THE STRUCTURE AND BEHAVIOR OF THE NUCLEI IN SPORES AND GROWING HYPHAE OF MUCORALES: I. MUCOR HIEMALIS AND MUCOR FRAGILIS

Abstract

The behavior of the nuclei in resting and germinating spores and in growing hyphae of Mucor hiemalis has been followed during life with phase contrast microscopy. The nuclei consist of a dense central body, the nucleolus, surrounded by a shell of variable shape composed of optically uniform material of low density. The nuclei have been seen to divide by constriction. One half of the nucleolus and one half-shell of the low density material pass to each daughter nucleus.All phases of nuclear division have been recognized in fixed and stained preparations. The nucleolus is readily stained by iron hematoxylin or gentian violet but is Feulgen-negative. The shell of low density has no marked affinity for hematoxylin and other basic or acid stains although it consists largely of granules and filaments which are Feulgen-positive. In these elements the chromosomes of the nuclei must somehow be contained but the size, shape, and behavior of individual chromosomes cannot yet be described. Division of the mass of chromatinic elements is direct and involves neither spindle nor metaphase plate. It is tentatively proposed that the chromosomes in the resting nucleus are already divided and segregated to opposite sides and that the constriction of the nucleus is merely the consummation of a kind of endomitosis initiated during the terminal stages of the previous division. Supporting observations have been made on Mucor fragilis, several other Mucorales, and two species of Saprolegnia. Uncertainties and controversies of the past can be explained in the light of the new findings.