1911.] Infusorian Micronucleus in Regeneration. 



341 



Two more experiments may be quoted, in both of which it is probable that 

 the posterior micronucleus divided. 



(1 0) Merozoon killed 4-|- hours after section. 



M, 3m : M, ra. 



I have never found in the stock culture an individual with so uneven a 

 distribution of micronuclei, so it seems probable that the cut animal had 

 nuclei : — 



M, 2m : M, ra. 



Division of the posterior micronucleus would then give the relation 

 observed. (Fig. 11.) 



(11) Merozoon killed 22 hours after section. 



In the hinder half were two micronuclei, one in the usual position close to 

 the meganucleus, and one right at the posterior border of the body. 



M, ra : M, rara. (Fig. 12.) 



I interpret this as a case where one daughter nucleus was carried at the 

 end of the spindle to the posterior margin of the body, and not anteriorly to 

 the side of the front member of the meganucleus. 



Discussion. — It is certain that division of the micronucleus can be caused 

 to occur during the processes of regeneration which follow operation. 



In the majority of cases in which mitosis occurred, it was the micro- 

 nucleus near the cut surface which divided, indicating that division is caused 

 by local formative processes rather than by the general condition of the cell. 

 The few puzzling cases in which the anterior micronucleus divided, have 

 a light thrown on them by experiment (9). Here the absorption and 

 subsequent regeneration of the adoral membranellae made the anterior region 

 a locality of constructive activity, whilst the regeneration of the hind end 

 proceeded with abnormal slowness. The absorption and subsequent regene- 

 ration of the adoral membranelke do not usually occur. In the great 

 majority of cases at no time is the animal without these organella? ; if any 

 reorganisation occurs it goes on by imperceptible stages. 



Now in more than 50 per cent, of the experiments there was no nuclear 

 increase, so regeneration can go on without causing the micronucleus to 

 divide. In these cases either the micronucleus was in a state different from 

 that obtaining in the successful experiments, or the conditions in the cell 

 were not such as to cause mitosis. 



Further research will show if there be a connection between the age of the 

 individual and the readiness of the micronucleus to divide. It may well be, 

 also, that the stimulus to division has to surmount some threshold before it 

 becomes effective. Before speaking of this, however, it is necessary to 



