80 CYTOKINESIS. 



Until the anaphase of this cleavage the spindle axis is a straight line, figs. 59, 

 66. In the telophase the mid-body, which marks the middle of the spindle, is car- 

 ried down toward the vegetal pole, while the centrosomes, spheres and nuclei are 

 moved up nearly to the animal pole, fig. 60. Finall}', in the resting stage the cen- 

 trosomes and spheres lie almost beneath the polar bodies, the nuclei lie just below 

 these, while the mid-body lies a little below, the middle of the plane of contact be- 

 tween the two daughter cells, fig. 61. In short, the spindle axis which was a single 

 straight line up to the anajahase, becomes bent on itself in the telophase and in the 

 resting period until its two halves lie close to each other on opposite sides of the 

 new cell wall. 



These movements of the structures which lie in the spindle axis are accom- 

 panied by general movements of the cell contents in the same direction. Thus the 

 cytoplasm which is at first spread out in the form of a cap at the animal pole, grows 

 deeper in the teloiDhase, and is carried down with the mid-body to the middle of the 

 cleavage plane ; at the same time the yolk is carried up at the peripherj' toward 

 the animal pole, figs. 61, 79, 80, 81. 



The movements in the first two cleavage cells are not, however, directly at 

 right angles to the plane of the first cleavage, but viewed from the animal pole they 

 are slightly dexiotropic, as is shown by the fact that the nuclei, spheres, and proto- 

 plasmic areas all move in a dexiotropic direction (fig. 82 and text fig. VIII). The 

 remains of the spheres of the first cleavage can be seen, until the anaphase of the 

 second cleavage, lying near the upper surface of the two blastomeres and close to 

 the wall between them, figs. 83 and 84 ; in this position they gradually fade out 

 into the cytoplasm, until at the close of the second cleavage no trace of them can 

 be seen. The rotation of cell substance indicated in fig. 82 continues until the 

 superficial extent of the protoplasmic area is smaller and its depth greater than is 

 indicated in that figure, and until the new centrosomes have taken their positions 

 at the poles of the greatly inflated nuclei. 



Such a change in the jjosition of these paiis could be brought about only by a 

 general I'otation of the entire cell bodj^ This general rotation precedes, accompanies 

 and follows the movements of the nuclei, centrosomes and spheres {cf. figs. 59, 60, 

 61), and is, in all probability, the cause of these movements. 



(2). Second Cleavage. — At the close of the first cleavage the centrosomes lie 

 above and on the outer side of the nuclei [i. ^., on the side away from the polar 

 bodies), figs. 82, 83. Here the centrosomes elongate and give rise to the daughter 

 centrosomes and central spindles, which stretch across the nuclei in the groove be- 

 tween the two germ halves, fig. 83, text fig. VIII, Avhile the remnants of the spheres 

 move into the furrow close under the polar bodies. In this position the definitive 

 spindles are formed, while the outlines of the vesicular nuclei, still filled with nuclear 

 sap, are visible on the side of the spindle next the polar bodies even as late as the 

 ■ nietakinesis, fig. 84. With the exception of the centrosomes and asters the entire 

 mitotic figure is lodged within that portion of the vesicular nucleus farthest removed 

 from the first cleavage plane, and into the spindle, thus located,>all the chromo- 



