THE PHENOMENA OF LIFE. 15 



chromoplasm of the daughter nuclei. The changes follow a closely 

 similar course in both plant and animal cells. The process has been 

 divided by different authorities into a varying number of stages, with 

 varying names, but for the sake of simplicity it seems best to accept the 



Achromatic spiral 



Fig. 13. Early stages of karyokinesis. A. The thicker primary fibres remain and the achro- 

 matic spindle appears. B. The thick fibres split into two and the achromatic spindle becomes longi- 

 tudinal. (WaldeyerO 



authority of Verworn and recognize two stages only a progressive one in 

 which the changes in the nucleus advance to a maximum, and a retro- 

 gressive one in which the resulting nuclear halves revert to the resting 

 state. 



Progressive stage. The resting nucleus becomes somewhat enlarged, 

 and the centrosome (according to those who regard it as lying normally 



Polar rctclia&on* 



central 

 jxcrUcfe' 



Fig. 14. Monaster stage of karyokinesis. (Rabl.) 



within the nucleus) migrates into the cell protoplasm. The centrosome 

 then divides into two daughter centrosomes which lie near the nucleus 

 but are separated by a considerable interval. Each is surrounded by the 

 radiating fibrils of the attraction sphere, and some of these fibrils pass 

 continuously from one centrosome to the other, forming the achromatic 

 spindle. At the same time (prophases) the intranuclear network be- 

 comes converted into a fine convoluted coil (spirem or skein) which may 

 be either continuous or else broken up into several threads. The thread: 



