CELLS. 



47 



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 therefore inexplicable in terms of matter and motion, so long, at 

 least, as we do not know the secret of protoplasm. 



On the other hand, Leuckart and Spencer have given a 

 general rationale of cell division. Why do not cells grow- 

 much larger ? why do they almost always divide at a definite 



Fig. 22. — Karyokinesis. — After Fleniming. 



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



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

 tudinal splitting of these (astroid stage). 



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

 (diastroid). 



Nuclear spindle, with chromatin elements at each pole, and 

 achromatin threads between. 



Division of the cell completed. 



limit of growth ? The answer is as follows : — Suppose a 

 young cell has doubled its original mass, that means that 

 there is twice as much living matter to be kept alive. But 

 the living matter is fed, aerated, purified through its surface, 

 which, in growing spherical cells, for instance, only increases 

 as the square of the radius, while the mass increases as the 



