Another point in the cultivation of yeasts has been elucidated by a number of workers, chief among whom perhaps is Hansen, namely, methods of obtaining _pure cultures_. We know, generally speaking, what this term means, and there is no difference in its meaning here to what is understood as its meaning with regard to bacteria. There is, however, some difference in the mode of securing it. It is only by starting with one individual cell that we can hope to secure a pure culture of yeasts. For the study of the morphology of yeasts under the microscope the problem was not a difficult one. It was comparatively easy to keep out foreign germs from a cover-glass preparation enough to perceive germination of spores and growth of mycelium. But when we require pure cultures for various physiological purposes, then a different standard and method are necessary. [Illustration: YEAST (SACCHAROMYCES CEREVISIÆ) × 1000] [Illustration: ASCOSPORE FORMATION IN YEAST (The capsule of the parent cell around the spores is invisible) × 1000] [Illustration: NITROGEN FIXING BACTERIA FROM ROOTLET-NODULES (SUBCULTURE) × 1000] [Illustration: BACILLUS OF TETANUS (From broth culture, showing spore formation) × 1000 _By permission of the Scientific Press, Limited_] Pasteur and Cohn adopted a practice based upon the fact that when organisms find themselves in a favourable medium they multiply to the exclusion of others to which the medium is less favourable. Hence if an impure mixture be placed under such circumstances there comes a time when those organisms for which the circumstances are favourable multiply to such an extent that they form an almost pure culture. The method is open to fallacy, and will rarely result in a really pure culture; and even if that be secured, it is quite possible that it will be to the exclusion of the desired culture. Hansen has devised a much improved process for securing a pure culture of yeast which depends upon dilution. We believe Lister was one of the first who, in the seventies, introduced some such plan as this. Hansen employed dilution with water in the following manner: