DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 217 



tions of Asterias, are apparently completely radial. In E differential re- 

 covery involves only the extreme apical region and perhaps the foregut. 

 Similar forms are produced by other agents with certain concentrations 

 and exposure periods; they also appear in eggs from animals kept too 

 long under laboratory conditions. All of them represent some degree of 

 secondary modification, differential tolerance, conditioning, or recovery 

 of a more or less extensive apical region, for they appear only in the lower 

 inhibiting concentrations or after return to water, and they develop from 

 gastrulae with inhibited apical ectoderm. 



The development of one or two ciliated bands in some of these forms 

 is of interest (Fig. 87, B, C, D). As in many echinoid ectoderms inhibited 

 to radial form, the bands have become transverse circles, that is, their 

 localization is different from the normal. A few individuals with three 

 bands, at least ventrally, have been observed (see Fig. 89, H). This locali- 

 zation of ciliated bands suggests approach to the larval pattern of the 

 crinoid with five bands and little development of ventrodorsahty in larval 

 form. 



Final stages of differentially inhibited forms with ventrodorsahty and 

 some degree of apical recovery are shown in Figure 88, A-D. In A, B, 

 and C the whole preoral region is represented only by an ectodermal 

 thickening, in D by secondary apical outgrowth; entoderm shows little 

 or no recovery, the foregut being relatively small {A, B, C) or not differ- 

 entiated (D). When these forms develop a ciliated band, it is transverse 

 (A, B). Special interest attaches to these modifications because they ap- 

 proach in form and proportions the echinoid prepluteus (see Fig. 73, B, C). 

 Since the apicobasal gradient of the starfish is apparently steeper than 

 that of the echinoid in earlier stages (Child, 1936a), it is perhaps signifi- 

 cant that with decrease of its steepness by differential inhibition there is 

 approach to echinoid form and proportions. These types evidently result 

 from a relatively high degree of differential inhibiting action between 

 apico ventral and other regions. They have been seen most frequently 

 with LiCl, which evidently has a high differential action, but they also 

 appear with other agents. 



With somewhat greater degree of differential recovery forms like E and 

 F of Figure 88 appear, often in the same lots as those of Figure 88, A-D. 

 In a given lot animals swimming free or at the surface show, in general, 

 greater recovery than those at the bottom or in aggregations. When the 

 primary inhibition is slight, forms resulting from differential recovery are 

 of the general type of Figure 88, G. The preoral region is enlarged; mouth 



