president's address. 679 



Some difference of opinion still exists with regard to the 

 derivation of the spindle — whether it is formed from the 

 acliroinatin network of the nucleus or from matter which passes 

 in from the extra-nuclear cytoplasm ; but the preponderance of 

 evidence and of authority would appear to be in favour of the 

 latter view — at least as regards the greater part of the spindle. 



Some cases which have been observed in the Protozoa seem at 

 first sight to point in an opposite direction ; in these instances the 

 nuclear membrane does not disappear on division, and the spindle 

 fibres appear within it ; but there is nothing to show that these 

 spindle fibres, growing out from the centrosomes, do not perforate 

 the nuclear membrane and thus reach the interior of the nucleus. 



These complicated changes in the chromosomes, centrosomes, 

 and atti'action-spheres that accompany cell-division are, as is 

 well known, not always observable when a cell divides. Besides 

 the complex mitotic cell-division, we find in many instances a 

 simple or amitotic form, in which the nucleus simply becomes 

 constricted into two parts, which separate from one another as the 

 cell divides. Now, as this amitotic form of nuclear division is 

 found to be constant in certain kinds of cells under certain condi- 

 tion's, the question naturally arises — To what is this due 1 Why 

 should one set of cells divide constantly in one way and others in 

 another ? What special purpose is served by the more complex 

 kind of division, and what are the conditions which render it 

 unnecessary 1 Various attempts to answer this question have 

 been made in the course of the past year. 



Flemming regards as admissible the conception that only the 

 mitotic form of nuclear division leads to physiological increase and 

 regeneration of cells, while the amitotic fragmentation of the 

 nucleus, with or without the division of the cells, represents either 

 a degeneration or an aberration, or perhaps in many cases aids in 

 the cellular metabolism by increase in the periphery of the nucleus. 



Ziegler* expresses somewhat similar views. To these he was 

 led first by the study of the periblast of bony fishes, in which 



* " Die biologische Bedeutung der amitotischen (direkten) Kerntheihing 

 ini Thierreich." Biol. Centralbl. 1891. 

 46 



