228 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



to water. The maximum inhibiting and modifying effect of a given con- 

 centration of LiCl is attained only gradually, presumably according to 

 rate of penetration. High concentrations are more rapidly effective than 

 low ; and the effects of the higher concentrations, even with short exposure, 

 are more persistent than those of low, perhaps because intracellular con- 

 centration remains above the threshold of effectiveness for a longer time. 

 Within certain hmits longer exposure to the lower effective concentrations 



Fig. 93, A-I. — Developmental stages of Dendr aster exogastrulae showing displacement 

 apically of primary mesenchyme {A-G), entodermization of prospective ectoderm {A-I), and 

 loss of epithelial character and dissociation of prospective entoderm (J5, D-F) and of basal 

 parts of entodermized ectoderm (G-I). A, LiCl m/50, 145 hr. from first cleavage, identical 

 forms in crowded lots; B, crowded 10 hr. from early blastula, 6 hr. water; C, LiCl m/30, 5 hr. 

 from first cleavage, 18 hr. water; D, E, LiCl m/60, 25 hr. from first cleavage; F, LiCl m/2S, 

 7 hr. from first cleavage, 14 hr. water; G, LiCl m/20, 9 hr. from first cleavage, 18 hr. water; 

 H, LiCl m/30, 18^ hr. from first cleavage; /, crowded 27 hr. from first cleavage. 



gives modifications essentially similar to those resulting from shorter ex- 

 posure to higher concentrations. 



Early stages of exogastrulae with different degrees of entodermization 

 and displacement apically of mesenchyme are shown in Figure 93. In ^ 

 of this figure, a rather long exposure to a relatively low concentration, the 

 apical region is obviously inhibited, and almost the whole basal half of 

 the ectoderm has become entoderm. With continued exposure the original 

 entoderm may invaginate more or less but is inhibited in further develop- 

 ment, the entodermized ectoderm becomes the external entoderm of the 

 exogastrula, and ventrodorsality is usually obliterated in the remaining 



