DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 229 



ectoderm (Fig. 91, D). This figure represents the most advanced stage 

 attained with continuous exposure to LiCl m/50. Evidently both invagi- 

 nated entoderm and entodermized ectoderm are inhibited in development, 

 and some dissociation of entoderm has occurred. After return to water 

 at the stage of Figure 93, ^, ectoderm may develop more or less ventro- 

 dorsality (Fig. 90, D, E), and entoderm may show some further develop- 

 ment. Figure 93, B, shows entodermization resulting from crowding in 

 early stages; after return to water, forms of the general type of Figures 90, 

 C, result. Evidently, entodermization is not a specific effect of lithium. 

 Figure 93, C, a short exposure to rather high lithium concentration, shows 

 extreme entodermization and greatly inhibited invagination, even after 

 18 hours in water. With further recovery, however, ectodermal ventro- 

 dorsality appears in many individuals, the most advanced attaining ap- 

 proximately the condition of Figure 90, D, with all gradations from this 

 form to completely radial forms like Figure gi,D. In all, the later develop- 

 ment of original entoderm and entodermized ectoderm remains inhibited. 



With higher concentrations or longer exposures the original entoderm 

 may lose, in part or wholly, its epithelial character and become a solid 

 cell mass from which cells dissociate internally or externally or both, 

 though in some individuals a thin external layer remains epithelial or re- 

 gains epithelial character after return to water (Fig. 93, D-G). Crowding 

 has a similar effect (Fig. 93, 5). With still more extreme inhibiting condi- 

 tions, either by long exposure or by high concentrations, loss of epithelial 

 character and dissociation progress acropetally in the entodermized ecto- 

 derm (Fig. 93, H, I). In the most advanced stages attained in recovery 

 from these effects more or less of the original entoderm may remain a 

 cell mass, internal or external, at the tip of the entodermized ectoderm, 

 or an external layer may constitute epithelium at the free end of the 

 exogastrular entoderm." 



In these forms it is impossible to determine how much of the exo- 

 gastrular entoderm develops from the original prospective entoderm; 

 but the earlier stages, such as E-G of Figure 93, indicate that the ex- 

 ternal entoderm may consist mostly, perhaps in some cases wholly, of 

 entodermized ectoderm. Dissociated entodermal cells do not usually ap- 

 pear to take any part in further development, though it is uncertain 

 whether they may function as mesenchyme in some cases of recovery. 

 Usually, however, with inhibition sufficient to produce entodermal dis- 

 sociation, little or no skeleton develops. 



'^ See Fig. 91, B, C, E, G, H , /, and probably also F; also Fig. 93, D, E, G. 



