CELL-DIVISION 



It becomes obvious in the light of what has been said 

 that nuclear and cytoplasmic division are separate phe- 

 nomena.^ Common usage has been loose in giving the term, 

 cell-division, the meaning of the division of the nucleus. 

 The statements made above, however, emphasize that we 

 must define cell-division more clearly and more exactly in 

 order to avoid the building up of theories on improper 

 grounds. Cell-division is to be defined as the division of 

 the cell-body. In what follows I shall adhere strictly to 

 this clear definition. 



Finding it impossible to relate division of the cytoplasmic 

 mass to the nucleus, we turn to the cytoplasm itself.- Here 

 we may recall three visible events, accompanying the 

 division-cycle in eggs: changes in shape of the egg, move- 

 ment of the cytoplasmic inclusions, and changes in the 

 behavior of the hyaline plasma-layer. The second and 

 third are not peculiar to eggs and will be discussed in detail. 

 Change in shape not being invariable is not given great 

 emphasis apart from its being a factor in surface-tension- 

 theories of cell-division taken up beyond. I speak first of 

 the movement of the cytoplasmic inclusions. 



Movements in the cytoplasm constitute a widely occur- 

 ring phenomenon, some cells exhibiting them to an extreme 

 degree. The simplest type is Brownian movement, so 

 extensively studied by physicists in inanimate systems. 

 This can be beautifully demonstrated in many cells espe- 

 cially by means of the darkfield in which the suspended 

 particles appear as bright points which shimmer on the 

 black background. Quite different from this phenomenon, 

 which is not peculiar to living cells, are the cyclical move- 

 ments in cells — often described for plant cells — a streaming 



1 Watase, i8gi. 



- Cj. Delage, 1895, P- 759- 



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