CHAPTER VII 



THE FORMS OF TISSUES OR CELL-AGGREGATES 



We pass from the solitary cell to 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 consider the effect of various forces; but, as in 

 the case of the soUtary 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 manifest itself in surfaces mininiae 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 approximates to an absolute minimum as nearly as 

 the circumstances and material exigencies of the case permit. 



There are certain fundamental principles, or fundamental equa- 

 tions, besides those we have already considered, which we shall need 

 in our enquiry; for instance, the case which we briefly touched 

 on (on p. 426) 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 Energy, 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 

 with a gas, a portion of the total 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 is constant for 

 these, save only in so far as it may be modified by changes of 

 temperature or of electrical charge. Equihbrium, which is the 

 condition of minimum potential energy in the system, will accordingly 



* See Clerk Maxwell's famous article on "Capillarity" in the ninth edition of 

 the Encyclopedia Britannica, revised by Lord Rayleigh in the tenth edition. 



