676 president's address. 



than the ordinary substance of the cell he explains by the suppo- 

 sition that, by the pressure exerted by the zone of microsomes 

 attracted towards the centre, the protoplasm of this central part 

 is in a more condensed condition than in other parts of the cell. 



Some objections to this view of the matter have been well 

 expressed by Fick.* It seems assailable on many grounds. 

 In the first place, it assumes the operation of a force for the 

 existence of which there is no evidence. This attraction between 

 the microsomes is not a molecular force — the microsomes being 

 bodies of (comparatively speaking) large size. What then is it? 

 But, even if we admit the operation of this force, it seems to me 

 very unlikely that it would act in the manner supposed. The 

 effect of such an attraction would inevitably be the formation of 

 groups having a compact central part. If the arrangement round 

 a central space occurred it would be an unstable condition, and 

 vei'y readily disturbed. In such a hollow sphere of microsomata, 

 kept in equilibrium by mutual pressure, moreover, there would 

 not, it appears to me, be any pressure exerted on the protoplasm 

 of the central space sufficient to increase its density. 



But the strongest evidence against this ingenious theory is 

 afforded by the observations that have been made, in the case of 

 cells of many different kinds, on the history of these bodies, and 

 their evident relations to the process of nuclear division. For 

 why should mere hollow spheres of granules of no functional 

 importance within the cell, but drawn together by some unknown 

 form of force, divide as the centrosomes are seen to do with 

 perfect regularity at a certain stage in the process of division of 

 the cell 1 



There is then good reason for the view that the centrosomes and 

 attraction-spheres, with the spindle fibres that proceed from them, 

 are of importance in the phenomenon of cell-division ; and accord- 

 ing to Rabl and other recent writers they form the actual vital 

 centres of the cell. 



* " Benierkungen zi; 0. Biirger's Erklarungsversuch der Attraktions- 

 sphiiren." Anat. Anz. VII. (1892). 



