THE CELL AND ITS CONSTITUENT STRUCTURES. 151 



so to speak, not yet become focussed. Finally, the general 

 radiation round the entire nucleus, figured by Guignard ^ 

 and others in the pollen mother cells of Lilitun, no longer 

 surprises us, although, on the assumption of the existence of 

 persistent centrosomes, whose supposed business it is to 

 manage these matters, the phenomenon strikes one as 

 odd. "^ 



Perhaps one might even go farther, and ask if the facts 

 really warrant us in thinking that the centrosome is more 

 than an indication of the position at which the forces are in 

 equilibrium ; perhaps the centrosome is no more the cause 

 of the forces than are the dead leaves and broken bits of 

 wood in any way effective in producing an eddy, the centre 

 of which they mark. The obvious objection that in many 

 cells the centrosomes endure from one division to another 

 by no means proves that they act, even here, as the directive 

 agents in initiating and presiding over each mitosis. We 

 know practically nothing as to the mechanical conditions of 

 protoplasmic contractility ; but it may perhaps be pointed 

 out that it is not necessary to suppose that the particles 

 which we may for the moment assume to be coerced to 

 form a centrosome, should disperse when in a medium of 

 approximate!}' the same density as themselves, even when 

 the conditions which caused theni to become aggregated, in 

 the first instance, are no longer operating ; and, secondly, I 

 may mention the fact that they are best seen in tissues in 

 which cell division is active, and thus one scarcely need 

 look for their dispersal during the short quiescent intervals. 

 Finally, there is the microcentrum of Heidenhein, in which 

 it would seem possible that such a scattering of the particles 

 is actually taking place. The fact of their division in cases 

 like that of the well-known Hermann's spindle" hardly helps 

 the matter one way or another. It will be remembered that 

 Hermann discovered that the spindle arose, in the Sala- 

 mander, as a concomitant of the divarication of the centro- 



^ "Nouvelles Etudes sur la Fecondation," An. Sci. Nat. {Bot.), 7" Serie, 

 t. xiv., 1891. 



- For figures and description see O, Hertwig, The Ct'// (Eng. trans.), 

 p. 185. 



II 



