MATERULS FOB A MONOGRAPH OF THE ASCONS. 551 



spicules to the other forms. This gap I shall endeavour to 

 fill, but will first consider in more detail the views advocated 

 by Sollas and Dreyer. 



Sollas's theory was put forward to explain the forms of 

 spicules originating within a single cell, and his explanation of 

 the triradiate system was based upon the assumption that it 

 developed in this manner. Since, however, the triradiate sys- 

 tem owes its origin to three mother-cells, it is clear that Sollas's 

 theory would not apply to it, still less to the quadriradiate. No 

 explanation based upon the principle of growth along lines of 

 least resistance would explain why the gastral actinoblast 

 should attach the monaxon spicule which it secretes to a tri- 

 radiate system. Sollas's theory is therefore reduced at the outset 

 to an explanation of, if anything, the simple monaxon sclerite 

 from which the more complex forms are put together. In the 

 case of monaxon sclerites secreted by a single cell, it seems to 

 me highly probable that the mechanical principle involved by 

 Sollas does operate largely in producing the form of the 

 sclerite. But the principle is, after all, only an explanation of 

 how the scleroblast lays down the sclerite, and not of why it 

 has that form and no other. It is as if one should try to 

 "explain" laughter by describing accurately the facial and 

 other muscles concerned, and the manner of their contraction. 

 If the lines of least resistance are such as to cause the secretion 

 of calcite to take an elongate, needle-like form while still 

 completely embedded in the scleroblast, this presupposes a 

 certain polarity in the cell ; but to what is the polarity itself 

 due? I am unable to understand how the analogy of a bone, 

 supposed to owe its form to the tension of the muscles and 

 other tissues, can give us any help in the present case ; for the 

 problem is to explain why the molecules of calcium carbonate 

 should be deposited more at the two opposite ends of the cell 

 than in the middle, and this can hardly be a matter simply of 

 pressure and tension on the sclerite itself. 



I cannot, therefore, consider Sollas's theory as more than ex- 

 planation, at most, of the mechanics of growth in the case of 

 the simplest skeletal elements, and not as a solution of the 



