fig. 32. Partheno- 

 genetic Variety of 

 Saccharomyces Lud- 

 wigii. Germina- 

 tion of Ascospores. 



to disappear. Thus in Sac- i 

 charomyces Ludwigii about 

 one-fourth of the spores 

 germinate without copula- 2 

 tion. With regard to the 

 yeast Johannisberg II, and 

 Willia Saturnus, parthe- 3 

 nogamyis observed in only 

 one-half of the ascospores. ^ 

 We have had opportunity 

 to study a variety of yeast 

 Saccharomyces Ludwigii, 5 

 arising from a culture of 

 Hansen's, which did not 

 offer any trace of parthe- 

 nogamy. The ascospores 

 formed long projections 7 

 which attempted to join 

 but never accomplished 

 this end. (Fig. 32.) 



All this shows in an 

 exact manner that the 

 yeasts make one of very 

 many examples of a group 

 in which sexuality is in the 

 act of retrograding and in 

 which one may follow each 

 step in the accomplishment of this phenomenon. 







Fig. 33. Scheme Representing the Development 

 of Forms of Yeasts. 



1, Sch. octosporus (isogamic); 2, Zygosaccharomyces Barkeri 

 (isogamic); 3, "Yeast 6" of Pearse and Barker (inter- 

 mediate forms between iso- and heterogamy); 4, Zygo- 

 sacch. chevalieri (heterogamy); 5, Nadsonia (heterogamy 

 and ascs resulting from the budding of eggs) ; 6, Yeast 

 of Rose (parthenogamy with traces of sexual attraction); 

 7, S. cerevisiae, (parthenogenesis); 8, S. Ludwigii (par- 

 thenogamy between spores). 



From this point of 



