1 68 FLORENCE PEEBLES. 



a new plane of division is laid down, and through fission two 

 individuals of equal size are produced; when the new plane is not 

 formed division takes place in the original plane and two cells 

 of unlike size are produced. What causes this difference? Why 

 does a fragment usually divide into two cells of equal size when 

 the posterior end is cut off and of unequal size when the anterior 

 end is removed? I have suggested that this is due to the posi- 

 tion of the nucleus. When the cut is made close to the anterior 

 end of the nucleus it is stimulated to divide at once while there 

 is no such stimulation when the cut is made through the end of 

 the body at some distance from the posterior end of the nucleus. 

 Popoff claims that the volume of the cytoplasm can be regulated 

 so that it accommodates itself to the volume of the nucleus. 

 In this instance, however, it does not seem to be a question 

 of balance between nuclear volume and cell volume. After 

 removal of one end the fragment contains a whole nucleus and 

 only two thirds to three fourths the normal volume of cyto- 

 plasm. After the first irregular division the small piece con- 

 tains one half of the original nucleus and from one fourth to one 

 third of the cytoplasm. Before the balance is established the 

 cell may divide again into a small and a larger cell, or into two 

 cells of the same size. \Vhen cut in half in the vegetative state 

 the surviving fragment contains all of the nucleus and one half 

 of the cytoplasm, yet these cell fragments divide in the normal 

 manner. 



It appears then that the ratio of the volume of the cyto- 

 plasm to the volume of the nucleus does not explain these 

 results, and we must, therefore, look to some other source for 

 light on this subject. If, as it appears, the division planes are 

 laid down shortly after fission is it not possible that irregularities 

 might result from the disturbance of these planes? I do not 

 believe that there is any visible plane present, but there is strong 

 indication of some such differentiation. If we consider a mature 

 Paramecium as possessing a plane through the center as indi- 

 cated by the double row of dots in Fig. 16 and two potential 

 planes in the middle of each half, at the single row of dots, there 

 are four normal individuals present in one cell. A cut made 



