RECONSTITUTIONAL PATTERNS IN EXPERIMENT 365 



By contraction of the cut ends they become circular in dorsal or ventral 

 aspect, new tissue merely fills in the contracted cut surfaces without out- 

 growth, and definitely directed locomotion is absent (Fig. 122, H). In 

 consequence of contraction of the cut surfaces the longitudinal muscles 

 function much like the circular muscles in a medusa, their contraction 

 decreasing the circular diameter of the piece. Since the ventral muscles 

 are stronger, the central regions are forced dorsally when they contract, 

 and in time a permanent change in form with more or less elongated dorsal 

 outgrowth results (Fig. 122, /). These forms finally appear completely 

 radial in form and motor activity. It is sug- 

 gested that outgrowth of tissue is absent in 

 these pieces because the whole piece is in- 

 volved equally or almost equally in the 

 changes following section; that is, the 

 pieces are so short that a differential be- 

 tween regions adjoining the cut surfaces 

 and other parts of the piece is practically 

 absent, the whole piece adjoins a cut sur- 

 face, and consequently there is no differen- 

 tial developmental behavior of any kind 

 except that finally resulting from the radial 

 muscular activity. 



Bipolar posterior ends may develop from short posterior pieces of cer- 

 tain triclad species that do not undergo fission (Morgan, 1904a; Lus, 

 1926). In these cases activation is not sufficient at either level of section 

 to determine dominance and independence of other parts, and reconstitu- 

 tion does not involve independent origin of a new pattern but develop- 

 ment of a subordinate part determined by other parts of the piece. Un- 

 doubtedly differences in physiological condition at different body-levels 

 may be concerned in determining whether bipolar heads or bipolar tails 

 develop, but lack of potency for head or tail development is not necessarily 

 involved. Bipolar heads or tails may be determined by a difference in 

 relation between the activated cells and other parts: if they become inde- 

 pendent, heads develop; if they remain subordinate, tails develop. 



Planarian pieces from the region lateral to the ventral nerve cords 

 show increase in frequency of head regeneration from the longitudinal cut 

 surface on the median side of the piece rather than from the anterior 

 transverse surface, with decreasing length of piece. The head may regen- 

 erate on the angle between the two cut surfaces (Fig. 123, A); or in very 

 short pieces, so strongly contracted that transverse and longitudinal cut 



Fig. 1 2T,. — Reconstitution of short 

 lateral planarian pieces (from Beyer 

 and Child, 1930). 



