342 



Mr. K. R. Lewin. The Behaviour of the [Sept. 15, 



summarise the evidence for regarding the micronucleus as an organella of 

 some independence during the asexual period, living in the plasma as it were 

 in a nutritive solution, and pricked on to division by some definite change in 

 its environment : — 



(a) The cell can live very well without the micronucleus, in the case of 

 Paramecium at least. (Lewin '11.) 



(b) The division of the micronucleus can occur independently of cell and 

 of meganuclear division, e.g., in depression (Popoff '09), in regeneration and 

 in conjugation. 



(c) The behaviour of the micronucleus in regeneration (in Stylonyehia) is 

 not of the nature of a regulation ; in fact, its effect may be increase of the 

 micronucleus above the normal number. 



These considerations justify, I think, the heuristic conception of the 

 micronucleus as living independently during the asexual cycle with the cell 

 as its environment ; it is independent in so far as it can lie outside that 

 narrower individuality of the organism which is jealously preserved by the 

 processes of organic regulation. 



Whether this view can be extended to the micronucleus at the time of 

 conjugation remains to be seen ; very possibly at this epoch in infusorian life 

 the organella in question loses its independence and merges in the regulated 

 individuality of the animal. Otherwise it is not at once clear how the great 

 regularity of nuclear organisation after conjugation is brought about. I may 

 point out that the measure of independence here conceptually accorded to 

 the micronucleus in no way precludes the possibility of its influencing and 

 being influenced by the cell in which it lies, just as infusoria in a culture 

 fluid influence it and it them. 



In this way I regard the micronucleus as being ripe for division, if not 

 throughout the whole of its resting phase, at least long before the cell is 

 ready for fission. There needs only to arise the appropriate stimulus, 

 i.e., the proper chemical or electrical condition of the circumambient plasma, 

 and mitosis will occur. 



At division, and in regeneration, this stimulus is associated with local 

 formative activity ; local, since in regeneration it is the nucleus near the 

 place of active formation of new parts which divides. 



In the artificial depression induced by Popoff in Stylonyehia the stimulus, 

 if it be the same, is associated with a disturbance of the normal metabolism, 

 such as occurs in " natural " depression. 



At conjugation the independent divisions of the micronucleus are of a 

 special type, owing either to a difference in the stimulus or to a change in 

 the nucleus itself. I cannot agree with Popoff when he compares the 



