DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 225 



ectoderm remaining is the tiny button at the apical pole. A thin-walled 

 "neck" connecting external entoderm and ectoderm develops in many 

 exogastrulae, apparently always as a secondary modification, not as a 

 direct effect of lithium (Figs. 90, A-E, and 91, A-C, E-G), and apparently 

 of ectodermal origin. Its possible significance is discussed later. 



Fig. 91, A-I. — More extreme types of Dendraster exogastrulae: final stages attained in 

 water after various exposures to LiCl. A, w/25, 7 hr. from first cleavage; B, m/30, i8§ hr. 

 from eight cells; C, m/25, 7 hr. from first cleavage; D, m/50, continuous from first cleavage; 

 E, m/30, 18^ hr. from first cleavage; F, m/25, 7 hr. from first cleavage; G, m/20, 9 hr. from 

 early blastula; H, I, m/20, 95 hr. from four cells. 



Exogastrular entoderm also shows a wide range of modifications. The 

 original entoderm may invaginate more or less but is inhibited in further 

 development (Figs. 90, D, and 91, D); or, with more extreme inhibition, 

 it may lose epithelial character and dissociate more or less completely 

 and the exogastrular entoderm develops in large part, perhaps in some 

 cases entirely, from entodermized ectoderm (Figs. 90, F, and 91, E-H). 

 It is often impossible to determine how large a part of the total entoderm 

 is original prospective entoderm. On the other hand, with relatively slight 



