MATERIALS FOR A MONOGRAPH OF THE ASCONS. 513 



oflF, namely, their poorness in the coarse granules characteristic 

 of the epithelium, is one that would not be very distinctive, 

 since the epithelial cells vary a good deal in this respect, and 

 it might be supposed that when once back in the epithelium 

 they would rapidly re-acquire the granules which they lost 

 when engaged in secreting the spicule. 



WhilCj however, it seems a reasonable supposition, iudircctly 

 supported in many ways, that the apical cell after leaving the 

 spicule ray returns to the flat epithelium, direct proof of this 

 is, from the nature of the case, very difficult, if not impossible, 

 to bring forward. Appearances such as that shown in PI. 38, 

 fig. 13, seem to show that the apical cell wanders away from 

 the spicule by amoeboid movement. Its destiuation could 

 only be determined with certainty by keeping it under ob- 

 servation when living, and this is scarcely possible. The 

 matter has to be studied by observing and comparing diff'erent 

 stages in preparations of preserved material, and by this 

 method it is not possible to obtain more than circumstantial 

 evidence. It can only be said, therefore, that the balance of 

 evidence makes it a matter of extreme probability that the 

 apical formative cell returns to the surface after leaving the 

 spicule, and forms part of the flat epithelium from which it 

 indirectly took its origin. 



The history of the spicule subsequent to the disappearance 

 of the apical cell is comparatively simple. The rays at this 

 period have a sharply conical form, whatever may be the form 

 of the complete spicule. Thus in coriacea the fully formed 

 spicule rays have the form seen in PI. 38, fig. 14, — cylindrical, 

 or very nearly so, for their proximal half, and then ta])ering 

 gradually to a blunt point. Contrasting fig. 14 with the figures 

 preceding it, especially figs. 9, 10, and 12, we see that the 

 spicule ray is built up to its full thickness at the base before it 

 has attained its full length, and that the formative cell remains 

 at or near the base until it has done its work there. It then 

 migrates slowly towards the tip, building up the spicule as it 

 goes, till finally in the fully formed spicule we find the 

 definitive spicule cell persisting at the extreme tip of the 



