CHAPTER VII 



THE FORMS OF TISSUES OR CELL-AGGREGATES 



We now pass from the consideration of the solitary cell to that 

 of cells in contact with one another, — to what we may call in 

 the first instance " cell-aggregates," — through which we shall be led 

 ultimately to the study of complex tissues. In this part of our 

 subject, as in the preceding chapters, we shall have to give some 

 consideration to the efEects of various forces ; but, as in the case 

 of the conformation of the solitary cell, we shall probably find, 

 and we may at least begin by assuming, that the agency of surface 

 tension is especially manifest and important. The effect of this 

 surface tension will chiefly manifest itself in the production of 

 surfaces mmimae areae : where, as Plateau was always careful to 

 point out, we must understand by this expression not an absolute, 

 but a relative minimum, an area, that is to say, which approxi- 

 mates to an absolute minimum as nearly as circumstances and the 

 conditions of the case permit. 



There are certain fundamental principles, or fundamental 

 •equations, besides those which we have already considered, which 

 we shall need in our enquiry. For instance the case which we 

 briefly touched upon (on p. 265) of the angle of contact between 

 the protoplasm and the axial filament in a Heliozoan we shall 

 now find to be but a particular case of a general and elementary 

 theorem. 



Let us re-state as follows, in terms of Energij, the general 

 principle which underlies the theory of surface tension or capillarity. 



When a fluid is in contact with another fluid, or with a solid 

 or a gas, a portion of the energy of the system (that, namely, 

 which we call surface energy), is proportional to the area of the 

 surface of contact : it is also proportional to a coefficient which 

 is specific for each particular pair of substances, and which is 

 constant for these, save only in so far as it may be modified by 



