288 



THE EVOLUTION THEORY 



When a cell is on the eve of dividing we observe first that the 

 chromatin grains, which have till then been scattered throughout the 

 network of the nucleus, approach each other and arrange themselves 



chrsl 



Fig. 74. Diagram of nuclear division, adapted from E. B. Wilson. A, resting cell 

 with cell-substance {zk), centrosphere (csph) which contains two centrosomes, nucleolus 

 (kk), and chromosomes {chr), the last distributed in the nuclear reticulum. B, the 

 chromatin united in a coiled thread ; the centrosphere divided into two and giving off 

 rays which unite the halves. C, the nuclear spindle {ksp) formed, the rays moi-e 

 strongly developed, the nuclear membrane {km) in process of dissolution, the chromatin 

 thread divided into eight similar pieces (chrs), the rays are attaching themselves to the 

 chromosomes. D, perfected nuclear spindle with the two centrospheres at the poles 

 {csph) and the eight chromosomes (chrs) in the equator of the spindle, all now longi- 

 tudinally split. E, daughter-chromosomes diverging from one another, but still 

 united by filaments, the centrosomes (cs) are already doubled for the next division. 

 F, daughter-chromosomes, quite separated from one another, are already beginning 

 to give off processes ; the cell-substance is beginning to be constricted. (?, end of the 

 process of division : two daughter-cells (fe) with similar nuclear reticulum {tk) and 

 centrospheres {csph), as in ^. 



