SEXUALITY 



19 



and Formation of the Asc in Schizosaccharo- 

 myces Pombe (in Stained Preparation). 



transformed into an asc which preserves always the form of a dumb- 

 bell. The ascospores, to the number of 4, originate in pairs in both 

 enlargements. 



Parthenogenesis, extremely rare in Sch. octosporus, is, on the 

 contrary, rather frequent in 

 Sch. Pombe and Sch. mellacei. 

 Sometimes two cells, already 

 united by a copulation canal, 

 form, without reabsorbing the 

 separating walls, a partheno- 



genetic asc; more often it is Fig. 19. Various^ Stages in the Copulation 

 an ordinary cell, which, with- 

 out trying to unite itself to 

 another, transforms itself directly into an asc. (Fig. 17, 4.) Sulc has 

 described a new species of Schizosaccharomyces, Sch. Aphalarae calthae, 

 found in the fatty tissue of the homoptera, which 

 seems to present a copulation analogous to that of 

 Sch. octosporus. Nakazawa has found a copulation 

 quite similar to that of Sch. Pombe and mellacei in 

 Sch. Sautanensis and formosensis which were iso- 

 lated by him from sugar products in Formosa. 



The sexual phenomena are present not only in 

 the Schizosaccharomyces; they have been described 

 also in a certain number of yeasts, multiplying 

 by budding. One observes not only isogamy but 

 heterogamy and intermediate forms between these 

 two methods. 



Barker, 1 in 1901, established the first of these in a new species 

 isolated from a solution containing ginger, for which he 

 created the genus Zygosaccharomyces. This yeast is known 

 today as Zyg. Barken. The copulation is isogamic and 

 occurs in the same manner as in Sch. Pombe and mel- Fig. 21. 

 lacei. The fusion is incomplete, and the asc which results 

 retains the form of two retorts united by a collar. The 

 mixture of the protoplasm and the nuclear fusion takes 

 place in the copulation canal. The ascospores, which 

 vary in number from two to four, develop in both enlarge- 

 ments of the asc or exceptionally in but one. (Fig. 21.) 



Recent work has shown that these sexual phenomena, which have 

 been regarded as rare at the time when they were first observed, are 



1 Barker, P. A conjugating yeast. Proc. of the Roy. Soc. 68, July 8, 1901. 

 On the spore formation among the saccharomycetes. Jour. Federated Insti- 

 tutes of Brewing, 13, 1902. 



Fig. 20. Copula- 

 tion and Formation 

 of the Asc in Sch. 

 mellacei. 



Q 



Dif- 

 ferent Stages 

 in the Copu- 

 lation and 

 Formation of 

 the Asc in 

 Zygosaccharo- 

 myces Barkeri 

 (afterBarker). 



