682 ON CONCRETIONS, SPICULES, ETC. [ch. 



The tetrahedral, or rather tetractineUid, spicule needs no further 

 explanation in detail (Fig. 313, D, E). For just as a triradiate 

 spicule corresponds to the case of three cells in mutual contact, so 

 does the four-rayed spicule to that of a solid aggregate of four cells : 

 these latter tending to meet one another in a tetrahedral system, 

 shewing four edges, at each of which three facets or partitions meet, 

 their edges being inclined to one another at equal angles of about 

 109° — the "Maraldi" angle. And even in the case of a single layer, 

 or superficial layer, of cells, if the skeleton originate in connection 

 with all the edges of mutual contact, we shall (in complete and 

 typical cases) have a four-rayed spicule, of which one straight limb 

 will correspond to the hne of junction between the three cells, and 

 the other three limbs (which will then be curved Hmbs) will corre- 

 spond to the three edges where the three cells meet in pairs on the 

 surface of the system. 



But if such a physical explanation of the forms of our spicules is 

 to be accepted, we must seek for some physical agency to explain 

 the presence of the solid material just at the junctions or interfaces 

 of the cells, and for the forces by which it is confined to, and moulded 

 to the form of, these intercellular or interfacial contacts. We owe 

 to Dreyer the physical or mechanical theory of spicular conforma- 

 tion which I have just described — a theory which ultimately rests 

 on the form assumed, under surface-tension, by an aggregation of 

 cells or vesicles. But this fundamental point being granted, we 

 have still several possible alternatives by which to explain the 

 details of the phenomenon. 



Dreyer, if I understand him aright, was content to assume that 

 the soHd material, secreted or excreted by the organism, accumu- 

 lates in the interstices between the cells, and is there subjected 

 to mechanical pressure or constraint as the cells get crowded 

 together by their own growth and that of the system generally. 

 As far as the general form of the spicules goes such explanation is 

 not inadequate, though under it we might have to renounce some 

 of our assumptions as to what takes j)lace at the surface of 

 the system. But where a few years ago the concept of secretion 

 seemed precise enough, we turn now-a-days to the phenomenon of 

 adsorption as a further stage towards the elucidation of our facts, 

 and here we have a case in point. In the tissues of our sponge, 



