Cell and Protoplasm. 107 



complex. It is usually marked by the somewhat dumb- 

 bell-like constriction of the nucleus as a whole ; without 

 complex preliminaries or manoeuvres, one cell becomes 

 two. In the great majority of cases it seems to be a 

 secondary process, and it certainly is not the usual mode 

 of cell-multiplication. In the usual mitotic process there 

 is an intricate interaction between nucleus and cell-sub- 

 stance, and a complex co-operation of the different 

 members of the " cell-firm", the centrosomes, the 

 chromosomes, the achromatin, and the general cell- 

 substance. One might compare it to the legal com- 

 plexities observable in the dissolution of partnership in 

 an old-established firm of several members with some- 

 what ravelled interests. 



According to Wilson's recent summary, in which he 

 seeks to strike a balance-sheet of many opinions and 

 observations, the centrosome is the organ of division 

 par excellence, "under its influence, in some unknown 

 manner, is organized the astral system, which is the 

 immediate instrument of division ", its rays becoming 

 associated with the chromosomes, which are certainly of 

 great importance, if they are not so exclusively essential 

 as some would make out. "Mitosis is due to the co-ordi- 

 nate play of an extremely complex system of forces 

 which are as yet scarcely comprehended." Its end, how- 

 ever, is clear; it is "to divide every part of the chro- 

 matin of the mother-cells equally between the daughter- 

 cells ". There are many peculiarities in different cases ; 

 there seem to be even individual variations; there are 

 certainly abnormalities here and there ; but in plant and 

 animal there is a fundamental similarity both of process 

 and result. 



The central corpuscles in animal cells seem to act as 

 if they were centres of force, and the indescribably fine 

 threads which pass from around them to the chromatin 

 bodies and elsewhere have been credited with motive 

 powers. But the cell-divisions in higher plants seem 

 to be accomplished without the presence of centrosomes. 

 The whole subject is beset with uncertainties. At the 

 same time, it can hardly be doubted that such sugges- 

 tions as Heidenhain's "tension-law" hold out some 



