492 THE FORMS OF TISSUES [ch. 



condition of theoretical and actual equilibrium. For on the one 

 hand, we see that now the four intercellular partitions do not meet 

 one another at all', but really impinge upon four new and separate 

 partitions, which constitute interfacial contacts not between cell 

 and cell, but between the respective cells and the intercalated 

 drop. And secondly, the angles at which these four little surfaces 

 meet the four cell -partitions will be determined, in the usual way, 

 by the balance between the respective tensions of these several 

 surfaces. In an extreme case (as in some pollen-grains) it may be 

 found that the cells under the observed circumstances are not truly 

 in surface contact: that they are so many drops which touch but 

 do not "wet" one another, and which are merely held together 



by the pressure of the surrounding envelope. But even supposing 

 that they are in actual fluid contact, the case from the point of 

 view of surface-tension presents no difficulty. In the case of the 

 conjoined soap-bubbles, we were dealing with similar contacts and 

 with equal surface-tensions throughout the system; but in the 

 system of protoplasmic cells which constitute the segmenting egg 

 we must make allowance for inequality of tensions, between the 

 surfaces where cell meets cell and where on the other hand cell- 

 surface is in, contact with the surrounding medium — generally water 

 or one of the fluids of the body. Remember that our general con- 

 dition is that, in our entire system, the sum of the surface energies 

 is a minimum ; and, while this is attained by the sum of the surfaces 

 being a minimum in the case ' where the energy is uniformly 

 distributed, it is not necessarily so under non-uniform conditions. 

 In the diagram (Fig. 176), if the energy per unit area be greater 

 along the contact surface cc', where cell meets cell, than along ca 



