1911.] Infusorian Micronucleus in Regeneration. 



divisions he observed with the micronuclear multiplication at conjugation, 

 for in his figures the mitoses appear to be quite of the ordinary type.* 



Resuming the discussion of the experimental results, it is now clear 

 what I mean by the stimulus having perhaps to surmount some threshold 

 before causing mitosis. It can easily be imagined that the grade of con- 

 structive activity must reach a certain degree of steepness before it or some 

 concomitant can start the micronucleus dividing. Hence, if it be assumed 

 that the grade of activity can be measured by the whole time taken by 

 regeneration, when the process goes on slowly, no mitosis would be expected. 



I suppose that the cases in which no mitosis occurred differed from the 

 successful experiments either in the age of the micronucleus, or in the 

 intensity of the constructive processes. Both factors may work together, 

 but that the latter is concerned is indicated in a measure by experiment (9). 

 Here there was the usual regeneration of the cirri and of the hind end of 

 the body taking place more slowly than usual. (Eegeneration is normally 

 accomplished in from three to five hours, and after seven the posterior 

 ciliation was not complete in this case.) The posterior micronucleus did 

 not divide. 



Anteriorly, the adoral membranellee, withdrawn between the third and 

 the fifth hour after section, were replaced by the seventh hour, indicating 

 considerable formative intensity. The anterior micronucleus did divide. 



One factor in the organisation of Stylonychia remains to be mentioned, 

 viz., the unknown influences which determine the situation of the micro- 

 nuclei in the cell. Normally, each lies by a member of the meganucleus. 

 Of the nature of the forces which bring this about nothing is known. 



After the increase in the number of micronuclei accompanying regeneration, 

 the supernumerary nucleus usually lies by the side of the meganuclear 

 element other than that by which its sister is found. The normal spindle- 

 length in mitosis in Stylonychia spans the distance between the two members 

 of the meganucleus. This regularity does not always obtain, as can be seen 

 from figs. 6 and 7. The failure of the forces to order the arrangement of 

 the micronuclei in the usual way is probably the cause of the unequal 

 distribution between the sister animals. 



Tig. 2 also indicates an irregularity, the anterior micronucleus lying on 

 one side of the meganucleus, and the posterior (in division) on the other, a 

 most unusual thing. Fig. 12 strongly suggests that the unknown forces 



* In Paramecium (in which Popoff observed similar phenomena) at least one would 

 expect to see the micronucleus in the characteristic " crescent " stage which precedes the 

 formation of the spindle for the first division in conjugation. Popoff figures several 

 mitoses, but does not refer to this stage. 



VOL. LXXXIV. — B. 2 C 



