84 



CYTOKINESIS. 



die axis shifts through an angle of about 45°, the upper pole of the spindle being 

 carried in toward the animal pole until the lobe of cytoplasm is pressed into 

 the angle between contiguous cells of the first quartette, fig. 93. In this position 

 the "equatorial" constriction occurs and the second quartette of micromeres is 

 separated from the macromeres. 



As the whole of the sphere substance of the second cleavage goes into the first 

 quartette of micromeres, so all the sphere substance of the third cleavage, remaining 

 in the macromeres, goes into the second quartette where it rapidly disa^^pears. 



In the telophase the cell contents of the second quartette move in a Iseotropic 

 direction until the centrosomes and spheres are carried from the extreme left to the 

 extreme right of each cell; at the same time the entire cell contents of the macro- 

 meres move in a dexiotropic direction until the nuclei, centrosomes, spheres and 



Fig. XXIV. Fig. XSV. 



Figs. XXIV, XXV.— Two stages in fourth cleavage of Crepidula showing the spreading of tlie olfl sphere substance 

 at the upper pole of the spindle and the lobing of the cytoplasm to form the second quartette of micromeres. 



cytoplasmic areas are carried to the right side of each cell (cf. figs. 93, 94, 95, text 

 figs. XIII, XIV). As a consequence of these movements in the daughter cells, the 

 spindle axes which were straight lines, until the telophase become bent at the mid- 

 bodies until finally the two halves of each spindle are nearly parallel with each 

 other. This movement of the cell contents and consequent bending of the spindle 

 axes is greater in the second c[uartette cells than in the macromeres, the rotation 

 in the former being through an angle of more than 90°. 



During these movements and throughout the succeeding rest period the centro- 

 somes and spheres are never overlaid by other cells; both in the second quartette 

 and in the macromeres they lie as near as possible to the animal pole without 

 moving under the first quartette cells, fig. 94, 95. If in some cases {e.^. fig. 

 95) they seem to be covered by the cells lying nearer the animal pole, this is 

 due merely to the overarching of these cells, as side views and sections show. 

 In all cases the centrosomes and spheres lie next to free surfaces of the cells. 



(5). J^i/i/i and Sixth Cleavages of the Macromeres. — While the cell contents 

 of the macromeres are moving from the left to the right side of each cell, the 



