CELLS. 



49 



\Ye are Air from being able to give even an approximate account of 

 the "mechanism" of cell division. Rapidly progressive research has 

 disclosed many mysteries, but it does not explain them. The nucleus 

 is resolved into a chromatin framework and an achromatin matrix, but 

 we know the nature of neither. The longitudinal division of each 

 loop shows how thorough is the partition of the chromatin substance. 

 The "central corpuscles," recently discovered, act like centres of force, 

 and the indescribably fine threads, which pass from around these to the 

 chromatin loops, have been credited with motive powers. Similarly 

 the threads of the nuclear spindle are believed by some to draw or drive 

 the chromosomes. But we do not know. The whole process is vital, 

 and cannot, at present at least, be re -described in terms of matter and 

 motion. 



On the other hand, Leuckart, Spencer, and Alexander 

 James have given a general rationale of cell division. Why 



c c 



FIG. 25. Karyokinesis. After Flemming. 



1. Coil stage of nucleus ; c.c., central corpuscle. 



2. Division of chromatin elements into U-shaped loops, and longi- 



tudinal splitting of these (astroid stage). 



3. 4. Recession of chromatin elements from the equator of the cell 



(diastroid). 

 q. Nuclear spindle, with chromatin elements at each pole, and 



achromatin threads between. 

 6. Division of the cell completed. 



do not cells grow much larger? why do they almost always 

 divide at a definite limit of growth ? The answer is as 

 follows : Suppose a young cell has doubled its original 

 volume, that means that there is twice as much living 



