LllAlTER XLVll. 



ANATOMY OF THE YEAST CELL. 



ii 249.— The Cell Membrane. 



In tlio fiisf of younger cells, actively enjiageil in metiibolisnt, 

 the cell membrane is very thin (only a few tenths of a ft) and 

 has nearly the same refractive power as the enclosed plasma. 

 It may be rendered deiirly visible and distinguishable by allow- 

 ing some strongly plasnjolytic reagent, e (j. dilute acids, to act 

 on the cell, whereby the contents are cau.sed to shrink. The 

 meud)rane can l)e exposed in a simpler manner by merely 

 pressing gently on the cover glass of the microscopical pre- 

 panition, so that the cells burst and the empty membmnes 

 become visible as jtale skin. The membrane of the jiernianent 

 cells described in § 245 and § 246 is, however, di.scerniV>le with- 

 out this preliminary treatment. According to the re.'-rearches 

 of II. Will (VIII.). the membrane in these cells attains a 

 thickness of 0.7 to o 8 /^, t.)r oven i /i occasionally. This thicken- 

 ing of the cell membnme is a precautionary niea.sure against 

 rtdveise influences, and occurs more paiticularly when the yeast 

 cells are obliged to develop in ov u[M)n a concentrate<l nutrient 

 medium, e.>/. .strong ale ("liockbier") wort or wort -gelatin. 

 C. Heckek (I.) determined the thickness of the cell membrane 

 of st(K'k yeast for tirdinary Munich l)eer, as 0.5 n, wiiereits that 

 taken from a vat of strong ale measured up to o 7 /x, ami 

 that from the still stronger Salvator beer 0.9 /i. The in- 

 crease in thickne.ss is accompanied by diminished penetnibility, 

 and consequently deciea.sed fermentative power. For this 

 leason the sedimental yea.st from strong ales is nirely usetl 

 again, or only for pitching worts of lower gravity (lager-beer). 



The influence of dilute acids (»•.(/. a i per cent, .solution 

 of osmic acid) or alkalis, on yeast cells, especially the per- 

 nument cells, is evidenced by stratification of the membrane, 

 which was first observed l)y II. Will (Vlll.) and then confirme<l 

 by O. C.\s.\r.K.\M)i (I.). Mostly the membnino is <livided into 

 two layers, or in .some instances int.o three or even more ; though 

 t". Heokeu (1.) never succeeded in finding more than two. The 

 outer of the.se gnidually beconu's iletiiclunl as the cell gn)ws 

 older, a phenomenon fir.st ob.serveil by P. LixnxKii (VI.) in the 



VOL. II. MS K 



