DOMINANCE IN RECONSTITUTION 357 



zation is determined. That range of dominance and scale of organization 

 differ with conditions in buds, as elsewhere, is highly probable; but ex- 

 perimental analysis concerned with this problem has not been undertaken. 

 In fission the length attained by an individual before appearance of a 

 fission zone may vary with development of the head and with external 

 conditions, and length of a new zooid when it first becomes distinguish- 

 able may also vary. However, the posterior zooid of a pair may not rep- 

 resent the entire polar axis in early stages; the axis may be determined 

 progressively from a dominant region, as in buds (p. 332). 



Questions of dominance and scale of organization in early embryonic 

 development will be taken up in later chapters, but attention is called to 

 a case of embryonic echinoderm reconstitution because it resembles so 

 closely the changes in scale in hydroid and planarian reconstitution. The 

 apical half of the sea-urchin embryo usually develops in water into an 

 entirely ectodermal blastula-like form; but when treated with hthium, it 

 may form entoderm and gastrulate.'^ The scale of organization deter- 

 mined in water is apparently too large for the apical half, but with decrease 

 of scale by hthium a complete individual develops. Short pieces of Tuhu- 

 laria and Corymorpha stem and short pieces of planarians show essentially 

 identical changes under inhibiting conditions, that is, scale of organiza- 

 tion decreases, so that a piece of a given length develops a larger part, 

 or the whole, of the polar pattern, instead of only the apical portion. 

 The basal half of the echinoderm embryo, however, can reconstitute an 

 apical region and become a complete larva. This is exactly what happens 

 in the hydroid stem pieces and in planarian and anneHd reconstitution. 

 Should we not attempt to account for all these essentially similar recon- 

 stitutions in essentially similar terms rather than by special hypotheses 

 for the sea-urchin embryo and others for hydroids, planarians, and an- 

 nehds? In many eggs and embryos scale of organization is more or less 

 fixed at the beginning of embryonic development, or certain cytoplasmic 

 regions must be present for complete development; these are evidently 

 beyond the stage of simple gradient pattern. 



CONCLUSION 



The evidence indicates that in both plants and animals origin of a new 

 dominant region is independent of other parts. Reconstitution of hy- 

 dranth or head is not determined by other parts of an isolated piece; on 

 the contrary, a certain degree of physiological or physical isolation from 



'S See pp. 506-S; also von Ubisch, 1925ft, 1929; Horstadius, 1936a. 



