26 



THE ESSENTIALS OF HISTOLOGY. 



becomes transformed into a sort of skein, formed apparently of one 

 long convoluted nTament ; the nuclear membrane and the nucleoli dis- 

 appear or are merged into the skein (fig. 22, b, c, d). Sometimes the 

 skein becomes looped in and out of a central space ; this form is termed 

 the rosette (e). 



FlG. 23. A DIVIDING CELL, SHOWING ATTRACTION PARTICLE AT EITHER POLE OF NUCLEUS 

 FROM WHICH THE GRANULES OF THE PROTOPLASM ARE SEEN RADIATING, AND WITH 

 WHICH ALSO THE SPINDLE-SHAPED SYSTEM OF ACHROMATIC FIBRES TRAVERSING THE 

 NUCLEUS IS CONNECTED. THE CHROMOSOMES, SIX IN NUMBER, JARE ARRANGED 

 ASTRALLY AT THE EQUATOR OF THE SPINDLE. (Rabl.) 



of 



orruxHc\ 

 spindle- I 

 wttk-centraL [ 

 /xx>rttek\ 



FIG. 24. A NUCLEUS AT A STAGE SIMILAR TO THAT SHOWN IN THE LAST FIGURE, BUP 



SEEN FROM ONE OF THE POLES INSTEAD OF IN PROFILE. THE SPINDLE IS REPRESENTED 

 FORESHORTENED. EIGHT CHROMOSOMES ARE REPRESENTED. (Rabl.) 



2. The filament breaks into a number of separate portions, often 

 V-shaped, and termed chromosomes. The number of chromosomes varies 

 with the species of animal ; in some animals the dividing nuclei may 

 contain at this stage only four chromosomes, in others 24 or more. 

 As soon as they become distinct they are usually arranged radially like 

 a star (aster, /, g). 



3. Each of the chromosomes splits longitudinally into two, so that 

 they are now twice as numerous as before (stage of cleavage, g, h). 



4. The fibres separate into two groups, the ends being for a time 

 interlocked (stage of metakmesis, i, j, k). 



5. The two groups pass to the opposite poles of the now elongated 

 nucleus and form a star-shaped figure (/) at each pole (dyaster). Each 

 of the stars represents a daughter-nucleus. 



6. 7, 8. Each star of the dyaster goes through the same changes as 



