MATERIALS FOR A MONOGRAPH OF THE ASCONS. 555 



intva-cellular secretion of the cell of the same nature as the 

 intercellular secretion which produces the gelatinous matrix of 

 the dermal layer. In the absence of precise observations on 

 this point, my statements with regard to the spicule sheath are 

 to be taken as provisional ; but I think it highly probable that 

 the spicule sheath represents a matrix secreted by the cell in 

 ■which the spicule is subsequently deposited. I imagine to 

 myself the spicule mother-cell secreting a minute drop or 

 vacuole of the substance, whatever it may be, of which the 

 sheath is composed, and in this vacuole the sclerite appearing 

 as a minute concretion. The sclerite may have had originally 

 a perfectly crystalline form, as Haeckel supposes, or it may 

 have been a simple globular concretion like the crystalline 

 substances deposited in an organic matrix in Rainey's ^ inter- 

 esting experiments. In either case it early assumed a definite 

 form in correlation with the functions imposed upon it, and as 

 it grew into a spicule, such as we are familiar with, the matrix 

 in which it was first deposited grew with it to form the 

 sheath. 



If this interpretation of the spicule sheath be correct, it is 

 obvious that the calcareous sclerite cannot be regarded, in 

 sponges at least, as having the value of a granule deposited at 

 a node of the protoplasmic framework, but rather as a simple 

 concretion within a vacuole, not subject to any force of the 

 nature of vesicular tension. This representation of the mode 

 in which the spicule is deposited is of course purely specu- 

 lative, and lacks as yet any support from actual observation 

 in the case of sponges. There are, however, instances of 

 sclerites deposited in this manner in botli animal and vege- 

 table cells.^ 



1 'British and Foreign Medico-Ciiirurg. Review,' xx, 1857, pp. 451 — 476; 

 see also Harting, ' Quart. Journ. Micr. Sci.' (u. s.), xii, 1872, pp. 118—123 

 and Ord, ibid., pp. 219—239, pis. xv, xvi. 



•' Compare, for instance, Leger's observations on the clinorhonibic crystals 

 of calcium oxalate in the cysts of Lithocystis Schneideri ('Ann. 

 Mag. Nat. Hist.' [6], xviii, 1895, p. 479). Sclerites are figured in cells by 

 Semon ('Mitth. Zool. Stat. Neapel,' vii, pi. ix, fig. 3) and Blochmann ('Die 

 Epithel-Frage bei Cestoden u. Trematoden,' Hamburg, 1896), but neither 



