THE DIVIDING CELL 



339 



in liquid, assume the direction of those of the cell, and meet the 

 mask nearly radially at every point (fig. 6). From this we must 

 infer that the boundary of the cytoplasm (" Haut schicht") is of 

 highly permeable matter. 



Moreover, in such a case the lines of force of the asters, 

 instead of passing away to meet at indefinite distances, pass into 

 the permeable envelope to meet therein. 



By this action of a permeable envelope it serves as a screen ; 

 it prevents similar forces in a field outside acting on the space 

 within, or interfering with an internal field of force : this screen 

 action is recognised by the practical electrician, who has found 

 that the best way to protect a building from lightning is to 

 screen it by a cage of connected wires which are conductors 



FlG. 6. — Axial section of a limited magnetic field of force ; the medium being powdered 

 magnetite in glycerine ; the effect of the permeable boundary in deflecting outwards the 

 astral rays is well shown with anastomoses and decussations of the chain : A, immediately 

 after the formation of the magnetic field ; B, after the action of a more intense field, 

 showing the sidling in of the outer chains of the spindle to leave a Biitschli's space on 

 either side. 



{i.e. inductors of infinite permeability). Our next deduction 

 from this screening action is that the centrosomes must at least 

 be bounded by a highly permeable layer, even if their whole 

 substance is not equally permeable, for no influence of the cell- 

 field's action on the alveolar internal structure of the centrosomes 

 is visible at any moment (fig. io). Similarly, the nuclear wall 

 must be highly permeable, and serves as a perfect screen so long 

 as it remains complete. 



Years ago, Leopold Errera, whose loss we all mourn, left 

 plants with their growing points in a very strong magnetic field, 

 and found that there was no distortion ; and this experiment 



