MORPHOLOGY AND PHYSIOLOGY OF GENUS URONYCHIA 387 



explained by assuming that they are due to changes in permea- 

 biUty of the cell. There is clearly a close resemblance between 

 the curve of variations in the resistance of Paramecium to KCN 

 and the curve of regenerative power of Uronychia. Both are 

 due to changes in the cytoplasm, and it may be that the sub- 

 stances which make the cells less susceptible to the action of 

 KCN are the same as those which increase the regenerative 

 power. 



It is possible that the change is a physical one, as Lund sug- 

 gests. Instead of a volutin-like substance being formed, the 

 cytoplasm changes with cell age, the age at which the changes 

 occur varying with different species, until a certain physical 

 state is reached in which enzymes originating from the micronu- 

 cleus are activated. These would be liberated as formed and 

 would not be stored in the micronucleus to be set free only at 

 division. The results on the whole seem to bear out the state- 

 ment of Morgan ('01) when he said that "the nucleus supplies 

 certain products of metabolism that must be present before the 

 protoplasm can successfully carry out its innate tendency to 

 complete the typical form." 



In Paramecium Calkins ('11) found that "there is strong evi- 

 dence of a division zone which lies in the center of the cell. If 

 the cell is cut anterior or posterior to this zone the fragment 

 divides in the original plane into a truncate abnormal form and 

 a normal form. The truncate form may divide again not through 

 its center but through the center of the cell were it perfect." 

 Lewin ('10) finds a somewhat similar condition in his merotomy 

 experiments on Paramecium, for he states that "since under 

 normal conditions no two sister merozoites were found to divide 

 there is a suggestion that possibly there exists in the cell a lo- 

 calized division center which passes on sectioning to one mero- 

 zoite leaving the other incapable of division." According to 

 these conclusions, the reason why division does not take place 

 in parts of protozoan cells is not necessarily the lack of one of 

 the cell components, such as the micronucleus, but rather that 

 in a protozoan cell there is a potential plane of division, and when 

 the cell is cut this zone can be present in only one of the frag- 



