September 7, 1900.] 



SCIENCE. 



565 



Fol, van Beneden and Plemming, whicli 

 showed that the division of the nucleus 

 and the cell was due to a series of very re- 

 markable changes, now known as indirect 

 nuclear and cell division, or karj'okinesis. 

 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 fibers. 

 The first stage in the process of nuclear 

 division consists in the threads arranging 

 themselves in loops and forming a compact 

 coil within the nucleus. The coil then be- 

 comes looser, the loops of threads shorten 

 and thicken, and somewhat later each looped 

 thread splits longitudinally into two por- 

 tions. As the threads stain when coloring 

 agents are applied to them, they are called 

 chromatin fibers, and the loose coil is the 

 chromosome (Waldeyer). 



As the process continues, the investing 

 membrane of the nucleus disappears, and 

 the loops of threads arrange themselves 

 within the nucleus so that the closed ends 

 of the loops are directed to a common cen- 

 ter, from which the loops radiate outwards 

 and produce a starlike figure (aster). At 

 the same time clusters of extremely deli- 

 cate 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 ar- 

 range 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 to- 

 wards its equator (metakinesis). By this 

 division of the chromatin fibers, and their 

 separation from each other to opposite poles 

 of the spindle, two star-like chromatin fig- 

 ures are produced (dyaster). 



Each group of fibers thickens, shortens, 

 becomes surrounded by a membrane, and 

 forms a new or daughter nucleus (di- 

 spirem) . 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 constric- 

 tion 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 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 chromo- 

 somes 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 chromosomes is con- 

 stant for each species, and that in the 



