to disappear, we find them again in succeeding generations. Notwithstanding these facts, it would be difficult to discover any very appreciable differences between the forms of the cells of any particular yeast and those of its aërobian ferment in course of development. So true is this, that the aërobian ferment of _saccharomyces pastorianus_ might even be caused to put on the forms of _dematium pullulans_, which we have had occasion to observe specially characterize this ferment after the cells have been subjected to a prolonged process of senescence.[107] This is evident from the following example, which will once again show the remarkable extent to which the forms of a particular organism may be varied by changes in composition of the nutritive medium:— On August 6th, 1873, we took some of the ferment _saccharomyces pastorianus_ from a flask of wort that had undergone fermentation, and sowed a scarcely perceptible quantity of it in another flask containing a saline medium, composed as follows:— Water containing about 10 per cent. of 150 c.c. (5-1/4 fl. sugar-candy oz.) Ash of yeast 0·5 gramme (8 grs.) Ammonic bitartrate 0·2 “ (3 grs.) Ammonic sulphate 0·2 ” (3 grs.) In the course of the following days the ferment began to develop, although with difficulty, the fermentation revealing itself by collections of bubbles appearing here and there on the surface of the liquid. We left the flask undisturbed till the 25th of November following. On that day we found a very white deposit of ferment covering the yeast-ash that had not been taken into solution, and a ring of aërobian ferment on a level with the surface of liquid; all the sugar had disappeared; the liquid contained 5·2 per cent. of alcohol, by volume, at a temperature of 15° C. (59° F.); and, lastly, in consequence of the purity of the materials employed, there was no trace of the formation of fungoid growths, whether of _mycoderma vini_ or of _mycoderma cerevisiæ_, on the surface of the liquid, or of vibrios or lactic-ferment below the surface.