22 MORPHOLOGY AND DEVELOPMENT OF YEASTS 



tion between a mother cell and a bud formed by that cell, which is 

 very small and still attached to the mother cell. About 25 per cent, 

 only, of the ascs result from an isogamic copulation between two 

 cells of the same dimensions. In Debaromyces tyrocolla, the copula- 

 tion is accomplished most often between the mother cell and its 

 bud and the isogamic copulation is very rare. According to Konoko- 

 tin, the genus Debaromyces may be represented by a heterogamic 

 copulation. In one other yeast, isolated by Guilliermond from oranges, 

 the Zygosaccharomyces Nadsonii, a heterogamic copulation has been 

 described, but with this yeast the heterogamy is the rule, and there 

 seems to be no instances of isogamy. 



Finally Nadson and Konokotin have discovered in the mucous 

 secretions of trees two species of yeasts (Nadsonia fulvescens and 

 elongata) in which the copulation always takes place by heterogamy 

 between an adult cell and one of the buds formed by it. All of the 

 contents of the male gamete go into the female gamete, but the 

 female gamete does not transform directly into an asc. It gives 

 birth, by budding, to a new cell into which its contents are poured, 

 and it is this cell which becomes the asc, usually including a 

 single ascospore. The authors think that the asc, formed by budding, 

 represents a rudiment of a sporophyte, and consequently have es- 

 tablished for these two species the genus Nadsonia (Guilliermondia) . 

 With Zygosaccharomyces it has been difficult to determine the parents 

 of the gametes which unite; the copulation is almost always ac- 

 complished between an adult cell and a young cell. In other yeasts 

 with heterogamic copulation, it takes place between a mother cell 

 and one of the cells formed by budding from it. In this case copula- 

 tion may be autogamic. 



Guilliermond l has isolated from the mucous secretions of chestnut 

 trees a new yeast, Zygosaccharomyces Pastori, 2 which presents a 

 heterogamic copulation effected between cells of unequal sizes. The 

 female cell is the adult while the other, the male, has not attained 

 its full development. Sometimes there is little difference between 

 the two cells. The contents of one always pour into the other 

 which is transformed into the asc. The asc may contain from 1 to 

 4 ascospores which are hat-shaped like those of Willia anomala. This 

 yeast produces only a few ascs; the cells, however, attempt to unite 



1 Guilliermond, A. Sur une nouvelle levure a copulation he'te'rogamique. 

 Comp. Rend. Soc. Biol. 1919. 



2 This yeast has not been fully described but presents a resemblance to Willia 

 anomala in having hat-shaped ascospores. Its cultural characteristics, however, do 

 not class it in the group Willia. Klocker has described an apiculate yeast in the 

 Saccharomyces apiculatus group in which the spores are hat-shaped. This form 

 ought not to be regarded as a characteristic of the group Willia 



