DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 203 



is, a part, or in extreme cases all, of the prospective ectoderm becomes 

 entoderm. If such forms are able to develop further after return to water, 

 they become exogastrulae with small or no ectoderm and large external 

 entoderm. With exposure to LiCl beginning in the later blastula stages. 



Fig. 76, A-K. — Differential inhibition. A-E, Arbacia, KCN, ethyl alcohol, etc. A, slight 

 axiation remaining; B, ventrodorsality not evident, apparently slight polarity; C, completely 

 anaxiate; D, E, partial differential death, apical region of early blastula killed, development of 

 basal region, but E anaxiate; F-I, Dendraster, extreme differential inhibition in high concentra- 

 tions of LiCl (e.g., m/20) continuing from two-cell stage to stage when susceptibility of prospec- 

 tive entoderm becomes higher than that of ectoderm; prospective entoderm and more or less of 

 the entodermized ectoderm becoming a solid cell mass from which cells dissociate externally or 

 internally or both; J, K, Strongylocentrotus franciscanus, showing inhibition of entoderm with 

 exposure to LiCl (m/40) beginning in later blastula stages {A-D, after Child, igiCxi; H, I, 

 from Child, 1940; /, K, from Child, 19366). 



about the time of increase in susceptibility of the entoderm, not only may 

 further entodermal development be inhibited (Fig. 76, J) but entoderm 

 may decrease in extent and thickness so that more or less of it becomes 

 indistinguishable from ectoderm (Fig. 76, K). Forms of this type suggest 

 that some of the prospective entoderm has been ectodermized. If this is 



