IV] STKUCTUEE OF THE CELL 197 



reality a "sphere of action" of certain more or less localised 

 forces ; and of these, surface-tension is the particular force which 

 is especially responsible for giving to the cell its outline and its 

 morphological individuality. The partially segmented differs from 

 the totally segmented egg, the unicellular Infusorian from the 

 minute multicellular Turbellarian, in the intensity and the range of 

 those surface-tensions which in the one case succeed and in the 

 other fail to form a visible separation between the "cells." Adam 

 Sedgwick used to call attention to the fact that very often, even 

 in eggs that appear to be totally segmented, it is yet impossible 

 to discover an actual separation or cleavage, through and through 

 between the cells which on the surface of the egg are so clearly 

 delimited ; so far and no farther have the physical forces effect- 

 uated a visible "cleavage." The vacuolation of the protoplasm in 

 Actinophrys or Actinosphaerium is due to localised surface-tensions, 

 quite irrespective of the multinuclear nature of the latter 

 organism. In short, the boundary walls due to surface-tension 

 may be present or may be absent with or without the dehmi- 

 nation of the other specific fields of force which are usually 

 correlated with these boundaries and with the independent 

 individuality of the cells. What we may safely admit, however, 

 is that one effect of these circumscribed fields of force is usually 

 such a separation or segregation of the protoplasmic constituents, 

 the more fluid from the less fluid and so forth, as to give a field 

 where surface-tension may do its work and bring a visible boundary 

 into being. When the formation of a "surface" is once effected, 

 its physical condition, or phase, will be bound to differ notably 

 from that of the interior of the cell, and under appropriate chemical 

 conditions the formation of an actual cell-wall, cellulose or other, 

 is easily intelligible. To this subject we shall return again, in 

 another chapter. 



From the moment that we enter on a dynamical conception 

 of the cell, we perceive that the old debates were in vain as to 

 what visible portions of the cell were active or passive, living or 

 non-living. For the manifestations of force can only be due to 

 the interaction of the various parts, to the transference of energy 

 from one to another. Certain properties may be manifested, 

 certain functions may be carried on, by the protoplasm apart 



