Evidence from Somatic Hist ogene sis 



the dermal epithelium and become arranged in a trefoil-like figure 

 (Figure 49). The nucleus of each cell then divides into two, in such 

 a way that one nucleus is placed more deeply and one more super- 

 ficially. Between each pair of sister nuclei a minute spicule ray 

 appears, the three rays being at first distinct from each other but 



FIGURE 50 SEE 49. 



soon becoming united at the center of the system (Figure 50 tr. 

 syst.}. As the rays grow in length the protoplasm of each actino- 

 blast becomes aggregated around each of the two contained nuclei 

 and finally more or less completely segmented off to form two 

 formative cells, of which the one placed more internally travels 

 to the tip of the spicule ray, while the other remains at the base 



FIGURE 51 SEE 49. 



b.f.c., basal formative cell, tr.syst., triradiate system. 



(Figure 51, b.f.c.}. The apical formative cell sooner or later dis- 

 appears, returning, apparently, to the epithelium. The basal 

 formative cell remains at the base of the ray (Figure 51), until 

 this portion is secreted to its full thickness. It then migrates 

 slowly outwards along the ray, and in the fully formed spicule is 

 found adherent to the extreme tip." 



