8o BIOLOGICAL LECTURES. 



trospheres, which are carried up toward the surface of the ^^g, 

 and the mid-body, which marks the middle point of the spindle, 

 is carried down into the egg, until finally the centrospheres 

 come to lie immediately under the cell membrane and above 

 the nuclei, while the mid-body is located near the centre of the 

 ^gg (Figs. 4-6). The currents which bring about this affect 

 the whole cell body, for not only are the centrospheres, nuclei, 

 and mid-bodies moved, but the yolk and protoplasmic areas 

 shift so that the protoplasm, which was spread out as a rather 

 broad area above the yolk, becomes narrower and deeper, the 

 yolk moving up at the periphery and down in the middle of the 

 Qgg (Figs. 3-5). Such a change of position could be brought 

 about only by a general movement of the entire cell body in the 

 direction of the arrows in Fig. 5. The movements in the two 

 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 protoplasmic areas all move in a dexiotropic direc- 

 tion (Fig. 6). The remains of the centrospheres 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 ; 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 origin of the centrosomes 

 of the second cleavage has not been traced in detail, but judging 

 by analogy with other cleavages it is almost certain that they 

 come from the inner portion of the centrospheres of the first 

 cleavage. The plane of divergence of the centrosomes is not 

 constant; in extreme cases the spindles may vary as much as 

 90° from the usual direction, and yet in all cases the spindle is 

 ultimately turned into a laeotropic position. In this case, there- 

 fore, as in the formation of the polar bodies and in many of the 

 later cleavages, the initial position of the mitotic spindle does 

 not determine the direction of the cell division. The flow of 

 cell substance indicated in Fig. 6 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 



