Ever since the year 1861 (see p. 92), this question of the possible transformation of the ordinary fungi, especially _penicillium_ and _mucor mucedo_, into yeast has engaged our attention. The results attained have been entirely negative; but hitherto only the conclusions of our work have been published, some account of which was given at the meeting of the _Société Philomathique_ of March 30th, 1861. The following extract is from the _Bulletin_ of the society:—“Meeting of March 30th, 1861. At this meeting a paper was read by M. Pasteur ‘On the supposed changes in the form and vegetation of yeast-cells, depending on the external condition of their development.’ It is well-known that Leuwenhoeck was the first to describe the globules of yeast, and that M. Cagnard-Latour discovered their faculty of multiplying by budding. This interesting vegetable organism has been the subject of a host of researches by chemists and botanists. The latter, from the days of Turpin and Kutzing, have almost unanimously regarded yeast as a form of development of various inferior vegetable types, especially _penicillium_. The studies of this subject which seem to have won most favour during the last few years are those of MM. Wagner, Bail, Berkeley, and H. Hoffmann. The researches of these botanists seem to strengthen and confirm the original observations of Turpin and Kutzing. M. Pouchet has, quite recently, expressed the same ideas, and has determined certain points in connection with them with much precision of detail. M. Pasteur has long studied this important question, which is so intimately connected with the essential nature of yeast and with those phenomena of the polymorphism of the inferior types of vegetable life, to which most of the remarkable works of M. Tulasne relate; he has, however, arrived at results that are altogether negative, and he declares that he was unable to detect the transformation of yeast into any of the _mucedines_ whatsoever, and, inversely, that he could never succeed in producing the smallest quantity of yeast from ordinary _mucedines_.” These same results we communicated to the _Société Chimique_ of Paris, at a meeting held April 12th, 1861. Throughout the investigation of which we have just indicated the conclusions, we insisted on the necessity of cultivating the separate organisms in a state of purity in all researches relating to these inferior forms of life, if we desire to attain to sure inferences about them; and the method of working, which we recommended, did not differ essentially from that adopted in the present work. Since then the study of these growths has been conducted with the utmost precautions; and other apparatus, perhaps as safe as those which we employ and better adapted than ours for the study of polymorphism of species, have been invented by botanists of great skill—M. de Bary, in Germany, and M. Van Tieghem, in France.