224 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



of Figure 74. With further increase of inhibiting action the ectoderm is 

 radial; the ciliated band, if it develops, is localized around the basal region 

 (Fig. 91, B-D); and if skeleton develops at all, it forms an irregular ring 

 of spicules, often subapical {B) instead of basal. In absence of skeletal 

 development a ring of mesenchyme may occupy the same position (C, D). 



Fig. 90, A-F. — Less extreme types of Dendraster exogastrulae. A, LiCl m/90, continuous 

 from early blastula; B, LiCl m/40, 14 hr. from early blastula; C, crowded 10 hr. from early 

 blastula; D, LiCl m/25, 5 hr. from first cleavage; E, m/70, continuous from first cleavage; F, 

 LiCl m/60, continuous from first cleavage. 



Further reduction of ectoderm is shown in Figure 91, E-G. Position of 

 the skeletal rods in E is probably not indicative of the original ventro- 

 dorsality, and form of the ectoderm appears to be determined by their 

 elongation. In F and G their direction is apicobasal; occasionally in such 

 forms a short armlikc outgrowth develops at the apical pole, apparently 

 induced by the skeleton. In Figure 91, H, the only part of the original 



