106 CYTOKINESIS. 



The orientation of the centrosome and spindle has reference therefore to two 

 different things, (1) the relation of these structures to the nucleus, (2) their relation 

 to the cell body. 



(1). Orientation of Centrosome and Central Spindle relative to the Nucleus. 

 — It is evident that some kind of connection exists at all stages of the cell cycle 

 between the centrosome and the nucleus. This is of course most evident during 

 division when the two are connected by the spindle fibres, but even during the rest 

 there must be some connection since the two lie in close contact and except in 

 abnormal cases do not separate. Whether this connection during the rest is in 

 the form of fibres (possibly- a persistence of those which previously connected centro- 

 some and chromosomes) or is the expression of some other mechanical action or of 

 a chemotropic attraction does not appear from my studies. The movement of the 

 initial spindle out of the sphere and into the groove between the germ halves of the 

 nucleus must be attributed to some such connection between the centrosome and 

 nucleus. Of course the- mere separation of the centrosomes until they lie as far 

 apart as the diameter of the nucleus may be attributed to the growth of the central 

 spindle. 



(2). Orientation of the Mitotic Figure relative to the Cell Body. — In the 

 early prophase the entire mitotic figure moves in a direction opposite to that of the 

 previous telophase ; or, in a word, the prokinesis is the reverse of the preceding telo- 

 kinesis. For example, in the telophase of the first cleavage the cell contents move 

 in a dexiotropic direction until nuclei, centrosomes and spheres are carried close to 

 the division wall and near to the free surface of the cells, fig. 82 ; in the following 

 prophase they move away from the division wall, in a Ifeotropic direction and deeper 

 into the cell {cf. figs. 82 and 84, also figs. 63 and 65). In the telophase of the second 

 cleavage the daughter nuclei and centrosomes move close to each other on opposite 

 sides of the division wall, fig. 86, and then into the apical angles of the cells, fig. 

 87 ; in the following prophase they move farther from the apical pole and a little 

 deeper into the cell, {cf. figs. 87 and 88). In the telophase of the third cleavage the 

 nuclei of the macromeres move in a Iteotropic direction until they come into contact 

 with the wall at the left of these cells, and toward the apical pole until they are 

 almost entirely covered by the overlying micromeres, fig. 91 ; in the proj^hase of 

 the next division of these cells the mitotic figures move away from the animal 

 pole and a little to the right, {cf. figs. 91 and 92). In the telophase of the third 

 cleavage the nuclei of the micromeres move to the right and as near to the animal 

 pole as possible, fig. 92 ; in the prophase of the next division of these cells the 

 mitotic figures move away from the animal pole and a little to the left, {cf. figs. 9.3 

 and 91). The same principle is shown in the first division of the second quartette 

 {cf. figs. 95 and 96). In fact it ma}^ be laid down as a general rule that the movement 

 in the prophase (prokinesis) is in the reverse direction of that in the telophase 

 (telokinesis), though usually not so extensive. 



Other movements of cell contents, which bring about peculiarities in the direc- 

 tion of division and hence peculiar types of cleavage (radial, spiral, bilateral), or 



