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Identification of the yeast nucleus, and particularly a resolution of its mitotic cycle, can scarcely be considered to have yielded unanimous agreement. In spite of a voluminous cytological literature, little has been established with certainty in regard to the identity, morphology, or numbers of chromosomes, or in regard to the orientation of the dividing nucleus with respect to the centrosomes during budding. Incertitude and divergences of opinion on these questions have resulted partly from difficulties inherent in obtaining suitable preparations of organisms so small as yeast for cytological analysis, and partly from the fact that cytological studies were undertaken in almost all instances on cultures unknown genetically as to the number of linkage groups and the degree of ploidy. Nagel (1946), DeLamater (1950), Lindegren and Rafalko (1950), Lietz (1951), and Winge (1951a) have considered various interpretations of yeast cytology at length; hence we shall omit a detailed historical discussion. The purpose of the present report, based on cytological procedures hitherto not used for yeast, is to describe single and dividing cells and offer evidence for a vesicular nucleus as well as to consider inadequacies apparent in some current viewpoints. It is sufficient to indicate here two major, opposing ideas: According to Lindegren (1949, 1952), following Wager and Peniston (1910), the chromosomes are located within the vacuole (the "nuclear vacuole") and exhibit a pattern of "mitotic" behavior thus far unknown in other organisms. This view is opposed by the majority of yeast cytologists who hold that the true nucleus is that structure identified by Lindegren as the centrosome; this is a Feulgen positive, sphere-like body of relatively massive proportions disposed adjacent to and outside the vacuolar wall. A few workers are in accord with the second view (Guilliermond, 1920; Beams et al., 1940; Sinoto and Yuasa, 1941) and believe the division of this body to be amitotic. However, well established genetical information is currently available to render this belief unwarranted. Conclusive proof that this extravacuolar body is the nucleus derives from the fact that its total volume and mean diameter increase in accord with the theoretical expectation for polyploids (Mundkur, 1953). This reinvestigation of a much-debated subject was undertaken in view of the availability of a polyploid series whose haploid, diploid, triploid, and tetraploid constitutions were clearly established genetically with the aid of 6-11 diagnostic markers by Lindegren and Lindegren (1951). Furthermore, a reexamination of the problem with the aid of the Altmann-Gersh freezing-drying technique, hitherto not exploited in yeast cytology (or perhaps in bacterial cytology), was deemed desirable in view of its many unambiguous qualities. Advantages of this technique are considered in detail by Gersh (1932, 1948) and recently by Bell (1952) and Danielli (1953). Suffice it to indicate here that (1) cessation of metabolic activity is almost instantaneous, (2) shifting of diffusible cell constituents is inappreciable, (3) there is little possibility of chemical alterations in cells leading to morphological artifacts or pseudostaining reactions such as those sometimes ensuing from the use of chemical fixatives, (4) shrinkage of cells is absent or negligible, (5) there is a greater preservation of cell inclusions than is possible when fixing solutions are used, and (6) direct infiltration of dehydrated cells with celloidin or paraffin is possible.

THE NUCLEUS OF SACCHAROMYCES: A CYTOLOGICAL STUDY OF A FROZEN-DRIED POLYPLOID SERIES