INDIRECT DIVISION. 



29 



not always, consists in the division of the centrosome and of the 

 attraction-sphere into two ; then the following changes take place 

 in the nucleus : The nucleoli and the secondary fibers disappear, 

 while the primary loops remain as chromosomes. These latter 

 become less twisted, forming a spirem or skein, and split into two 

 sets, forming a dispirem or double skein, thus doubling the number 



FIG. 13. 



FIG. 14. 



FIG. 15. 



FIG. 16. 



FIG. 



FIGS. 13-17. Mitotic cell-division of fertilized whitefish eggs Coregonus albus 



(Huber). 



FIG. 13, Cell with resting nucleus, centrosome, and centrosphere to the right 

 of the nucleus; FIG. 14, cell with two centrospheres, with polar rays at opposite 

 poles of nucleus ; FIG. 15, spirem ; FIG. 16, monaster ; FIG. 17, metakinesis stage. 



of chromosomes (Fig. 10, 11). The number of chromosomes is 

 subject to considerable variation in different animal cells. In some, 

 four have been seen, in others as many as twenty-four. 



The achromatic spindle (Fig. 6) now appears. This consists 

 of a spindle-shaped structure, at each end of which is a centro- 

 some, the two having been formed from the original centrosome 

 of the cell. These are connected by achromatin fibers i. e., fibers 

 which are not colored by the staining-material used in the study 



