DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 227 



Is generally more inhibited than ectoderm ; and if exogastrulation occurs, 

 the entoderm is usually relatively small (Fig. 92, A-C). Figure 92, C, 

 is an entexogastrula in which partial invagination preceded e vagina tion, 

 but entoderm is evidently inhibited in development. Forms resulting from 

 exposure of Strongylocentrotus to lithium in late pregastrular stages are 

 shown in Figure 92, D-I . It was suggested in an earlier paper (Child, 

 19366) that in cases like D-F of Figure 92 more or less ectodermization of 

 prospective entoderm has taken place. Cells in the blastocoel do not in- 

 dicate extensive dissociation of entoderm. 



DEVELOPMENT OF ECHINOn) EXOGASTRULAE 



The final forms of many exogastrulae give little information concerning 

 their developmental physiology: for such information it is necessary to 

 look to the earlier stages, as modified by different concentrations and ex- 

 posure periods. Since the pioneer work of Herbst it has been known that 

 lithium retards and, in sufficient concentration, completely stops develop- 

 ment and kills. That entodermization of prospective ectoderm may occur 

 in connection with exogastrulation was also observed by Herbst. Lithium 

 is believed by Runnstrom and his co-workers to have a specific action 

 on the postulated vegetative concentration gradient. In recent papers 

 it is maintained that lithium inhibits the animal kind of metabolism and 

 increases the vegetative kind." We have to inquire whether, or to what 

 extent, the development of exogastrulae supports this hypothesis. 



Normally the primary mesenchyme after immigration and before bi- 

 lateral aggregation is localized in a more or less definite ring or circle on 

 the inner surface of the ectoderm near its junction with the prospective 

 entoderm. As is well known, this ring may be localized much farther api- 

 cally in developing exogastrulae; that is, the ectodermal level which local- 

 izes it is displaced apically. This shift apically of certain unknown con- 

 ditions in the ectoderm is usually associated with entodermization of the 

 more basal levels of prospective ectoderm, though slight apical shift is 

 apparently possible without appreciable entodermization. Lithium has 

 still another effect: it inhibits development of entoderm. That it ento- 

 dermizes ectoderm and also inhibits entoderm is not the paradox that it 

 may appear to be, but is very simply interpreted. And, finally, there is 

 evidence indicating that some ectodermization of prospective entoderm 

 may occur under certain conditions, particularly in recovery after return 



" See, e.g., Lindahl, 1936, p. 339, etc., and references to earlier papers by Runnstrom; 

 also Runnstrom, 1928Z), i93Sa. 



