362 PATTERNS AND PROBLEMS OF DEVELOPMENT 



As regards establishment of a dominant region and induction of other 

 parts by it, reconstitution in planarians, nemerteans, and anneUds does 

 not differ essentially from that in coelenterates, except that regeneration 

 of new tissue from the cut surface is a characteristic feature. In absence 

 of inhibiting conditions the activation following section and isolation re- 

 sults in outgrowth of tissue, apparently more or less embryonic in char- 

 acter and undergoing differentiation. Usually the new dominance develops 

 from this regenerating tissue at the higher end of that part of the original 

 polar gradient present in the piece, and the tissue becomes wholly or in 

 part a head. The length of outgrowth and the postcephalic parts develop- 

 ing from it differ in different forms and in planarian pieces from different 

 body-levels of the original individual (Figs. 17-22 [pp. 43~46]). In some 

 planarians the outgrowth may be greatly decreased by inhibiting condi- 

 tions, so that the head develops in large part by reorganization posterior 

 to the level of section (Fig. 72, H [p. 191]; Fig. 119, D [p. 352]). Under 

 these conditions planarian head reconstitution approaches the type of 

 reconstitution of hydranth in Tuhularia and Corymorpha. Anterior out- 

 growth of new tissue and head development are increasingly inhibited 

 with decrease in length of piece in several planarian species; and in pieces 

 below a certain length, which differs in a definite way with level of origin 

 of piece, size and physiological condition of animal, and external condi- 

 tions, there is complete inhibition of head development, but development 

 of a posterior end is possible at the same level under the same conditions 

 (pp. 180-90). In absence of the tissue outgrowth from which head de- 

 velops, httle or no reorganization of body-levels anterior to level of origin 

 of the piece takes place, but all parts posterior to that level can still de- 

 velop. For example, pieces from the prepharyngeal region, cut short 



ence and movement in opposite directions of two formative substances are postulated by 

 Hammerling, but it seems evident that dominance and a gradient are present along the axis 

 and that determination of new dominances and gradients by activation following section are 

 concerned in the reconstitution of these plants. Moreover, the postulated movement of forma- 

 tive substances in opposite directions from the nucleus, which is situated near the basal end, 

 in unipolar reconstitution of pieces, seems to require the presence of an axial differential of 

 some sort. The fact that nonnucleated pieces possess considerable capacity for reconstitution 

 appears to support this view. Whatever the situation as regards movement of substances, 

 orderly movement in opposite directions seems to require presence of a pattern of some sort, 

 and the data of reconstitution of these algae suggest that that pattern is a gradient. Sub- 

 stances from the nucleus may, of course, play an important part in determining physiological 

 condition and the character of later differentiation of parts, but how they can determine the 

 general spatial pattern and axiate order and the localization of secondary axes as lateral 

 branches does not appear. 



