will be found that they are giving rise to minute buds, which rapidly grow, assume the size of the parent Torula, and eventually become detached; though, generally, not until they have developed other buds, and these yet others. The Torule thus produced by gemmation, one from the other, are apt long to adhere together, and thus the heaps and strings mentioned, as ordinarily occurring in yeast, are produced. No Torula arises except as the progeny of another; but, under certain circumstances, multiplication may take place in another way. The Torula does not throw out a bud, but its protoplasm divides into (usually) four masses, termed ascospores, each of which surrounds itself with a cell-wall, and the whole are set free by the dissolution of the cellwall of the parent. This is multiplication by endogenous division. As each of the many millions of Torule which may thus be produced from one Torula has the same composition as the original progenitor, it follows that a quantity of Protein, Cellulose and Fat proportional to the number of Torulæ thus generated, must have been produced in the course of the operation. Now these products have been manufactured by the Torula out of the substances contained in the fluid in which they float and which constitute their food. To prove this it is necessary that this fluid should have a definite composition. Several fluids will answer the purpose, but one of the simplest (Pasteur's solution) is the following. Ammonium Tartrate (CH(NH),0). Potassium Phosphate (KH,PO.). In this fluid the Torula will grow and multiply. But it will be observed that the fluid contains neither Protein nor Cellulose, nor Fat, though it does contain the elements of these bodies arranged in a different manner. It follows that the Torula must absorb the various substances contained in the water and arrange their elements anew, building them up into the complex molecules of its own body. This is a property peculiar to living things. The Torula being alive, the question arises whether it is an animal or a plant. Although no sharp line of demarcation can be drawn between the lowest form of animal and of vegetable life, yet Torula is an indubitable plant, for two reasons. In the first place, its protoplasm is invested by a continuous cellulose coat, and thus has the distinctive character of a vegetable cell. Secondly, it possesses the power of constructing Protein out of such a compound as Ammonium Tartrate, and this power of manufacturing Protein is distinctively a vegetable peculiarity. Torula then is a plant, but it contains neither starch nor chlorophyll, it absorbs oxygen and gives off carbonic anhydride, thus differing widely from the green plants. On the other hand, it is, in these respects, at one with the great group of Fungi. Like many of the latter, its life is wholly independent of light, and in this respect, again, it differs from the green plants. Whether Torula is connected with any other form of Fungi is a question which must be left open for the present. It is sufficient to mention the fact that under certain circumstances some Fungi (e.g. Mucor) may give rise to a kind of Torula different from common yeast. The fermentation of the sugar is in some way connected with the living condition of the Torula, and is arrested by all those conditions which destroy the life of the Torula and prevent its growth and reproduction. The greater part of the sugar is resolved into Carbonic anhydride and Alcohol, the elements of which, taken together, equal in weight those of the sugar. A small part breaks up into Glycerine and Succinic acid, and one or two per cent. is not yet accounted for, but is perhaps assimilated by the Torula. This is the more probable as Torule will grow and multiply actively in a solution in which sugar and Ammonium Nitrate replace the Ammonium Tartrate of the former solution, in which case the carbon of the Protein, Cellulose and Fat manufactured, must be obtained from the sugar. Moreover, though oxygen is essential to the life of the Torula, it can live in saccharine solutions which contain no free oxygen, appearing, under these circumstances, to get its oxygen from the sugar. It has further been ascertained that Torula flourish remarkably in solutions in which sugar and pepsin replace the Ammonium Tartrate. In this case, the nitrogen of their protein compounds must be derived from the pepsin; and it would seem that the mode of nutrition of such Torula approaches that of animals. LABORATORY WORK. Sow some fresh baker's yeast in Pasteur's fluid' with The sugar is to be omitted when Pasteur's fluid "without sugar” is ordered. Pasteur himself used actual yeast ash; the above constituents give an imitation ash, which, with the ammonium salt and sugar, answers all practical purposes. sugar and keep it in a warm place: as soon as the mixture begins to froth up, and the yeast is manifestly increasing in quantity, it is ready for examination. A. MORPHOLOGY. I. 2. 3. a. Spread a little out, on a slide, in a drop of the fluid, and examine it with a low power ( inch objective, Hartnack, No. 4) without a cover-glass. Note the varying size of the cells, and their union into groups. Cover a similar specimen with a thin glass and examine it under a high power ( objective. Hartnack, No. 7 or 8, Oc. 3 or 4). Note the size (measure), shape, surface and mode of union of the cells. b. Their structure: sac, protoplasm, vacuole. a. Sac; homogeneous, transparent. B. Protoplasm; less transparent; often with a few clear shining dots in it. y. S. Vacuole; sometimes absent; size, position. The relative proportion of sac, protoplasm, and vacuole in various cells. Draw a few cells carefully to scale. Run in magenta solution under the cover-glass. (This is readily done by placing a drop of magenta solution in contact with one side of the cover-glass, and a small strip of blotting paper at the opposite side.) a. Note what cells stain soonest and most deeply, and what part of each cell it that stains: the sac is unaffected; the protoplasm stained; the vacuole unstained, though it frequently appears pinkish, being seen through a coloured layer of protoplasm. 4. 5. 6. Burst the stained cells by placing a few folds of blotting paper on the surface of the cover-glass and pressing smartly with the handle of a mounted needle: note the torn empty and colourless, but solid and uncrushed transparent sacs; the soft crushed stained protoplasm. Repeat observation 3, running in iodine solution instead of magenta. The protoplasm stains brown; the rest of the cell remains unstained. Note the absence of any blue coloration; starch is therefore not present. Treat another specimen with potash solution, running it in as before: this reagent dissolves out the protoplasm, leaving the sac unaltered. 7. [Sow a few yeast-cells in Pasteur's solution in a moist chamber and keep them under observation from day to day; watch their growth and multiplication.] 8. [Endogenous division: take some dry German yeast; suspend it in water and shake so as to wash it. Let the mixture stand for half an hour: pour off the supernatant fluid, and, with a camel's hair pencil, spread out the creamy deposit in a thin layer on fresh cut potato slices or on a plate of plaster of Paris, and place with wet blotting paper under a bell-jar: examine from day to day with a very high power (800 diam.) for ascospores, which will probably be found on the eighth or ninth day.] B. PHYSIOLOGY. (Conditions and results of the vital activity of Torula.) Sow a fair-sized drop of yeast in— I. a. b. C. Distilled water. 10 per cent. solution of sugar in water. |