DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 237 



differentiations except those of the extreme apical region and perhaps the 

 skeleton develop in other than their original prospective regions or cells. 

 Anal arms, if they appear, develop from various levels, all apical to the 

 prospective levels. If ventrodorsahty is not obliterated and a ciliated 

 band develops around the ventral side, all except its apical levels develop 

 from other cells than normally. When ventrodorsal ity is obliterated and 

 the band develops around the basal ectoderm (Fig. 91, B, C, D), its locali- 

 zation is, in large part, different from normal. The scale of ectodermal 

 organization may be greatly decreased, so that the apical half or even less 

 of the prospective ectoderm may approach pluteus form. This decrease 

 in scale is similar to that occurring in hydroid reconstitution : a piece of 

 Tubularia or Corymorpha stem so short that it gives rise only to a hy- 

 dranth or the apical part of a hydranth under natural conditions may, 

 under inhibiting conditions, develop a complete hydranth on a smaller 

 scale and stem, or in Corymorpha, hydranth, stem, and basal holdfast 

 region (pp. 344-49). A similar decrease in scale of organization results 

 from inhibiting conditions in planarian reconstitution (pp. 349-54). The 

 evidence indicates that in hydroids and planarians the decrease in scale 

 results from depression and decrease in length of a quantitative physio- 

 logical gradient, and at present convincing grounds for regarding the de- 

 crease in echinoderms as fundamentally different do not appear. The 

 small, thin-walled "neck" connecting ectoderm and entoderm in many 

 exogastrulae is evidently a reconstitution, and its possible significance is 

 an interesting question.^^ It is apparently ectodermal in origin; and after 

 return to water the entoderm often separates from it, leaving it attached 

 to the ectoderm. Usually, if not always, it is a secondary modification, not 

 a direct effect of the primary inhibition. Since it develops from what 

 would have been the anal region in an entogastrula, it might be regarded 

 as a proctodeum, developing under the experimental conditions though 

 absent or not developing appreciably in the entogastrula. On the other 

 hand, if the attached end of the entodermized ectoderm is physiologically 

 apical, it might perhaps be regarded as representing more nearly a stomo- 

 deum induced by the entoderm, particularly in the forms with radial 

 ectoderm and basal ciliated band, in which the flattened basal ectoderm 

 seems to be more or less Hke the normal ventral region in certain respects 

 (Fig. gi, B,C). It shows the highest rate of dye reduction after return to 

 water (Child, 19366), and the presence of the ciliated band about it sug- 

 gests that it approaches the ventral region in physiological condition. 

 '" Fig. 90, B, C, D; Fig. 91, .4, B, C, E, G. 



