DIFFERENTIAL DEVELOPMENTAL MODIFICATION. I 169 



or control is evidently lost in these forms; locomotion is limited to an in- 

 definite rolling-about instead of definitely directed with apical end in 

 advance, as in normal planulae. If returned to water, these forms may 

 live 2 weeks or more without further development and without any indi- 

 cation of a developmental pattern. Apparently they are unable to develop 

 unless a new pattern is determined in them (see pp. 425-26). Except for 

 the more frequent emigration of cells in LiCl the same modifications ap- 

 pear as with other agents used. All of them "entodermize"; that is, the 

 higher gradient levels are more or less completely inhibited down to the 



Fig. 58, A-D. — Normal and inhibited development of axes in Phialidium. A, normal at- 

 tachment of planula by end originally apical, and development of hydranth-stem axis from end 

 originally basal; B, C, inhibited planulae with some degree of differential tolerance or differ- 

 ential recovery, giving rise to stolon axes; D, hydranth-stem axis developing from planula 

 stolon after recovery (from Child, 19256). 



level characteristic of the basal region in normal individuals, so that im- 

 migration of cells is not limited to the basal region but occurs farther api- 

 cally or, in the spherical forms, equally from all parts of the wall, and 

 some cells may emigrate instead of immigrating. The entodermization re- 

 sembles that occurring in echinoderms, and the emigration of cells is prob- 

 ably comparable to echinoderm exogastrulation (see chap. vi). 



With lower concentrations of inhibiting agents which do not completely 

 obliterate polarity, retarded elongation of the planula occurs. The normal 

 planula in good condition swims actively and attaches by the apical end 

 (Child, 1925a), and the first hydranth develops from the original basal 

 end (Fig. 58, A). The slightly inhibited planulae do not swim free but 



