94 CYTOKINESIS. 



this cone has an angle of about 130°. The cell membrane 1 begins to protrude 

 over the base of this cone, the interior of which is filled with a clear non-stain- 

 ing substance ; as the cell membrane protrudes, the space between it and the centro- 

 some increases, for although the pole of the spindle moves into this protrusion, it 

 moves more slowly and to a less extent than does the cell substance. During this 

 period of protrusion the base of the cone withdraws from the cell membrane, and 

 at the same time its angle decreases until finally it ceases to touch the membrane 

 and becomes an irregular sphere, fig. 73. 



That a similar elongation is taking place at the deeper pole of the spindle is 

 shown by the facts : (1) that a protrusion of cytoplasm surrounding this pole is 

 thrust down into the yolk, which, at the same time, is moved out of the axis of the' 

 spindle and up at the sides toward its equator, text figs. XIX-XXIII; (2) in abnor- 

 mal eggs it frequently happens that a protrusion of the cell membrane takes place 

 opposite the deeper pole of the spindle, as well as at its apical pole, only in this 

 case the protrusion at the lower pole is filled with yolk and frequently ruptures the 

 egg membrane altogether, text figs. XXX, XXXI. This protrusion at the lower 

 pole, in connection with that at the upper one, shows that the surface tension is 

 lessened at points on the cell membrane opposite the poles of the spindle, and further 

 that this is associated with movements of the cell substance from the equatorial to 

 the polar areas of the spindle. 



Fig. XXX. Fig . xxxl 



Figs. XXX, XXXL— Abnormal stages in the formation of the second and third quartettes of Crepidula, showing a 

 lobe of cell substance at both poles of certain spindles. e 



But while this polar flow of cell substance usually takes jolace in the spindle 

 axis, it does not follow that the spindle or any part of it is the cause of the flow. 

 On the other hand it sometimes happens in abnormal cleavages that the spindle 

 does not move into its proper position, but remains in the area of one of the future 

 daughter cells ; in such cases the cell elongates in its usual direction, but not in the 

 spindle axis, and the constriction and division of the cell body may be wholly typical 

 1 In gasteropod eggs the cell membrane is merely a denser layer of protoplasm. 



