VII] OF THE POLAE FURROW 313 



as is in all probability the actual case, 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 through- 

 out the system; but in the system of protoplasmic cells which 

 constitute the segmenting egg we must make allowance for an in- 

 equality of tensions, between the surfaces where cell meets cell, and 

 where on the other hand cell-surface is in contact with the sur- 

 rounding medium, — in this case generally water or one of the fluids 

 of the body. Remember that our general condition 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. 

 118) if the energy per unit area 

 be greater along the contact 

 surface cc' , where cell meets cell, 

 than along ca or ch, where cell- 

 surface is in contact with the surrounding medium, these latter 

 surfaces will tend to increase and the surface of cell-contact 

 to diminish. In short there will be the usual balance of forces 

 between the tension along the surface cc', and the two opposing 

 tensions along ca and ch. If the former be greater than either 

 of the other two, the outside angle will be less than 120° ; and if 

 the tension along the surface cc' be as much or more than the 

 sum of the other two, then the drops will stand in contact only, 

 save for the possible effect of external pressure, at a point. This is 

 the explanation, in general terms, of the peculiar conditions 

 obtaining in Nostoc and its allies (p. 300), and it also leads us to 

 a consideration of the general properties and characters of an 

 "epidermal" layer. 



While the inner cells of the honey-comb are symmetrically 

 situated, sharing with their neighbours in equally distributed 

 pressures or tensions, and therefore all tending with great accuracy 



