572 POPULAR SCIENCE MONTHLY. 



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 fibers. 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 op- 

 posite pole, and the free ends of each loop are directed toward its equa- 

 tor (metakinesis). By this division of the chromatin fibers, and their 

 separation from each other to opposite poles of the spindle, two star- 

 like chromatin figures are produced (dy aster). 



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

 &jT~rnbrane, 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 Flem- 

 ming — 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 preexisting cell. 



A repetition of the process in each of these cells leads to the forma- 

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

 different 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. 



In the study of karyokinesis, importance has been attached to the 

 number of chromosomes in the nucleus of the cell. Flemming had seen 

 in the Salamander twenty-four chromosome fibers, which seems to be a 

 constant number in the cells of epithelium and connective tissues. In 

 other cells, again, especially in the ova of certain animals, the number 

 is smaller, and fourteen, twelve, four and even two only have been de- 

 scribed. The theory formulated by Boveri that the number of chromo- 

 somes is constant for each species, and that in the karyokinetic figures 

 corresponding numbers are found in homologous cells, seems to be not 

 improbable. 



In the preceding description I have incidentally referred to the ap- 

 pearance in the proliferating cell of an achromatic spindle-like figure. 

 Although this was recognized by Fol in 1873, it is only during the last 

 ten or twelve years that attention has been paid to its more minute 

 arrangements and possible signification in cell-division. 



