Viscosity Changes of Protoplasm 191 



times see a slight thickening in the equatorial plane. When it con- 

 tracts and meets in the center to form the stalk which connects for 

 a time the two daughter cells, it exhibits marked thickness. Since the 

 theory is relatively new, no particular attention has been given to 

 this region with the idea that there might be such a thickening of 

 the gel layer which then automatically contracts and pinches the 

 cell in two. 



Schechtman's (1937) observations on the cleavage of the eggs 

 of Triturus torosiis, according to my interpretation of them, not his, 

 lend support to the idea that cleavage is due to the contraction of a 

 thickened equatorial band of the gel layer. His observations and 

 interpretations are here quoted verbatim. They are followed by my 

 interpretation and comments (p. 222) : "Cleavage is indicated by 

 a contraction of the egg cortex at the site of the future furrow. This 

 is a contraction in the sense that the cortex becomes thickened and 

 bulges toward the interior. At the same time the surface of the 

 egg is displaced toward the site of the thickening." This is exactly 

 what one would expect when the thickened equatorial band of the 

 gel layer begins to contract. "The mid-portion of the contracted 

 cortex begins to expand within one to two minutes after the above 

 contraction (at temperatures ranging from 22° to 26° C). The 

 pigment of this expanding portion is rearranged in irregular streaky 

 lines, plainly indicating that the cytoplasm is being stretched. The 

 surface of the stretched material sinks below the general egg sur- 

 face much as does the surface of a fluid material stretched between 

 relatively firm supports." The continued contraction of the band 

 at the bottom of the furrow has stretched the adjoining gel layer, 

 especially its superficial pigmented surface, and thus the pigment is 

 pulled into streaky lines which extend into the furrow. The com- 

 parison with surface of fluid material is superfluous, since this 

 involves a gel layer only, not a fluid one. "Chambers gives other 

 evidence that this zone is liquid in his observation of Brownian 

 motion and his micro-dissection experiments." Chambers' evidence 

 that this zone is liquid does not hold for Triturus (Schechtman) 

 and Arbacia (Marsland) . "The stretched cortical material ('the 

 primary furrow') has a lower concentration of pigment per unit 

 surface and therefore, appears lighter than the rest of the upper 

 hemisphere." The cortical material stretched by the contraction of 

 the equatorial band naturally has a lower concentration of pigment 

 per unit area than it had before. "A secondary furrow appears at 

 about the center of the primary furrow. It gives evidence of addi- 



