VISCOSITY CHANGES DURING MITOSIS 441 



DISCUSSION 



In the last few years the importance of protoplasmic viscosity 

 changes has been increasingly realized. Of all the properties 

 of protoplasm the fact that it is a viscous fluid is perhaps its 

 most general characteristic. That a viscous colloidal fluid like 

 protoplasm would probably undergo marked viscosity changes 

 was obvious to biologists long ago. They tried to associate 

 many activities of protoplasm with changes in its consistency. 

 In spite of the numerous theories that were proposed, actual 

 attempts at measuring viscosity have until recently been almost 

 totally lacking. Without doubt, if marked changes in pro- 

 toplasmic viscosity occur, they must play an important part 

 in the mechanics of many vital processes. There is now actual 

 evidence to show that great viscosity changes do occur 

 in protoplasm. 



In addition to the results obtained on marine ova, Seifriz 

 ('18, '20) and Leblond ('19) have shown viscosity changes in 

 plant protoplasm and Bayliss ('20) has shown similar changes 

 in Amoeba. Various methods have been used. The gravity 

 method was employed by Heilbronn ('14) and later by F. and 

 G. Weber ('17). Leblond and Bayliss used the presence of 

 Brownian movement as a criterion of viscosity. 



All of the methods are not equally reliable. To my mind the 

 microdissection method particularly is open to question. Kite 

 ('13) was the originator of the method. According to Kite, 

 all egg protoplasm was a highly viscous gel which scarcely varied 

 its consistency. After my demonstration of the fluidity of un- 

 fertilized sea-urchin egg protoplasm and the change to a gel 

 after fertilization ('15), Chambers was able to confirm these 

 facts with microdissection. At first I was inclined to consider 

 Chambers' work as a valuable support to my views of pro- 

 toplasmic viscosity change. In my 1920 paper this point of 

 view is brought out. But the recent publications of the micro- 

 dissectionists have made me doubt the value of the method. 

 Compare, for example, recent statements of Chambers and 

 Seifriz. Chambers ('19) says: ''The time of appearance of the 

 amphiaster until completion of cleavage lasts from 10 to 



