STUDIES IN SPICULE FORMATION. 39 



the result being that the nucleus becomes lodged in a depres- 

 sion to one side of the plate (fig. 25). The further develop- 

 ment of the spicule consists of the enclosure of the nucleus 

 by the extension of the calcite, as shown in figs. 26 — 28. 

 Two arms of calcareous matter extend round the nucleus on 

 either side, meet, and finally fuse, and the adult plate, in 

 consequence, contains a central perforation, in which the 

 nucleus is imprisoned (fig. 28). Occasionally, when the per- 

 foration is larger than usual, a secondary ingrowth of calcite 

 occurs (fig. 28&) ; occasionally also the two arms of calcite 

 first overlap each other instead of fusing immediately (figs. 

 27a and 276), and sometimes, but rarely, secondary pairs of 

 arms are formed, which tend to emulate the first pair in their 

 direction of growth (fig. 28a). This last feature (as also that 

 represented by fig. 29) proves, however much it may appear 

 to the contrary, that the presence of the nucleus is not the 

 only stimulus giving rise to the peculiar mode of extension of 

 the calcite just described. In fact, here, as in the cases 

 described in the foregoing parts of this paper and elsewhere, 

 the nucleus, with its associated mass of cytoplasm, probably 

 has very little to do with the direction of growth of the 

 calcite — with the form of the spicule — and must largely be 

 discounted as a factor in the production of spicular forms. 



Theoretical Considerations and Previous Work. 



From the results described above, and from the figures of 

 young spicules in the various classes of echinoderms provided 

 by Agassiz (1), Ludwig (7), Seeliger (9), Fewkes (2, 3), 

 Theel (11, 12), and many others, we may assume what has, 

 indeed, been already implied, viz. that in Ophiuroidea, 

 Asteroidea, Echinoidea, and Crinoidea, the typical mode of 

 scleroblastic development of the spicules is that described 

 above for Amphiura elegans, i.e. the spicule originates 

 as a triradiate structure contained within a single cell. From 

 the figures of these and other authors, on the other hand, we 

 may also assume that the typical mode of development of 



