GROWTH 99 



and it is sometimes hard to decide whether all that 

 \\c observe existed in the living cell or are artifacts 

 produced by our manipulations. The spindle is 

 the physical expression of forces operating within 

 the cell, but whether as cause or as effect is not easy 

 to say, and there have not been wanting theorists 

 who interpreted the phenomena from either point 

 of view. Comparisons have been instituted be- 

 tween the spindle and the lines of force depicted 

 when a horseshoe magnet is held under a layer of 

 iron filings, and the claim has beeji made that the 

 spindle we see is but the arrangement of the particles 

 of protoplasm in accordance with lines of a force, 

 itself as invisible as magnetism. On the other 

 hand, dividing cells have been dissected in the 

 living condition, and it has been amply demonstrated 

 that the spindle and the asters are real and tangible 

 things, and that the separation of the chromosomes 

 is accompanied by the contraction of these threads. 

 It has even been suggested that the spindle fibers 

 are the result of chemical changes in the semiliquid 

 cytoplasm analogous to the formation of the ropy 

 fibrin in the clotting of the fluid plasma of the 

 blood. 



Cell multiplication is to be looked upon, not as an 

 end in itself, but as a readjustment of the physico- 

 chemical relation of two sorts of protoplasm con- 

 tained in nucleus and cytoplasm, following an 

 increase in substance. Considering the tissue as a 

 whole, this growth is a continuous process ; from the 

 standpoint of the cells as physiological units it is 



