EMBRYONIC RECONSTITUTIONS 507 



In the isolated apical half, or an, or an^, the primary gradient is not partly 

 obliterated or reversed by the secondary, resulting from basal activation ; 

 consequently, the scale of polar organization becomes larger than in nor- 

 mal development and larger than the apical half or the fourths, and they 

 develop as more or less extreme apical partial forms. These apical partial 

 forms are essentially similar in origin to the apical partial forms in the 

 reconstitution of Tuhularia and Corymorpha pieces. Short pieces of these 

 forms with a single gradient reconstitute unipolar apical partial forms 

 which usually have a much larger scale of organization than bipolar partial 

 forms developing from pieces of the same length with two opposed gra- 

 dients (Fig. 116 [p. 346]). The later the developmental stage at which 

 isolation of apical halves or fourths occurs, the less is the change in scale 

 of organization from normal development (Horstadius, 1936a). This, of 

 course, is to be expected if there is a secondary basal activation and altera- 

 tion of the primary gradient. 



That the apical overdevelopment of apical halves and fourths is really 

 due to the large scale of organization determined in the isolates, rather 

 than to restriction of potency for entoderm and mesenchyme, is shown by 

 the fact that after temporary exposure to differentially inhibiting condi- 

 tions (lithium salts) apical halves may develop primary mesenchyme and 

 entoderm, gastrulate, develop skeleton, and become plutei; or in some 

 cases they may even become exogastrulae (von Ubisch, 1925a, b, 1929; 

 Horstadius, 1936a). Runnstrom and Horstadius regard lithium as exert- 

 ing a specific effect in increasing the vegetal gradient or substance, but it 

 has been shown that after the secondary basal activation occurs lithium 

 produces modifications in the opposite direction and that other agents be- 

 sides lithium produce entodermization (pp. 228-33). Exposure to lithium 

 salts involves two factors — the direct, differentially inhibiting action, and 

 the recovery after return to water, which may also be differential. In 

 early stages hthium inhibits the higher apical levels of apical halves more 

 than the basal levels; consequently, scale of organization of these regions 

 is decreased and extent of extreme apical development is therefore less 

 than without the inhibiting action. Basal regions of the apical halves 

 are also inhibited to a lesser degree, but sufficiently to bring them down 

 to the gradient-levels of presumptive entoderm and mesenchyme in early 

 stages, that is, entodermization of presumptive ectoderm results. With re- 

 covery after return to water apical dominance is inadequate to prevent 

 the secondary basal activation, and mesenchyme formation and gastrula- 

 tion take place much as in the normal individual. With sufficient inhibi- 



