16 



THE ANIMAL CELL 



[CH. II. 



to the other (fig. 17). In some cells there is at first one long, 

 much twisted thread, which subsequently breaks up into segments. 

 The loops are called chromosomes. 



3. Each loop becomes less convo- 

 luted and splits longitudinally into two 

 sister threads, and the achromatic 

 spindle appears (fig. 18, A and B). 



4. The equatorial stage; monaster. 

 The nucleus has now two poles, those 

 of the spindle ; and at each pole there 

 is a polar corpuscle or centrosome. 

 The division of the centrosome of the 

 original cell, and then of the attraction 

 sphere into two, usually precedes the 



commencement of changes in the nucleus ; the two attraction spheres 

 become prominent in cell division, and the connecting achromatic 



FIG. 17. Early condition of the skein 

 stage vie wed at the polar end. l.c.f., 

 Looped chromatic filament ; i.f., irre- 

 gular filament. (Rabl.) 



Achromatic spindle --x?- 



FIG. 18. Later condition of the skein stage in karyokinesis. A, The chromosomes become less cor- 

 voluted and the achromatic spindle appears. B, The chropnosomes split into two and the ac'h^o. 

 matic spindle becomes longitudinal. (Waldeyer.) 



spindle is probably also formed from them or from the achromatic 

 material of the nucleus. 



At this stage the nuclear Pole of spmni 



Outer granular 



zone. 



. Split fibres. 



Inner clear zone. 



- Polar corpuscle. 



FIG. 19. Monaster stage of karyokinesis. 

 (Waldeyer.) 



membrane is lost, and thus cell 

 protoplasm and nuclear sap 

 become continuous; the proto- 

 plasmic granules are arranged 

 radially from the polar corpuscles. 

 The star-like arrangement of 

 these granules is much better 

 marked in embryonic cells, indeed 

 the lines present very much the 

 appearance of fibrils (see fig. 20). 

 The V - sna P e d chromosomes 

 sink to the equator of the 

 spindle, and arrange themselves 

 from it. 



so as to project horizontally 



