CELL DL VI SI ON. 



47 



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

 cube. The surface growth ahvays lags behind the increase 

 of mass. Therefore, when the cell has, let us say, quadrupled 

 its original mass, but by no means quadrupled its surface, 

 difficulties set in, waste begins to gain on repair, anabolism 

 loses some of its ascendancy over katabolism. At the limit 

 of growth, then, the cell divides, halving its mass and gaining 

 new surface. Of course surface may be increased by out- 

 flowing processes, just as that of leaves by many lobes ; and 



"^N^ 



"-^ 



Fir,. 5.— Karyokinesis. {Afti;r Flemming.) 



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



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

 longitudinal splitting of these (astroid stage). 



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

 (diastroid). 



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

 achromatin threads between. 



6. Division of the cell completed. 



division may occur before the limit of growth is reached, 

 but as a general rationale, applicable to organs and bodies 

 as well as to cells, the suggestion of Leuckart and Spencer 

 is very helpful. 



(5) Protoplasm. — Morphological as well as phy.siological analysis 

 passes from the organism as a whole to its organs, thence to the tissues, 

 thence to the cells, and finally to the protoplasm itself. But although 



