CYTOKINESIS. 83 



way around the periplieiy as in the preceding cleavages, and ultimately separating 

 the first group of micromeres from the macromeres, fig. 90 and text figs. XXI-XXIII. 



In this division there is a diiferential distribution of the sphere substance, the 

 whole of the sphere remnants of the preceding division passing into the micromeres 

 while no portion of them goes into the macromeres. During division each cell 

 becomes more nearly spherical than in the resting period, and especial!}' in the late 

 anaphase, when the equatorial constriction is occurring, the daughter cells (both 

 macromeres and micromeres) are so nearly spherical that they touch neighboring 

 cells only by relatively small surfaces, fig. 90. In the telophase and rest they again 

 flatten against one another, fig. 91. 



During the telophase of this cleavage the cell contents rotate in a dexiotropic 

 direction in the upper cells (micromeres), and in a la^otropic direction in the lower 

 cells (macromeres). Even before the telophase this movement is presaged by the 

 dexiotropic lobing of the cytoplasm in each cell preparatory to the formation of the 

 micromeres ; in the telophase it appears in the bending of the spindle axis and in 

 the rotation of the nuclei, centrosomes and spheres. In fig. 90 the earliest bend in 

 the spindle axis is indicated, the middle of the spindle in three of the cells being 

 displaced slightly to the right. Sections through an egg of this stage show that the 

 spindle axes are also bent at the middle toward the surface of the egg, fig. 73. The 

 dexiotropic rotation of the substance of the micromeres continues until the daughter 

 nuclei are carried from the left to the right sides of the cells, though the spheres 

 being in the angles of the cells nearest the animal pole are unable to move through 

 any considerable arc {cf. figs. 90, 91). At the same time the substance of the mac- 

 romeres rotates to the left, until the nuclei, centrosomes, spheres and cytoplasmic 

 areas are carried to the extreme left sides of these cells. Throughout the whole 

 of this movement the centrosomes and spheres never move under the cells of the 

 first quartette, but they always lie on the outer margin of these cells and in contact 

 with a free surface of the macromeres, fig. 91; the nuclei on the other hand are 

 partially or wholly overlaid by the micromeres. 



(4). Fourth Cleavage. — In this position the centrosomes and central spindles 

 for the fourth cleavage of the macromeres arise fi'om the mother centrosomes, the 

 spindles lying over the upper and outer portions of the nuclei and in the groove be- 

 tween the germ halves as in the preceding cleavages, figs. 74, 75, 91, 92. The 

 initial position of these spindles is very different from their final position ; at first 

 their axes are nearly at right angles to planes bisecting each macromere in a radial 

 direction, and the two poles of each spindle are at nearly the same horizontal level. 

 Then the left pole of each spindle rises until it lies immediately under the remnants 

 of the sphere at the surface of the macromere, Avhile the right pole sinks towards 

 the center of each macromere. When the left pole of the spindle approaches the 

 old sphere substance the latter is spread into a ring with dense periphery and clear 

 center, fig. 92, text figs. XXIV, XXV, XXIX, as in the preceding cleavage. Then 

 the cell elongates in the direction of the spindle axis by the formation of a lobe of 

 cytoplasm in the region of the old sphere substance, and at the same time the spin- 



