two sides. The principal difference consists in the relative freshness and the budding going on in individuals in the right-hand half. There is a simple means of transforming the small, disjointed forms of the yeast as it occurs in a deposit, at the end of a fermentation, back into the long, tubular, pear-shaped forms peculiar to the germination of the germ-cells, which exist amongst the particles of dust spread over the surface of fruits. Plate X. illustrates the result of the process. For this purpose we must effect as complete an exhaustion as possible of the ferment _saccharomyces pastorianus_, by leaving it to itself for a very long time, without aliment, in contact with pure air, in a damp state; or, better still, in presence of sweetened water. We cultivate some yeast in wort, in one of our two-necked flasks, and then carefully decanting the fermented liquid through the right-hand neck, leave the deposit of yeast on the sides of the flask. The glass stopper which closes the india-rubber tube must be replaced, and the moist yeast be left thus, in contact with pure air. The cells will steadily continue their activity, and so gradually age, without meanwhile losing their vitality. We use the word _age_, as we have already observed, because the period of rejuvenescence in the case of such a yeast is so much the slower the longer the plant has remained in that state. Under these conditions the yeast rarely dies. It becomes attenuated and shrivelled but still preserves its vitality, that is, the power of reproducing itself after a lapse of several months or even several years. In the end, however, it dies, a fact which is proved by the cells, when sown in a nutritive medium, remaining inert. To exhaust yeast, without destroying it, sweetened water is preferable. Having decanted the beer, we substitute in its place water sweetened with 10 per cent. of pure sugar. By effecting the substitution in the following manner, we escape the risk of introducing germs from floating particles of dust, which would nullify all experiments of this kind. We prepare, then, a flask containing sweetened water, free from all foreign germs, which we attach to the other flask [_i.e._, two of M. Pasteur’s flasks with two necks, one straight and wide, the other bent and narrow (Fig. 8)]. This is done by taking the india-rubber tube off the flask containing yeast, and removing the glass stopper from the other india-rubber tube attached to the flask containing the sweetened water; then, introducing the right-hand neck of the yeast flask into the india-rubber tube connected with the other, we raise the latter flask so as to pour the sweetened water on to the yeast. At the same time an assistant passes the flame of a spirit-lamp over the bent part of the curved tube attached to the water flask, with the object of destroying the vitality of the germs in the floating particles of dust, which enter the flask in proportion as it is emptied into the other.