THE MECHANISM OF MITOSIS 



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covery that the ''central granule " of the Heliozoa is to be identified 

 with the centrosome and plays the same role in mitosis (Fig. 41). 

 In these animals the axial filaments of the radiating pseudopodia con- 

 verge to the central granule during the vegetative state of the cell, 

 thus forming a permanent aster which Schaudinn's observations prove 

 to be directly comparable to that of a leucocyte or of a mitotic figure. 

 There is in this case no doubt of the contractility of the rays, and a 



B 



.' 







W/:i/^ 



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Fig. 50. — Pigment-cells and asters from the epidermis of fishes. [ZiMMERMAN'N.] 



A. Entire pigment-cell, from Bleimius. The central clear space is the central mass of the aster 

 from which radiate the pigment-granules; two nuclei below. D. Nucleus (//) and aster after ex- 

 traction of the pigment, showing reticulated central mass. C. Two nuclei and aster with rod- 

 shaped central mass, from Sargus. 



strong, if indirect, argument is thus given in favour of contractility in 

 other forms of asters.^ The contraction-hypothesis is beautifully 

 illustrated by means of a simple and easily constructed model, devised 

 by Heidenhain ('94, '96), which closely simulates some of the phenom- 

 ena of mitosis. In its simplest form the model consists of a circle, 

 marked on a flat surface, to the periphery of which are attached at equal 



1 For an interesting discussion and develoj ment of the contraction-hypothesis see 

 Watase, '94. 



