MITO'SIS (Neo-Lat., from Gk. aurae, ?altos, thread). The phenomena accompanying the di vision of the nucleus of cells, a term proposed by Fleming in IS:82 and superseding •karyokinesis.' In cell-division the scat of the changes is the nucleus. in which portion of the cell the processes of eell-division originate. After the nucleus sub divides the entire cell divides into halves, form ing two new cells. occurs not only in the division of cells during growth. but the nucleus subdivides, forming the first steps in reproduction or fertilization of all organisms. When the nucleus is about to divide, the chro matic granules forming part of the nuclear sub stance, and previously scattered throughout the central mass of the nucleus, become arranged in a row, forming a long thread, which extends through the nucleus in an irregular spiral (spireme) and then divides into portions ('chromosomes') of fairly equal length. The chromosomes are shaped like long loops, which afterwards become shortened, thus giving rise to short loops, straight rods, or rounded granules. As a rule the number of chromosomes is constant for each species of plant or animal, and also for successive series of cells during growth. By the time the process has reached this stage a special mechanism appears, which has till now remained concealed in the cell-substance. This serves to divide the chromatin into two equal parts, to separate the resulting halves from one another, and to arrange them in a regular man ner. At the opposite poles of the longitudinal axis of the nucleus two clear bodies—the 'Cen t rOsOlneS: each surrounded by a clear zone, the so-called 'sphere of attraction'—now become visi ble. These were first discovered by Fol. by Van Iteneden, and also by Boveri. who recognized their They appear to possess a great power of attraction over the vital particles of the cell, so that these become arranged around them like a series of rays. At a certain stage in the preparation for division, the substance of the cell body and of the nucleus gives rise to delicate fibres or threads; these fibres are motile, and, after the disappearance of the nuclear mem brane, seize the chromosomes with wonderful cer tainty and regularity, and in such a way that each element is held on either side by several threads from each pole. The chromatin elements
thus immediately become arranged in a fixed and regular manner, so as to lie in the equatorial of the nucleus. The eentrosomes and threads or astral fibres (aster) called the 'spin dle• together form the 'aluiddoster.' The chro matin elements next split longitudinally, thus becoming doubled. as discovered by Fleming. This splitting is completed by the two halves being gradually drawn farther apart toward the op posite poles of the nuclear spindle, until they finally approach the centre of attraction or een trosome, which has now fulfilled its object for the present, and retires into the obscurity of the tell substance, only to become active again at the next cell division. Each separate half of the nucleus now ^oustitutu s a daughter nucleus in which it. breaks up, and is scattered in minute granules in the nuclear network. The holly of the cell then IliVides, sill/Wing two new cells. has pointed out that the whole complex but wonderfully exact apparatus for the division of the nucleus exists for the purpose of divid ing the chromatin substance in a fixed and regu lar manner. not merely quantitatively. but also in respect of the different qualities which must be contained in it. It will be remembered that the particles OT chromosomes are be lieved to lie bearers of heredity. .The mechanism of mitosis is thus far u»known. The problem may be solved, as Wilson states, through chemi cal resea reit.
Amrrosts. :Mitosis is the indirect division of the nucleus: whene the nucleus elongates and di rectly divides t h rough its total mass the process is called This appears to occur ceptionally. but is known to take place in anueba, in leueocytes, and has been observed in the sperm-cells and eggs of batrachians and some in sects; but more commonly in pathological tissues.