Viscosity Chaiiges of Protoplasm 187 



are not Brownian movements. Since the movements continue until 

 after the chromosomes become arranged in the metaphase plate 

 as oscillations, they are probably produced by the same mechanism 

 that is responsible for the oscillation. Perhaps invisible fibers, or 

 spindle threads, form and gel sufficiently during late prophase to 

 exert tension on the chromosomes to which they are attached. Varia- 

 tions in the viscosity and contractile tension of these gel-like threads 

 which extend one from each chromosome to each pole would move 

 the chromosomes into the median plate, and there produce the 

 oscillations which move them individually in short paths in and out 

 of the median plate toward the poles. There are no visible bands 

 or fibers, but the fact that the chromosomes move individually a 

 little in and out of the metaphase plate would seem to indicate that 

 some sort of gel material exists and is also responsible for pulling the 

 chromosomes to the poles. 



The nucleoli split up into chromosome-like material almost 

 immediately after the chromosomes begin to move. The splitting is 

 probably in some way connected with the movements of the chromo- 

 somes to which the compound nucleoli are presumably connected. 

 Eve.7its leading to cell cleavage. (Figs. 5-11.) 



At the beginning of prophase the cell processes and pseudopodia 

 begin to retract, and the cell tends to assume a spherical form. The 

 withdrawal of the processes is often incomplete. Withdrawal of 

 pseudopodia is a common phenomenon and is probably due to an 

 increase in the viscosity of the gel layer, which results in an increase 

 of its contractile tension and a shortening or complete withdrawal 

 of the pseudopod. It is similar to the changes which go on at the 

 posterior end of the ameba or slime mold where contraction and 

 solation go hand-in-hand. 



The tendency of the cell to assume a spherical form may be due 

 to an increase in the viscosity or thickness of the superficial gel 

 layer until it becomes fairly uniform over the body of the cell and 

 exerts a uniform contractile tension in all directions. There is con- 

 siderable evidence, as will be noted later, that the gel layer of eggs 

 increases in viscosity and thickness and becomes more rigid and 

 stable before and during cleavage. 



The metaphase cell is approximately bilateral, with the chromo- 

 somes in the equatorial plane. The spindle and poles are surrounded 

 by a considerable zone of endoplasm with mitrochondria, fat 

 globules, and granules. The mitochondria are only approximately 

 bilaterally distributed. The fat globules vary in number. They may 



