4o6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



a similar loss of epithelial character with obHteration of gradient by dif- 

 ferentially inhibiting conditions (chap. vii). However, complete reversal 

 of gradient and dominance is possible in Corymorpha and many other 

 coelenterates and in planarians without isolation of cells. Even if it should 

 be found that a gradient is necessary for the origin of epithelial order, its 

 persistence is evidently not always essential for maintenance of epithehal 

 character. 



EFFECTS OF OTHER PARTS ON DOMINANT REGIONS 



Although a new dominant region originates independently of other 

 parts in many plants and animals, its scale of organization and in some 

 forms its morphological pattern may be affected by other parts. In recon- 

 stitution this effect is usually inhibition. 



In pieces of Tubularia or Corymorpha stem below a certain length, but 

 still two or three times the length of the hydranth primordium developing 

 on longer pieces with distal end at the same level, the primordium length 

 decreases with decrease in length of piece (Driesch, 1899; Child, i907e). 

 Driesch regarded this decrease as indicating a teleological relation between 

 length of piece and primordium length. Actually, however, the primor- 

 dium length decreases less rapidly than length of piece, so that, with suffi- 

 cient decrease in length of piece, only a hydranth develops; and in still 

 shorter pieces, apical partial forms (Fig. 113, A-I [p. 334])- Primordium 

 length does not adjust to length of piece, either with the aid of an entel- 

 echy, as Driesch maintained, or in any other way; but the activation fol- 

 lowing section at the proximal end of the piece, even though not sufficient 

 to determine a hydranth there, does determine, at least temporarily, a 

 gradient, as dye reduction shows; and in pieces below a certain length this 

 opposes the distal gradient and decreases scale of organization of the distal 

 primordium somewhat. That this is the case is readily shown by delaying 

 proximal section for different periods after distal section. Driesch's teleo- 

 logical assumptions are not only unnecessary but fail entirely to account 

 for the apical partial forms. 



Certain planarian species show an inhibiting action on head develop- 

 ment originating from the posterior end in pieces below a certain length 

 and increasing in effectiveness with decrease in length of piece at a given 

 body-level. In the discussion of differential inhibition of the planarian 

 head (pp. 177-90) it was pointed out that this physiological inhibiting 

 factor originates at the posterior cut end; that it is effective only below a 

 certain length of piece, which differs in a definite graded manner with 



