ORIGIN OF THE CLEAVAGE CENTROSOMES. 



225 



solved into chromosomes which lie near those of the egg. In 

 all cases there are two spindles present which are usually united 

 into a tetraster though they may be more or less independent of 

 each other. In some eggs yolk spherules separate the two 

 spindles so that a tetraster is not formed (Figs. 9, 10), while in 

 others an incomplete tetraster is formed (Figs. 6, 7) ; in still 



FIG. 9. 



FIG. 10. 



FIG. ii. 



FIG. 12. 



FIGS. 9-12. NaCl eggs of C. plana, viewed from the animal pole; the polar 

 bodies lie above the spindles in all the figures. Fig. 9. Egg and sperm spindles 

 united at one pole. Fig. 10. The two spindles quite separate. Figs, n and 12. 

 Complete tetrasters in different phases of the separation of the chromosomes. 



others the tetraster is complete, each of the four poles being 

 united to the other three (Figs. 8, 1 1, 12). 



Although I have not seen the genesis of the sperm spindle 

 there seems to be no reason for doubting that it is formed essen- 

 tially as the egg spindle is and that the two are at first wholly 

 independent ; only later do secondarily formed fibers connect one 



