220 L. V. HEILBRUNN 



All of the above tables tell the same story. During the mitotic 

 process there is a marked increase in viscosity, which is later 

 followed by a decrease. These changes in viscosity are very 

 considerable. Soon after fertilization and again before the first 

 cleavage, a hyaline zone can be made to appear distinctly through 

 a large part of the egg, if the centrifuge handle is turned thirty 

 times in twenty-five seconds. On the other hand, at about 

 twenty minutes after fertilization, even if the centrifuge handle 

 is turned fifty times in thirty seconds, the hyaline zone either 

 does not appear at all or shows in only a small region of the egg. 

 In the latter case the force exerted is approximately twice as 

 great. The increase in viscosity is at least twofold and is almost 

 certainly far more than this. Such a marked viscosity increase 

 is beyond much doubt due to a gelation with the cytoplasm. 



As the tables show the gelation reaches its height slightly 

 prior to the time that the pre-spindle first becomes visible. The 

 appearance of the spindle is then followed by a decrease in vis- 

 cosity. These facts suggest a definite time relation between the 

 viscosity changes and the formation of the spindle. 



During the second mitotic division of the egg, there is a parallel 

 series of viscosity changes. As the first cleavage occurs, and 

 apparently even before the cells are fully separated, there is a 

 renewed stiffening of the cytoplasm. This is in preparation for 

 the second mitosis. This gelation lasts ten or fifteen minutes, 

 and then the cytoplasm becomes more fluid again. No attempt 

 was made to follow similar changes in later divisions of the egg. 



The fact that gelation occurs during mitosis is indicated by 

 various other observations. In his book on the cell, Flemming 

 pointed out that during the process of division the refractive index 

 of the cell rises. With this fact he correlated the increased stain- 

 ing capacity of fixed preparations of dividing cells, as first ob- 

 served by van Beneden (75). Flemming ('82) apparently believed 

 that these observations indicated a stiffening of the cytoplasm. 

 In a later paper ('91) he states that the refractive index of a di- 

 viding connective-tissue cell increases, and then in the anaphase 

 begins to decrease again. Similar observations have been made 

 by Levi ('16), apparently without any knowledge of Flemming' s 

 work. 



