EMBRYONIC RECONSTITUTIONS 525 



region. Separation of halves by ligature in later cleavage, blastula, or even 

 early gastrula stages shows much the same differences, according to plane 

 of separation. After median separation the inner sides of the two indi- 

 viduals are, in general, increasingly less developed than the outer sides, the 

 later the stage of separation. The left member of the pair has normal situs 

 viscerum, but in the right member situs inversus is not rare.'^ Contrary 

 to the earlier conclusions of A. Brachet from experiments with local 

 cautery, ligature and removal of parts showed that early anuran embryos 

 have essentially the same capacity for reconstitution as the urodele (Vogt 

 und Bruns, 1930; G. A. Schmidt, 1933). 



These reconstitutions in the early stages of amphibian development in- 

 dicate that the dorsal inductor region is in some way localized but that, 

 within it, relations of dominance and subordination and capacity for re- 

 constitution are present, and the symmetry pattern, if definitely present in 

 these stages, may undergo extensive reconstitution. However, the fact that 

 new dorsal inductors may originate in any part in inverted and partly in- 

 verted eggs (pp. 428-30) indicates that determination of the inductor re- 

 gion in early stages has probably not proceeded very far in specific differ- 

 entiation. 



RECONSTITUTIONS IN THE AMPHIBIAN GASTRULA AND 

 LATER STAGES 



The discussion of amphibian inductors in the preceding chapter was 

 largely concerned with reconstitution in consequence of altered relations 

 to an inductor. It was shown there that an inductor may determine the 

 course of development of other parts but that its own development may 

 be largely or wholly independent of them and may depend on relations of 

 dominance and subordination within the inductor region itself. When a 

 region, field, or organ system has attained a certain stage of determina- 

 tion, chemodifferentiation, or physiological stability, reconstitution of a 

 different region, field, or organ system from it does not occur under the 

 usual experimental conditions, though we do not know that it might not 

 occur under other conditions. In amphibian development progressive limi- 

 tation or restriction of developmental potencies under known experimen- 

 tal conditions becomes evident in gastrula and later stages, but extensive 

 reconstitution is still possible within various organ primordia and fields 

 when parts are removed. Some organ systems, like legs and tails of uro- 

 deles, are capable of regeneration even in the adult. Reconstitution in an 



'•» Spemann und Falkenberg, 1919; Ruud und Spemann, 1922. 



