MATERIALS FOR A MONOGRAPH OP THE ASOONS. 505 



cells of the epithelium. Like them they are packed with 

 large opaque granules, some appearing quite dark, others of a 

 much paler tint in preparations. The spicule mother-cells, in 

 fact, only differ from the dermal epithelial cells in their posi- 

 tion and shape. Instead of being spread out into thin plates 

 they form compact lumps, and hence acquire a rather denser 

 and darker appearance. From all other cell elements they 

 are easily distinguished. They are much smaller and less 

 opaque than the contracted pore-cells, and their nuclei are 

 slightly smaller and stain more deeply. On the other hand, 

 their granules and the structure of their nuclei distinguish 

 them at once from the various kinds of wandering cells, 

 from which they differ so markedly that it will be sufficient 

 to refer to PI. 38, fig. 10, a.m. c.^, am. c.^, and to the description 

 of the wandering cells given above. 



I have just spoken of these cells as "spicule mother-cells," 

 but this term is to be avoided if it conveys the idea that one 

 such cell produces a whole spicule, for, as we shall see, each 

 of the cells in question is responsible for one ray only of the 

 spicule. The commonly used term " scleroblast " is perhaps 

 a sufficiently vague term, meaning only a cell which secretes a 

 spicule, and not committing us to any theory or opinion as to 

 whether the cell so termed secretes the whole or part of a 

 spicule. It is, how^ever, better on the whole to invent a term 

 which connotes accurately the relations of these cells to the 

 future spicule, and they may be conveniently termed '' actino- 

 blasts.'' 



Sometimes these skeletogenous cells are found singly (PI. 38, 

 fig. 10, act. bl.), or at least too far from any others for their 

 connection with them to be apparent. More usually they are 

 met with in trios (PI. 38, figs. 1 — 3), and, as a rule, in such 

 close contact that their apposed surfaces become more or less 

 flattened against one another by mutual pressure, giving a 

 figure like a trefoil. Trios may, however, be found in which 

 there are considerable intervals between the component cells 

 (fig. 1) ; and this fact, taken in connection with the not infre- 

 quent occurrence of isolated ' actinoblasts,' show that in many 



