PATTERNS OF DEVELOPMENTAL PROGRESS 713 



than in their normal environment; that is, more or less reconstitution 

 occurs, indicating that their differentiation is not wholly independent 

 (pp. 529-35). The insect embryo is not a mosaic in early stages but, at 

 least as regards ability of larger regions to continue differentiation inde- 

 pendently for a time, approaches mosaic condition rather early, though 

 integrating factors are essential in metamorphosis. 



According to the earlier predeterministic theories of development, every 

 differentiation is essentially a self-differentiation; but experiment has 

 made it evident that this is far from true. At present the capacity for self- 

 differentiation of a part is generally regarded as a condition appearing in 

 the course of development, earlier in some forms and in some parts than 

 in others. Weiss has called it an "autonomization" (1926a) or "emanci- 

 pation" of parts from the whole (1935, 1939)- The emancipation is rarely, 

 if ever, complete. Transplanted or isolated self-differentiating parts very 

 commonly show some departure from the normal; they may show some 

 reconstitution, developing more than they would normally; they may 

 develop less than normal; their development may continue for only a 

 short time, and differentiation is often less complete than normally; even 

 if histological differentiation is normal, departure from normal morphol- 

 ogy is frequent. Moreover, whatever the degree of emancipation at a 

 certain stage, it is not permanent. 



Perhaps of even greater significance for an adequate conception of de- 

 velopment than occurrence of independent differentiation is its absence, 

 except in the dominant region, in some of the simpler animals. Reconsti- 

 tution of wholes from isolated blastomeres, parts of blastulae and of 

 planulae, and also from pieces of the mature individual proximal to the 

 hydranth takes place in many coelenterates. The hydranth of Tubularia 

 or Corymorpha and of various other forms, or even its extreme apical re- 

 gion, can differentiate quite independently of other parts; but no other 

 region of the hydroid body is capable of independent differentiation at 

 any stage of life. Medusa buds and probably some gonophores, after 

 attaining a certain stage, are capable of more or less independent differen- 

 tiation; but they, like the hydranth, are dominant regions. As regards 

 potentialities of parts of the planarian embryo, nothing is known ; but in 

 postembryonic reconstitution the head can develop quite independently 

 of other parts, but no other part of the body is capable of independent 

 differentiation. A hydranth or part of a hydranth or a planarian head, 

 when isolated, does not give rise to other parts; but other parts recon- 

 stitute hydranth or head, and these induce development of other parts. 



