STUDIES IN SPICULE FORMATION. 325 
Fic. 9 shows well the young ‘tetrahedron ’’—the three-cornered granule ; 
also seen in Figs. 11 and 22 (Plate II). 
Fie. 15 shows well the position of the spicule relative to the wall of the 
larva. 
Fic. 16 shows two young triradiates dissimilarly aurientated (p. 316); also 
Fig. 7. 
PLATE 19. 
Fries. 17, 20, and 22 illustrate the production of two large triradiates in 
the longitudinal strand. In Fig. 25 as many as three triradiates have been 
formed. 
Fic. 19 shows half a dozen granules produced at the centre of the longi- 
tudinal strand, 
Fies. 21 and 25 illnstrate the rare occurrence of small triradiate spicules 
in the course of the blastoporic ring. In Fig. 25 the cells have deserted the 
spicule after forming it. In Fig. 22 they are still present. 
Fies. 22, 24, and 26 show the typical fully-formed triradiate spicules with 
their actinoblasts. As is evident, the precise position of the scleroblasts is in 
no way constant, at least at this stage, for the given form of spicule—the 
relation is only a general one. 
Fig. 25 is remarkable for the number of independent deposits which have 
arisen in the blastoporic ring. The ring is viewed from the posterior end of 
the larva. 
Fic. 26.—Spicule formation well advanced. The recurrent apophyses have 
not yet met in the mid-ventral line. 
Fies. 27, 28 show the junction of the recurrent apophyses. Despite the 
completeness of the blastoporic ring, the apophyses are not contiguous in the 
same line. 
Fic. 29 shows the “mound” form which the actinoblast of the adult larval 
skeleton assumes (p. 320). 
