REGENERATION IN PARAMECIUM CAUDATUM. 



157 



cell (Fig. 2, B) and a normal posterior cell (C). The smaller cell 

 may grow rapidly to the normal size and divide into two equal 

 cells, or it may form another small anterior and large posterior 

 cell. At times the anterior cell finally produces a monster. 



2. Removal of the Posterior End.- The removal of the posterior 

 end of the cell through the region back of the mouth (Fig. I, Z) 

 produces less disturbance than cutting off the anterior end. This 

 result would be expected as there is little danger of injury to the 

 nucleus which lies in front of the cut. The results from this 

 experiment are given in the next table. 



TABLE II. 



The percentage of fragments which regenerated and divided 

 in the normal manner is much larger than that of Table I. Here 

 sixty-two per cent, after recovery from the operation divided 

 normally, forming a race of the original size, while only thirty-four 

 per cent, of the cells in Table I. regenerated the lost end before 

 dividing. In twenty per cent, of the fragments the division was 

 irregular, resulting in a large anterior and a small posterior cell. 

 This result stands out in sharp contrast to the fifty per cent, 

 dividing irregularly in Table I. The probable explanation of 

 this result is to be found in the fact that when the anterior end is 

 removed the nucleus is either exposed directly to the surrounding 

 medium, or is separated from it by a thin film of cytoplasm, and 

 may be thus stimulated to divide in the plane already laid down 

 before a re-arrangement of symmetry is effected. The result is 

 that one daughter cell contains twice as much cytoplasm as the 

 other, while both have the same amount of nuclear material. 

 When the posterior end is removed (Fig. I, Z) there is an equal 

 amount of cytoplasm back and front of the nucleus, so that a 

 new division plane forms, and when fission takes place each half 

 gets an equal amount of cytoplasm. The later history of the 

 small cells formed by irregular division will, I think, support 

 this suggestion. 



