IX] OF THE SKELETON OF SPON.GES 683 



wherever two cells meet, there we have a definite surface of contact, 

 and there accordingly we have a manifestation of surface-energy; 

 and the concentration of surface-energy will tend to be a maximum 

 at the lines or edges whereby such surfaces are conjoined. Of the 

 micro-chemistry of the sponge-cells our ignorance is great; but 

 (without venturing on any hypothesis involving the chemical details 

 of the process) we may safely assert that there is an inherent prob- 

 abihty that certain substances will tend to be concentrated and 

 ultimately deposited .just in these hues of intercellular contact and 

 conjunction. In other words, adsorptive concentration, under 

 osmotic pressure, at and in the surface-film which bounds contiguous 

 cells, and especially in the edges where these films meet and intersect, 

 emerges as an alternative (and, as it seems to me, a highly preferable 

 alternative) to Dreyer's conception of an accumulation under 

 mechanical pressure in the vacant spaces left between one cell and 

 another. 



But a purely chemical, or purely molecular, adsorption is not the 

 only form of the hypothesis on which we may rely. For from the 

 purely physical point of view, angles and edges of contact between 

 adjacent cells will be loci in the field of distribution of surface- 

 energy, and any material particles whatsoever will tend to undergo 

 a diminution of freedom on entering one of those boundary regions. 

 Let us imagine a couple of soap bubbles in contact with one another ; 

 over the surface of each bubble tiny bubbles and droplets glide in 

 every direction ; but as soon as these find their way into the groove 

 or re-entrant angle between the two bubbles, there their freedom 

 of movement is so far restrained, and out of that groove they have 

 little tendency, or little freedom, to emerge. A cognate phenomenon 

 is to be witnessed in microscopic sections of steel or other metals. 

 Here, together with its crystalline structure, the metal develops a 

 cellular structure by reason of its lack of homogeneity; for in the 

 molten state one constituent tends to separate out into drops, while 

 the other spreads over these and forms a fihny reticulum between 

 — -the disperse phase and the continuous phase of the colloid chemists. 

 In a poHshed section we easily observe that the little particles of 

 graphite and other foreign bodies common in the matrix have 

 tended to aggregate themselves in the walls and at the angles of 

 the polygonal cells — this being a direct result of the diminished 



