DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 201 



stages to LiCl in concentrations slightly above the range in which differ- 

 ential tolerance or conditioning is possible. The long period of exposure 

 makes the inhibition practically irreversible within the developmental 

 period, though there is some further development after return to water 

 and perhaps slight differential recovery, but differential inhibition is evi- 

 dently predominant. These forms, however, are not specific for lithium 

 but have been seen with other agents having a high differential action — 

 for example, CUSO4, HgCl2, and very similar forms, though with differen- 

 tiation more inhibited, are produced by KCN. In the individual of Fig- 

 ure 75, A and B, there is evidently extreme apical inhibition, the oral lobe 

 being completely absent; the skeletal rods converge toward what is ap- 

 parently the ventral side, but arms do not develop, and the ciliated band 

 is apparently more or less transverse. Figure 75, C and D, shows a more 

 extreme case of this type, with further approach to radial form but still 

 with some evidence of ventrodorsality in position of the skeletal rods. In 

 lots subjected to the same conditions forms like EI of Figure 75, also 

 occur; in these forms apicobasal pattern is almost completely obliterated 

 and only a single skeletal rod is present, apparently transverse and in the 

 same plane as the ciliated band, suggesting a possible relation. But 

 whether, or to what extent, position of arms 180° apart, when they de- 

 velop, is determined by prospective arm areas in the ectoderm or merely 

 by elongation of the skeletal rod is uncertain. Elongation of a rod can in- 

 duce development of an arm or armlike outgrowth in other than the posi- 

 tion of the original arm area. In forms with excess of skeleton supernu- 

 merary arms are often formed, three-, four-, and five-armed forms result- 

 ing; and in the apparently anaxiate ectoderm of extreme exogastrulae a 

 skeletal rod may induce a short armlike outgrowth in various positions, 

 even at what was originally the apical pole. It seems possible, therefore, 

 that the arms of forms like E and F of Figure 75 result from skeletal 

 elongation. If this is the case, they may be without definite relation to the 

 original ventrodorsality and constitute a bilaterality independent of it. 

 With less elongation of the skeletal rod bilaterality is less evident (Fig. 75, 

 G, H). That the original ventrodorsality and the resulting bilaterality 

 may be completely obliterated is suggested by absence of stomodeum and 

 by completely radial form of entoderm. These forms are very different 

 from the wide-angled forms resulting from secondary modifications de- 

 scribed in the following section and appear only under rather extreme 

 inhibiting conditions, but they probably do involve some slight recovery, 

 for development of ciliated band, skeleton, and arms occurs only after 



