YEAST-LIKE FUNGI OF HUMAN INTESTINAL TRACT 17 



etc., aid in the separation of species. In the yeast-like organisms under con- 

 sideration there are certain" difficulties encountered in attempting to separate 

 the species by the various characters given. The yeasts are primarily unicellular 

 organisms, even when an apparent mycelium is formed. These individual cells 

 tend to assume a certain fairly uniform shape under a definite set of condi- 

 tions (Fig. 1). When 2 species are examined side by side under the microscope 

 it usually is easy to see that they are different. But when a number of cells 

 of 1 of these species are measured a wide variation- is observed, and when com- 

 pared with the other species these variations cause the measurements to over- 

 lap to such an extent that it is impossible to distinguish, from description, 

 which of the 2 is under consideration. A very large number of measurements 

 possibly would reveal a constant difference between the 2 species, but the neces- 

 sary measurements would be too laborious and uncertain for practical use. It 

 is highly desirable that a system of species differentiation be devised which is 

 practical for the average laboratory worker. 



The difficulties of differentiating species arising from this great variation 

 in the size and form of the cells has been recognized by such workers as 

 Hansen, Will, and Geiger. They maintain, as I do, that the morphologic char- 

 acters have little analytic value when taken alone. 



An attempt was made to use internal structures such as size, number, and 

 position of granules, the relative refraction of the protoplasm, the size, num- 

 ber, and location of the vacuoles, etc., as a basis for separating species. It was 

 found, however, that these internal structures varied according to the age of 

 the cell and the medium in which it was placed. At one stage a small vacuole 

 with a single dancing particle within it was present; a few hours later this 

 vacuole would be much larger and there might be several dancing particles 

 in it, or the vacuole might seem to disappear entirely, due, probably, to the 

 change in the refractive index of the solution. Thus no satisfactory means of 

 separation could be found by a study of internal structure. 



The form of the cells is 'fairly constant in some species. For example, 

 Saccharomyces cerevisiae differs from Saccharomyces pastorianus in a con- 

 stant manner, and one would have no difficulty in separating these 2 species 

 under the microscope. But the number of possible forms the yeasts can assume 

 is very limited, as is indicated by Figure 1. Furthermore, a single species 

 may have cells of all of these forms in a single field of the microscope (Plate 3, 

 Fig. 10) . This is especially true of the 'wild' yeasts under consideration. 



The manner of budding is constant in many species. If a given species is 

 cultivated under a fairly wide range of conditions the buds may uniformly 

 appear at some definite point on the cell. But here again there are only a 

 few possibilities of differences. A large number of species may bud in the 

 same manner and again there are forms which may vary in their manner of 

 budding under varying conditions. The tendency to form elongated cells of 

 a hyphal character is constant in certain species, but is present in a large num- 

 ber of species. 



All of the characters mentioned, when used alone or in combination, are 

 of value in the description of a species ; but the general statement can be made 

 that no constant morphologic character or combination of characters can be 

 found which is uniformly reliable in differentiating single species, on account 

 of the great tendency to vary among species and the limitation of possibilities 

 in such simple organisms. 



From this discussion it is evident that morphologic characters must be used 

 in connection with biochemical properties and cultural peculiarities in the dif- 

 ferentiation of asporogenic yeast-like species in the same manner in which the 



