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 durable cells (Dauernzellen) ; he regards them as resistant organs which serve to perpetuate the species over unfavorable periods. In this, they are similar to the ascospores. Perhaps they may be regarded in the same light as cysts, or clamydospores, which have been observed so frequently in the Endomyces.

When these durable cells are placed again in favorable circumstances, their membrane is broken and budding takes place giving rise to spherical and elongated yeasts, be they separated or in groups.

Sporulation is a form of resistance which allows the yeast to remain viable, even though active budding has stopped. It plays an important role in the hibernation of yeasts, permitting them to pass the winter in the ground of vineyards where they are deposited in the autumn. Sporulation is observed in old cultures where food is scarce, also in certain solid media such as carrots or gelatin which are not very favorable for budding. It is especially easy to secure sporulation by submitting the yeasts to starvation after they have been able to build up sufficient reserve products necessary for the formation of ascospores.

We shall take up in a following chapter the details which determine sporulation; therefore this question will not receive attention at this time.

Internal, or ascospores, were observed for the first time by Schwann in 1839 and described by Seynes. They were regarded by some authors, notabty Van Tieghm, as resulting from a sort of encystment resulting from some pathological process. Brefeld considered these cells, which bear spores, as sporangia or cysts. On the contrary, Rees, de Bary,  and later Hansen, likened the sporangia of yeasts to the asc of the Ascomycetes and regarded the yeasts as a group of fungi. This opinion has been entirely confirmed by our investigations on the cytological phenomena of the formation of ascospores, and especially by the discovery in certain yeasts of a copulation in the origin of the asc. It is definitely admitted today.

Sporulation is indicated by any cell, either yeast cell or a cell constituting a rudimentary mycelium. In this way certain yeasts (S. Ludwigii, Pichia membranaefaciens) are able to form ascospores in mycelial cells developing on the surface of old cultures. Each cell, then, seems able to develop into an asc.