CYTOKINESIS. 89 



la?otropic movement has carried the centrosomes and spheres frcm the extreme left 

 to the extreme right of each cell, fig. 96, the cleavage begins. The mother centro- 

 some becomes spindle shaped and gives rise to the daughter centrosomes and central 

 spindle. These stretch over the outer and upper sides of the nuclei, most probably 

 in the groove between the germ halves. The right and upper pole of the spindle 

 lies immediately under the old sphere substance, all of which goes into the right 

 daughter cell, where it i-apidly disintegrates and disappears, fig. 96, text fig. XXXI. 

 The cell elongates in the spindle axis and the equatorial constriction appears as 

 usual ; the division is approximately equal, though the right cell product slightly 

 overlaps the left, and therefore appears larger in surface view, fig. 97. 



In the telophase of this division the contents of the daughter cells rotate, as is 

 usual, in opposite directions, the upper or right hand cells to the right, the lower 

 ones to the left. This movement continues until the centrosomes and spheres are 

 carried to that part of each cell nearest the animal pole, and until the mid-body is 

 carried downwai'd and outward between the daughter cells, figs. 97, 98, text fig. XV. 

 This rotation is greater in the left cells than in the right ones, owing to the fact 

 that the latter are jsartly covered by the basal cells under which the centrosomes 

 and spheres do not move ; in the posterior quadrant the basal cell is smaller than 

 the other three and does not ovei'lap the second quartette cells to the same extent, 

 and correspondingly the centrosomes and spheres in the second quartette cells of 

 this quadrant are free to move to the ajjical side of each nucleus, figs. 97, 98. 



In the second division of the second quartette cells the left hand cells divide 

 nearly equally into upper and lower products, and the right hand cells divide very 

 unequall}', the upper products being the small tip cells of the arms of the cross, figs. 



99, 100. The posterior tip cell is larger than the other three, and this is jorobably 

 associated with the fact that the adjoining basal cell is smaller than in the other 

 quadrants and does not overlap the second quartette cells to the same extent, fig. 



100. The divisions in both the right and left cells are heotropic in direction, and 

 in both the sphere substance of the mother cells passes into the uppermost of the 

 daughter cells. The movements of the contents of these daughter cells in the 

 telokinesis are in all respects typical, i. e., they are lajotropic in the upper products 

 and dexiotropic in the lower ones. 



(8). Subdivision of the Third Quartette. — The third quartette divides rela- 

 tively late, and only the first division of these cells will be described here. This 

 division is peculiar, because one of the cells, 3d, divides in a dexiotropic direction, 

 Avhereas the other three cells of the quadrant divide in a la^otropic direction. Since 

 these cells were formed by a dexiotropic cleavage, the sub-division of 3d in a dexio- 

 tropic direction is a violation of the rule of alternation in successive cleavages, i. e.. 

 it is a case of reversal. At the time of the division, however, this reversal is slight, 

 the spindle being almost exactly radial ; after the division the lower cell moves to 

 the left, so that the position of the daughter cells is such as would result from a 

 very decided dexiotropic cleavage. In the other quadrants the spindles are from 

 the first decidedly la^otropic in direction. The interest in this reversed cleavage is 



12 JOUEN. A. N. S. PHILA., VOL. XII. 



