Page:The yeasts (1920).djvu/43

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19
SEXUALITY


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 n O Q created the genus Zygosaccharomyces. This yeast is known jL ? (j A today as Zyg. Barken. The copulation is isogamic and occurs in the same manner as in Sch. Pombe and met- Fig. 21. Dif- 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

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. of the Asc in Sch. ferent Stages in the Copu- lation and Formation of the Asc in Zygosaccharo- myces Barkeri (afterBarker).