ADDRESS. 13 



kinesis. The changes within the nucleus are of so complex a character that 

 it is impossible to follow them in detail without the use of appropriate 

 illustrations. I shall have to content myself, therefore, with an elemen- 

 tary sketch of the process. 



I have previously stated that the nucleus in its passive or resting stage 

 contains a very delicate network of threads or fibres. The first stage in 

 the process of nuclear division consists in the threads arranging them- 

 selves in loops and forming a compact coil within the nucleus. The coil 

 then becomes looser, the loops of threads shorten and thicken, and some- 

 what later each looped thread splits longitudinally into two portions. As 

 the threads stain when colouring agents are applied to them, they are 

 called chromatin fibres, and the loose coil is the chromosome (Waldeyer). 



As the process continues, the investing membrane of the nucleus dis- 

 appears, and the loops of threads arrange themselves within the nucleus 

 so that the closed ends of the loops are directed to a common centre, from 

 which the loops radiate outwards and produce a starlike figure (aster). 

 At the same time clusters of extremely delicate lines appear both in the 

 nucleoplasm and in the body of the cell, named the achromatic figure, 

 which has a spindle-like form with two opposite poles, and stains much 

 more feebly than the chromatic fibres. The loops of the chromatic star 

 then arrange themselves in the equatorial plane of the spindle, and 

 bending round turn their closed ends towards the periphery of the nucleus 

 and the cell. 



The next stage marks an important step in the process of division of 

 the nucleus. The two longitudinal portions, into which each looped thread 

 had previously split, now separate from each other, and whilst one part 

 migrates to one pole of the spindle, the other moves to the opposite pole, 

 and the free ends of each loop are directed towards its equator (meta- 

 kinesis). By this division of the chromatin fibres, and their separation 

 from each other to opposite poles of the spindle, two star like chromatin 

 figures are produced (dyaster). 



Each group of fibres thickens, shortens, becomes surrounded by a 

 membrane, and forms a new or daughter nucleus (dispirem). Two nuclei 

 therefore have arisen within the cell by the division of that which had 

 previously existed, and the expression formulated by Flemming — omnis 

 nucleus e nucleo — is justified. Whilst this stage is in course of being 

 completed, the body of the cell becomes constricted in the equatorial plane 

 of the spindle, and, as the constriction deepens, it separates into two parts, 

 each containing a daughter nucleus, so that two nucleated cells have 

 arisen out of a pre-existing cell. 



A repetition of the process in each of these cells leads to the formation 

 of other cells, and, although modifications in details are found in diflferent 

 species of plants and animals, the multiplication of cells in the egg and in 

 the tissues generally on similar lines is now a thoroughly established fact 

 in biological science; 



