MODIFICATIONS OF MITOSIS 57 



C. Modifications of Mitosis 



The evidence steadily accumulates that the essential phenomena 

 of mitosis are of the same general type in all forms of cells, both 

 in plants and in animals. Everywhere, with a single important 

 exception (maturation), the chromatin-thread splits lengthwise through- 

 out its whole extent, and everywhere an achromatic spindle is formed 

 that is in some manner an agent in the transportal of the chromatin- 

 halves to the respective daughter-cells. The exception to this general 

 law, which occurs during the preparation of the germ-cells for their 

 development and constitutes one of the most significant of all cyto- 

 logical phenomena, is considered in Chapter V. We have here only 

 to glance at a number of modifications that affect, not the essential 

 character, but only the details of the typical process. 



I. Varieties of the Mitotic Figiwe 



All of the mitotic phenomena, and especially those involved in the 

 history of the achromatic figure, are in general most clearly displayed 

 in embryonic cells, and especially in the egg-celP (Fig. 24). In 

 the adult tissue-cells the asters are relatively small, the spindle 

 relatively large and conspicuous. The same is true of plant-cells 

 in general where the very existence of the asters was at first 

 overlooked. Plant-mitoses are characterized by the prominence of 

 the cell-plate (Fig. 25), which is rudimentary or often wanting in 

 animals, a fact correlated no doubt with the greater development 

 of the cell-membrane in plants. With this again is correlated the 

 fact that division of the cell-body in animal-cells generally takes place 

 by constriction in the equatorial plane of the spindle ; while in plant- 

 cells the cell is usually cut in two by a cell-wall developed in the 

 substance of the protoplasm and derived in large part from the cell- 

 plate. 



The centrosome and centrosphere appear to present great varia- 

 tions that have not yet been thoroughly cleared up and will be more 

 critically discussed beyond.^ They are known to undergo extensive 

 changes in the cycle of cell-division and to vary greatly in different 

 forms (Fig. 108). In some cases the aster contains at its centre 

 nothing more than a minute deeply staining granule, which doubtless 



^ A very remarkable modification of the achromatic figure occurs in the spiral asters, 

 <liscovered by Mark ('81) in the eggs of Umax, the astral rays being curved as if the entire 

 aster had been rotated about its centre. The meaning of this phenomenon is unknown. 



- See p. 224. 



