THE MITOTIC CYCLE 



they recovered from this state as the solute entered the cells, and the 

 inverse of the 'deplasmolysis time' was used as a measure of the rate 

 of penetration (Figure 50) . In the early mitotic divisions of these cells, 

 permeability to sucrose is at a maximum during metaphase and appears 

 to fall during anaphase. During meiosis, Stern finds that in the pre- 

 leptotene stages, sucrose enters so readily that plasmolysis cannot be 

 induced by this solute. Further investigations of this kind would be of 

 much interest, particularly if another means of measuring permeability 



Prophase 



Telophase 

 tiefaphase - Anaphase 



Figure 50 Permeability changes during 

 mitosis of pollen cells of Trillium erectum. 

 Time for deplasmolysis in sucrose solu- 

 tions (inversely proportional to per- 

 meability) plotted against stage of 

 mitosis. ^ I'D Molar; q 1*5 Molar. 

 From Stern*** {By courtesy. Trans. R. Soc. 

 Canada) . 



were also used. The deplasmolysis method is not entirely without 

 objection (Weber^^'). Shimakura^^^ found that sucrose very readily 

 enters the pollen mother cells of Tradescantia. 



Viscosity 



The implications of these variations in permeability we shall later 

 discuss in relation to our next topic in mitotic physiology, namely the 

 changes in protoplasmic viscosity within the dividing cell. There are 

 numerous descriptions of a decrease in the viscosity of cells at different 

 phases of mitosis, based either on the centrifugation of readily stratifi- 

 able cells, or the observation of changes in the extent of Brownian 

 motion of cytoplasmic granules. A third method relies on subjective 

 impressions of cytoplasmic fluidity during microdissection. Most 

 observers agree that viscosity is low during metaphase or anaphase. 

 Heilbrunn^^^ maintains that in the eggs ofArbacia and Cumingia and of 

 Chaetopterus (Heilbrunn and Wilson^"") 'the appearance of the mitotic 

 spindle is preceded by an increase in protoplasmic viscosity and is 

 followed by a decrease in protoplasmic viscosity'. He says *it is as though 

 the spindle were coagulated out of the protoplasm'. Fry and Parks^*^^ 

 put the minimum in Arbacia and Cumingia at an earlier point; their 

 results are more in agreement with the subjective observations of 

 Chambers. ^"2 ^"^ To the criticisms of Fry and Parks, Heilbrunn has 

 replied with some acerbity (Heilbrunn and Wilson^^^). It is probable 

 that the difficulties of timing the mitotic cycle in marine eggs are 



136 



