4o8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of the body. These stimuli apparently undergo a decrement in effective- 

 ness with transmission anteriorly; consequently, posterior section beyond 

 a certain distance from the anterior section has no effect on head forma- 

 tion. However, the decrement may be only apparent and is perhaps ac- 

 tually a block rather than a decrement. In pieces from anterior body- 

 levels in which activation and development at the anterior end of a piece 

 is more rapid than farther posteriorly, posterior section must be at a very 

 short distance from the anterior end to be at all effective in inhibiting 

 head development. At more posterior levels of the anterior zooid it is 

 effective over a greater distance, perhaps because less effectively blocked 

 by the less intense anterior activation. This relation to body-level and 

 length of piece is shown in Figure 66 (p. 182) ; it is evidently an expression 

 of differences in condition at different gradient-levels. 



Demonstrative evidence that the physiological factor inhibiting head 

 regeneration is associated with effects of posterior section is provided by 

 the results of different periods of delay of either anterior or posterior sec- 

 tion (Child and Watanabe, 1935a). Pieces of Dugesia dorotocephala, in- 

 cluding the region XP (Fig. 139, A), develop 100 per cent normal heads; 

 but pieces XY , with anterior ends at the same level but only 1/8 or less 

 of the body length of animals 16-20 mm. long, are almost or quite 100 

 per cent acephalic. If anterior section at X is made first, the posterior 

 section at Y later, head frequency increases with increasing delay of the 

 posterior section (Fig. 139, B). Even i or 2 hours' delay is usually suffi- 

 cient to increase head frequency. With 12 hours' delay of posterior section 

 head frequency increases from almost complete acephaly to almost 100 

 per cent normal, and with 24 hours delay it is 100 per cent normal. 



For delay of anterior section pieces AY are cut, and later pieces XY 

 are taken at intervals from successive lots. Figure 140 shows the change 

 in head frequency with delay of anterior section up to 96 hours. There is 

 usually a slight decrease in frequency with 12-24 hours' delay. Appar- 

 ently not merely the posterior cut surface but the cell activity following 

 section plays a part in inhibiting head regeneration. Comparison of Fig- 

 ures 139 and 140 shows that with delay of anterior section the inhibiting 

 effect of posterior section persists over a much longer period than with 

 delay of posterior section. 



Inhibition of head development in relation to length of piece and body- 

 level occurs in different degree in different planarian species." Even in 



" Cf . Child, 1913c, 1914J; SiviCkis, 1923; Buchanan, 1933; Watanabe, 19356; Child and 

 Watanabe, 1935a; Abeloos, 1930. 



