RECONSTITUTIONAL PATTERNS IN EXPERIMENT 387 



gradual reorganization of the nervous system, progressing from, and in- 

 duced by, the dominant region of the graft. Doubtless a similar, though 

 less extreme, reorganization is induced posterior to the graft; but since 

 it is in the direction of the original polarity, its progress is not so readily 

 shown. According to Miller, the epithelium and pigment of heteroplastic 

 grafts are, in most cases, gradually replaced by epithehum and pigment 

 characteristic of the host. To what extent other tissues are replaced is 

 not known. 



The extensive experiments of Okada and Sugino with a Japanese pla- 

 narian species show essentially the same relations of dominance and in- 

 duction by the grafts, but they have also shown that the head region an- 

 terior to the eyespots and pieces from the lateral head region, including 

 an auricle, are capable of dominance and induction and that postcephalic 

 pieces from the prepharyngeal region can induce a pharynx even without 

 head regeneration. Both the anterior and lateral pieces of the head include 

 some of the central nervous system, and the inducing capacity of pre- 

 pharyngeal pieces is what might be expected from the fact that isolated 

 prepharyngeal pieces that remain completely acephalic reconstitute all 

 parts posterior to their level of origin in the parent body (p. 180) but 

 nothing anterior to that level. 



Working with European planarians, Br^ndsted (1939) finds that en- 

 tire, fully developed heads grafted on anterior cut surfaces of planarian 

 pieces do not induce reorganization. He maintains that the new tissue 

 of regeneration, the blastema, is the inducing agent. With the American 

 species, however, very little new tissue is formed in many cases in which 

 the grafts induce, and in the experiments of Santos and Miller no evident 

 relation between amount of new tissue and induction has been found. It 

 is entirely possible, however, that fully developed heads of some planarian 

 species are not active enough to induce reorganization. 



MIRROR-IMAGING IN VARIOUS RECONSTITUTIONS 



The two opposed components of bipolar forms of Tubularia, Corymor- 

 pha, planarians, and annelids are, in general, mirror images of each other 

 because, although their polarities are opposed, the radial, ventrodorsal, 

 or bilateral patterns are essentially the same. Under certain conditions 

 reconstitution may result in triplication of axes. The actinian Harenadis 

 affords an excellent example. A partial transverse section involving both 

 body wall on one side and one side of the esophagus results in union of 

 the cut surfaces of body wall and esophagus both distal and proximal to 



