124 MORPHOLOGY AND LIFE-HISTORY OF YEASTS. 



both Sacchawmyretes and 7ion-Saccharomycetes. Of these latter 

 we shall deal (in the last section), under the generic name Myco- 

 (lerma, with a special group, the species of which are widely 

 distributed and grow spontaneously on the surface of wine or 

 beer when the latter ai'e exposed to the air, a quick-growing, 

 wrinkled skin (mould film) being formed. Air being highly 

 essential to Mycoderma, these organisms normally grow ex- 

 clusively in the form of a superficial film on the nutrient 

 solution. Owing to this peculiarity they are the cause of 

 disturbance in researches into film-formation in the true 

 yeasts, when the latter are not grown as pure cultures but 

 contaminated with the very abundant Jlycoderma. On this 

 account objection may be raised against the reports of Reess 

 (the first to observe the production of films by true Saccharo- 

 myrete^), and also those of Pasteur (III.). In 1876 the latter 

 characterised as aerobic or mould yeasts the film developing ou 

 the surface of fully fermented wort. He halted between two 

 opinions with respect to this phenomenon, one being that it was 

 a special (i.e. aerobic) condition of development of the beer yeast 

 residing at the bottom of the fermented liquid ; whilst the other 

 looked on the film as composed of extraneous cells undesirably 

 present with the sowing. It was not until Hansen applied pure 

 cultures to these investigations that a decision could be formed 

 on this point. 



The question of the convertibility of bottom-fermentation 

 yeast into top-fermentation yeast, and vice vet'sa, is also touched 

 by the foregoing explanations. Pasteur was of opinion that 

 the " aerobic " cells constituting the film that had developed 

 it at the close of primary fermentation in his cultures inocu- 

 lated with (impure) bottom-fermentation yeast, were capable 

 of exciting top-fermentation when transferred to a fresh 

 nutrient solution. He even gave a recipe by means of which 

 the brewer could prevent any such undesirable conversion of 

 the stock yeast. This theoretically and practically important 

 question was afterwards taken up by Hansen, who found that 

 the descendants of the films of all the species of bottom yeasts 

 examined by him in this connection, invariably produced nothing 

 but bottom -yeast cells when inoculated into fresh nutrient 

 solutions, even when exposed to a temperature (26° C.) very 

 favourable to the progress of top-fermentation. 



The true Saccliawiaycetes can be separated — though not very 

 sharply — into two groups : one of which does not form films 

 until the primary fermentation is terminated and the sedi- 

 mental yeast has all come down ; whilst in the other group 

 growth proceeds on the surface fi'om the commencement, and 

 indeed in many cases exclusively so. An instance of this latter 

 kind is afforded by the Sacrharomyces Diemhrana'faciens, first 

 discovered by E. Chr. Hansen (VIII.) in the mucinous 



