Viscosity Changes of Protoplasvi 193 



nistic." The moment one assumes that the advance of the furrow 

 is due to cortical growth by an "inhibition process" or to a flow, 

 insurmountable difficulties are encountered in explaining the streaks 

 of the granules and of the dye materials. Schechtman recognized 

 that "the mechanism of cell division has certain features in common 

 with the sol-gel transformations generally regarded as important in 

 the formation of pseudopodia in certain amebae", but his interpre- 

 tation of the important facts which he contributed differ, as already 

 noted, from mine. 



The material and the type of experiment employed by Schecht- 

 man would seem to be unusually favorable for an analysis of cell 

 cleavage. These experiments should be repeated again and again 

 with a continuous series of marks across the equatorial line. One 

 would expect that marks exactly on this line would be carried in the 

 constriction band to the very bottom of the furrow. 



The following quotations from Marsland (1939) on "The Mechan- 

 ism of Cell Division" indicate that he recognized that an equatorial 

 band played an important role in cell cleavage (p. 21) : "At the time 

 of cleavage the superficial protoplasm of the egg, especially in the 

 region of the incipient furrow, becomes firmly set and displays the 

 properties of a rigid gel." The hydrostatic pressure effect upon 

 dividing egg cells (Arbacia) led him to the following conclusion 

 (p. 22) : "These results indicate that a gelation reaction by which 

 the plasmagel in and near the walls of the furrows undergoes aug- 

 mentation at the expense of the subjacent plasmasol may account 

 for the formation and progress of the furrow. According to this view, 

 the intrusion of the furrow is a process closely analogous to the 

 extrusion of a pseudopodium." The following quotations from his 

 discussion are significant (p. 20) : "These experiments give strong 

 support to the view that a gelation of the cortex of the egg, especially 

 in the equatorial region where the furrow will form, plays an essential 

 role in the mechanism of cell division." "Without question, just 

 before the appearance of the furrow, this part of the protoplasm 

 becomes firmly set and assumes a rigidity which is several times 

 greater than before." (p. 21) : "The view that a plasmagel girdle 

 in the equator of the cell undergoes augmentation from the sub- 

 jacent plasmasol on either side bordering the incipient furrow, and 

 thus, is pushed inward toward the equator is further supported by 

 the present experiments." (p. 21) : "Although it is evident that the 

 plasmagel over the entire surface of the egg undergoes a setting 

 process just prior to and during cleavage, some evidence has 



