DIVISION OF NUCLEI, S. It is accepted that a nucleus never arises de novo, but only by the division of a preexisting nucleus. The principal structures concerned in division are the chromo somes, the linin, the spindle (achromatic figure), and the centrosome. The last structure. how ever, is not so constantly present as in animals, and many believe that it does not exist at all in the fern-plants (pteridophytes) and plants (spermatophytes). In the cells which produce pollen-grains the spindle first appears as a weft of fibres surrounding the nucleus. which become grouped into cones forming a mul tipolar spindle, that gradually becomes bipolar. In ordinary vegetative cells the spindle i. bi polar from the start. making, its appearame as a pair of dome-shaped caps at opposite poles of the nucleus. The chromatin is generally re as the most important part of the nu cleus, as it is believed to be the morphological basis of heredity. division the chromatin assumes the form of a narrow band which divides into a definite number of more or le,. elongated pieces. the chromosomes. The number of chromo somes is constant in :1 given and the number in the sexless generation (sporophytel is always double the number found in the sexual generation (gametophyte). The reduction in
number takes place during the formation of spores. 11 hen the nucleus of the sperm fuses with that of the egg, the double 'mintier is re stored. The splitting of the chromosomes in the vegetative divisions, and perhaps also in the division: concerned in the reduction of chromo som•s. is longitudinal. Some of the fibres of the spindle become attached to the chromosomes, half the number being drawn to one pole and half to the other, while a series of granule:, the be ginning of a new •ell-wall. appear: on the fibres which stretch between the young nuclei. The wall becomes completed as the fibres disappear. In the formation of the endosperm of seeds, and in some other eases, nuclear division may take place Nvithout being follinveil by the formation of a cell-wall—a phenomenon called free nuclear division. See FERTILIZATION; CYTOLOGY.