CYTOKINESIS. 81 



somes are drawn, leaving only the darkly staining nuclear sap in the remaining 

 portion of the mother nucleus, fig. 84. 



At this time the nuclei and cytoplasmic areas still preserve their dexiotropic 

 positions in each cell, fig. 83, text fig. VIII ; with the breaking of the nuclear mem- 

 brane, however, the spindles and cytoplasmic areas shift into a la3otropic position, 

 fig. 84, and at the same time the surface extent of the cytoplasm becomes greater, 

 and the blastomeres, which had become so flattened against each other that they 

 were nearly perfect hemispheres, again become more and more spherical in shape. 

 The spindles, cytoplasm and entire cell then elongate in the direction of the spindle 

 axis ; a constriction appears first at the animal pole and then entirely around the 

 periphery in the equator, and the cell divides as in the preceding cleavage. 1 



In the telophase of the second cleavage the nuclei, spheres and centrosomes 

 again move toward the animal pole while the middle of the spindle and the mid- 

 body are carried down and away from the animal pole toward the middle of each 

 cleavage plane, fig. 86. The direction of this rotation is Inotropic in the two blas- 

 tomeres which lie at the higher level (A and C, fig. 86) and dexiotropic in the two 

 lower ones (B and D, fig. 86). This movement continues until the centrosomes 

 and spheres are carried into the inner angles of the cells, immediately below the 

 polar bodies and until the daughter nuclei, which at first lie very near each other 

 on opposite sides of the second cleavage planes, fig. 86, are swung out into the 

 centers of the cytoplasmic areas, fig. 87. 



(3) Third Cleavage. — The centrosomes and central spindles then appear as 

 in the preceding cleavage, viz, : on the upper and outer sides of the nuclei and in 

 the grooves between the germ halves, figs. 70, 71, 88. The spindles do not at first 

 occupy similar positions in the four cell, but are often dexiotropic in the two lower 

 blastomeres (the ones meeting in the polar furrow) and frequently Inotropic in the 

 upper ones, fig. 88. As the cleavage advances, however, the spindles are all turned 

 into a dexiotropic direction. All this time the remains of the spheres of the pre- 

 ceding cleavage occupy the central angles of the cells, and when the upper pole of 

 the spindle moves up under them, their substance is spread in the form of a ring 

 with dense periphery and clear center, figs. 87-90 and text figs. XIX-XX. The 

 spindle and cytoplasm, then elongate, as in the preceding cleavages, and although 

 the cell as a whole does not elongate symmetrically, i. e., at the two poles, it does 

 elongate by the formation of a lobe of protoplasm over the upper end of each spin- 

 dle. This lobe becomes more and more prominent, and into it the upper pole of the 

 spindle moves. Then the equatorial constriction of the cell begins, forming all the 



I The equatorial constriction sometimes 'appears as a broad irregular depression in the cytoplasm 

 beneath the polar bodies (fig. 85). At the margins of this depression there may be serrated processes of 

 cytoplasm which project for a short distance over the depression. I have seen this phenomenon in 

 only a few eggs, and do not know whether it is a normal one or not. It reminds one of the threads and 

 "filose processes" observed by G. F. and E. A. Andrews ('97, '98) in various forms of protoplasm. In 

 this same preparation (fig. 85) the cytoplasm adjoining the first cleavage on each side of this depression 

 is elevated into a ridge ; it looks as if the formation of the depression and the elevation of the ridge 

 were parts of the same process. 



II JOUEN". A. N. S. PHILA., VOL. XII. 



