342 FORM AND STRUCTURE OF THE CELL [ch. 



cellular 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 effectuated a visible "cleavage." 

 The vacuolation of the protoplasm in Actinophrys or Actinosphaerium 

 is due to localised surface-tensions, quite irrespective of the multi- 

 nuclear 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 delimination of the other specific fields of force which 

 are usually correlated with these boundaries and with the inde- 

 pendent 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 bouhdary 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 inteUigible. 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 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 from the 

 nucleus; but the interaction of the two is necessary, that other 

 and more important properties or functions may be manifested. 

 We know, for instance, that portions of an Infusorian are incapable 

 of regenerating lost parts in the absence of a nucleus, while nucleated 

 pieces soon regain the specific form of the organism: and we are 

 told that reproduction by fission cannot be initiated, though 



