90 CYTOKINESIS. 



increased, because it is one of the very first examples of bilateral cleavage in this 

 egg. Owing to this reversal in quadrant D, the position of the third quartette cells 

 in quadrants D and C is bilaterally symmetrical with reference to the plane separat- 

 ing the two mesentoblast cells, fig. 100. If this reversal had not occurred, the 

 position of these cells would have been radially symmetrical, as in the other quad- 

 rants and as in all preceding cleavages. The causes of this reversal, therefore, have 

 more than ordinary interest. In the formation of the third quartette the cell con- 

 tents of 3d, as well as of all the other cells of this quartette, rotate in the telophase 

 in a dexiotropic direction, and to about the same extent in all the cells, figs. 96, 97, 

 98, text figs. XV, XVI. The cause of this reversal cannot, therefore, be found in 

 the absence or the reversal of the usual cell movements during the preceding telo- 

 phase and rest, On the other hand, the cap of micromeres in quadrant D is so 

 lifted from the macromeres by the formation of the mesentoblast cell that a space 

 is left between the micromeres and the yolk, and into this space the lower product 

 of the division of 3d pushes, fig. 100. This is not, therefore, so much a case of 

 reversed cleavage as it is one of displacement of daughter cells. Such displacement 

 may occur irrespective of the direction of division or of the movements of cell 

 contents. 



Further divisions have been followed in detail up to a late stage in the cleav- 

 age, but as they illustrate merely the principles which have been already described, 

 no account is given of them here. 



III. Analysis of Movements During Cell Division. 



The movements within cells during the cycle of division may be classified under 

 three heads : (1) Movements in Metakinesis, (2) Movements in Telokinesis, (3) 

 Orientation of Centrosomes and Spindles. The first of these has been treated to a 

 limited extent in Part I; however, only those features are there described which 

 are of importance in understanding nuclear division ; it will now be in order to con- 

 sider these movements in their relation to the general cell movements. 



1. The Movements in Metakinesis are of two kinds — movements in the spindle 

 and aster, coincident movements in the cell body, (a) Movements in Spindle and 

 Aster. As everybody knows the chromosomes, which may be widely scattered 

 through the nuclear cavity, are first drawn into the equatorial plate of the spindle 

 and then separated in the metakinesis, the daughter chromosomes moving toward 

 the poles of the spindle as far as the spheres. Here the movements of the chromo- 

 somes cease (except in the single case of the maturation divisions where the chromo- 

 somes at the outer pole are pushed right on through the sjmere, see pp. 19, 76), and 

 here, in contact with the spheres, the chromosomes become vesicular and fuse to 

 form the daughter nuclei. 



The movement of the chromosomes into the equatorial plate is accompanied by 

 a condensation or contraction of the linin network, which is at first uniformly distri- 

 buted throughout the nucleus {cf. text figs. XVII, XVIII, with figs. 57, 65 and 84), 

 and at the same time the greater part of the nuclear sap is squeezed out of this 



