52 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



few days; but, although tentacle development is not completely inhibited 

 at the most proximal levels, the full normal number, sixteen, is not usually 

 attained, even after 3 or 4 weeks, and the tentacles farthest proximal ap- 

 pear last and develop very slowly. Experiment on another species of 

 stalked scyphozoan {Thaumatoscyphus distinctus) shows a delay of recon- 

 stitution at proximal levels of an obhque cut surface more nearly like 

 that in the scyphistoma than in Haliclystus (Hanaoka, 1935). In these 

 forms there is not only a difference in time of appearance of marginal 

 structures at the different levels of the oblique section but a continuing 

 dominance of the distal region which greatly retards reconstitution at the 

 more proximal levels or, in Haliclystus, completely inhibits it. Similar 



_ CD 



Figs. 25, yl-D.— Reconstitution after oblique section. Fig. 25, A, early, and B, late, stages 

 of distal reconstitution in Cerianthiis solitaries; C, anterior and posterior planarian regenera- 

 tion after oblique section; D, development of both head and posterior end on oblique posterior 

 cut end of short piece. 



differences in rate on an oblique cut end appear in the actinians Ceriantkus 

 solitarius (Child, 1904a) and C. aestuarii (Child, 1908), as indicated in 

 Figure 25, ^ and B. In both species development at the most distal level 

 is more rapid, and that at the proximal level less rapid, than on transverse 

 cut surfaces at corresponding levels. 



On strongly oblique anterior cut ends of planarian pieces head regen- 

 eration is localized on the more anterior part; and the more anterior side 

 of the head, including the eyespot, usually develops more rapidly than 

 the other side (Fig. 25, C, /)). This asymmetry constitutes further evi- 

 dence of difference in rate of head development at different body-levels. A 

 regenerating posterior end is localized on the more posterior part of an 

 oblique posterior cut surface (Fig. 25, C). The posterior end is a subordi- 

 nate part and is determined by levels anterior to it (pp. 339-40). Physio- 

 logical dominance of more anterior levels is undoubtedly concerned in 



