CYTOKINESIS. 93 



daughter nuclei, centrosomes and spheres are at first absolutely equal in the two 

 cells, but the astral rays are always proportional in quantity to the volume of the 

 daughter cells. Later, as the astral rays disappear, the spheres grow, becoming in 

 the end proportional in size to the volume of these rays. These facts favor the 

 conclusion that the substance of the astral rays flows into the spheres during the 

 later stages of mitosis. 



(b) Movements in the Cell Body. — While these movements are taking place 

 in the spindle and asters other coincident movements are apparent in the cell body, 

 which lead to the elongation of the cell in the spindle axis and to its ultimate con- 

 striction at right angles to this axis. 



The elongation of the cell in the spindle axis takes place in every division, 

 even though this axis, when fully elongated, may not be as long as the greatest 

 diameter of the cell. This elongation of the cell may be symmetrical at the two 

 poles, as is the case in all equal cleavages, or it may occur chiefly or entirely at one 

 pole, as is true in very unequal divisions. 



There are many reasons for believing that this elongation of the cell is due to 

 a flow of cell substance into the polar areas from the equatorial region of the spin- 

 dle. The most important of these evidences are found in unequal cleavages in 

 which the elongation of the cell takes place chiefly or entirely at one pole. If one 

 considers either of the maturation divisions or the formation of the first, second or 

 third quartettes of Crepidula, one perceives that the cell as a whole does not elon- 

 gate in the direction of the spindle axis until one pole of the spindle has come close 

 to the cell membrane. After the sphere, and in the case of the maturation divi- 

 sions, the centrosomes also, have been flattened against the cell wall, the latter pro- 

 trudes from the general outline of the cell, and into this protrusion the sphere sub- 

 stance, the pole of the spindle and some of the cell substance passes. The elonga- 

 tion of the cell in this case is brought about by this protrusion at one pole of the 

 spindle, and a study of the steps by which this is accomplished shows that there is 

 (1) the movement within the cell which carries the spindle to a peripheral position 

 and presses one pole against the wall, (2) a rapid growth of the cell wall over the 

 pole of the spindle, especially in the area where the sphere is pressed against the 

 wall, (3) a consequent diminution of surface tension at this point and a movement 

 of the pole of the spindle and the cell substance into the protrusion thus formed. 

 If the peripheral movement of the spindle is strong, it may be thrust into this pro- 

 trusion as far as possible, as in the case of the maturation divisions ; if it is less 

 strong the growth of the cell wall and the outflow of cell substance may outrun the 

 movement of the spindle, as is the case in the formation of the first three quartettes. 

 Successive steps in the elongation of the cell preparatory to the separation of the 

 quartettes are shown in text figs. XIX-XXV. It will be seen by these figures that 

 during the prophase and metaphase the peripheral centrosome lies close to the cell 

 membrane, and that the aster is pressed against the cell membrane in the form of a 

 cone, the base and periphery of which are formed of the old sphere substance 

 (sphere remnants of a previous cell cycle). In the metaphase of the third cleavage 



