238 THE president's address. 



be differentiated from the surrounding mesogloea. In the 

 specimen shown in fig. 7, however, the appearances are some- 

 what different, the body being irregular, with a fairly distinct 

 nucleus in the middle, and seem to support the view that it is 

 an entire cell with which we are dealing. 



The nature of this cell is somewhat problematical. Others 

 similar to it are found in the adjacent mesogloea, and I have by 

 no means always found one in immediate association with the 

 developing spicule. I venture to interpret it, provisionally at 

 any rate, in the light of my recently published observations on 

 the secretion of silica in the genus Collosclerophora,* of which 

 I gave an account to the Club last year. It will be remembered 

 that in that genus small masses of gelatinous silica are secreted 

 by certain scleroblasts (silicoblasts) not unlike the cells in ques- 

 tion. These masses of silica shrink up on dehydration and can 

 be made to swell out again by the addition of water. They do 

 not assume the definite form characteristic of ordinary spicules, 

 but are rounded, sausage-shaped or kidney-shaped bodies en- 

 closed in thin-walled vesicles formed either as precipitation 

 membranes or by condensation of the surrounding mesogloea. 

 I would suggest that the normal function of such cells is to bring 

 up supplies of silica to the developing spicules, where it is 

 utilised by the formative cells in building up the spicule around 

 the protorhabd, and that in Collosclerophora the protorhabd 

 and formative cells have disappeared, leaving the secondary 

 or accessory silicoblasts, as they may conveniently be termed, 

 to discharge their silica into the surrounding mesogloea in the 

 form of colloscleres. This hypothesis accords very well with 

 what is known of the disappearance of entire spicule-categories 

 in the phylogeny of many tetraxonid sponges, a subject to 

 which I referred at some length in my last Presidential 

 Address. 



It would seem then, if these views are correct, that three 

 factors may be concerned in the growth of a siliceous sponge- 

 spicule : (1) the protorhabd, which is responsible for growth in 

 length and serves as a foundation upon which silica is de- 

 posited ; (2) the formative cells, which are responsible for the 



♦ Proc. Royal Soc, B, vol. 89, p. 315, 1916. 



