88 CYTOKINESIS. 



spindles, as a study of fig. 97 will show. The spindles here lie very nearly in the 

 middle of the cells, but the equatorial constrictions show plainlj?- not only that the 

 division will be unequal, but also that the basal cell in the posterior ai-m of the 

 cross will be smaller than the other three. The cause of the inequality of this 

 division probably lies, not in the position of the spindle, but in the activities of 

 the cytoplasm ; and this suggests that the inequality of the preceding cleavage was 

 not caused by the eccentric position of the spindles, but that both the position of 

 the spindles and the inequality of the cleavage are the results of cytoplasmic 

 activities. 



In the telophase of this cleavage the nuclei remain near the middle of the 

 daughter cells, but the spheres in the apicals rotate through an angle of from 90° 

 to 180°. In fig. 98 the spheres in these cells are close to the new cell wall between 

 the daughter cells, i. e., they have rotated over the nuclei into this new position, and, 

 therefore, away from the animal pole ; that this movement is not merely in a 

 vertical plane is shown by the position of the spheres in fig. 99, where it is evident 

 that the rotation is also dexiotropic, as it should be, since these cells were formed 

 by a dexiotropic cleavage. In the basal cells the spheres move toward the free 

 surface of the cell and a little to the left, figs. 98, 99, but this movement is never 

 extensive ; in no case do the spheres move to that portion of the cell which is 

 nearest the animal pole, but they invariably remain on the side of the nucleus 

 farthest removed from that pole. In both of the daughter cells of this cleavage, 

 therefore, the movements ai'e very unusual since the spheres do not move in the 

 telophase as close as possible to the animal pole. The middle of the spindle axis is 

 bent to the right, as it should be following a dexiotropic cleavage (see text fig. XVI). 



Especial interest attaches to the cell movements in reversed cleavage or cases 

 in which two successive divisions are in the same direction. Such an instance occurs 

 in the first division of the basal cells of the cross. These cells were formed by 

 dexiotropic division of the cephaloblasts, and to preserve the law of alternation they 

 should divide in a Ifeotro^aic direction, but they all divide dexiotropicall}^, though 

 the posterior and smaller one does not divide until long after the others (it is still 

 undivided in fig. 100). The reversed cleavage of these cells is associated with the 

 fact that during the preceding rest the centrosomes and spheres remain on the 

 outer sides of the nuclei and do not move to that portion of the cell nearest the 

 animal pole, fig. 99. Therefore the reversal is due to the limited extent of the cell . 

 movements, and not to reversed rotations of the cell contents. 



At the close of this division of the basals the contents of the upper cells rotate 

 to the right, while those of the lower cells rotate to the left, fig. 100. This is the 

 typical cell movement following a dexiotropic cleavage, and accordingly we may 

 expect to find the subsequent cleavage of these cells entirely typical, an expectation 

 Avhich is fully realized. 



(7). Sub-divisions of the Second Quartette. — The second quartette cells were 

 formed by a la^otropic cleavage, and consequently the rotation within them is in a 

 laeotropic direction; this rotation has been fully described on p. 84. When this 



