MATERIALS FOB A MONOGRAPH OF THE ASCONS. 507 



their limits are indistinguishable towards the interior. The 

 young triradiate spicule appears in the central portion of the 

 sextet, and is so placed that each of its rays corresponds to 

 one of the three pairs of formative cells (PI. 38, figs. 5, 6, 

 coriacea ; PI. 40, figs. 31, 32, embryo of reticulum; and 

 PI. 41, fig. 48, contorta). The exact relation of the spicule 

 ray to its two formative cells is difficult to determine in surface 

 views. So far as can be made out by carefully focussing the 

 different structures, the ray lies between the two nuclei. It 

 is, however, impossible to discover by this method exactly 

 which of the two formative cells is concerned with the first 

 appearance of the spicule ray. In the absence of direct evi- 

 dence on this point there are some grounds for believing it to 

 be the inner formative cell whicii secretes the minute ray at 

 its first appearance, both on account of the subsequent rela- 

 tions which we shall shortly describe between this cell and 

 the ray, and for another reason which may be mentioned here. 

 In many preparations made by the osmic acid and picro- 

 carmine method the spicule-secreting cells are remarkable for 

 the fact that their nuclei stain much more brightly than the 

 nuclei of other cells of the dermal layer. Not only is this the 

 case with the nuclei of separate cells, but it even occurs, as we 

 shall see, when a cell contains two nuclei, one of which is 

 destined to be the nucleus of a scleroblast (figs. 18, 18 a, 19, 

 19 a). This interesting reaction to staining fluids is not the 

 rule in preparations made by this method, and I am unable to 

 state upon what it depends, or to give any directions for 

 obtaining this result ; but in a series of preparations mounted 

 from a specimen of the sponge which I denote for the present 

 as Clathrina, sp. dub., the deeper colour of the nuclei in 

 the spicule-secreting cells was very obvious (see PI. 39, figs. 

 17—21).! 



Fig. 17 shows a sextet in which the cells are still perfectly 

 distinct from one another, and no sign of the spicule is to be 



' This colour reaction suggests a couuection between the nucleus and the 

 secretion of the calcareous matter, for which we shall find further evidence 

 when discussing the growth of the adventitious gastral ray. 



