RECONSTITUTIONAL PATTERNS IN EXPERIMENT 363 



// 



enough to be acephalic (/ of Fig. 121, A), develop pharynx and mouth 

 and all parts posterior to the mouth (Fig. 121, 5); but acephalic pieces 

 from a level just posterior to the mouth (7/ of Fig. 121, .4) do not develop 

 mouth, pharynx, or prepharyngeal region (Fig. 121, C). However, if there 

 is even rudimentary head development 

 (anophthalmic [p. 179]) in pieces from 

 postoral levels, the anterior part of the 

 piece reorganizes into a prepharyn- 

 geal, pharyngeal, and oral region (Fig. 

 121, D). Moreover, such forms, even 

 though head development never goes 

 beyond this stage, are distinctly more 

 like normal animals in motor behavior 

 than are acephalic forms. 



Many investigators have recorded de- 

 velopment of bipolar forms from short 

 planarian pieces.'' Such forms may con- 

 sist merely of heads alone or of heads 

 and more or less of the anterior post- 

 cephalic region, and sometimes a phar- 

 ynx develops in one or both axes. Oc- 

 casionally also one or two posterior ends 

 develop from lateral regions, usually 

 after more or less elongation in conse- 

 quence of the opposed locomotor activ- 

 ity of the two heads (Fig. 122). Both 

 heads may be normal; or one, usually 

 the posterior, may be more or less chf- 

 ferentially inhibited (Fig. 122, E). Oc- 

 casionally both head and posterior end 

 develop from a posterior cut surface, 

 particularly if it is somewhat obhque (Fig. 122, G; also Fig. 25, D [p. 52]). 

 Bipolar forms may apparently develop from any body-level in sufficiently 

 short pieces, but in the species with a definite postoral fission zone they 

 seem to appear most frequently from the region of the fission zone, with 

 pieces of given length. In this region there is a shght rise in gradient- 

 level from the posterior end of the anterior zooid to the anterior end of 



4 See, e.g., Morgan, 1898, 1900a, 1904(7, h; Bardeen, 1902, 1903; Child, 1911&; P. Lang, 

 1913a; Rustia, 1925; Lus, 1926; and other authors. 



Fig. 121, A-D. — Development of 

 acephalic planarian forms from different 

 body-levels. .-1 , outline indicating levels 

 of sections / and //; B, prepharyngeal 

 acephalic form from level /, pharynx and 

 mouth reconstituted; C, postpharyngeal 

 acephalic form from level //, does not 

 reconstitute pharyn.x or prepharyngeal 

 region; D, postpharyngeal piece with 

 rudimentary anophthalmic head, recon- 

 stitutes pharynx, mouth, and prepha- 

 ryngeal region {B-D, from Child, 1911c, 

 1929&). 



