202 PATTERNS AND PROBLEMS OF DEVELOPMENT 



return to water. In the absence of skeletal development completely radial 

 individuals with apparently transverse ciliated band are frequent in the 

 same experimental lots (Fig. 75, /). This series of forms has been obtained 

 with Arbacia, Strongylocentrotus , and Dendraster. 



As degree of inhibition increases, there is progressively less evidence of 

 axiate pattern, the skeleton is represented by a few spicules or does not 

 develop, and mesenchyme cells are scattered instead of being localized 

 bilaterally in the regions of arm development. The particular form of 

 larva resulting depends largely on the stage at which exposure begins and 

 the rate at which inhibition occurs in relation to rate of development. 

 With relatively rapid inhibition, beginning at the two-cell stage and re- 

 turn to water before the increase in susceptibility of entoderm associated 

 with gastrulation, ectodermal development may attain sKght ventrodor- 

 sality (Fig. 76, yl) or be completely radial, and more or less regional ento- 

 dermal differentiation may occur. Although these forms probably repre- 

 sent a slight degree of differential recovery, they are mentioned here be- 

 cause they represent chiefly considerable degrees of differential inhibition. 

 Other somewhat similar forms are described below under differential re- 

 covery. With sufficient inhibition involving the early gastrula entoderm 

 is inhibited, develops incompletely, or may become spherical, separate 

 completely from the closed blastopore, and lie free in the blastocoel. 

 Ectoderm may show slight polarity or be completely anaxiate (Fig. 76, 

 B, C). Forms of this sort, returned to water, may live for 2 weeks or more. 

 Completely anaxiate forms like Figure 76, C, do not show definitely di- 

 rected locomotion but roll about in all directions. With still more extreme 

 inhibition in early blastula stages more or less cytolysis and disintegration 

 progress basipetally from the apical pole. With return to water at the 

 proper time the disintegration is arrested, the cells come together apically, 

 and invagination of entoderm may take place (Fig. 76, D, E). Some of 

 these forms remain completely anaxiate; others show slight differential 

 recovery (pp. 208-10). With these relatively extreme inhibiting condi- 

 tions continuing from early stages to the stage when entodermal suscepti- 

 bility increases, the entoderm tends to lose its epithelial character and be- 

 come a solid cell mass, from the surface of which cells dissociate. This 

 dissociation may begin either without invagination or during its early 

 stages (Fig. 76, F-I). Cells may also dissociate from regions apical to 

 the prospective entoderm, in some cases from all levels, as in Pkialidium 

 (pp. 167-69). In many of these cases, particularly with LiCl but also 

 with other agents, there is entodermization of prospective ectoderm; that 



