THE YEAST CELL in a Mendelian manner in S. cerevisiae by S. bayanus hybrids. Saccharomyces bayanus does not ferment galactose but otherwise resembles S. cerevisiae in fermentative ability. Saccharomyces pastorianus is responsible for bitter taste, disagreeable odor, and turbidity in beer. It is a bottom fermenting yeast producing cylindrical cells. Saccharomyces validus is a top fermenting yeast with long cylindrical thread-shaped cells producing turbidity in beer. Saccharomyces turbidans grows well over a wide temperature range producing elliptical cells and a very turbid broth. It is also undesirable In beer because of the turbidity. CRITERIA FOR DISTINGUISfflNG HAPLOPHASE AND DIPLOPHASE Winge (1935) distinguished clearly between haplophase and diplophase yeast cultures, and we have corroborated his observation, adding the concept that genes controlling mating type specificity segregate at meiosis to produce two kinds of haplophases. I shall summarize all the arguments, Winge* s reinforced by ours, for distinguishing haplophase and diplophase. Over four-fifths of the cultures which one encounters are easily characterized by microscopic examination. They are either obviously haploid or diploid, as shown simply by size, shape and aggregation of the cells. The reasons for classifying them are as follows: a. The large vegetative cell, which we call "legitimate diploids" (fig. 1-7), produce viable 4-spored asci. These spores germinate to produce smaller, usually round, cells which we call "haploid." The latter multiply vegetatively, generally maintaining their specific cell-shape and size. b. Two of these smaller cells may fuse to produce a zygote which on budding produces a large "diploid" cell capable of vegetative multiplication. While the large cell is undergoing vegetative reproduction, it retains its characteristic ellipsoidal shape and size. Under certain conditions this diploid cell can be induced to sporulate. Spores from it in turn produce haploids and the process can be repeated indefinitely. o. The large cells which we recognize as diploids are extraordinarily stable in their genetical characteristics when they are grown under conditions in which sporulation does not occur. Transferring the cultures every forty-eight hours in broth is generally sufficient to maintain the vegetative diplophase. Colonies produced by plating out are not sectored; the plates do not show colonial variants. However, when haplophase (single ascospore) cultures of any age are plated out, a variety of colonial variants appears on the plate or the giant colonies are sectored. These facts are consistent with the view that the large cells are diploid, thus minimizing the num-LIFE CYCLES 1-9 ber of variants which may appear, while in the haplophase most variants become apparent and are easily discovered. d. When the diploid cells spomlate to produce haploid cells, tetrad analysis of the 4 spores has proved that a reduction division occurs. Hybrids heterozygous for as many as 12 alleles have been analyzed, all of which segregated regu...

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