CELLS IN DIVISION 



responsible for these differences in results. A minimum viscosity at 

 anaphase is found by Carlson^*'* in the grasshopper neuroblast, and 

 between metaphase and anaphase by Kato^^^ in the pollen mother 

 cells of the lily; the results of these two workers were based on observa- 

 tions of Brownian movement. Kostoff^os centrifuged the flower buds 

 of Nicotiana and finds that here also the minimum viscosity is between 

 metaphase and anaphase in both of the meiotic divisions. Zimmer- 

 MANN^°^ places the minimum in the Alga Sphacelaria at metaphase. 

 Lewis^"^^ states that in the ova of vertebrates the movement of granules 

 in the cytoplasm markedly increases during prophase. 



VEP ^ery Early Prophase 

 tP Farly Prophase 

 MP Midd/e Prophase 

 LP Lafe Prophase 

 PM Promefaphase 



M Metaphase 



k Anaphase 



TEL Te/ophase 



INT Interphase 



yery 

 tow 



Kr 



Figure 51. Cytoplasmic viscosity at various points (Figure 25) in the neuroblast 



of ChoTtophaga viridifasciata during mitosis, judged from observations of 



Brownian movement. From Carlson^"* {By courtesy, Biol. Bull.). 



The measure of agreement between these various observers is more 

 noteworthy than are their points of difference. It is to be expected that 

 the general viscosity of a sea-urchin ^gg would rise in late anaphase 

 when the asters are attaining their full size ; it is not obvious why there 

 should be a similar increase in a plant cell without asters and in which 

 the cell divides by a wholly different method. Centrifugation gives an 

 indication of general cytoplasmic viscosity; in the grasshopper neuro- 

 blast Carlson^"* find that Brownian movement becomes more pro- 

 nounced during anaphase at each of six sites in the cell (Figure 51). 

 The absolute values of viscosity which are compared by these two 

 methods are probably of the same order; they refer to the lower end of 

 the whole range of protoplasmic viscosities. The centrifugal forces 



K 137 



