54 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



incredible. From their situation they can in no way aid in the separation 

 of the chromosomes. 'It is, however, very evident that they may be of 

 material use in the later phenomena of cell division. Indeed, it can be 

 plainly seen that they may be of use in the elongation of the cell during 

 the anaphase. As the chromosomes move apart the centrosomes also 

 diverge, but still maintain a fixed distance from the cell walls at the poles 

 of the cell. This is accompanied by an elongation of the cell. I believe 

 that this lengthening of the cell is accomplished in the following manner. 

 The centrosomes are united with the cell membrane in the equatorial 

 region by means of astral rays. As they move apart (influenced 

 probably by some repellent force which they exert upon each other, or 

 possibly by the action of the central spindle), the rays which are inserted 

 in the cell membrane at the equator are put under greater tension, and 

 therefore pull with greater force, than rays farther away from the axis of 

 the spindle. This naturally lessens the diameter of the cell in the 

 equatorial region and also necessarily causes the diameter corresponding 

 with the axis of the spindle to increase proportionately. With the 

 elongation of the cell the centrosomes come to lie upon the cell membrane, 

 and the force exerted then by the astral rays which are attached farthest 

 from the equator comes to act approximately parallel to the cell mem- 

 brane, and thus the power which they exert upon this structure is mini- 

 mized. This condition is most obvious in those cases where both ends of 

 the cell have become conical. 



During the metaphase the fibres attached at the region of the poles 

 are finer and less well developed than those nearer the equator. As the 

 equatorial diameter diminishes, the cytoplasm formerly in this region is 

 forced away, and moves to the ends of the cells, thus causing the elonga- 

 tion of the cell. Of course, this also has the effect of further lessening 

 the force applied at the pole, because the number of rays is relatively 

 less for the amount of cytoplasm. However, the force exerted by the 

 fibres which are attached in the equator is still applied at a sufficient 

 angle to cause a further lessening of the cell diameter at the point of 

 application. 



It is, I believe, the universal conception that the cell membrane is a 

 flexible, elastic structure, which is capable of being much altered by 

 very slight irregularities in the forces acting upon it. When a force 

 applied at several points of such a structure acts in the direction of any 

 fixed point outside the membrane, the effect is to cause the points of 

 application to approach one another. Thus, when the astral rays at- 

 tached in the equatorial region become active, the effect is for the 



