230 PATTERNS AND PROBLEMS OF DEVELOPMENT 



With these more extreme lithium ejffects mesenchyme is locahzed near, 

 or even at, the apical pole, indicating entodermization of most of the 

 ectoderm (Fig. 93, E-G)\ with complete entodermization it may not be 

 definitely localized (Fig. 93, H, I). Occasionally two circles of mesen- 

 chyme appear at different levels (Fig. 93, £), and in a crowded lot indi- 

 viduals with three circles have been observed 



With concentrations of LiCl high enough to stop development in rather 

 early stages, particularly with sea-water solutions, which are hypertonic, 

 entodermization of ectoderm is apparently complete, and the basal region 

 becomes extremely thick, with partial or progressive obliteration of the 

 blastocoel (Fig. 94). Under these conditions many individuals become 

 spherical solid cell masses. With return to water before lethal effect, forms 



Fig. 94, /I -Z).— Entodermization with continuous exposure to high concentrations of LiCl 

 from early cleavage. A, B, m/25, 26 hr.; C, D, m/20 in sea water (hypertonic). 



like A and B of Figure 94 may become exogastrulae like Figure 91, E-H, 

 even though they appear earlier to be completely entodermized.'^ 



Exogastrulae of Strongylocentrotus, Arbacia, and Echinarachniiis, and, 

 so far as can be determined from the work of others, those of other 

 echinoid species, do not differ essentially from Dendraster exogastrulae. 

 Differences in proportions of certain parts, particularly in the secondary 

 modifications, apparently result from differences in steepness of the gradi- 

 ents and in degree of activation of entoderm in different species; but as 

 regards the general types of modification, there is a high degree of simi- 

 larity. 



ASTEROID EXOGASTRULATION 



General characteristics of asteroid and echinoid exogastrulation are 

 similar, but there are minor differences resulting from absence in the star- 

 fish of mesenchyme formation preceding gastrulation and from the later 



'3 In the hypertonic solutions (Fig. 94, C, D) there is little or no dissociation, probably be- 

 cause of hypcrtonicity; but after return to water it may be e.xtensive or more or less complete. 



