DETAILS OF MITOSIS 



85 



with the facts described in plants, it indicates that the forces involved 

 in spindle-formation are far more complex than Van Beneden's and 

 Boveri's hypothesis would lead one to suppose.^ 



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. 152). In some cases the aster contains at its centre 

 nothing more than a minute deeply staining granule, which doubtless 



A 



Fig- 36- — Division of spore-mother-cells in Equisetum, showing spindle-formation. [Osterhout.] 



A. Early prophase, "kinoplasmic"fibrillae in the cytoplasm. B. Multipolar fibrillar figure invad- 

 ing the nuclear area, after disappearance of the nuclear membrane. C. Multipolar spindle. 

 D. Quadripolar spindle which finally condenses into a bipolar one. 



represents the centrosome alone. In other cases the granule is sur- 

 rounded by a larger body, which in turn lies within the centrosphere 

 or attraction-sphere. In still other cases the centre of the aster is 

 occupied by a large reticular mass, within which no smaller body can 

 be distinguished {e.g. in pigment-cells) ; this mass is sometimes called 

 the centrosome, sometimes the centrosphere. Sometimes, again, the 

 spindle-fibres are not focussed at a single point, and the spindle 



1 See p. 276 for the peculiar spindles, devoid of asters, observed during the maturation of 

 the egg in certain forms. Cf. also Morgan's experiments on the artificial production of asters 

 and centrosomes, p. 307. 2 ggg p ^04. 



