CYTOKINESIS. 91 



mass of chromatin and linin into the peripheral portion of the nucleus, and when 

 the nuclear membrane dissolves, into the cell body. The intra-nuclear spindle, 

 although containing a considerable quantity of interfilar substance, is much denser 

 than the surrounding nuclear sap; the radiations of this denser material which 

 surround the equator of the spindle (see p. 18) are apparently due to the fact that 

 in the shrinkage of the linin reticulum some of the fibres of the latter remain 

 attached peripherally, and thus cause a stellate appearance of the spindle when seen 

 in cross sections. 



I have already indicated (p. 38) that the cause of the movements of the chromo- 

 somes in the metakinesis cannot be found exclusively in the contraction of the 

 mantle fibres, though this may form an important factor ; it is probable that this 

 movement is associated with chemotropic attraction between the centrosomes and 

 spheres (Strasburger, Wilson et a/.). At the time when the chromosomes are being 

 separated the interfilar substance of the spindle aggregates at the two poles, thus 

 contributing to the growth of the spheres. It is probable that this movement is in 

 the nature of diffusion streams, and that the cause of the movement lies primarily 

 in the chemotropic influence of the centrosome. It is scarcely possible that this 

 interfilar substance could be moved by the activity of the spindle fibres, and the 

 fact that the chromosomes move to and partially surround the spheres indicates that 

 their movement may be associated with the same factor which is active in the move- 

 ment of the interfilar substance. 



In the astral radiations the movements are also in the nature of diffusion 

 streams, as was pointed out in the first part of this work (p. 49). In the growth of 

 the aster the denser substance of the alveolar walls (hyaloplasm) is aggregated toward 

 the centrosome. while the more fluid alveolar contents and all cytoplasmic inclusions, 

 such as yolk, are moved farther and farther from the centrosome. The mechanical 

 principles involved in this process have been worked out in detail by Rhumbler 

 ('96). But in addition to this movement there is probably, in the earlier stages of 

 mitosis, a diffusion of nuclear substance from the sphere along the astral radiations. 

 This is indicated by the fact that these radiations stain like the central area of the 

 aster, into which nuclear sap has escaped, and much more deeply than the hyalo- 

 plasm of the cell. 



In the prophase of the third, fourth and fifth cleavages the upper pole of the 

 spindle lies immediately under the old sphere substance, which at this stage forms 

 a compact, lenticular mass immediately below the cell membrane, figs. 71, 72. In 

 the metaphase this sphere substance is spread for a considerable distance under the 

 cell membrane, its periphery being marked by a thickened ring of this material. 

 As this substance spreads, the rays which go to its periphery remain large and deep- 

 staining, thus forming a kind of "antipodal cone" 1 (Van Beneden), the apex of 

 which lies at the centrosome and its base at the cell wall (text figs. XIX-XXIX). 

 Within this cone the rays are faint and stain little, and the interfilar spaces are 



1 This name is used merely as a convenient descriptive term and without intending to homologize 

 the structure observed by Van Beneden with the one here described. Rhumbler (1901) has called a 

 similar structure in nematode eggs the " Polfontaine." 



